Vehicle dynamics terminology
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of the vehicle to changes in steering angle associated with changes in lateral acceleration. This sensitivity is defined for a level road for a given steady state operating condition by the Society of Automotive Engineers (SAE) in document J670 and by the International Organization for Standardization (ISO) in document 8855. Whether the vehicle is understeer or oversteer depends on the rate of change of the understeer angle. The Understeer Angle is the amount of additional steering (at the road wheels, not the hand wheel) that must be added in any given steady-state maneuver beyond the Ackermann steer angle. The Ackermann Steer Angle is the steer angle at which the vehicle would travel about a curve when there is no lateral acceleration required (at negligibly low speed).
The Understeer Gradient (U) is the rate of change of the understeer angle with respect to lateral acceleration on a level road for a given steady state operating condition.
The vehicle is Understeer is the understeer gradient is positive, Oversteer if the understeer gradient is negative, and Neutral steer if the understeer gradient is zero.
Car and motorsport enthusiasts often use the terminology informally in magazines and blogs to describe vehicle response to steering in a variety of manoueuvres.
Test to determine understeer gradient
Several tests can be used to determine understeer gradient: constant radius (repeat tests at different speeds), constant speed (repeat tests with different steering angles), or constant steer (repeat tests at different speeds). 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 U to vary over the range of testing. It is possible for a vehicle to show 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 gradient
Many properties of the vehicle affect the understeer gradient, including tyre cornering stiffness, camber thrust, lateral force compliance steer, self aligning torque, lateral weight transfer, and compliance in the steering system. Weight distribution affects the normal force on each tyre and therefore its grip. These individual contributions can be identified analytically or by measurement in a Bundorf analysis.
In contrast to limit handling behavior
Great care must be taken to avoid conflating the understeer/oversteer behavior with the limit behavior of a vehicle. The physics are very different. They have different handling implications and different causes. The former is concerned with tire distortion effects due to slip and camber angles as increasing levels of lateral acceleration are attained. The latter is concerned with the limiting friction case in which either the front or rear wheels become saturated first. It is best to use race driver's descriptive terms "push (plow) and loose (spin)" for limit behavior so that these concepts are not confused.
Description of limit handling characteristics
While much of this article is focused on the empirical measurement of understeer gradient, this section concentrates on road performance.
Push (plow) can typically be understood as a condition where, while cornering, the front tyres become saturated (unable to produce additional traction force) before the rear. Since the front tyres cannot provide any additional lateral force and the rear tyres can, the front of the vehicle must follow a path of greater radius than the rear.
The opposite is true if the rear tyres become saturated before the front. The front tyres will continue to provide the lateral force necessary to keep the front of the vehicle on the desired path. The rear tyres must instead follow a path with a larger radius. The result is that the rear tyres will swing outward relative to the front of the vehicle. This turns the vehicle toward the inside of the curve. If the steering angle is not changed (i.e. the steering wheel stays in the same position), then the front wheels will trace out a smaller and smaller circle while the rear wheels continue to swing around the front of the car. This is what is happening when a car 'spins out'. A car susceptible to being loose is sometimes known as 'tail happy', as in the way a dog wags its tail when happy and a common problem is fishtailing.
In real-world driving (where both the speed and turn radius may be constantly changing) several extra factors affect the distribution of traction and the tendency to plow or spin. These can primarily be split up into things that affect weight distribution to the tyres and extra frictional loads put on each tyre.
The normal (vertical) load distribution of a vehicle in steady state will affect handling. If the center of mass is moved forward, the understeer gradient tends to increase due to tyre load sensitivity. When the center of mass moved is rearward, the understeer gradient tends to decrease. Longitudinal load transfer is proportional to the magnitude of longitudinal acceleration and the height of the center of mass. When braking, additional normal load is applied to the front wheel and an equal magnitude of normal load is removed from the rear tyres. The normal load on the tire and the coefficient of friction determine the maximum traction force that can be generated. When accelerating, some of the normal load transfers to the rear from the front, changing the maximum traction that can be achieved at eight end.. In extreme cases, the front tyres may completely lift off the ground meaning no steering input can be transferred to the ground at all.
Tyres transmit lateral and longitudinal forces to the ground. the total traction force is the vector sum of the lateral and longitudinal forces. If the resultant desired traction force required in any operating condition exceeds the tyre's available traction force (a function of the normal force and coefficient of friction), then the tyre is saturated.
While weight distribution and suspension geometry have the greatest effect on measured understeer gradient in a steady-state test, power distribution, brake bias and front-rear weight transfer will also affect which wheels lose traction first in many real-world scenarios.
This section needs additional citations for verification. (October 2019)
When an understeer vehicle is taken to the grip limit of the tyres, 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 tyres, it becomes dynamically unstable with a tendency to spin. Although the vehicle is unstable in open-loop control, a skilled driver can maintain control past the point of instability with countersteering and/or correct use of the throttle or even brakes; this is done purposely in the sport of drifting.
If a rear-wheel-drive vehicle has enough power to spin the rear wheels, it can initiate oversteer at any time by sending enough engine power to the wheels that they start spinning. Once traction is broken, they are relatively free to swing laterally. Under braking load, more work is typically done by the front brakes. If this forward bias is too great, then the front tyres may lose traction, causing understeer.
Understeer gradient is one of the main measures for characterizing steady-state cornering behavior. It is involved in other properties such as characteristic speed (the speed for an understeer vehicle where the steer angle needed to negotiate a turn is twice the Ackermann angle), lateral acceleration gain (g's/deg), yaw velocity gain (1/s), and critical speed (the speed where an oversteer vehicle has infinite lateral acceleration gain).
- SAE International Surface Vehicle Recommended Practice, "Vehicle Dynamics Terminology", SAE Standard J670, Rev. 2008-01-24
- International Organization for Standardization, "Road vehicles – Vehicle dynamics and road-holding ability – Vocabulary", ISO Standard 8855, Rev. 2010
- International Organization for Standardization, "Passenger cars – Steady-state circular driving behaviour – Open-loop test methods", ISO Standard 4138
- T. D. Gillespie, "Fundamentals of Vehicle Dynamics", Society of Automotive Engineers, Inc., Warrendale, PA, 1992. pp 226–230
- Milliken, William F.; Milliken, Douglas L. (1994). Race Car Vehicle Dynamics. SAE International.