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.
Photo Gallery: Suspension Tech - Sport Compact Car Magazine
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