Understeer and oversteer are vehicle dynamics terms used to describe the
sensitivity of a vehicle to steering. Simply put, oversteer is what occurs
when a car turns by more than the amount commanded by the driver. Conversely,
understeer is what occurs when a car steers less than the amount commanded by
the driver. Automotive engineers define understeer
and oversteer based on changes in steering angle associated with changes
in lateral acceleration over a sequence of steady-state circular turning tests.
Car and motorsport enthusiasts often use the terminology more generally in
magazines and blogs to describe vehicle response to steering in all kinds of
maneuvers. Vehicle dynamics terminology
Standard terminology used to describe understeer and oversteer are defined by
the Society of Automotive Engineers in document J670 and by the International
Organization for Standardization in document 8855. By these terms,
understeer and oversteer are based on differences in steady-state conditions
where the vehicle is following a constant-radius path at a constant speed
with a constant steering wheel angle, on a flat and level surface.
Understeer and oversteer are defined by an understeer gradient K that is a
measure of how the steering needed for a steady turn changes as a function of
lateral acceleration. Steering at a steady speed is compared to the steering
that would be needed to follow the same circular path at low speed. The
low-speed steering for a given radius of turn is called Ackermann steer. The
vehicle has a positive understeer gradient if the difference between
required steer and the Ackermann steer increases with respect to incremental
increases in lateral acceleration. The vehicle has a negative gradient if the
difference in steer decreases with respect to incremental increases in
lateral acceleration. Understeer and oversteer are formally
defined using the gradient K: if K is positive, the vehicle shows understeer;
if K is negative, the vehicle shows oversteer; if K is zero, the vehicle is
neutral. Several tests can be used to determine
understeer gradient: constant radius, constant speed, or constant steer.
Formal descriptions of these three kinds of testing are provided by ISO.
Gillespie goes into some detail on two of the measurement methods.
Results depend on the type of test, so simply giving a deg/g value is not
sufficient; it is also necessary to indicate the type of procedure used to
measure the gradient. Vehicles are inherently nonlinear
systems, and it is normal for K to vary over the range of testing. It is
possible for a vehicle to be understeer in some conditions and oversteer in
others. Therefore, it is necessary to specify the speed and lateral
acceleration whenever reporting understeer/oversteer characteristics.
= Contributions to understeer= Many properties of the vehicle affect
the understeer gradient, including tire cornering stiffness, camber thrust,
lateral force compliance steer, self aligning torque, lateral load transfer,
and compliance in the steering system. These individual contributions can be
identified analytically or by measurement in a Bundorf analysis.
Limit conditions When an understeer vehicle is taken to
the grip limit of the tires, where it is no longer possible to increase lateral
acceleration, the vehicle will follow a path with a radius larger than intended.
Although the vehicle cannot increase lateral acceleration, it is dynamically
stable. When an oversteer vehicle is taken to
the grip limit of the tires, it becomes dynamically unstable with a tendency to
spin out. Although the vehicle is unstable in open-loop control, a skilled
driver can maintain control past the point of instability with
counter-steering, and/or correct use of the throttle or even brakes; this can be
referred to as drifting. However, at some limit in lateral acceleration, it
is not physically possible for even the most skilled driver to maintain a steady
state and spinout will occur. Related measures
Understeer gradient is one of the main measures for characterizing steady-state
cornering behavior. It is involved in other properties such as characteristic
speed, lateral acceleration gain, yaw velocity gain, and critical speed.
References