Index of performance

Since I deal with some top engineers in very senior formulae, I sometimes get asked some really interesting questions regarding simulation. For example, recently ChassisSim’s US Dealer asked me whether I was able to figure out stability index from yaw rate? The answer to this question is not only yes, but also that there are some other very interesting things that spring up when we consider this.

But before I get into how it all works, it would be worthwhile taking a little time to provide a quick recap of what the stability index is. In a nutshell the stability index measures the moment arm nondimensionalised by wheelbase between the centre of gravity and the centre of the lateral forces. This is illustrated in Figure 1.

This can be calculated through the vehicle model or it can be calculated from race data. Here we’re going to discuss calculating, or at least approximating, the stability index from logged yaw rate data. To frame this discussion appropriately, this is not about calculating the stability index to within one