Long Corners

Long corners, as in an arc of more than 90 degrees, are more common in autocross than road-racing, but this is one case where the conventional wisdom is the same: approach on a tangent line, brake in a straight line to take the section at the maximum speed for the minimum radius and follow the minimum radius around the corner. Take a look at the figure below.


The typical construction in autocross is a gate that limits entry to some extent, a series of pointed cones set into an arc, more or less circular, followed by a shortish exit that leads to another turn.

The classic way to take such a corner is shown by the solid line with straight-line braking starting at A in plenty of time to match the radius. If you misjudge the braking and go past the minimum radius you will definitely lose time. Better to let off the brake a little early than push out beyond the minimum radius. This is the way I was taught in Evo school.

What you definitely don’t want to do is shown by the dashed line. This is intended to illustrate a late-apex approach, except that instead of braking early in a straight line I’ve shown an attempt at combination turning and trail braking in the curve starting at C. Even so, the extra distance traveled and the slow speed necessary to negotiate the small radius before the exit will kill your time. There isn’t enough acceleration zone before the next feature to overcome the time lost in the late-apex corner.

Any other path that takes a larger radius than the minimum will also be slow, all else equal. See this blog post to understand why.

The dotted line is what I’ve gotten into the habit of doing and what I see some others  do. I really don’t know if it is any better than the classic technique or not. It shows an attempt at delaying braking by initially going wide, then braking and turning into a slide that scrubs speed down to meet the minimum radius. Does it save any time on entry? The more I think about it the less I like it. Even if it does work in theory, time-wasting mistakes are very easy to make. I’d love to hear any good ideas on whether there is any chance this technique saves time. It’s a very complex and difficult situation to paper analyze.

Watch This!

The most interesting autocross video I’ve ever seen is a split screen of Matthew Braun and Geoff Walker, both driving Geoff’s S2000CR in STR on day 2 of the 2014 Wilmington Championship Tour. You can find it here on youtube.

I’ve been lucky enough to meet both of these guys and I can tell you they are both great drivers and great persons. Many of you know, or know of, Matthew. He’s been a fixture on the national circuit for a long time with (I think) two jackets and many Nationals trophies. Geoff, from nearby Nashville, is a very solid up-and-comer that I’ve been chasing for years. He and Matthew drove Geoff’s car in STR last year at Nationals. Both trophied, with Matthew in 2nd and Geoff only 0.6s (over two days) back in 8th. On to the video.

I’ve seen some side-by-side comparisons, but I’ve never seen anything done quite as well as this. Matthew is on the top half of the screen, Geoff on bottom half. The action is shown at about half-speed, so you have time to take in what’s happening. (Still, I’ve watched it countless times.) At three points along the way the video stops, the car behind is allowed to catch up and the delta time and total elapsed time is displayed. Really cool.

But the driving, Holy Toledo Pro-Solo! There may have never been as good a comparison of two very different styles. Geoff is smooth as butter. The car is always in perfect position. And, believe me, he is fast! Matthew is a wild man by comparison, but utterly perfect. The first ten seconds is enough to tell the story. Go ahead and watch it. I’ll wait right here.

Each of the offset cones in the first 10 seconds of the run is the same story, and really is the story. Matthew is traveling faster, turning the wheel much faster, farther and earlier (has to be earlier because his velocity is greater) and the back end slides out approaching each cone. Then he counter-steers, catches the tail while hitting the throttle and zooms on to the next cone. Meanwhile Geoff is smooth and controlled with, near as I can tell, very little sliding going on. (I’ve seen Geoff slide that S2000CR, so I know he is sometimes more ragged, but not here. Maybe he was, for whatever reason, just being slightly safe on that run. I don’t know. I’ve been wanting to ask him but he hasn’t been racing much this year.) After 11.7 seconds on-course, Matthew is 0.265s ahead. Matthew’s total lead over Geoff on day 1 was less than 0.265 seconds.

The two drive almost exactly the same line, in the same car, on the same tires, yet by the end Matthew is over 0.8 seconds in front. The only “mistake” I see anywhere is that Geoff gets a little late in the slalom around 13 seconds into the run. Just a little bit. But, you can see it costs him some time as the slalom ends and Matthew gets on the throttle earlier.

Go watch it again!

180-Degree Turn-Arounds Revisited

In a recent post here we talked about saving time in 180 degree turn-arounds by taking the tightest possible circular path. I feel like I owe it to the reader to expand on that discussion as soon as possible because, really, it left out an important consideration.

