There’s no cheating when it comes to handling.
Understanding the principles of a vehicle’s suspension will give you a better idea as to what the vehicle, as well as the tires, are going through. When cornering, various forces affect a turn and change the direction of the vehicle, while also putting the tires under tremendous force.
CENTRIFUGAL FORCE AND CENTRIPETAL FORCE
Centrifugal force pulls a vehicle toward the outside of the corner during a turn. This same action requires that an opposing and equal force counters the effect to ensure that stable cornering is achieved. In short, centrifugal force pushes a car in the direction of momentum and the tires grip the road to stabilize your ride; this is the gripping power that we call “cornering force.”
CORNERING FORCE (CF)
This is the lateral frictional force generated by a cornering tire, acting in opposition to the centrifugal force. Tire force is generated by tire slip, and in the case of cornering, tire force is proportional to slip angle.
SLIP ANGLE (SA)
Slip angle describes the deformation of the tire contact patch; this deflection of the patch deforms the tire in a fashion akin to a spring. The slip angle is measured in how much the tire’s contact patch has been twisted or sheered in relation to the wheel during cornering. This cornering force increased in proportion to the occurrence and increases in the slip angle. During normal driving, the slip angle is approximately 2 to 5 degrees with a maximum of 10 degrees.
SELF-ALIGNING TORQUE (SAT)
Self-Aligning Torque is the torque that occurs at the contact patch of the tires due to the friction entailed in cornering. This produces a major impact on the tire contact patch.
CORNERING POWER (CP)
CP is the rate at which cornering force (kgf) increases as the slip angle increases. This in turn determines the driver handling, and the higher the CP, the more cornering ability and stability the vehicle will have.
TIRE AND CORNERING FORCE/POWER
• Wet surfaces decrease the friction thus reducing overall cornering force.
• As load increases, cornering force increases, but the force will gradually decline if the load exceeds the recommended value for the tire.
• Tire stiffness will increase cornering power. Please remember that it’s dangerous when the inflation pressure or the rim width exceeds the maximum approved level.
Oversteer, in an automobile, occurs when the rear tires have a loss of traction during a cornering situation. This in turn causes the rear of the vehicle to head toward the outside corner. A more technically correct definition is that oversteer is the condition when the slip angle of the rear tires exceeds that of the front tires. Rear-wheel-drive cars are generally more prone to oversteer, in particular when applying power in a tight corner.
Understeer is a term for a car handling condition during cornering in which the circular path of the vehicle’s motion is of a greater diameter than the circle indicated by the direction its wheels are pointed. In general, understeering of a vehicle will cause slightly better stability and control than oversteer.
Neutral steer is a cornering condition in which the front and rear slip angles are roughly the same. Although seemingly an ideal state of balance, perfect neutral steer is not as stable as slight understeer.
Centering & Offset
Get a better understanding of the wheels on your car.
Offset is the location of the flat mounting surface of a wheel relative to the wheel’s centerline. Negative offset means that the mounting surface is toward the center of the car, positive offset means that it’s toward the outside of the car, or the wheel is “pulled in” toward the center. Offset affects many things other than just whether the wheel has the appearance of “sticking out” past the fender. The wrong offset can cause rubbing problems when the suspension is compressed or the wheel is turned. Offset affects the steering geometry’s scrub radius, possibly leading to problems with torque steer or self-centering characteristics.
Offset also affects the suspension’s motion ratio, which directly determines the effective spring and damper rates. Potentially, in a very heavily loaded vehicle, or with extreme changes in offsets, wheel bearing life can be affected, but this is more often talked about by truck people than by small car enthusiasts. It’s very, very important that the proper offset wheels be used.
While not directly a matter of offset, brake caliper clearance is a related issue. If you have, or plan to have big brakes on your car, be sure that your wheels, or the wheels that you’re going to use, will fit over the calipers. Spacers are available to solve the problem if they don’t, but it’s best to get a wheel with enough dish to meet your offset specs and still fit your brakes. Consulting the wheel and brake manufacturers ahead of time is wise. Many aftermarket brake companies even have templates of their brakes available that you can easily check against any wheel.