Transient Response

As autocrossers we all know, or think we know, that Transient Response (TR, for short) is important to getting around the course quickly. Because slaloms and other types of offset features. In no other form of motorsport is the ability to quickly change direction quite so critical.

I recently became convinced that TR is actually much more important than we generally think. So important that it’s the number one thing that causes course dependency. Course dependency is when one particular car or type of car is inordinately favored by the course design. Let me give you an example of course dependency.

A few years ago a friend bought one of the first Porsche Macan Turbos in our area. It had been classed in B-Street, which was also the class for my 2000 Corvette FRC. I will readily admit that this friend was a faster driver than me. We had co-driven several times. He was always half a second faster. In my own car, in his car, it didn’t matter.

But he made a mistake. He informed me that he had bought autocross tires for the Macan and intended to beat me and my Corvette at the next event. What he didn’t know was that I was the course designer. I told him he had just made a huge mistake. I told him that I would design a course where there was no way he could win. He scoffed, confident in his and the Macan’s abilities.

I was good to my word and designed a transitiony course. No pinch points, just a lot of transitions. I was 2nd to an S2000. He didn’t even get 3rd. He was 4th in our local B-Street class. AFAIK, those tires have never been used since. It was his wife’s daily driver and no way would she put up with stiff, howling RE71Rs.

The Macan accelerated faster than the Corvette. It probably pulled as much, or very close to as much, lateral-G in a sweeper. It was slightly wider, but like I said there were no pinch points. What it couldn’t do was transition as fast as a Corvette or S2000. The CG is just too high.

Here’s what I’ve come to realize: in the Street classes a car’s peak lateral-G capability is of zero importance in determining speed through a slalom. Not less than what we thought previously, I mean zero.

In a slalom all that counts is TR and car width.

For instance, it doesn’t even matter what tires you are on or how wide they are, except in so far as the tires affect TR. As long as those tires can deliver the lateral capability that the TR capability allows the car to reach then they are sufficient. In Street class that means about 0.9G to 1.0G of lateral acceleration.

(More to come.)

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s