COMMON HANDLING PROBLEMS
When any wheel leaves contact with the road there is a change in handling, so the suspension should keep all four wheels on the road in spite of hard cornering, swerving and bumps in the road. It is very important for handling, as well as other reasons, not to run out of suspension travel and “bottom” or “top”.
It is usually most desirable to have the car adjusted for a small amount of under steer, so that it responds predictably to a turn of the steering wheel and the rear wheels have a smaller slip angle than the front wheels. However this may not be achievable for all road and weather conditions, speed ranges, or while turning under acceleration or braking.
The most important common handling failings are:
- Under steer – the front wheels tend to crawl slightly or even slip and drift towards the outside of the turn. The driver can compensate by turning a little more tightly, but road-holding is reduced, the car’s behavior is less predictable and the tires are liable to wear more quickly.
- Over steer – the rear wheels tend to crawl or slip towards the outside of the turn more than the front. The driver must correct by steering away from the corner, otherwise the car is liable to spin, if pushed to its limit. Over steer is sometimes useful, to assist in steering, especially if it occurs only when the driver chooses it by applying power.
- Bump steer – the effect of irregularity of a road surface on the angle or motion of a car. It may be the result of the kinematic motion of the suspension rising or falling, causing toe-in or toe-out at the loaded wheel, ultimately affecting the yaw angle (heading) of the car. It may also be caused by defective or worn out suspension components. This will always happen under some conditions but depends on suspension, steering linkage, unsprung weight, angular inertia, differential type, frame rigidity, tires and tire pressures. If suspension travel is exhausted the wheel either bottoms or loses contact with the road. As with hard turning on flat roads, it is better if the wheel picks up by the spring reaching its neutral shape, rather than by suddenly contacting a limiting structure of the suspension.
- Body roll – the car leans towards the outside of the curve. This interferes with the driver’s control, because he must wait for the car to finish leaning before he can fully judge the effect of his steering change. It also adds to the delay before the car moves in the desired direction. It also slightly changes the weight borne by the tires as described in weight transfer.
- Excessive load transfer – On any vehicle that is cornering, the outside wheels are more heavily loaded than the inside due to the CG being above the ground. Total weight transfer (sum of front and back), in steady cornering, is determined by the ratio of the height of a car’s center of gravity to its axle track. When the weight transfer equals half the vehicle’s loaded weight, it will start to roll over. This can be avoided by manually or automatically reducing the turn rate, but this causes further reduction in road-holding.
- Slow response – sideways acceleration does not start immediately when the steering is turned and may not stop immediately when it is returned to center. This is partly caused by body roll. Other causes include tires with high slip angle, and yaw and roll angular inertia. Roll angular inertia aggravates body roll by delaying it. Soft tires aggravate yaw angular inertia by waiting for the car to reach their slip angle before turning the car.
Lowering the center of gravity will always help the handling (as well as reduce the chance of roll-over). This can be done to some extent by using plastic windows (or none) and light roof, hood (bonnet) and boot (trunk) lid materials, by reducing the ground clearance, etc. Increasing the track with “reversed” wheels will have a similar effect. Stiffer springs and/or shocks, both front and rear, will generally improve handling. Light alloy (mostly aluminum or magnesium) wheels improve handling as well as ride comfort. Moment of inertia can be reduced by using lighter bumpers and wings, or none at all.
|Component||Reduce Under-steer||Reduce Over-steer|
|Weight distribution||Centre of gravity towards rear||Centre of gravity towards front|
|Front shock absorber||Softer||Stiffer|
|Rear shock absorber||Stiffer||Softer|
|Front sway bar||Softer||Stiffer|
|Rear sway bar||Stiffer||Softer|
|Front tyre selection¹||Larger contact area²||Smaller contact area|
|Rear tyre selection||Smaller contact area||Larger contact area²|
|Front wheel rim width or diameter||Larger²||Smaller|
|Rear wheel rim width or diameter||Smaller||Larger²|
|Front tyre pressure||Lower pressure||Increase pressure|
|Rear tyre pressure||Increase pressure||Lower pressure|
|Front wheel camber||Increase negative camber||Reduce negative camber|
|Rear wheel camber||Reduce negative camber||Increase negative camber|
|Front height (because these usually affect camber and roll resistance)||Lower front end||Raise front end|
|Rear height||Raise rear end||Lower rear end|
|Front toe in||Decrease||Increase|
|Rear toe in||Decrease||Increase|
1) Tire contact area can be increased by using wider tires, or tires with fewer grooves in the tread pattern. Of course fewer grooves has the opposite effect in wet weather or other poor road conditions.
2) These also improve road holding, under most conditions.