Transient Response 2

In the earlier post I wrote that “in the Street classes a car’s peak lateral-G capability is of zero importance in determining speed through a slalom.” What?

The analysis was done a while back (see J-Rho covering it in part 5 of his series Comparative Vehicle Dynamics) that all the techno-nerdy autocrossers (like me) know about where someone (I won’t mention his name) derived the time around cones in a slalom using basic physics. In his derivation it came down to how much the car must move left and right and the peak grip (lateral-G capability) of the car. Using his formula you can calculate the time lost by leaving more space around the cones, for instance, and you can compare one car to another based on width and peak lateral acceleration capability.

The only problem is that it doesn’t work.

Am I saying that the math (for sinusoidal curves) was wrong? No. What I am saying is that it is a useless, misleading, nonsensical simplification that totally missed and therefore obscured what really matters since it was done over 15 years ago. Let me prove it. With data.

Soon after acquiring a GPS data device I noticed something weird. I will illustrate it with values from recent runs, but it was clear from the very first run I ever took with data.

In my last autocross in the Corvette the course contained both slaloms and sweepers, as usual. In five different sweepers the data shows the car pulling sustained lateral acceleration values of 1.250, 1.083. 1.084, 1.125 and 1.260 Gs. In various slalom sections the car pulled peak levels of 0.676, 0.774 and 0.552 Gs.

In my most recent autocross I co-drove a 2008 Cayman and in various sweepers achieved 1.299, 1.080 and 1.276 Gs. In the slaloms the car pulled 0.720, 0.435, 0.469 and 0.627 Gs.

[Update: I’ve come to realize that my data device was missing the momentary peaks of lateral G. Those peaks were still less than the steady-state capability however. As we will learn later, it takes Street Touring preparation level for most cars to even momentarily achieve peak steady-state lateral-G in a slalom, so the point stands.]

See the pattern?

For a long time I thought I was just pitiful at driving slaloms. It made me work really hard to improve. I got faster! The pattern didn’t change, however. I’m so stupid!

If a car can’t achieve it’s peak lateral acceleration capability within a slalom then peak lateral acceleration capability is not important and is no predictor of slalom speed. Any mathematical formula that claims to be able to predict the time from cone to cone in a slalom that uses peak lateral acceleration capability is just. plain. wrong.

We need to start over and rethink it. I’ll do that in the next installment.

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.)