Understeer

[Tony]

Understeer is a condition when during cornering the circular path of the vehicles motion is greater than the circle indicated by the direction that the wheels are pointed. This circumstance allows the front tyres to lose traction during a turn, causing the car to either not turn as tightly as expected or continue in a straight line.

 

Manufacturers today tend to configure Understeer into the design by default, seemingly it is thought that if the car Understeers slightly it tends to be more stable if a violent change of direction occurs, improving safety, this i don't entirely agree with

 

Under all high speed conditions cornering with pneumatic tyres generates amplification of the slip angle (side slip) velocity this is in reply to the direction in which the wheels are pointed. If the slip angle of the front and rear wheels is equal, the car is in a neutral steering state, if the slip angle of the front wheels exceeds that of the rear, the vehicle is said to be understeering.

 

All vehicles understeer or oversteer at different times based on road conditions, speed and available traction. The design of the vehicle, will tend to produce a particular 'terminal' condition when pushed to the limit of available traction, then the 'Terminal Understeer' refers to a vehicle which, as a function of its design, tends to understeer when cornering loads exceed tyre traction.

 

Terminal handling conditions are a function born from vehical length and front/ rear weight distribution this is the polar moment of inertia and front and rear tyre traction. Further modified by the relative roll stiffness front and rear, which affects the outward weight transfer during cornering.

 

A front heavy car with low rear roll stiffness albeit soft springs or insufficient sway tension will have a tendency for terminal understeer. The front tyres being more heavily loaded will reach the limits of grip before the rear tyres, and thus develop larger slip angles.

 

Although understeer and oversteer can cause a loss of control, many manufacturers design their cars for terminal understeer in the belief that it is easier for the average driver to control than terminal oversteer. Unlike terminal oversteer, which requires several steering corrections, understeer can often be reduced simply by reducing speed.