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.
Photo Gallery: Suspension Geometry - Sport Compact Car Magazine
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