Part 4: More lessons in geometry
This series began with handling basics, and moved to more difficult subjects involving suspension geometry. This month, there's more of the same. However, unlike previous installments, the suspension fundamentals we discuss this time are less tunable but just as important. Who knew paying attention in geometry class could make your car faster?
Toe steer
Like bump steer, which we discussed in the previous installment, toe steer can adversely affect your car's handling. Toe steer is a product of suspension components of different lengths moving through different arcs at the same time. The resulting changes in toe in the rear suspension can cause unpredictable handling.
Early sport compact suspension designs like the semi-trailing-arm type found in mid-'80s BMWs, Mk I and II Toyota Supras and the Datsun 510 have severe toe steer. Because of this geometry, some semi-trailing arm suspensions toe out under roll, which can cause severe trailing-throttle oversteer. Others, like those found on the Porsche 928 and FC RX-7, use links or bushings of different hardness to force the suspension to passively toe in under roll, which creates understeer at the limit.
These designs work well if they're operating the way the engineers intended them to. It's when speed junkies like us try to out-engineer the engineers that problems develop. This usually happens when lowering the car.
One easy solution to reduce toe change in lowered, semi-trailing-arm suspensions is to increase the spring rate to reduce wheel travel (effectively increasing the overall wheel rate). The less the wheel moves, the less toe can change. Obviously, overstiffening the rear suspension has its own drawbacks, but it worked well on the Datsun 510 and BMW 3 Series, both of which had a long and successful racing heritage.
Many current sport compacts use multilink rear suspensions designed to sweep through complex camber and toe angles to maximize street performance, comfort and stability. However, this combination of camber and toe can conspire against an enthusiast who's willing to accept the compromises of a more aggressive suspension setup.
Aftermarket tuners like Whiteline offer offset bushings for cars like the EVO that relocate the pivot points to reduce toe steer. If kits are unavailable for your application, a racecar fabricator can reposition the trailing arms and other links to a corrected position on severely lowered cars. It's also common to slot control arm mounting points to alter their path of travel and correct toe on lowered cars.
Excessive lowering can make toe steer worse by placing the suspension links in a static position and range of travel they were never designed for. A beam axle suspension with trailing arms, as found in the rear of many small front-wheel-drive cars is a good example of where this might occur.
At stock ride height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car's wheelbase during compression and droop. Since each arm swings equally in different directions, the axle's toe doesn't change because each end of the beam axle is pulled the same distance forward.
If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don't move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer.
Anti-dive and anti-lift
Anti-dive and anti-lift are tricks that can be applied to a car's front suspension geometry to control brake dive and acceleration lift.
Lift and dive can be mitigated by carefully locating suspension pivot points to take advantage of the "force reaction" on the chassis created by acceleration or deceleration. This avoids the need to increase spring rates to reduce pitching-an important issue for ride quality in all street cars.
Anti-dive helps prevent the nose of the car from diving during hard braking. Most cars have some degree of anti-dive designed into the stock suspension geometry. Anti-dive utilizes deceleration forces to increase the front wheel compression rate and reduce brake dive.
By changing the angle of the suspension links, the amount of anti-dive can be manipulated. However, excessive anti-dive can hinder performance by causing the front suspension to stiffen while braking for a corner, which can cause understeer. In extreme amounts, anti-dive geometry will cause wheel hop and caster changes under braking.
Most racecar suspensions have much less anti-dive and anti-lift than street car suspensions. On racecars, the stiff suspension is used to control body motion instead of redirecting braking or acceleration forces. Most kits available for serious wrenchers to alter anti-dive and anti-lift work to reduce these factors.
On front- and all-wheel-drive cars, this same geometry also resists front-end lift under acceleration. Subaru's WRX is notorious for large amounts of anti-dive and anti-lift, causing nonlinear steering response. Bushings reducing anti-dive and anti-lift are designed to give more natural steering response and improve corner-exit traction on WRXs by eliminating these compromises.
Anti-lift geometry greatly affects launch traction for front-wheel-drive drag cars, yet only recently have tube-frame pro class front-wheel- drive drag racers begun to consider using front-end anti-lift geometry.
Whiteline has suspension mounts and bushings to tune the anti-lift and anti-dive out of many popular all-wheel- and front-wheel-drive cars. Kits are available for all WRXs, the Sentra SE-R, many Celicas, the Mazda 323 and the Galant/ Eclipse. A few Japanese tuners offer subframe mounts for the Nissan S13, S14, Z32 300ZX, R32, 33 and 34 Skyline to change the height of the front instant center.
Anti-squatAnti-squat is the exact same geometry applied at the back axle of a rear-wheel-drive car. Unlike the front of the car, small amounts of anti-squat are generally a good thing for a rear-drive car since it allows for softer rear suspensions without the excess squat.
Cars like the Nissan R32 Skyline GT-R, Z32 300ZX and S13 240SX have a great deal of anti-squat in their rear suspension geometry. This makes them transition to on-throttle oversteer very rapidly because anti-squat, like anti-dive, significantly increases the wheel rate. This is why the S13 works so well for drifting.
Extreme anti-squat can cause wheelspin and rear wheel hop under power, which is why the Z32 is notorious for launching poorly at the dragstrip.
Rear-wheel-drive drag racers have made a science of anti-squat tuning to maximize rear-wheel traction. Drag cars often have so much anti-squat geometry that the back of the car actually lifts when launching, driving the tires into the ground. Drag cars have adjustable four-link rear suspensions,so this percentage is tunable for the amount of bite desired in different conditions (see sidebar on page 102 to learn how to calculate anti-lift, anti-dive and anti-squat).
Ackerman angleWhen cornering, the inside and outside wheels have to travel different distances and different arcs. If both wheels are at the same steering angle, then one or both tires would be scrubbing.
Think of Ackerman as dynamic toe-out, which increases toe-out as the steering wheel is turned. This gives the quick turn-in advantage of having toed-out alignment while turning without the handling and tire-wear drawbacks of static toe-out in a straight line.
Just about all cars have Ackerman built into their steering geometry. It is, for all practical purposes, non-adjustable. The Ackerman angle can only be changed by moving the steering rack. A simpler solution is to just dial in some static toe-out, which will multiply the Ackerman effect that's already engineered in the car's steering geometry.
