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.[3] Gillespie goes into some detail on two of the measurement methods.[4]
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.
Limit handling characteristics
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.