This article was first published in 2008.
Here we cover the definitions of some of the words
used to describe car handling and suspension. If you don't know how to describe
it, it's a lot harder to get someone to help you fix it.
Understeer is when the cornering car does
not turn as sharply as the amount of steering lock indicates it should. In other
words, the front of the car is sliding. A car that understeers off the road does
so with the front still facing the original direction. Understeer is safer than
oversteer (see below) and so all car manufacturers set their cars up to
understeer when grip levels are exceeded.
Oversteer is where the car turns to a
greater degree than the steering indicates it should. In other words, the rear
of the car is sliding. A car that oversteers off the road is spinning, so it may
hit sideways or tail-first (or even do a 360 degree spin, hitting nose first!).
An oversteering car can be felt to be rotating around you.
Lift-off oversteer occurs when the throttle
is abruptly raised mid-corner. This normally applies to front-wheel drive cars
but will occur in any car with high rear roll stiffness. A car that lift-off
oversteers will normally tuck-in if the throttle lift isn’t so great –
that is, the front will stop understeering.
Turn-in oversteer is when the car initially
turns-in more than the steering angle requested. This is very disconcerting
because, as with turn-in understeer, it’s not immediately apparent what can be
done to stop it.
Bump steer occurs when the wheels change their toe
angles (the direction they’re pointed in) as the suspension moves up and down.
It’s generally most easily felt on turn-in, when a suspension with toe-in on
bump will have turn-in oversteer (it will twitchily turn-in more than expected)
and a suspension with toe-out on bump will have turn-in understeer.
A car with linear roll is progressive in its roll,
with the angle of roll being directly related to how hard the car is cornering.
A car with non-linear roll may rapidly lean on turn-in but then not lean any
further as the cornering load increases. Roll linearity is seldom mentioned but
it is very important in giving the driver the correct signals as to what is
Steady State Understeer/Oversteer
When corning at a steady (or near steady) speed,
can the car be edged into oversteer (or understeer) and then easily brought back
to a neutral state? If it can, it has a good steady state understeer/oversteer
A skidpan is an ideal test environment for
assessing this trait. For example, a car might be moved into a just
understeering state by going a little faster and then applying more steering
lock, or it might be edged into a just oversteering state by applying a little
more power (in a rear wheel drive car) and then unwinding a little steering
This describes how rapidly the car responds to
driver inputs. A twitchy car will respond very rapidly to steering and power
inputs. In a road car this tends to be tiring and at times disconcerting.
However, a car that is the extreme opposite to ‘twitchy’ will feel dull and
unresponsive. Stiff bump damping and stiff sway bars are two characteristics
that will make a car twitchy.
A car that is on the edge of sliding (either
understeer or oversteer) and can then be moved into a slightly sliding state by
a variation in power is said to be being throttle-steered. Throttle steering
requires a very well balanced car with an engine that has a linear torque
response (so not a little engine with a big turbo!).
While individual preferences vary, a car with good
overall handling is one that is controllable, predictable and progressive... and
has high grip levels. And, note, in that order of priority!