Camber curveOne drawback of independent suspension design is its inability to maintain a consistent contact patch as the car's body rolls in a turn. Suspension engineers counter with designs that gain negative camber under roll, which increases cornering grip.
Multilink and A-arm suspensions are designed with shorter upper links or different rotation points so the upper components sweep in a tighter arc than the lower links. The different arcs make the wheel gain negative camber as the suspension compresses.
However, there can be drawbacks. Too much camber gain can cause side scrub. Side scrub occurs as a control arm sweeps through its range of motion and pulls the tire laterally, which adds further traction load on the tire. This can cause instability over bumps, especially if the bumps only affect one side of the car.
Placing the suspension links for negative camber gain also affects the roll center location. Fortunately, it's easy to find a good compromise between roll center location, negative camber gain and minimal side scrub. Companies like SPL offer adjustable suspension links, which allow for camber curve and roll center location adjustments.
On MacPherson strut-equipped cars, the wheel will gain negative camber under roll as long as the lower control arm is positioned less than 90 degrees relative to the strut axis. Unfortunately, many enthusiasts with MacPherson strut-equipped cars lower their cars too much and make this angle greater than 90 degrees. Beyond 90 degrees, the suspension will gain positive camber instead of negative as it compresses, significantly compromising grip (see illustration below).
To counter this problem there are aftermarket control arms that lower the outer pivot to restore a proper camber curve and roll center on an aggressively lowered, strut-equipped car. SPL sells control arms for the front suspension of the S13 and S14. Several suppliers sell block spacers to place between the lower control arm and ball joint to correct the roll center and camber curve on MacPherson strut-equipped cars.
Corner balancing
Corner balancing is the adjustment of weight distribution at each wheel. Ideally, the cross-weight percentage is the same diagonally between the car's corners. This is done so a car's understeer/oversteer balance is the same in a right- or left-hand corner. Corner weight can be adjusted on any car with a height-adjustable suspension. There are applications for nearly all popular performance cars nowadays. For older or non-mainstream cars, companies like Ground Control sell parts to make any car's coil-over suspension height-adjustable.
Corner weights are set by adjusting the suspension ride height at each corner while the car is on four linked electronic scales. The scales display the weight supported by each wheel. With the driver in the car, the spring perches are adjusted to achieve the desired cross-balance. Raising the perch increases the weight at that corner; lowering decreases it.
Weight also tends to be transferred diagonally across the car when changing perch height. Through trial and error, the weights should be adjusted to be as close to equal as possible from side to side and diagonally.
The complexities of modern suspension design, from anti-lift geometry to camber curves, are fundamentals that must be understood before making effective changes to a vehicle's suspension. For more information on suspension design and tuning, look for the "Making It Stick" series on the Web at www.sportcompactcarweb.com
Next, we'll dive into the mystery of dampers, discussing design, operation and valving strategies that work together to make or break a car's handling.
Calculate anti-dive, anti-lift and anti-squat
1) Find the center of gravityThe calcuated center of gravity (C.G.) usually ends up 15 to 20 inches above ground-a few inches higher than the plane of the crankshaft-in a typical sedan. The horizontal location is approximately a foot or so forward of the middle of the wheelbase on a front-engine, rear-wheel-drive car. On a rear- or mid-engine car, it's a foot or so aft of the middle of the wheelbase. On a front-wheel-drive car it is about even with the driver's seat. Draw a vertical line from the C.G. to the ground.
2) Find the instant center
Calculating anti-lift and anti-dive requires finding the side-view instant center for the front suspension. Anti-squat calculations require finding the side-view instant center for the rear suspension. To find either instant center, draw lines through the suspension pivots of the upper and lower control arms at their attachment points to the chassis. On MacPherson strut cars, draw the upper line from the upper strut mount perpendicular to the axis of the strut. These lines should converge somewhere in the middle of the car between the wheels. This intersection is the side instant center.
3) Find the percentage
Anti-dive, anti-lift and anti-squat are expressed as percentages of the C.G. height. Draw a line from the center of the tire's contact patch up to the instant center intersecting the line from the C.G. to the ground. This line represents the force vector where the acceleration or deceleration force acts on the mass of the car. To calculate the percentage of anti-lift, anti-dive, or anti-squat, compare the overall height of the C.G. to the distance between this intersection point and the ground. For example, if the force vector intersects the line between the ground and C.G. at one-quarter of its height, the suspension has 25 percent anti-dive.
Part 3: It's all in the geometry
In the first two parts of this series we covered relatively basic suspension tuning techniques. Now it's time to bury ourselves in suspension geometry. Making changes on this fundamental level is what racecar and suspension engineers do for a living. But we've found that with the more popular performance cars in this market, there are parts available that will allow you to make these changes.
Roll Center
Roll center is the virtual pivot point in space that a car rotates around when subjected to cornering forces. The roll center is significant because its location determines how a car will handle and what factors must be considered when tuning its suspension.
To find a car's roll center, first locate the "instant centers" of its front and rear suspension. The instant center is the point in space around which the suspension's links rotate. Locating your car's instant centers can be done by measuring its suspension and creating a scale drawing. Measure how high the pivot points are above the ground and know the exact dimensions of the control arms.
To find the instant centers on a car with upper and lower control arms, draw lines from the center of the ball joint through the inner pivots of the upper and lower control arms and extend them inward toward the center of the car until they meet. Now draw a line from the center of the tire's contact patch to the instant center on both sides of the car. The point where these two lines intersect is the roll center.
For a car with a MacPherson strut suspension, the upper line is made by drawing a line 90 degrees from the strut axis, starting at the upper mounting point of the strut (see illustration below).
Roll center affects many critical elements of a car's handling. The most critical are steering input, body roll, balance and mechanical grip.
The center of gravity location (CG) for each end of the car can be found by jacking the car up a known distance on each side while it's on corner scales, and observing the change in corner weights. This data can then be fed into an equation to give you the coordinates of the CG.
Since most people don't have a perfectly flat surface and expensive corner scales, it's usually safe to estimate the CG for the front suspension around crankshaft height in a front-engine car. In the rear, it's usually at the floor of the trunk.
The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center. In a rear- or mid-engine car, these approximations apply to the opposite end of the car.
The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.
The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement.
And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.
On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds.
