Stabilising influence

One of the biggest criticisms levelled at lap time simulation packages is that handling is a total afterthought, and that they can even come up with set-ups that are undriveable. I will admit that I have fallen into this trap on many occasions, and about two years ago I wrote an article that discussed this in depth. Then, upon rereading it recently, I saw there were a couple of key themes that should not only be reviewed, but also explored in greater depth.

The reason this situation exists is that we in the business have done an appalling job of defining racecar stability, primarily because what drives it is very poorly understood. But the good news is, once we address this, then a lot of things fall into place.

So, to kick off this discussion, while refreshing everyone’s memory, the stability index (stbi) is the primary driver for understeer and oversteer. It has its origins in aircraft longitudinal dynamics and it’s a measure of the moment arm between the centre of gravity and the centre of the tyre forces, as shown in Figure 1.

The neutral point is the location of the sum of the lateral forces. With the stability index