Suspension Definitions
Important Definitions, and how they apply to the Mustang
In the coming months we will be putting a series of technical articles on the website to explain the fundamentals of suspension dynamics and ways to improve the handling of your Mustang. To help you get the most from these articles we have compiled a list of important terms you will need to know to fully understand your suspension.
Camber: The tilt of the tire relative to vertical. If the top of the tire leans inward (towards the engine) the car has Negative-Camber. If it leans outward, that is Positive-Camber. For more about adjusting camber, see Steeda Alignment Basics.
Caster: is the how far the contact patch of the tire follows behind the imaginary line where the steering axis of the wheel intersects the ground. (Think of the front wheel of a shopping cart, where the wheel follows behind the steering axis.) For more about adjusting caster, see Steeda Alignment Basics.
Toe: Toe-in or Toe-out describes the alignment of the front wheels relative to each other the same way you would describe your feet. Toe-in means they are closer together at the front than the rear. For more about toe, see Steeda Alignment Basics.
Bump-Steer: A change in toe caused by the suspension moving up or down. Bump-steer is built into the geometry of the suspension and steering system, and occurs independently of inputs from the steering wheel. The effect of bump-steer is for the wheel to toe-in or toe-out when the suspension moves up or down. This toe change, or "steering" occurs any time the suspension moves, whether it is from body roll, brake dive, or hitting a bump in the road. Bump steer is undesirable because the suspension is steering the car instead of the driver. Small amounts of bump-steer can be eliminated by installing offset rack-bushings. More severe bump steer problems require a "bump-steer kit" .
Roll Center: The point the body of the car rotates about when the car rolls in a turn. When the car rolls, the roll center is the center of the rotation. The roll center is not necessarily a physical point on the car, but rather is a theoretical point determined by the intersection of imaginary lines drawn through attachment points of the suspension, such as the control arm bushings, ball joints and tire contact patches. The front and rear suspension each have their own roll center.
Roll Axis: The line between the front & rear roll centers. In most cases you want this line to be parallel to the ground. In other words, you want the front and rear roll centers to be at or near the same height. If the roll axis is not parallel to the ground, body roll will create a yawing moment as the car tries to twist around the roll axis. Usually this can be felt as the car pitching forward or backwards and twisting as the car rolls.
For example, a typical lowered Mustang with otherwise stock suspension will have a rear roll center at 16 inches above the ground and a front roll center below ground level. As the car rolls, the weight will shift forward and sideways as the car rotates around the diagonal line between the low front roll center and the high rear roll center. The cure for this is to raise the front roll center with the X2 balljoint and lower the rear roll center with the 5-Link suspension. With these modifications the front and rear roll centers can be set at closer to the same height, creating a roll axis nearly parallel to the ground. With a level roll axis, the body can roll without twisting or pitching, keeping the forces on the tires more evenly distributed for better grip.
Spring Rate: The force necessary to compress a spring. Usually expressed in pounds-per-inch. For example 300 lb/in spring will compress 1 inch when 300 lbs are applied to it, 2 inches when 600 lbs are applied to it, etc.
Motion Ratio: The relative amount of motion that will occur at one point of a system as the result of moving another part of the system. Specifically for suspension design, the amount the spring seat will move as a result of the wheel moving. For example on the stock Mustang the spring is half way between the wheel and the control arm pivot, resulting in a motion ratio of 0.5. If the wheel moves up 1 inch the spring moves .5 times that amount, or ½ inch.
Wheel Rate: The "spring rate" at the wheel. This is the "real" spring rate of the vehicle, because this is what is actually acting upon the road, regardless of what the type of suspension system or springs are used. A car with a wheel rate of 100 lbs/in at all 4 wheels will sink 1 inch if 100 lbs is placed on over each wheel on the body of the car. To find the wheel rate you multiply the spring rate times the square of the motion ratio. For example on a stock suspension Mustang with a 450 lb/in spring the front wheel rate is 450 x 0.5squared, or 450 x (0.5 x 0.5), = 112.5 lb/in. Twisting bushings and binding of the suspension components will also alter the wheel rate, but if the suspension is designed properly their effects will be minimal relative to the effect of motion ratio and spring rate. Understanding wheel rate becomes critical when evaluating alternative suspension designs such as coil-overs vs. stock spring locations.
Roll-Steer: The effect of the rear axle "steering" to one side as a result of the car leaning over (rolling) in a corner. It is caused by the rear control arms pulling the axle backward and forward as they swing through their arc when the axle moves up or down. To minimize roll-steer on a Mustang, the lower rear control arms should be as close to level as possible.
Understeer: When the front tires lose traction before the back tires during a corner. More technically, understeer occurs when the slip angle of the front tires is greater than the slip angle of the rear tires. An understeering car will not turn as sharply as the driver intends. To compensate, the driver will have to turn the wheel sharper or slow down, or both. When pushed too far, understeering cars plow off the road straight-ahead.
Oversteer: When the back tires lose traction before the front tires during a corner. Also, when the slip angle of the rear tires is greater than the slip angle of the front tires. To compensate for oversteer the driver will have to reduce the steering angle (straighten out the wheels). Extreme cases of oversteer will require the driver to turn the wheel in the opposite direction to recover. When driven beyond their limit, oversteering cars tend to spin. A well know racer once quipped "Understeer means you hit the wall head first, oversteer means you hit the wall back-end first."
Loose: Oversteer
Push: Understeer
Load Transfer: When the load (weight) that is normally on one of the wheels is transferred to another wheel because of the effects of acceleration and inertia.
A car cornering on three wheels or a drag race car pulling a wheelie are extreme examples of load transfer. It is not necessary for the body of the car to move (roll, pitch) for weight transfer to occur. Even if the car is held perfectly flat by huge swaybars, the wheels on the outside of a corner will carry more load than the wheels on the inside. However, load transfer can be increased because weight has been physically moved, such as when the body of the car leans to one side. Load transfer can work for you, as in drag racing where you want the weight shifted to the rear wheels for traction, or it can work against you, as in road racing where you want to keep the weight more evenly distributed for better control in corners.
Lateral: Sideways. That is, left-to-right or right-to-left on the car.
Longitudinal: Front to back, in line with the "long way" of the car.
Bind: When the suspension movement is restricted by some undesirable conflict. Usually caused when the geometry of the system forces a part to move in a direction that is physically impossible unless something “gives,” such as using a bolt or pin where a balljoint is needed. (A pin only allows rotation in one plane. A balljoint allows full circular movement.)
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