The mass and roll center locations can be used to predict a car's natural handling characteristics. If the front and rear roll centers are plotted and a line is drawn between them, the line indicates the roll axis of the car. The roll axis is the axis that the car rolls around in a turn.
The mass axis is a line drawn between a car's front and rear centers of gravity, which can be determined using the method discussed above. Mass axis can be roughly plotted by drawing a line through the center of gravity points in the front and rear of the car. Since there isn't already a preexisting engineering term for this axis, we'll call it the Mike axis.
When the roll axis and the Mike axis are plotted next to each other, the distance and slope between the two are useful in determining a car's natural handling tendency.
If the space between the two lines is greater in the front of the car, with an upward sloping Mike Axis, the car will tend to understeer due to greater weight transfer to the outside wheels at that end of the car. Front-engine, front-wheel-drive cars strongly exhibit this trait. Conversely, if space is greater in the rear of the car, with a downward-sloping Mike axis the car will tend to oversteer.
Front-engine, rear-wheel-drive cars will tend to have a Mike axis that slopes up toward the front of the car. Front-wheel-drive cars will usually have a Mike axis that slopes upward at a steeper angle. Rear-engine cars will have a downward-sloping Mike axis. Since the roll axis on a well-designed car tends to slope downward toward the front of the car, it's easy to see why front-heavy cars tend to understeer and rear-engine cars tend to oversteer.
Roll center can be adjusted by using aftermarket control arms with adjustable pivot points on virtually every Nissan Z car ever built, Nissan's S13 and S14, and Toyota's AE86, to name a few. Whiteline and SPL both make this kind of control arm. Or, if you're ambitious, it's not impossible to find a fabricator capable of modifiying control arms to suit your needs.
Remember, if you can adjust roll center, you can reduce the roll couple and lower the center of gravity effectively. This is an effective way to change your car's dynamic balance by reducing roll couple and weight transfer. But most importantly, it's critical to remember that overlowering a car will create more problems than it solves.
Bump Steer
Steering precision and stability-both of which are affected by bump steer-are the next victims of overlowering. Bump steer is steering input created by the suspension moving through its stroke in response to bumps and roll. It's caused by the suspension's control arms moving in different arcs than the steering linkage as the suspension follows its stroke.
It's fairly easy to design a suspension system that doesn't have bump steer. To do so on a double-wishbone-type suspension, the steering tie rods must lie between two vertical lines drawn between the upper and lower control arm's pivots while pointing at the suspension's instant center.
In order for a MacPherson strut suspension to have no bump steer, the tie rods must lie in line with the lower contol arm with the inner tie rod end in plane with the inner pivot of the control arm (see illustration on this page).
However, most production cars have the steering rack placement compromised by packaging constraints so steering tie rod location is often less than optimal. Lower the car and the problem gets worse.
What can you do to reduce bump steer? Many cars have aftermarket parts available to relocate the tie rod ends of the steering linkage. Tie rod ends with spherical bearings and spacers can be tuned to reduce bump steer by placing the tie rods at a more favorable angle.
SPL and Whiteline make bump steer reduction kits for popular cars like the 240SX, 300ZX and EVO. If these parts are not available for your favorite car, they can be easily fabricated.
By learning what effects changes in suspension geometry have on a car's behavior, you can tune and adjust your suspension to work like you want. Understanding these geometry traits and making them adjustable is a powerful tool when trying to eke out the last bit of cornering performance.
If you're a racer, autocrosser, drifter or just a hard-core canyon carver, these tools will give you a significant edge.
Next month the science continues with damper tuning.
Overlowering:Don't do it
Almost everybody does it. Lowering your car is paramount to improving its handling. The key, however, is to lower it just enough to gain the benefits it creates without suffering the potential drawbacks.
The aftermarket does little to help us in this regard. Nearly every company that makes suspension components, even very reputable ones, spews out thousands of sets of lowering springs that are both too low and too soft for optimal handling. Why do they do this? Are the engineers at these companies incompetent? Is it a conspiracy to make our cars suck? No, the enthusiast is to blame.
The majority of enthusiasts want a low ride height to fill the ugly gap in their stock wheel wells. They also won't accept a ride that, for the most part, is a lot harsher than stock. Macho or not, most enthusiasts don't drive hard enough or well enough to realize that their cars actually handle worse than stock, mistaking reduced roll for better handling.
The original Nissan Sentra SE-R is a typical example of a car with suspension geometry that doesn't allow lowering more than an inch. But the problem isn't limited to the SE-R.
The first problem with lowering the SE-R is that it only has about 2 inches of compression travel at the stock ride height in the front suspension. Let's say you lower the car the typical 1.5 inches. That leaves a half inch of travel before you hit the bump stops. Your typical aftermarket lowering spring might only up the spring rate a paltry 20 percent or so, which isn't nearly enough to keep the car off the bump stops with only a half inch of travel.
The result is poor ride quality and sub-standard handling. As the car leans in a corner, the suspension will settle onto the bump stop. As the bump stop compresses, the spring rate ramps up infinitely, which causes massive weight transfer and relentless understeer.
But it gets better. On the SE-R, the lower control arms are positioned so they begin to point upward as the car is lowered. Now when the car rolls in a corner, the outside tire goes into positive camber. And, if you've been reading this series, you know that is just about the least effective way to corner.
Believe it or not, it gets worse. With the lower control arms pointing upward, the instant center starts to drop rapidly and the roll couple greatly increases. The bigger roll couple causes more weight to transfer to the outside wheels and more body roll.
Finally, the steering tie rods start to point upward more radically because they are shorter than the lower control arm and positioned out of place in the lowered chassis. This causes toe-out when the wheels deflect, making the steering twitchy and the car feel unstable.
The SE-R exhibits just about every problem overlowering can cause and when combined, those problems will ruin its handling. Fortunately, SE-R guys tend to be pretty hard-core and they have taken the issue of making functional drop-in lowering springs into their own hands. This isn't the case with every car. Even worse, this situation is not unique to the Sentra. There are lowering springs available that are capable of causing these or similar problems on just about any car.
What can you do to work with the drawbacks of overlowering or avoid it completely?
Make sure your car doesn't use the bump stops under maximum cornering load. The easy way to detect this problem is with a zip-tie telltale on the shock shaft. If the zip-tie is pushed up flush or into the bump stop after a hard turn, then your car is using the bump stops every time you corner hard.
