Re: understeer/oversteer definition

[Carlyle Fitzgerald <clfitz@inetone.net>]

On Fri, 12 Mar 1999, Andrei Kogan wrote:

> Hello,
> Can someone tell me what the definition of the oversteer/understeer terms
> is? How
> does understeering/oversteering affects the behavior of a car on turns

I'll try!

Understeering means the car is steering less than the input from the
steering wheel would lead you to expect.  Some people call this "plowing",
since the car wants to continue going straight ahead despite the driver's
attempt to turn.

Oversteer, of course, means that the car steers *more* than is called for
by steering input.  Some drivers refer to this as being "loose", although
I have no idea why.

> and also what
> does it depend on ( that is, what are the factors determining whether the
> car will oversteer/understeer).

All kinds of things!  Front/rear weight distribution, steering geometry,
tire pressure, and on and on.

Front-wheel drive cars understeer partly because most of the weight is in
the front--the engine and transaxle.  Rear-wheel drive cars understeer for
the same reason, although there's proportionally less weight in front.
Actually, the *biggest* reason cars understeer is that's the behavior most
drivers are comfortable with.  Can you imagine what would happen if your
average driver attempted to negotiate a turn and the car broke into a
slide?  That's what would happen with an oversteering car; with
understeer, though, you just crank the wheel a little more.  Providing the
pavement is dry and you're not going too fast, the car will steer.

> What's the normal behavior of a car with proper wheel alignment, and is
> there typically a difference in the degree
> of the oversteer/understeer between say a mass-market car and  a
> race/performance oriented car?

Most new cars understeer to varying degrees.  Racing cars I'm not at all
sure about, since I'm not a big fan of racing.

Older Porsche 911s could be induced to oversteer *very* easily; the motor
was in the back, for one thing, but there was also a change in suspension
geometry brought about by going into a turn too fast and then braking.  I
believe what happened was that the rear wheels actually steered the car's
rear end toward the outside of the turn, which put a lot of Porsches in
the weeds, much to the surprise of their drivers!

In short, think of oversteer as the rear of the car sliding toward the
outside of the turn; think of understeer as the opposite.

Hope this helps!

Carlyle
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