Design And Innovation

Figure 4 tells you a wealth of information about what the driver is feeling at the seat. If the car is understeering, the slope of the slip angle curve at the front of the tyre is significantly less than what is at the rear. Looking at equation (3), the first part of the stability index decreases and the rear component of the stability index increases. Consequently, the stability index will drop well below zero. The converse is the case for oversteer. Either way, the car will be telling you something.

To further explore this, let's evaluate some numbers. The first table I'm going to present has some typical F3 numbers that will show the combined traction circle radiuses front and rear. This is shown in table 1.

The next table we are going to discuss is a spread of normalised slip angle derivatives based on figure 4. This is shown in table 2.

The derivatives of table 2 can be found by evaluating the derivatives of a curve fit of the points in figure 4.

So let's first consider the case where the car is understeering. Let's say the front slip angle is five degrees and the rear slip angle is four degrees. Using equation (4) and the derivatives from table 2, the stability index is shown here (above right, top).

Let's now reverse the case and consider the oversteer case, where the front slip angle is four degrees and the rear slip angle five. Again, evaluating equation (4), :he index is shown here

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