If you must run low, do it racecar style. Get short-bodied high-end coil-over shocks or struts with higher rate springs. Independently adjustable ride height and spring preload are also critical. Suspension components with these features are designed to work at low ride heights. Many popular performance cars have kits to adjust and correct roll centers, camber curves and bump steer.
If you can't get a decent rate drop-in spring for your car, Ground Control makes kits for many cars allowing the use of Eibach 2.5-inch ERS racing springs, which come in nearly an infinite selection of rates and lengths. With Ground Control's threaded spring perches, you can also adjust the ride height.
If you can't do this, run short, soft progressive microcellular urethane bump stops so the wheel rate will ramp up gradually if the bump stops are used. Koni makes excellent bump stops.
If you have a MacPherson strut suspension, be especially aware of short travel and suspension geometry problems. MacPherson strut cars usually have a very small lowering window. It is typically best to run these cars at close to the stock ride height unless you significantly modify many other components.
Part 4: More lessons in geometry
This series began with handling basics, and moved to more difficult subjects involving suspension geometry. This month, there's more of the same. However, unlike previous installments, the suspension fundamentals we discuss this time are less tunable but just as important. Who knew paying attention in geometry class could make your car faster?
Toe steer
Like bump steer, which we discussed in the previous installment, toe steer can adversely affect your car's handling. Toe steer is a product of suspension components of different lengths moving through different arcs at the same time. The resulting changes in toe in the rear suspension can cause unpredictable handling.
Early sport compact suspension designs like the semi-trailing-arm type found in mid-'80s BMWs, Mk I and II Toyota Supras and the Datsun 510 have severe toe steer. Because of this geometry, some semi-trailing arm suspensions toe out under roll, which can cause severe trailing-throttle oversteer. Others, like those found on the Porsche 928 and FC RX-7, use links or bushings of different hardness to force the suspension to passively toe in under roll, which creates understeer at the limit.
These designs work well if they're operating the way the engineers intended them to. It's when speed junkies like us try to out-engineer the engineers that problems develop. This usually happens when lowering the car.
One easy solution to reduce toe change in lowered, semi-trailing-arm suspensions is to increase the spring rate to reduce wheel travel (effectively increasing the overall wheel rate). The less the wheel moves, the less toe can change. Obviously, overstiffening the rear suspension has its own drawbacks, but it worked well on the Datsun 510 and BMW 3 Series, both of which had a long and successful racing heritage.
Many current sport compacts use multilink rear suspensions designed to sweep through complex camber and toe angles to maximize street performance, comfort and stability. However, this combination of camber and toe can conspire against an enthusiast who's willing to accept the compromises of a more aggressive suspension setup.
Aftermarket tuners like Whiteline offer offset bushings for cars like the EVO that relocate the pivot points to reduce toe steer. If kits are unavailable for your application, a racecar fabricator can reposition the trailing arms and other links to a corrected position on severely lowered cars. It's also common to slot control arm mounting points to alter their path of travel and correct toe on lowered cars.
Excessive lowering can make toe steer worse by placing the suspension links in a static position and range of travel they were never designed for. A beam axle suspension with trailing arms, as found in the rear of many small front-wheel-drive cars is a good example of where this might occur.
At stock ride height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car's wheelbase during compression and droop. Since each arm swings equally in different directions, the axle's toe doesn't change because each end of the beam axle is pulled the same distance forward.
If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don't move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer.
Anti-dive and anti-lift
Anti-dive and anti-lift are tricks that can be applied to a car's front suspension geometry to control brake dive and acceleration lift.
Lift and dive can be mitigated by carefully locating suspension pivot points to take advantage of the "force reaction" on the chassis created by acceleration or deceleration. This avoids the need to increase spring rates to reduce pitching-an important issue for ride quality in all street cars.
Anti-dive helps prevent the nose of the car from diving during hard braking. Most cars have some degree of anti-dive designed into the stock suspension geometry. Anti-dive utilizes deceleration forces to increase the front wheel compression rate and reduce brake dive.
By changing the angle of the suspension links, the amount of anti-dive can be manipulated. However, excessive anti-dive can hinder performance by causing the front suspension to stiffen while braking for a corner, which can cause understeer. In extreme amounts, anti-dive geometry will cause wheel hop and caster changes under braking.
Most racecar suspensions have much less anti-dive and anti-lift than street car suspensions. On racecars, the stiff suspension is used to control body motion instead of redirecting braking or acceleration forces. Most kits available for serious wrenchers to alter anti-dive and anti-lift work to reduce these factors.
On front- and all-wheel-drive cars, this same geometry also resists front-end lift under acceleration. Subaru's WRX is notorious for large amounts of anti-dive and anti-lift, causing nonlinear steering response. Bushings reducing anti-dive and anti-lift are designed to give more natural steering response and improve corner-exit traction on WRXs by eliminating these compromises.
Anti-lift geometry greatly affects launch traction for front-wheel-drive drag cars, yet only recently have tube-frame pro class front-wheel- drive drag racers begun to consider using front-end anti-lift geometry.
Whiteline has suspension mounts and bushings to tune the anti-lift and anti-dive out of many popular all-wheel- and front-wheel-drive cars. Kits are available for all WRXs, the Sentra SE-R, many Celicas, the Mazda 323 and the Galant/ Eclipse. A few Japanese tuners offer subframe mounts for the Nissan S13, S14, Z32 300ZX, R32, 33 and 34 Skyline to change the height of the front instant center.
Anti-squatAnti-squat is the exact same geometry applied at the back axle of a rear-wheel-drive car. Unlike the front of the car, small amounts of anti-squat are generally a good thing for a rear-drive car since it allows for softer rear suspensions without the excess squat.
Cars like the Nissan R32 Skyline GT-R, Z32 300ZX and S13 240SX have a great deal of anti-squat in their rear suspension geometry. This makes them transition to on-throttle oversteer very rapidly because anti-squat, like anti-dive, significantly increases the wheel rate. This is why the S13 works so well for drifting.
Extreme anti-squat can cause wheelspin and rear wheel hop under power, which is why the Z32 is notorious for launching poorly at the dragstrip.
