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Causes of Oversteer and Understeer

If we start with a neutral steering car --- one with the front and rear traction limits equal --- we can create a table of the effects of accelerating and braking. For acceleration, the effect depends on whether the front or rear wheels are driven. Most modern road cars driven through the front wheels. The results also depend on whether or not the weight transfer compensates for the additional traction required for braking/accelerating.

The likely effects of acceleration and braking are summarised in the following table.

To understand the entries in this table, draw the traction circles for the balanced car, and add the cornering forces. Now redraw the traction circle allowing for the weight transfer, add the vectors showing the driving/braking forces, and determine whether the vector sum of the forces is closer to the traction limit at the front or rear of the car (closer at front leads to understeer, at rear leads to oversteer). The entry marked assumes that most of the braking force is applied at the front wheels.

Richard Bower
Thursday October 8 16:09:30 BST 1998