Understeer and Oversteer

When discussing the handling characteristics of a vehicle, the words "oversteer" and "understeer" come up all the time. Here's a basic description of what those terms mean, and a little bit of theory about why particular cars exhibit either oversteer or understeer.

Definitions:

"Understeer", also known as "push", and "dammit, why won't the car turn?", happens when a vehicle doesn't turn as quickly as the angle of the front wheels would suggest. Turning the steering wheel further just makes the tires slip more. An understeering vehicle wants to point  to the outside of a turn, plowing ahead instead of where the wheels are aimed.

"Oversteer", AKA "loose", or "OH S*&T!", is when the vehicle wants to turn too far, with the back end sliding around and, in extreme cases, trying to pass the front. An oversteering vehicle feels like it's about to spin, and frequently does if the driver isn't skilled enough to "catch" it.

An easy way to remember the difference is that Understeer is when you see what you're about to hit through the windshield, but Oversteer means you see it in the mirrors...

A little theory:

There are a lot of different dynamics that cause under- or oversteer; the front-to-rear weight bias of the car, the presence or absence of anti-sway bar(s), which wheels are doing the work of accelerating the car, and even the size and type of tires.

Most cars come from the factory with a bias towards understeer. That's because it's generally thought that understeer is easier for the average driver to cope with than oversteer, which is probably true. The instinctual reaction for a driver in a sliding car is to lift off the throttle and hit the brakes, which will transfer weight to the front end and increase traction there, helping an understeering car to recover control. Doing the same thing in a car that's oversteering will usually make the situation worse by unloading the rear tires and further reducing their traction.

For this reason, you'll almost never see a factory-stock car with a rear anti-roll bar, but no bar on the front. Without getting into a lot of advanced car dynamics, I'll just say that putting a "swaybar" or anti-roll bar on one end of the car (or replacing an existing one with a stiffer bar) will tend to give the opposite end of the car more traction. Thus, most cars come equipped with a front bar but none in the rear, or if they have both, the front bar will be considerably stiffer. This preserves the tendency to "safely" understeer once the limits of handling are reached.

It's possible for a vehicle to exhibit both understeer and oversteer at different times. A classic example of this is the first-generation Porsche 911 Turbo. With the weight of the engine over the back axle, a big tire size difference front-to-rear, and an engine that produces a sudden hit of power when the turbo spools up, the old 911 Turbos were notorious for their evil handling characteristics, going from power-off understeer to power-on oversteer at the speed of thought. Porsche has managed to breed these traits out of the newer 911's through careful chassis tuning, redistribution of weight, and the adoption of an all-wheel-drive system for the newest Turbos.


Source: http://www.techweasel.com/articles/understeerandoversteer.htm