Rear-wheel-drive drag racers have made a science of anti-squat tuning to maximize rear-wheel traction. Drag cars often have so much anti-squat geometry that the back of the car actually lifts when launching, driving the tires into the ground. Drag cars have adjustable four-link rear suspensions,so this percentage is tunable for the amount of bite desired in different conditions (see sidebar on page 102 to learn how to calculate anti-lift, anti-dive and anti-squat).
Ackerman angleWhen cornering, the inside and outside wheels have to travel different distances and different arcs. If both wheels are at the same steering angle, then one or both tires would be scrubbing.
Think of Ackerman as dynamic toe-out, which increases toe-out as the steering wheel is turned. This gives the quick turn-in advantage of having toed-out alignment while turning without the handling and tire-wear drawbacks of static toe-out in a straight line.
Just about all cars have Ackerman built into their steering geometry. It is, for all practical purposes, non-adjustable. The Ackerman angle can only be changed by moving the steering rack. A simpler solution is to just dial in some static toe-out, which will multiply the Ackerman effect that's already engineered in the car's steering geometry.
Camber curveOne drawback of independent suspension design is its inability to maintain a consistent contact patch as the car's body rolls in a turn. Suspension engineers counter with designs that gain negative camber under roll, which increases cornering grip.
Multilink and A-arm suspensions are designed with shorter upper links or different rotation points so the upper components sweep in a tighter arc than the lower links. The different arcs make the wheel gain negative camber as the suspension compresses.
However, there can be drawbacks. Too much camber gain can cause side scrub. Side scrub occurs as a control arm sweeps through its range of motion and pulls the tire laterally, which adds further traction load on the tire. This can cause instability over bumps, especially if the bumps only affect one side of the car.
Placing the suspension links for negative camber gain also affects the roll center location. Fortunately, it's easy to find a good compromise between roll center location, negative camber gain and minimal side scrub. Companies like SPL offer adjustable suspension links, which allow for camber curve and roll center location adjustments.
On MacPherson strut-equipped cars, the wheel will gain negative camber under roll as long as the lower control arm is positioned less than 90 degrees relative to the strut axis. Unfortunately, many enthusiasts with MacPherson strut-equipped cars lower their cars too much and make this angle greater than 90 degrees. Beyond 90 degrees, the suspension will gain positive camber instead of negative as it compresses, significantly compromising grip (see illustration below).
To counter this problem there are aftermarket control arms that lower the outer pivot to restore a proper camber curve and roll center on an aggressively lowered, strut-equipped car. SPL sells control arms for the front suspension of the S13 and S14. Several suppliers sell block spacers to place between the lower control arm and ball joint to correct the roll center and camber curve on MacPherson strut-equipped cars.
Corner balancing
Corner balancing is the adjustment of weight distribution at each wheel. Ideally, the cross-weight percentage is the same diagonally between the car's corners. This is done so a car's understeer/oversteer balance is the same in a right- or left-hand corner. Corner weight can be adjusted on any car with a height-adjustable suspension. There are applications for nearly all popular performance cars nowadays. For older or non-mainstream cars, companies like Ground Control sell parts to make any car's coil-over suspension height-adjustable.
Corner weights are set by adjusting the suspension ride height at each corner while the car is on four linked electronic scales. The scales display the weight supported by each wheel. With the driver in the car, the spring perches are adjusted to achieve the desired cross-balance. Raising the perch increases the weight at that corner; lowering decreases it.
Weight also tends to be transferred diagonally across the car when changing perch height. Through trial and error, the weights should be adjusted to be as close to equal as possible from side to side and diagonally.
The complexities of modern suspension design, from anti-lift geometry to camber curves, are fundamentals that must be understood before making effective changes to a vehicle's suspension. For more information on suspension design and tuning, look for the "Making It Stick" series on the Web at www.sportcompactcarweb.com
Next, we'll dive into the mystery of dampers, discussing design, operation and valving strategies that work together to make or break a car's handling.
Calculate anti-dive, anti-lift and anti-squat
1) Find the center of gravityThe calcuated center of gravity (C.G.) usually ends up 15 to 20 inches above ground-a few inches higher than the plane of the crankshaft-in a typical sedan. The horizontal location is approximately a foot or so forward of the middle of the wheelbase on a front-engine, rear-wheel-drive car. On a rear- or mid-engine car, it's a foot or so aft of the middle of the wheelbase. On a front-wheel-drive car it is about even with the driver's seat. Draw a vertical line from the C.G. to the ground.
2) Find the instant center
Calculating anti-lift and anti-dive requires finding the side-view instant center for the front suspension. Anti-squat calculations require finding the side-view instant center for the rear suspension. To find either instant center, draw lines through the suspension pivots of the upper and lower control arms at their attachment points to the chassis. On MacPherson strut cars, draw the upper line from the upper strut mount perpendicular to the axis of the strut. These lines should converge somewhere in the middle of the car between the wheels. This intersection is the side instant center.
3) Find the percentage
Anti-dive, anti-lift and anti-squat are expressed as percentages of the C.G. height. Draw a line from the center of the tire's contact patch up to the instant center intersecting the line from the C.G. to the ground. This line represents the force vector where the acceleration or deceleration force acts on the mass of the car. To calculate the percentage of anti-lift, anti-dive, or anti-squat, compare the overall height of the C.G. to the distance between this intersection point and the ground. For example, if the force vector intersects the line between the ground and C.G. at one-quarter of its height, the suspension has 25 percent anti-dive.
Part 4: More lessons in geometry
This series began with handling basics, and moved to more difficult subjects involving suspension geometry. This month, there's more of the same. However, unlike previous installments, the suspension fundamentals we discuss this time are less tunable but just as important. Who knew paying attention in geometry class could make your car faster?
Toe steer
Like bump steer, which we discussed in the previous installment, toe steer can adversely affect your car's handling. Toe steer is a product of suspension components of different lengths moving through different arcs at the same time. The resulting changes in toe in the rear suspension can cause unpredictable handling.
Early sport compact suspension designs like the semi-trailing-arm type found in mid-'80s BMWs, Mk I and II Toyota Supras and the Datsun 510 have severe toe steer. Because of this geometry, some semi-trailing arm suspensions toe out under roll, which can cause severe trailing-throttle oversteer. Others, like those found on the Porsche 928 and FC RX-7, use links or bushings of different hardness to force the suspension to passively toe in under roll, which creates understeer at the limit.