Earlier this year I competed at the SCCA Georgia Match Tour in Moultrie. The results are here. That course had three 180-degree turn-arounds, all different, and only one of which was the situation discussed in the earlier post, i.e. with the entrance, exit and path tightly constrained. I want to talk about the first of those three turn-arounds because it was the opposite: both the entrance and the exit were wide open and the path from entry to exit was also unconstrained. That is, the driver was free to go wide on the entrance and go wide on the exit and as deep or shallow as desired. You’ll have to take my word for it that this was the case, because the ability to go wide depends not only on how the turn-around is constructed but also on the course design leading into and leading away from the turn-around. In any case, this is the kind of feature one finds on big, National-type courses.

The first turn-around at Moultrie was more like the following figure. You could take any path you wanted from the entrance to the exit, constrained only by a far-away boundary at the rear.


The red circle is the tight path discussed in the previous post. I didn’t mention it then, but that path takes 3.63 seconds from B to A if the car corners at 1G lateral. What if you enter wide and take the black circle path? That path has a radius of 68 feet. Neglecting for the moment what happens before B and after A, that path takes 2.0 seconds from B to A. Yeah, wow.

The black path is not only shorter from B to A, it’s much faster and that’s why I took it absolutely as wide as I could at Moultrie, as did most other folks.

Of course, that’s not the whole story. Going wide on the entrance costs time. I will stipulate (and you will have to give me the benefit of the doubt) that going wide on the exit did not cost any time, thanks to the higher velocity at the exit, which put the engine at a higher torque point, allowing faster acceleration beyond the exit, with plenty of time to maneuver for the next feature. In any case, let’s say going wide cost .63 seconds. It wasn’t nearly that much, but let’s assume it was. (For one thing, it means I didn’t have to brake as much because the path was going to be a faster one.) Even if it did cost .63 seconds, the wide (black) path still saves one full second over the red “tight” path.

Now we know why we go wide and shallow through almost every “corner” if we can.

Cars & Coffee – Huntsville

I went to Huntsville Cars & Coffee this morning. (First Saturday of the month, Books-A-Million at the corner of the Parkway and 72.)

Of particular interest were two cars. The first a ’74 BMW 2002, bought new by the present owner, until recently a daily-driver, in great shape and with, get this: somewhere around a million miles. The owner lost track years ago at 875,000! Four engine rebuilds over its life. Two Mikuni side-drafts on it right now.

The second car had just been imported from Japan under the 25-year rule: a 1990 Nissan Skyline GTR. Right-hand drive, twin-turbo, 4-wheel drive with rear-wheel steering. I’d never seen one in person.

An amazing contrast is how simple the first car was and how complicated the second.

Saving Time: Take a Smaller Radius?

Most experienced autocrossers think that, all else equal, a smaller, slower radius takes less time than a larger, faster radius. You know that, right? Are you sure? Let’s make sure and find out how exactly much time we’re talking about.

The figure below shows one typical autocross situation, a slow 180 degree turn-around. Let’s assume the car is 6′ wide, corners at 1G and the cones create an 40′ inside radius with a path width of the SCCA minimum 15′. So, the center of the car travels on a 40+3=43′ radius on the red circle and a (40+15)-3=52′ radius on the green path. I won’t bore you with the math, but at 1G the car on the red path travels at 25.4 mph while the same car on the green path travels at 27.9 mph.


Given this is a 180 degree turn-around, we are interested in the time difference if we enter at A and exit at B. Once again I won’t bore you with the math. The inner, red path saves 0.37 seconds over the green path. The total time for the outer path is only 4.0 seconds, so the difference is almost 10%.

Hey, you were right!

Why is this? Because the length of the path increases faster than the speed. The red path is 135.1′ while the green path is 163.4′. So, the path distanced increased 21% while the velocity only increased 10%.

Now, if the course designer gave you extra room, and you were foolish enough to take it, you’d lose even more time. For instance, just 10′ more radius allows the car to increase speed to 30.4 mph and add another .37 seconds to the time from A to B.

Another way of looking at this: You know a tighter radius is better, but by mistake you enter the turn-around 5 mph too fast. (30.4 – 25.4 = 5)

You just lost three quarters of a second in this one feature. That’s called “forever” in autocross. Still want to wait to the last possible second to hit the brakes?