What is oversteer / understeer?
Why does my car do that?
They say you steer as much with the throttle as the steering wheel. What does that mean?
Welcome back. To briefly summarize our last column, by changing the balance of your car
over its wheels, we can modulate the traction, and thus the handling characteristics of
the car. (Don't you hate it when the entire prior day's lesson can be distilled into one
sentence ? like why did I spend so much time studying it then?) Accelerating shifts more
weight, and therefore traction, to the rear wheels, while slowing transfers more weight to
the front tires. This has a direct bearing on how the car will corner.
Understeer occurs when car turns less than would normally be caused by a set deflection
of the steering wheel, a condition also referred to as "plowing" or "pushing," since the
car pushes towards the outside of the curve. Understeer routinely occurs in 2 situations.
In the first, a vehicle without antilock brakes is braking hard enough to lock the front
brakes. Since the front tires in this situation are not turning, there is no response to
steering input and the car keeps sliding forward. Obviously, the proper response is to
ease off the brakes and allow the wheels to resume rotating to allow steering inputs.
Matt shows how extreme weight transfer can promote understeer. This
photo was taken at our 2005 Charity Autocross at SIR. Photo Credit: Kelly Jo
Volin
The other situation where understeer is likely is when cornering forces exceed the traction
of the tires. This may mean too high of a speed in a sweeping turn or applying too much
throttle for the curve. In the first case, the tires just don't have enough traction to
provide the lateral acceleration required. The solution is to demand less cornering force
from the tires (i.e. take the corner slower) or increase the traction. Remember from last
time, traction can be improved by avoiding debris on the track, or perhaps by finding an
area where some rubber build up has made the track stickier, or by increasing the downforce
on the tire (i.e. trail braking, an advanced technique ? more on this later). If this occurs
in a sweeper, my suggestion is to consider taking the corner slower or changing your line
from a single apex to a double apex ? I'll address this more later.
In the second understeer situation, understeer may occur as the driver is rolling on
the throttle in the corner, causing some weight to shift off the front tires and onto
the rear tires. As the weight shifts off the front tires, there is less grip to provide
lateral acceleration or turning force. The inexperienced driver will try to correct by
turning the wheel more, but to no avail. As Rink Reinking was fond of pointing out to
me, if what you're doing is ineffective, doing more of it isn't likely to help ? try
something else! Decreasing the steering angle may allow the tires to once again resume
turning and begin to provide some lateral force. More important, however, is to ease
off the throttle (I mean to lift minimally). This causes weight to shift to the front,
increasing grip. Caution! If too much weight shifts forward, it sets
you up for the next condition, oversteer.
Oversteer occurs when the car turns more sharply than normal for a
set deflection of the steering wheel. As with understeer, this is commonly seen in
two settings. I alluded to the more common situation, "trailing throttle oversteer,"
above. When Rink repeats his "Don't lift in a turn" mantra, he's protecting you from
this. Lifting off the throttle shifts weight onto the front tires and increases their
ability to make the car turn, while shifting weight off the rear tires, giving them less
grip to resist breaking loose. If this occurs after the car has started rotating, the
result may be a tendency for the rear of the car to slide towards the outside of the curve,
referred to as "getting loose." The appropriate counter measure is a GENTLE
application of throttle to shift more weight to the rear and provide more grip. The goal
is to accomplish this without causing either understeer or spinning the rear tires which
reduces traction in the rear even more.
Spinning the rear tires in a corner is the other setting in which oversteer occurs since
a spinning tire has the least traction. This is referred to as "throttle oversteer" and
is more likely to happen to cars with limited slip differentials, as both rear tires may
spin. With a normal or open differential, only the inside rear tire on the turn is likely
to spin, and the non-spinning outside rear wheel will tend to prevent the back end from
sliding. This is also where a more powerful car (like most Porsches) is more likely to
have problems than a less powerful car.
If you've driven the track with the Arizona Region, you may have seen Glenn, one of their
instructors, in his 911 (referred to as his tricycle since it usually has only 3 wheels
on the ground). Glenn plays the throttle like a virtuoso, deliberately causing oversteer
to rotate the car. Once in a corner, he abruptly lifts off the gas, breaking the back end
loose. The instant the car is pointing the desired direction, he's back on the throttle to
set the rear end and lock the car in at that angle. Caution! Don't try this one at home,
kids. This is an advanced driving technique that almost guarantees a spinout at the worst
possible location if done improperly! I would also point out that although oversteer and
drifting is impressive to watch and fun to do, it is usually not the fastest way around a
corner.
In a properly executed high-speed turn, the car will be balanced between understeer and
oversteer. If you apply a little more throttle, it will begin to understeer. If you
lift off the throttle a little, the car will begin to oversteer. When the car is balanced
between these limits, it is said to be in a "4 wheel drift," and the gas pedal is used for
course corrections while keeping the steering wheel steady. This is the fastest the car
will hold on this line. You can see from the preceding discussion how important the
throttle is to the steering capabilities of the car, even before this limit is reached.
This explains the statement in the introduction above.
Next issue, I will address how to adjust your line through the corners for your car and
the track conditions. Until then, keep the shiny side up and the dirty side down
(unless you're a Concours person, then keep the shinier side up and the slightly less
shiny side down!)
Greg
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