These designs work well if they're operating the way the engineers intended them to. It's when speed junkies like us try to out-engineer the engineers that problems develop. This usually happens when lowering the car.
One easy solution to reduce toe change in lowered, semi-trailing-arm suspensions is to increase the spring rate to reduce wheel travel (effectively increasing the overall wheel rate). The less the wheel moves, the less toe can change. Obviously, overstiffening the rear suspension has its own drawbacks, but it worked well on the Datsun 510 and BMW 3 Series, both of which had a long and successful racing heritage.
Many current sport compacts use multilink rear suspensions designed to sweep through complex camber and toe angles to maximize street performance, comfort and stability. However, this combination of camber and toe can conspire against an enthusiast who's willing to accept the compromises of a more aggressive suspension setup.
Aftermarket tuners like Whiteline offer offset bushings for cars like the EVO that relocate the pivot points to reduce toe steer. If kits are unavailable for your application, a racecar fabricator can reposition the trailing arms and other links to a corrected position on severely lowered cars. It's also common to slot control arm mounting points to alter their path of travel and correct toe on lowered cars.
Excessive lowering can make toe steer worse by placing the suspension links in a static position and range of travel they were never designed for. A beam axle suspension with trailing arms, as found in the rear of many small front-wheel-drive cars is a good example of where this might occur.
At stock ride height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car's wheelbase during compression and droop. Since each arm swings equally in different directions, the axle's toe doesn't change because each end of the beam axle is pulled the same distance forward.
If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don't move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer.
Anti-dive and anti-lift
Anti-dive and anti-lift are tricks that can be applied to a car's front suspension geometry to control brake dive and acceleration lift.
Lift and dive can be mitigated by carefully locating suspension pivot points to take advantage of the "force reaction" on the chassis created by acceleration or deceleration. This avoids the need to increase spring rates to reduce pitching-an important issue for ride quality in all street cars.
Anti-dive helps prevent the nose of the car from diving during hard braking. Most cars have some degree of anti-dive designed into the stock suspension geometry. Anti-dive utilizes deceleration forces to increase the front wheel compression rate and reduce brake dive.
By changing the angle of the suspension links, the amount of anti-dive can be manipulated. However, excessive anti-dive can hinder performance by causing the front suspension to stiffen while braking for a corner, which can cause understeer. In extreme amounts, anti-dive geometry will cause wheel hop and caster changes under braking.
Most racecar suspensions have much less anti-dive and anti-lift than street car suspensions. On racecars, the stiff suspension is used to control body motion instead of redirecting braking or acceleration forces. Most kits available for serious wrenchers to alter anti-dive and anti-lift work to reduce these factors.
On front- and all-wheel-drive cars, this same geometry also resists front-end lift under acceleration. Subaru's WRX is notorious for large amounts of anti-dive and anti-lift, causing nonlinear steering response. Bushings reducing anti-dive and anti-lift are designed to give more natural steering response and improve corner-exit traction on WRXs by eliminating these compromises.
Anti-lift geometry greatly affects launch traction for front-wheel-drive drag cars, yet only recently have tube-frame pro class front-wheel- drive drag racers begun to consider using front-end anti-lift geometry.
Whiteline has suspension mounts and bushings to tune the anti-lift and anti-dive out of many popular all-wheel- and front-wheel-drive cars. Kits are available for all WRXs, the Sentra SE-R, many Celicas, the Mazda 323 and the Galant/ Eclipse. A few Japanese tuners offer subframe mounts for the Nissan S13, S14, Z32 300ZX, R32, 33 and 34 Skyline to change the height of the front instant center.
Anti-squatAnti-squat is the exact same geometry applied at the back axle of a rear-wheel-drive car. Unlike the front of the car, small amounts of anti-squat are generally a good thing for a rear-drive car since it allows for softer rear suspensions without the excess squat.
Cars like the Nissan R32 Skyline GT-R, Z32 300ZX and S13 240SX have a great deal of anti-squat in their rear suspension geometry. This makes them transition to on-throttle oversteer very rapidly because anti-squat, like anti-dive, significantly increases the wheel rate. This is why the S13 works so well for drifting.
Extreme anti-squat can cause wheelspin and rear wheel hop under power, which is why the Z32 is notorious for launching poorly at the dragstrip.
Rear-wheel-drive drag racers have made a science of anti-squat tuning to maximize rear-wheel traction. Drag cars often have so much anti-squat geometry that the back of the car actually lifts when launching, driving the tires into the ground. Drag cars have adjustable four-link rear suspensions,so this percentage is tunable for the amount of bite desired in different conditions (see sidebar on page 102 to learn how to calculate anti-lift, anti-dive and anti-squat).
Ackerman angleWhen cornering, the inside and outside wheels have to travel different distances and different arcs. If both wheels are at the same steering angle, then one or both tires would be scrubbing.
Think of Ackerman as dynamic toe-out, which increases toe-out as the steering wheel is turned. This gives the quick turn-in advantage of having toed-out alignment while turning without the handling and tire-wear drawbacks of static toe-out in a straight line.
Just about all cars have Ackerman built into their steering geometry. It is, for all practical purposes, non-adjustable. The Ackerman angle can only be changed by moving the steering rack. A simpler solution is to just dial in some static toe-out, which will multiply the Ackerman effect that's already engineered in the car's steering geometry.
Camber curveOne drawback of independent suspension design is its inability to maintain a consistent contact patch as the car's body rolls in a turn. Suspension engineers counter with designs that gain negative camber under roll, which increases cornering grip.
Multilink and A-arm suspensions are designed with shorter upper links or different rotation points so the upper components sweep in a tighter arc than the lower links. The different arcs make the wheel gain negative camber as the suspension compresses.
However, there can be drawbacks. Too much camber gain can cause side scrub. Side scrub occurs as a control arm sweeps through its range of motion and pulls the tire laterally, which adds further traction load on the tire. This can cause instability over bumps, especially if the bumps only affect one side of the car.
Placing the suspension links for negative camber gain also affects the roll center location. Fortunately, it's easy to find a good compromise between roll center location, negative camber gain and minimal side scrub. Companies like SPL offer adjustable suspension links, which allow for camber curve and roll center location adjustments.
