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The Traction Circle at each Wheel

In Part IV, we considered the traction circle as applying to the car as a whole. To understand why a car skids, we need to look at the traction circles of each of the wheels individually. This is because weight transfer affects the grip offered by each of the wheels.

For example, consider a car that shares its weight equally between the wheels when it is stationary. During braking, the weight of the car shifts towards the front, and the traction circle of the front wheels grows at the expense of the rears'. As we discussed in Part III, we need to make sure that there is less braking effort at the rear than the front.

Now consider what happens if the car is cornering. During cornering the weight shifts to the outer wheels. With our equally weighted car the outside front and rear tyres will have equally large traction circles, and will provide most of the cornering force.

  
Figure 11: The effect of braking during a fast corner. The braking force transfers weight from the rear wheels to the front, resulting in a rear wheel skid.

But what happens if we panic and brake part way though a fast corner? In addition to the lateral (side-to-side) weight transfer, there is now also weight transfer to the front (Fig. 11). The traction circle at the rear shrinks and suddenly the cornering force that is required at the back of the car lies outside the available traction. If this happens quickly, the rear wheels will lose their grip, and the car is launched into a rear wheel skid.

This is just one example. Depending on how much weight is transfered, and how much braking/accelerating force is applied, we might have cause the front wheels to lose grip. The nature of the skid depends heavily on whether the front or rear wheels provide the driving force, and the distribution of the wheels' traction when the car is static.



next up previous
Next: Oversteer and Understeer Up: Skids and Why Previous: Aims of this



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