The Many Variables of Handling

by Ben Pila

Handling of a vehicle is determined by many variables. Some of these are tires, center of mass (a.k.a. center of gravity), chassis torsional stiffness, suspension geometry, suspension components (springs, shocks, anti-roll bars), etc.

There are many things that most people do not wish to change due to various reasons. For example, most people do not want to strip the interior of the car to lower the center of mass. It is understandable as it would make a very impractical street car. Installing roll cage is not a really good idea for a street car, either. Changing the suspension geometry is not recommended without thorough understanding of the vehicle dynamics. Changing suspension components is the easiest way to alter the handling characteristics of the vehicle. Tires Tires are black rubber things that provide the only contact with the road surface. Friction between tires and the road surface creates acceleration, braking (deceleration), and cornering. Most of you learn in high school physics that:

Ff = Cf * Fn

where Ff is the friction, Cf is the coefficient of friction, and Fn is the normal force. Well, ladies and gentlemen, I got news for you. This equation only applies to very special cases where both surfaces are very hard and have reasonably smooth finish. This equation certainly does not apply for tires.

Tire's frictional property can not be expressed as an equation really well. If I use the above equation, Cf will decrease with increase in Fn. As normal force increases, coefficient of friction decreases. As a result, friction force does not increase as much as increase in normal force. Weight Transfer Weight transfer is a shifting of loading on tires due to acceleration. Increasing speed, braking, and cornering cause weight transfer. For transverse acceleration, weight transfer can be expressed as:

dW = (m * h * a) / t

where dW is the resulting weight transfer due to acceleration a (in m/s2) for metric unit and G in U.S. unit when lb. Used for mass), m is the mass of the vehicle (kg for metric and lb. for U.S. unit), h is the height of center of mass, and t is the track width. For longitudinal weight transfer, use wheel base instead of t.

This equation calculates the total weight transfer but does not calculate how much weight is transferred by front or rear wheels. It required more complicated equation and a lot more details and assumption to calculate the weight transfer by each wheel, and I am not about to get into that.

Just remember that, if you make on end of the car stiffer, there will be more weight transfer at the end and lose some traction capability due to increase loading on that end. Springs Springs are energy storing devices. They store energy by deflection. Most springs are constant rate springs, i.e., amount of the force that is stored is proportional to the deflection of the sprint. It is usually expressed as:

F = k * X

where F is the force (in lb., at least in the U.S.), k is the spring constant (or spring rate) in force per deflection (lb./inch), X is the deflection (in).

There are also progressive springs. Progressive spring's spring constant (k) increases with deflection.

OK, enough about what springs are. We can now talk what springs do in a car. Springs absorb bumps, limits the motion of the vehicle due to acceleration, braking, cornering, etc. Shocks Shock absorbers dampen the motion of suspension. Shock absorbers do not absorb impacts: springs do. Shock absorbers are dampers.

Shock absorbers also control the transient motion of the vehicle. Shock absorbers control HOW the car nose dive when brake is applied. Springs control HOW MUCH car nose dives. Shock absorbers control HOW the car does into roll when the steering is applied. Springs and anti-roll bar control HOW MUCH car rolls.

There are some adjustable shocks available. Adjusting the extension of the shock is called "rebound", and adjusting other direction is "bump". Anti-Roll Bars Anti-roll bars are also known as roll bars, sway bars, anti-sway bars, etc. Anti-roll bars connect the right and left wheel. They resist roll by twisting themselves, acting as torsion springs. Tuning Springs, Shocks, & Anti-Roll Bars You have to understand a few terms before I can explain how to tune suspension.

Understeer ("Push" for circle track people) - Handing characteristic that, when the car is turning on the constant radius circle, front end of the car pushes to the outside of the circle with increasing speed. Turning radios of the car increases with increasing speed, or more steering is necessary with increasing speed. It is caused by the front end of the car having less traction than the rear end.

Oversteer ("Loose") - Handling characteristic that, when the car is turning on the constant radius circle, the rear end of the car pushes to the outside of the circle with increasing speed. Turning radius of the car decreases with increasing speed, or less steering is necessary with increasing speed. It is caused by the front end of the car having more traction than the rear end.

Neutral - Ideal condition that is rarely achieved. The car turns on the constant radius circle with constant steering angle with increasing speed. As strange as it seems, some racing cars can do this, until you lose traction and get into severe oversteer.

Entry - Imaging that you are approaching a long sweeper. Turn-in is when you turn the steering and wait for the car to "settle" to a steady state roll angle.

Steady State - Your car is finished turning in. You are giving the car constant steering angle and throttle angle.

Corner Exit - You are beginning to get on the gas pedal and unwind the steering. Symptoms & Solutions Entry Understeer - Increase the rebound or the rear shocks. Decrease the bump of the front shocks. Increase the rear brake bias. Brake earlier.

Entry Oversteer - Decrease the rebound of the rear shocks. Increase the bump of the front shocks. Decrease the rear brake bias.

Steady State Understeer - Stiffen rear springs and/or anti-roll bar. Soften the front springs and/or anti-roll bar.

Steady State Oversteer - Stiffen the front springs and/or anti-roll bar. Soften the rear springs and/or anti-roll bar.

Exit Understeer - Increase rebound of front shocks. Decrease bump of rear shocks. Stiffen rear springs/anti-roll bar. Soften front springs/anti-roll bar.

Exit Oversteer - Decrease rebound of front shocks. Increase bump of rear shocks. Soften rear springs/anti-roll bar. Stiffen front springs/anti-roll bar.

Car is slow to respond to driver's input - Stiffen the car by springs, anti-roll bars, and shocks. Increase tire inflation pressure.

Car hops over the bump - Soften the car. Decrease tire inflation pressure.

As you can see, changing one thing can alter all three stages of handling. You will have to make a compromise that best suits your requirement. There are also more variables such as ride height of the vehicle, wheel alignment, tires, driving style, corss weight, etc., that can alter the handling of a vehicle.