There are a number of aspects to your car's handling and cornering ability that we need to understand in order to fully appreciate our discussion on suspension tuning. We'll begin by looking at the various conditions and properties that affect your car's handling, and the different characteristics of road holding.
Understeer, Oversteer, Bump Steer and Roll Steer
The terms understeer, oversteer and bumpsteer refers to the car's steering characteristics that are affected by the car's suspension settings. Understeer and oversteer occurs under cornering conditions, while bumpsteer (and roll steer) occurs when negotiating rough road conditions.
- Understeer occurs when the front wheels of the car tends to lose grip and drifts towards the outside of the turn, giving the impression that the car is straightening, or not turning in enough. The suspension setting on most stock cars tend to favor understeer. This is because understeer is seen as a safer option than oversteer for the average driver as the correction is instinctively accomplished by reducing speed or turning more into the corner.
- Oversteer occurs when the rear wheels the car tends to lose grip and drifts towards the outside of the turn, giving the impression that the car is turning in too much. The driver must counter oversteer by straightening the car a little otherwise the car will spin if it is pushed to its limit. Oversteer on rear wheel drive and all wheel drive (AWD) cars is sometimes useful for the experienced driver who can use the power to the rear wheels to balance the steering through the corner, allowing the driver to come out of the corners at higher speeds.
- Bump steer results from the suspension geometry occurs when the toe angle of the front wheels change as the suspension moves in an upward (bump) or downward (drop) motion as it negotiates the bumps over a rough road surface.
- Roll steer is similar to bump steer but occurs when the toe angle of the front wheels change as the suspension moves in an upward due to body roll.
For good handling and road holding, the car should not have a strong tendency for either understeer or oversteer, but should rather be at a point between the two, a point often referred to as neutral steer. Achieving neutral steer is one of the aims of suspension tuning, and is usually accomplished by increasing the traction on the wheels that lose grip.
Weight Distribution and Load Transfer
Weight distribution and load transfer also affects the car's road holding. Load transfer is often, but incorrectly referred to as weight transfer due to their close relationship, however, load transfer is the effect of inertia or centrifugal force on the car's center of gravity rather than the actual transfer of weight. When a car accelerates, the traction force between the tire and the road surface moves the car forward. This force is acting at the road surface. But the car is also subject to an inertia force that acts against the traction force. However, the inertia force acts against the car's center of gravity, which is located above the road surface. The effect of these two forces, one pushing the car forward at the road surface, and the other pushing the car back at the higher center of gravity, causes load transfer to the rear of the car and results in the tendency for the front of the car to lift. This is called fore-and-aft load transfer or longitudinal load transfer and occurs during acceleration and braking. There is also lateral load transfer that occurs during cornering when centrifugal force acts sideways at the car's center of gravity, in a direction away from the center of the corner. Again, the effect of traction force at the road surface, and the centrifugal force at the higher center of gravity causes the load to be transferred from the inside tires to the outside tires.
So why is load transfer a problem? It's got to do with traction and tire grip. While the load transfer is always proportional so that the amount of load transferred from the front of the car is the same amount of load that is transferred to the back of the car under acceleration, the amount of traction or grip that is lost due to reduced load is not proportional to the amount of traction or grip gained by increased load. The amount of grip lost is always more than the amount of grip grained.
Load transfer is amplified by the height of a car's center of gravity, and by the amount of body roll of the car. It can thus be countered by lowering the car's center of gravity, which can be accomplished by lowering the car, and by reducing the car's weight. Load transfer can also be minimized by increasing the stiffness of the car's suspension, which will reduce body roll, and by increasing the car's track width and its wheelbase, in other words, increasing the distance between the wheels.
Body roll is another important consideration in suspension tuning. A car with excessive body roll affects the amount of power a driver can use during cornering as the angle of roll affects the camber of the wheels, which in turn affects the grip of the tires. As mentioned earlier, a large amount of body roll will also amplify lateral load transfer, particularly on a car with a high center of gravity, and will adversely affect tire grip. For these reasons, a race car will have very little body roll, but this translates to a much harsher and less comfortable ride. On production cars, ride comfort is usually more important, which is why most production cars have too much body roll.