Oversteer/understeer

When I worked at the theatre, I bought Road & Track and Car & Driver magazines. They are two magazines that I’d read cover to cover. The letters, editorials, news, reviews, technical sections … everything. As far as magazines go, they’re the best value to me.

Early on I noticed they’d often refer to a car’s handling in terms of oversteer, understeer, or neutrality. It took me a long time to figure out what these terms really meant, because they never defined them. My interest in auto racing is what really cleared them up for me.

If you navigate a corner too quickly, the tires will lose their grip on the road. Without some correction, you will not make your way out of the corner safely. How the car behaves in this circumstance is what the terms define.

Imagine the front tires lose their grip first. You’re turning the steering wheel as much as is required to navigate the corner, but they’re not gripping the road as they should so the car isn’t turning as much as the angle of the front wheels indicate it should. This is understeer. In NASCAR terms, the car is tight or pushing. “Pushing like a snow plough” is a particularly descriptive phrase. The consequence is the car will take a much wider turn than it should based on the steering wheel position because the front wheels aren’t in full contact with the road.

Now imagine the rear wheels lose their grip first. Again, you’re turning the steering wheel as much as is required to make the turn, but the rear wheels break loose. The back end of the car swings out and the car turns more quickly than the angle of the front wheels indicate it should. This is oversteer. In NASCAR terms, the car is loose. The consequence is the car can take nice tight turns, but if you’re not careful, the back end will come all the way around and you’ll be facing the wrong direction.

The third possibility is that the front and rear tires all lose traction at the same time. The car then just slides toward the outside of the turn, with no oversteer or understeer. The turn will be wider than planned, but not as wide as an understeer condition. Neutral handling is an unusual condition because it’s such a delicate balance. Even if a car were configured to be neutral, the weight of a passenger, or even a change in the amount of fuel in the tank could upset the handling balance and move the car to oversteer or understeer.

I suppose that since oversteer and understeer are opposites, it should come as no surprise that the corrective measures are opposite as well. Letting off the gas while understeering causes a weight transfer toward the front of the car. This can be enough to get the tires to reestablish their grip on the road. If not, gentle application of the brakes can do the trick. Excessive braking will lock the front wheels, and then you’ve got no steering control. It may seem counterintuitive, but if you’re in an understeer condition, you want to gently apply more throttle. This causes a weight transfer to the rear, and may get the tires to ‘bite’ into the pavement and reestablish their grip. Too much throttle will spin the tires too quickly, negating any grip the tires had. The back end will come around and the car will spin.

Neutral handling is the safest, but since it’s a delicate balance, passenger cars are generally designed to understeer. A car that won’t steer as much as the driver wants is not a good thing, but it’s easier to control than a car with a back-end that’s eager to step out and set the car spinning. Racers generally prefer a car that will oversteer because it can take corners better, as long as the driver can control the car and prevent a spin.

An old racer’s joke also serves as an excellent mnemonic. It describes the car’s handling characteristics in terms of how your car will leave the track if you lose control in a turn: If the car understeers, the front of the car will leave the track first. If the car oversteers, the back of the car will leave the track first. If the car is neutral, the side of the car will leave the track first.

It’s not a simple quality to control, however. Handling is influenced by the suspension, tires, weight distribution, aerodynamics, and probably a bunch of other things I’m forgetting. The good news is that this aspect of your car’s handling only comes into play when you’re cornering at, or past the limit of the tires’ adhesion. Drive safely on the street and you’ll never know if your car oversteers or understeers.

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