Understeer and oversteer describe two ways that a driver can lose tight control of a car—often when driving on slick roads, especially when cornering. Understeer, sometimes called plowing, occurs when the car doesn’t turn as much as a driver expects based on the movement of the steering wheel. It happens most often in front- and all-wheel drive cars but can also happen with rear-wheel drive—because so many passenger vehicles are tuned for slight understeer.
Oversteer, also called fish-tailing is just the opposite: the driver turns the steering wheel slightly, and the back of the car cuts loose.
There are two ways to correct these problems. The first is to modify the suspension tuning, and the second is to hone your driving skills. If you have a race-prepared car and own a good set of hand tools and floor jacks, then modifying the suspension is a good option. Your toolkit needs to include a good tire pressure gauge, tire pyrometer for measuring tire temperatures, and tire toe plates.
To fix understeer, consider the following suspension modifications:
- Raise front tire pressure and reduce rear tire pressure.
- Reduce front ride height.
- Install shorter front tires and taller rear tires.
- Reduce front shock compression and increase rear shock compression.
- Increase front camber—the angle between the vertical axis of the wheels and the vertical axis of the vehicle. Also, increase toe out—how the front of the wheel points away from the centerline of the vehicle.
- Reduce front roll-bar stiffness and increase rear roll bar stiffness.
- Increase front track.
New driving skills can also resolve the problem. If you go into a corner too fast or find yourself skidding on a wet road, lift off the throttle and apply a little bit of braking going into the corner. The key word here is “little.” If you dynamite the brakes, you’ll upset the chassis and send yourself into a 360. Light braking through the first part of the corner should bring the vehicle back under control.
Some drivers like a bit of oversteer or looseness in the chassis. If you’re not a drifter at heart, following are some chassis modifications to reduce oversteer.
- Increase front shock compression and install a bigger front roll bar.
- Increase rear wheel camber and toe-in; install a smaller or less stiff rear sway bar.
- Reduce rear ride height.
If you find your rear-wheel drive car fish-tailing on icy winter roads, adding some weight in the back is an easy fix. Throw some sandbags in the trunk. If you get stuck on a slippery patch, some sand around the wheels will create traction to help you move the vehicle forward.
If you are driving a car close to its limits on dry pavement, you may experience oversteer if you go into a corner too fast, brake mid-corner, or lift off the throttle mid-corner. The fix is to countersteer: turn the steering wheel in the direction of the skid. Counter-intuitive as this may seem, doing so will straighten out the wheels to regain control of the vehicle.
Since you cannot get out of oversteer by braking and your window of time to make this correction is limited, practice is important. Take your time to practice steering correction so that you learn the limits of your car.