On MacPherson strut-equipped cars, the wheel will gain negative camber under roll as long as the lower control arm is positioned less than 90 degrees relative to the strut axis. Unfortunately, many enthusiasts with MacPherson strut-equipped cars lower their cars too much and make this angle greater than 90 degrees. Beyond 90 degrees, the suspension will gain positive camber instead of negative as it compresses, significantly compromising grip (see illustration below).
To counter this problem there are aftermarket control arms that lower the outer pivot to restore a proper camber curve and roll center on an aggressively lowered, strut-equipped car. SPL sells control arms for the front suspension of the S13 and S14. Several suppliers sell block spacers to place between the lower control arm and ball joint to correct the roll center and camber curve on MacPherson strut-equipped cars.
Corner balancing
Corner balancing is the adjustment of weight distribution at each wheel. Ideally, the cross-weight percentage is the same diagonally between the car's corners. This is done so a car's understeer/oversteer balance is the same in a right- or left-hand corner. Corner weight can be adjusted on any car with a height-adjustable suspension. There are applications for nearly all popular performance cars nowadays. For older or non-mainstream cars, companies like Ground Control sell parts to make any car's coil-over suspension height-adjustable.
Corner weights are set by adjusting the suspension ride height at each corner while the car is on four linked electronic scales. The scales display the weight supported by each wheel. With the driver in the car, the spring perches are adjusted to achieve the desired cross-balance. Raising the perch increases the weight at that corner; lowering decreases it.
Weight also tends to be transferred diagonally across the car when changing perch height. Through trial and error, the weights should be adjusted to be as close to equal as possible from side to side and diagonally.
The complexities of modern suspension design, from anti-lift geometry to camber curves, are fundamentals that must be understood before making effective changes to a vehicle's suspension. For more information on suspension design and tuning, look for the "Making It Stick" series on the Web at www.sportcompactcarweb.com
Next, we'll dive into the mystery of dampers, discussing design, operation and valving strategies that work together to make or break a car's handling.
Calculate anti-dive, anti-lift and anti-squat
1) Find the center of gravityThe calcuated center of gravity (C.G.) usually ends up 15 to 20 inches above ground-a few inches higher than the plane of the crankshaft-in a typical sedan. The horizontal location is approximately a foot or so forward of the middle of the wheelbase on a front-engine, rear-wheel-drive car. On a rear- or mid-engine car, it's a foot or so aft of the middle of the wheelbase. On a front-wheel-drive car it is about even with the driver's seat. Draw a vertical line from the C.G. to the ground.
2) Find the instant center
Calculating anti-lift and anti-dive requires finding the side-view instant center for the front suspension. Anti-squat calculations require finding the side-view instant center for the rear suspension. To find either instant center, draw lines through the suspension pivots of the upper and lower control arms at their attachment points to the chassis. On MacPherson strut cars, draw the upper line from the upper strut mount perpendicular to the axis of the strut. These lines should converge somewhere in the middle of the car between the wheels. This intersection is the side instant center.
3) Find the percentage
Anti-dive, anti-lift and anti-squat are expressed as percentages of the C.G. height. Draw a line from the center of the tire's contact patch up to the instant center intersecting the line from the C.G. to the ground. This line represents the force vector where the acceleration or deceleration force acts on the mass of the car. To calculate the percentage of anti-lift, anti-dive, or anti-squat, compare the overall height of the C.G. to the distance between this intersection point and the ground. For example, if the force vector intersects the line between the ground and C.G. at one-quarter of its height, the suspension has 25 percent anti-dive.
Part 3: It's all in the geometry
In the first two parts of this series we covered relatively basic suspension tuning techniques. Now it's time to bury ourselves in suspension geometry. Making changes on this fundamental level is what racecar and suspension engineers do for a living. But we've found that with the more popular performance cars in this market, there are parts available that will allow you to make these changes.
Roll Center
Roll center is the virtual pivot point in space that a car rotates around when subjected to cornering forces. The roll center is significant because its location determines how a car will handle and what factors must be considered when tuning its suspension.
To find a car's roll center, first locate the "instant centers" of its front and rear suspension. The instant center is the point in space around which the suspension's links rotate. Locating your car's instant centers can be done by measuring its suspension and creating a scale drawing. Measure how high the pivot points are above the ground and know the exact dimensions of the control arms.
To find the instant centers on a car with upper and lower control arms, draw lines from the center of the ball joint through the inner pivots of the upper and lower control arms and extend them inward toward the center of the car until they meet. Now draw a line from the center of the tire's contact patch to the instant center on both sides of the car. The point where these two lines intersect is the roll center.
For a car with a MacPherson strut suspension, the upper line is made by drawing a line 90 degrees from the strut axis, starting at the upper mounting point of the strut (see illustration below).
Roll center affects many critical elements of a car's handling. The most critical are steering input, body roll, balance and mechanical grip.
The center of gravity location (CG) for each end of the car can be found by jacking the car up a known distance on each side while it's on corner scales, and observing the change in corner weights. This data can then be fed into an equation to give you the coordinates of the CG.
Since most people don't have a perfectly flat surface and expensive corner scales, it's usually safe to estimate the CG for the front suspension around crankshaft height in a front-engine car. In the rear, it's usually at the floor of the trunk.
The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center. In a rear- or mid-engine car, these approximations apply to the opposite end of the car.
The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.
The often-overlooked disadvantage to lowering is that roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement.
And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.
On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds.
The mass and roll center locations can be used to predict a car's natural handling characteristics. If the front and rear roll centers are plotted and a line is drawn between them, the line indicates the roll axis of the car. The roll axis is the axis that the car rolls around in a turn.
The mass axis is a line drawn between a car's front and rear centers of gravity, which can be determined using the method discussed above. Mass axis can be roughly plotted by drawing a line through the center of gravity points in the front and rear of the car. Since there isn't already a preexisting engineering term for this axis, we'll call it the Mike axis.
When the roll axis and the Mike axis are plotted next to each other, the distance and slope between the two are useful in determining a car's natural handling tendency.
If the space between the two lines is greater in the front of the car, with an upward sloping Mike Axis, the car will tend to understeer due to greater weight transfer to the outside wheels at that end of the car. Front-engine, front-wheel-drive cars strongly exhibit this trait. Conversely, if space is greater in the rear of the car, with a downward-sloping Mike axis the car will tend to oversteer.
