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).
Instant Center
Center of gravity
Roll Couple
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.
Relocated ball joints
This Sentra racecar has had the lower ball joint relocated downward to correct the roll center and the camber curve. Chevy parts from Coleman Racing were used to fabricate this with a minimum of expense. This is an example of how these principles can be inexpensively and easily applied on any car.
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.
These front and rear control arms from SPL allow the adjustment of the camber curve and roll center by shimming the location of the spherical bearing downward. The spherical bearing replaces the stock ball joint. These arms are available for the 300ZX, S13/S14, EVO VIII and several other cars.
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.