Suspension Glossary
Geometry: the virtual connection points where the frame, control arms, spindles, and tie rods intersect.
Packaging: the real-world implementation of the geometry. Packaging always requires geometry compromises: a simple example is that the engine limits the length of the control arms.
Bump: when the tire is moving upwards towards the body of the car.Rebound is the opposite of bump.
Driver feedback: describes how a suspension responds when the driver turns, accelerates or stops. Predictable driver feedback is more important than great geometry. If the car responds in a comprehensible way, a good driver can learn how to make it go fast.
Roll center: a key element of suspension predictability. It is the pivot point where the body leans over in a turn. A low roll center allows the body to roll easily and one that is high will inhibit body roll. A roll center that is too high can cause the car to roll suddenly near the traction limit of the tires. Not good. Modern high performance designs use a roll center somewhere around 2 or 3 inches off the ground.
Dynamic roll center: how the roll center moves relative to the body as it leans over. A well-designed suspension will have the roll center at apex be within a fraction of an inch of the roll center at the entrance and exit of a turn.
Side scrub: where the tire wants to move sideways in relation to the car. It is caused by camber gain. Modern radial tires allow more camber gain because they are more tolerant of side scrub, but excessive side scrub can make a car feel very unstable.
Camber gain: refers to the ability of the top of the tire to "tip-in" as the body rolls toward it. The effect is to keep the tire flatter to the road surface during body roll. This may be the biggest difference between older and new suspension designs. Nevertheless, too much camber gain causes side scrub and it reduces the tire contact patch and the brake's effectiveness during hard braking (since the suspension is compressing as the car noses down). A good design rule of thumb for camber gain is to limit it to 1 or 2 degrees per inch of bump.
Bumpsteer: when suspension movement, usually bump, causes steering movement. Many musclecar suspension designs (Gen I F-bodies being particularly notorious) came from the factory with bumpsteer. It takes careful consideration of the tie-rod and steering arm geometry to minimize bumpsteer (no design can remove it entirely). An acceptable upper limit is about 0.02 inch of tie rod movement per 1 inch of suspension movement. Sweet spot: where the geometry of a suspension is at its most ideal. Most suspensions have about 4 inches of sweet spot: 2.5 inches for bump and 1.5 inches for rebound from ride height.
Scrub radius: the difference between the center of the contact patch and the actual swivel point of the spindle. Minimizing scrub radius by using highly backspaced wheels allows longer control arms to be used. A small scrub radius also reduces steering effort and resultant driver fatigue. A barometer for scrub radius is that it should never be more than 1 inch plus half the rim width, i.e. an 8-inch rim should have a scrub radius less than 5 inches.
Ride height: the position of the body relative to the tires. A common way to improve a car's handling (and looks) is to lower the ride height. This achieves a lower center of gravity (always good) with stiffer springs (usually good). Lowering the car too much can cause problems since most suspensions are designed for 2 to 2.5 inches of bump travel. If you lower your car 1.5 inches, you may only have an inch of reasonable bump geometry left before undesirable side effects occur.
Motion ratio: the ratio of the shock/spring movement to the wheel movement. If the spring were mounted in the same place as the ball joint, the spring would move the same amount as the wheel, having a motion ratio of 1.0. As the spring is brought to the inside of the control arm for packaging reasons, its stiffness has to increase to compensate for the leverage loss. Typical motion ratios are 0.65 to 0.75. That's why front springs are usually a lot stiffer than back springs, since live rear ends have a motion ratio near 1.
Ackerman angle: the difference in turning radius between the inside and outside front tires in a turn.
 The caliper has just enough...  The caliper has just enough clearance for the bolts holding the three-piece wheels together. Wilwood's tech support and huge product line allowed me to pick a rotor and hat to make the fitment possible. |
 The dimension between the...  The dimension between the tie-rod end and steering arm is critical to minimize bumpsteer. I made the spacer from 4130 tubing. The zinc-coated bushings allow full rod-end movement so binding doesn't occur during full bump or rebound. These are the street setting versions. For the track geometry, different spacers are used. |
 The hub is recessed into the...  The hub is recessed into the aluminum upright so there can be more material "surrounding" it. Aluminum is a marginal material for such a critical piece, so the more beef the better. Using a late-model hub means no more packing wheel bearings as part of rotor replacement. Each upright took over 40 hours of machine time to make from a huge chunk of 7075 alloy. |
 The path from the ididit steering...  The path from the ididit steering column to the rack is a long and tortuous one. It takes four U-joints to make the journey though the Stainless Works headers. |
 The bolt-on steering arms...  The bolt-on steering arms bow out slightly to implement Ackerman angle. They are made from 4140-steel. |
 The lower coil-over mount...  The lower coil-over mount and sway bar mount are the same distance from the ball joint pivot point so that their motion ratio is the same. |
 The upper attaching point...  The upper attaching point to the sway bar end link has two settings to allow quick adjustment to the sway bar stiffness. |
Editor's Note:
When we first got this transcript from reader John Parsons, we were at odds with running it. We asked ourselves "Who is this guy? What is his experience? And does he race, or does he merely want to?" Yet there are some tantalizing (and accurate) technical details in this piece that shed some important light on the mechanics of suspensions, so we elected to publish it-doing anything else would be a shame. The car you're about to read about, II Much, has been a long time in the making and has been the subject of much discourse on g-Machine websites like Pro-Touring.com. (You may even remember a news item we ran on it in June 2004.) We must stress to our readers and advertisers that as this is published, Parson's Chevy II has yet to turn a wheel in anger-i.e. we're still waiting for some proof of concept.
It's important for us to note that aftermarket manufacturers of performance suspensions have decades of experience in building suspensions and racing them. Unless you are a chassis engineer like Mark Stielow or Art Morrison, it's not advisable to start cutting up a proven set of components. However, if you like living on the wild side and just can't ignore the siren song of that welder sitting in the corner, be our guest and start thrashing!--Johnny Hunkins
WHY THREE-PIECE MODULAR WHEELS?
Kinesis forges all three-pieces of their modular wheel. A 3,500-ton press exerts 40,000 psi to push the aluminum center into place, removing porosity and aligning the grain into a radial shape. The result is a part that is lighter and stronger than a corresponding cast piece.
Lighter is better when it comes to wheels: they are unsprung weight. Unsprung weight is vehicle mass that isn't controlled by the springs. Wheels, tires, control arms and spindles are all unsprung weight. The weight acts like a pendulum and the more of it there is, the harder the suspension has to work. That makes for sluggish handling, difficult suspension tuning and parts that wear out faster. Wheels also act like flywheels, storing kinetic energy as the car is driven down the road. A heavier wheel stores more kinetic energy, which has to be absorbed by the brakes to stop the car. Just a few pounds can make a significant difference in stopping distance.
Kinesis wheels can be taken apart and pieces replaced if they get damaged or if the backspacing needs to be changed. Since I'm building a maximum-effort front suspension, I need to use the lightest and strongest wheels I can find for ultimate handling and braking performance. Like many things in the automotive world, the performance capabilities of these wheels really translate into jaw-dropping good looks.