Front-engine, rear-wheel-drive cars will tend to have a Mike axis that slopes up toward the front of the car. Front-wheel-drive cars will usually have a Mike axis that slopes upward at a steeper angle. Rear-engine cars will have a downward-sloping Mike axis. Since the roll axis on a well-designed car tends to slope downward toward the front of the car, it's easy to see why front-heavy cars tend to understeer and rear-engine cars tend to oversteer.
Roll center can be adjusted by using aftermarket control arms with adjustable pivot points on virtually every Nissan Z car ever built, Nissan's S13 and S14, and Toyota's AE86, to name a few. Whiteline and SPL both make this kind of control arm. Or, if you're ambitious, it's not impossible to find a fabricator capable of modifiying control arms to suit your needs.
Remember, if you can adjust roll center, you can reduce the roll couple and lower the center of gravity effectively. This is an effective way to change your car's dynamic balance by reducing roll couple and weight transfer. But most importantly, it's critical to remember that overlowering a car will create more problems than it solves.
Bump Steer
Steering precision and stability-both of which are affected by bump steer-are the next victims of overlowering. Bump steer is steering input created by the suspension moving through its stroke in response to bumps and roll. It's caused by the suspension's control arms moving in different arcs than the steering linkage as the suspension follows its stroke.
It's fairly easy to design a suspension system that doesn't have bump steer. To do so on a double-wishbone-type suspension, the steering tie rods must lie between two vertical lines drawn between the upper and lower control arm's pivots while pointing at the suspension's instant center.
In order for a MacPherson strut suspension to have no bump steer, the tie rods must lie in line with the lower contol arm with the inner tie rod end in plane with the inner pivot of the control arm (see illustration on this page).
However, most production cars have the steering rack placement compromised by packaging constraints so steering tie rod location is often less than optimal. Lower the car and the problem gets worse.
What can you do to reduce bump steer? Many cars have aftermarket parts available to relocate the tie rod ends of the steering linkage. Tie rod ends with spherical bearings and spacers can be tuned to reduce bump steer by placing the tie rods at a more favorable angle.
SPL and Whiteline make bump steer reduction kits for popular cars like the 240SX, 300ZX and EVO. If these parts are not available for your favorite car, they can be easily fabricated.
By learning what effects changes in suspension geometry have on a car's behavior, you can tune and adjust your suspension to work like you want. Understanding these geometry traits and making them adjustable is a powerful tool when trying to eke out the last bit of cornering performance.
If you're a racer, autocrosser, drifter or just a hard-core canyon carver, these tools will give you a significant edge.
Next month the science continues with damper tuning.
Overlowering:Don't do it
Almost everybody does it. Lowering your car is paramount to improving its handling. The key, however, is to lower it just enough to gain the benefits it creates without suffering the potential drawbacks.
The aftermarket does little to help us in this regard. Nearly every company that makes suspension components, even very reputable ones, spews out thousands of sets of lowering springs that are both too low and too soft for optimal handling. Why do they do this? Are the engineers at these companies incompetent? Is it a conspiracy to make our cars suck? No, the enthusiast is to blame.
The majority of enthusiasts want a low ride height to fill the ugly gap in their stock wheel wells. They also won't accept a ride that, for the most part, is a lot harsher than stock. Macho or not, most enthusiasts don't drive hard enough or well enough to realize that their cars actually handle worse than stock, mistaking reduced roll for better handling.
The original Nissan Sentra SE-R is a typical example of a car with suspension geometry that doesn't allow lowering more than an inch. But the problem isn't limited to the SE-R.
The first problem with lowering the SE-R is that it only has about 2 inches of compression travel at the stock ride height in the front suspension. Let's say you lower the car the typical 1.5 inches. That leaves a half inch of travel before you hit the bump stops. Your typical aftermarket lowering spring might only up the spring rate a paltry 20 percent or so, which isn't nearly enough to keep the car off the bump stops with only a half inch of travel.
The result is poor ride quality and sub-standard handling. As the car leans in a corner, the suspension will settle onto the bump stop. As the bump stop compresses, the spring rate ramps up infinitely, which causes massive weight transfer and relentless understeer.
But it gets better. On the SE-R, the lower control arms are positioned so they begin to point upward as the car is lowered. Now when the car rolls in a corner, the outside tire goes into positive camber. And, if you've been reading this series, you know that is just about the least effective way to corner.
Believe it or not, it gets worse. With the lower control arms pointing upward, the instant center starts to drop rapidly and the roll couple greatly increases. The bigger roll couple causes more weight to transfer to the outside wheels and more body roll.
Finally, the steering tie rods start to point upward more radically because they are shorter than the lower control arm and positioned out of place in the lowered chassis. This causes toe-out when the wheels deflect, making the steering twitchy and the car feel unstable.
The SE-R exhibits just about every problem overlowering can cause and when combined, those problems will ruin its handling. Fortunately, SE-R guys tend to be pretty hard-core and they have taken the issue of making functional drop-in lowering springs into their own hands. This isn't the case with every car. Even worse, this situation is not unique to the Sentra. There are lowering springs available that are capable of causing these or similar problems on just about any car.
What can you do to work with the drawbacks of overlowering or avoid it completely?
Make sure your car doesn't use the bump stops under maximum cornering load. The easy way to detect this problem is with a zip-tie telltale on the shock shaft. If the zip-tie is pushed up flush or into the bump stop after a hard turn, then your car is using the bump stops every time you corner hard.
If you must run low, do it racecar style. Get short-bodied high-end coil-over shocks or struts with higher rate springs. Independently adjustable ride height and spring preload are also critical. Suspension components with these features are designed to work at low ride heights. Many popular performance cars have kits to adjust and correct roll centers, camber curves and bump steer.
If you can't get a decent rate drop-in spring for your car, Ground Control makes kits for many cars allowing the use of Eibach 2.5-inch ERS racing springs, which come in nearly an infinite selection of rates and lengths. With Ground Control's threaded spring perches, you can also adjust the ride height.
If you can't do this, run short, soft progressive microcellular urethane bump stops so the wheel rate will ramp up gradually if the bump stops are used. Koni makes excellent bump stops.
If you have a MacPherson strut suspension, be especially aware of short travel and suspension geometry problems. MacPherson strut cars usually have a very small lowering window. It is typically best to run these cars at close to the stock ride height unless you significantly modify many other components.
