Oversteer. Understeer. What’s the difference?

Hey there, time traveller!
This article was published 10/03/2006 (7229 days ago), so information in it may no longer be current.

WE read about vehicle handling in almost every vehicle review. We experience vehicle handling every time we drive. What is this vehicle handling? Why do some vehicles handle “good” and others are “poor”? Can you change vehicle handling? Let’s look at what the designers and engineers are striving for when they design any vehicle.

For some drivers, the perfect-handling vehicle would be neutral. This means that when going around a corner, neither the front end nor the rear end of the vehicle would depart from the cornering arc. If you drive too fast into the corner, the whole vehicle would slip sideways instead of just one end breaking loose.

Some race car drivers, such as rally or sprint car drivers, might prefer oversteer. Oversteer occurs when the back end of the vehicle tries to come around or slide sideways on a corner, making the vehicle steer more into the turn. Rear-wheel drive vehicles tend to oversteer easier under power, as the back wheels lose traction. This can occur quickly. When a vehicle spins on a slippery road, this usually happens because conditions created an oversteer situation.

Many front-wheel drive vehicles on our roads are designed with understeer in them. When a car understeers, the front end of the car tries to push sideways on the corner while the back end remains stable.

It feels like the vehicle doesn’t want to turn. The natural driver’s reaction when understeer occurs is to back off the power, which causes the vehicle to slow down and usually transfers more weight onto the front wheels. This gives more traction to the front wheels and the vehicle then turns.

Front-wheel drive cars can oversteer, too. Put winter tires on the front and all-season tires on the back and your vehicle is a prime candidate for oversteer every time you slow down as the front tires grip and the rear slip. Using tires to tune the vehicle handling characteristics is an old but relevant technique.

Suspension design, spring rates, shock absorbers, sway bars, tires and even tire pressures all play an important part in how a vehicle handles.

Wider tires provide more traction on pavement. Look at high-powered vehicles such as the Viper, Corvette or Porsche 911. They have wider tires on the back so they don’t oversteer as easily when power is applied in a corner. But like any rear-wheel drive vehicle, apply too much power and the tires will lose traction. Instant oversteer, and you had better be ready to correct by reducing power and steering opposite to the slide.

Vehicles with most of the weight in the front tend to understeer. It would be impractical to put larger tires on the front than the rear, so designers use spring rates, sway bar sizes and suspension bushing stiffness to fine tune the vehicle handling. All keep the vehicle from rolling or leaning on corners.

Holding the tires vertical keeps more tread in contact with the road, but go too stiff with either springs or sway bars and the tires can’t move with road variations. This can cause exactly the opposite effect you want.

Most drivers will want to stay with the factory suspension. It can take a lot of trial and error before you get handling exactly the way you want it by changing suspension. Just look at how hard it is for professional race car teams to set suspensions exactly right.

We can change vehicle handling in much cheaper ways. Gas-filled shock absorbers and struts are better at controlling tire bounce. Installing premium units will improve both ride comfort and vehicle handling.

Tire pressure is critical. One or two PSI can make a difference in a street-driven vehicle. Higher pressures make the tire more stable, so handling improves at the cost of ride comfort. Lower pressures allow the vehicle to sway more. Using different pressures between front and rear tires can tune your vehicle handling, but never go below minimum recommended pressures.

The biggest change in the handling of a vehicle can come from different tires. Low-profile tires make steering feel more crisp because there is less flex in tire sidewalls.

There are many regular vehicles also sold with sport packages. Often the difference in the handling between these two vehicles is due to the sport package having wider tires with a low-profile sidewall. Just a change in tires can make it feel like a totally different vehicle.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net