The Really Weird Thing About Modern American Autocross – Part 1

The really weird and wonderful thing about Modern American Autocross, particularly as opposed to road-racing, and to a greater extent than Rallycross, is that often the driver must decide where the track is and what path to take. This seems to take a peculiar type of brain-power, or at least gobs of experience, in addition to driving skill.

I’ve talked about having to choose the track and path in one of my early posts titled No Corners, No Straights. Now I’m going to try to use a little brain-power to analyze the question:  what radius to take around an offset cone?

It’s also true that the drivers in American autocross are themselves pretty weird, and pretty wonderful. By and large the srsbsns autocrossers are fantastically friendly and helpful to the others coming up in the sport and they share a very strong, fiendishly intelligent camaraderie with each other. This sets a certain tone of friendly challenge where the most common thing is four guys in the same class donate parts, grab tools and pitch in to repair a competitor’s broken car in grid just so they can beat him! A lot has to do with the amateur nature of the sport and the type of folks it attracts. Some people think it strange that we race for  a $3.50 trophy, or maybe a Tee-shirt, but they are missing something important. A lot of the sport’s amateur charm is only possible because there is no large monetary prize at stake.

Of course not everyone in the sport is so cool all the time. For example, I see a clear difference between the srsbsns autocrossers, who I define as those with a burning desire to get to the top echelon of the sport, whether they are there yet or not, and the business-serious drivers. Some of the bad-feeling controversies of late in the sport seem to me fueled by the latter. Not exclusively, by any means, and certainly not in every case. (Those fiendishly intelligent racers are often very quick to tell each other when they are being stupid.) But, by business-serious I mean those that have somehow contrived to make a living in a manner connected to autocross. They used to be just srsbsns racers, but now have a significant monetary stake in what happens so have become business-serious as well. That always skews things, both for individuals and for institutions. (Witness big-time college football.) But, I digress.

I’m going to go out on a limb and assume that historically there has been a movement within American autocross course design. (I could be wrong about this historical movement, but you’ll get my point just the same.) The movement I’m thinking about has been along a continuum from “specified-track” course design to “unspecified-track” course design. Most of us have seen what I call a “specified-track” design one time or another, or at least as a section of a course. This is where two lines of cones are set out to create what looks like the borders of a road-course, like this:

"Specified Track" Course Design

“Specified-Track” Course Design

In a specified-track course, the width of the course is more-or-less constant and there is little choice of line. There are a few clubs around that still do this, but not many. The majority in the sport have moved on to a more challenging type of course design and use specified-track components sparingly, maybe for safety purposes or for variety.

In unspecified-track course design the cones may be few and far between so the choices of path become almost infinite. The driver must decide what line to take based on either knowledge, experience or both. Here’s the same course as above, but with less of a specified track:

Unspecified Track Course Design

Unspecified-Track Course Design

Say you are running the course above. As you approach the pointed cone in the very middle and have to turn right, what radius do you take around it?  Assume it’s a 90 degree angle from approach to departure. Unlike the way it’s drawn above, most people would say, first of all, that you should get the turning done by the time you pass the cone. That makes the cone the exit of the “corner”, not the apex, allowing you to accelerate early down to the gate at the lower right.

Here’s the situation I’ve created in order to analyze this question, except I have you turning 90 degrees left around a pointer cone & gate:


time in curve 1

3 Possible Paths from A to C Around Gate at B

Imagine you are in your car at A. You have the choice to take the small 10′ radius turn, the medium 25′ radius turn,  the big 50′ radius turn or anything in-between. Which path is best? How wide do you take the corner if there is no specified track width to create a limit?

It is commonly believed that the proper path and corner radius is influenced by the acceleration capability of the car. Dennis Grant was a strong proponent of this school of thought. (See his Far North Racing website, the Autocross To Win section.) In a car with a high  power-to-weight ratio that can also get the power down (think Super Street Modified, for example) it is believed you should tend to “drive diamonds” which means tight radii around the cones. In an H-Street econobox that can’t get out of it’s own way you should “maintain momentum” and take a wider arc, so they say. I wanted to see if I could prove the truth of this and get a feel for how much it matters. I’ve never seen such a proof in a book or anywhere on-line.  I’m sure there’s a simulator out there that could crank this out in an instant, but I don’t have access to one. I’ve worked the problem out with a combination of spreadsheet and  graphics. I’ve set it up like this:

-At A all cars are traveling 50 mph and automatically orient toward the chosen radius. They continue at this speed until they must brake for the turn. All cars brake the same (1.0g) and corner the same (1.2g). This is not quite true across the autocross classes (the classes with aero and slicks corner much harder, for instance) but it’s a simplification I had to make.

-Each car brakes at 1g in a straight line until it hits the tangent of the chosen radius. So, a big radius means you have to brake earlier, reducing the time spent at 50 mph. But, a big radius also means you brake less and later because the speed in the curve will be higher, which means you are going faster at B when you hit the gas again. A big radius also means the car will travel a longer path. See how complicated this is?

-The car then travels around the chosen radius at a constant speed dictated by 1.2 g cornering capability. Again, the bigger the radius, the faster the speed and the longer the path.

-At B each car accelerates at it’s capability, from the constant cornering speed dictated by the radius, in a straight line for 75 feet.

-I calculate the total elapsed time from A to C for cars with 2nd gear acceleration capability from .1g (maybe an HS econobox) to .6g, maybe an STU Corvette, for radii that vary from 5 feet to 55 feet.

Results in the next post!






TGPR Part 2: Anatomy Of An Agricultural

No one was going to kiss me at the end.

On Sunday I went off at TGPR for the second time in nine years of driving that track during 14 track days in three different cars. Both times were in the Corvette and for exactly the same reason, but not at the same corner. Neither off occasioned any damage, but I’m getting tired of it. I don’t want it to ever happen again. I”m going to analyze what happened so maybe I can learn my lesson. Maybe others can learn something too.

I plan to present data analysis of the cornering technique options at this track, which will have relevance to autocross, but that will follow later. Need to get this done first.

While I consider TGPR to be a very safe track, I know of at least two cars that have gone off and been totaled. These were both PDX-group cars like me, not Time-Trial racers. In neither case was there a serious injury. In both cases, I’m told, the cars went off at very high speed, travelled a long distance from the pavement on wet grass and eventually hit something. In one case it was a drainage culvert, in the other it was a tire wall of some sort, intended to stop a car before it reaches the treed edge of the property. (It worked.)

My incidents were not so bad. In both of my offs I effectively “drove” off the corner, going straight and under control (more or less) into the grass at the edge of the pavement after not being able to make the corner due to insufficient or late braking on corner entry. The first time I was probably going 15 mph when I went off. Not so this time.

Here’s the 1.4 mile track map with the way I number the corners. I went off in corner 1.

TGPR Track Map

TGPR Track Map

I’d been intentionally driving the corners in slightly different ways during the previous five sessions. I’d study the data in between sessions and compare the different techniques. Each session would consist of two warm-up laps, then two or three fast laps, a lap or two at reduced pace to allow the engine oil temps and tire temps to come down, then two or three more hard laps and then a cool-down before pitting. All this was traffic permitting, of course. I’d then come in, usually before the checkered came out, and study the data.

Using this method I was incrementally reducing lap times while some of the Time-Trialers were saying the track was going away. I don’t know about that, but air temps were increasing as the afternoon went on, so engine power was decreasing in any case.

So, let me stop right here and recognize that there may be many out there who don’t think what I was doing should be happening in a PDX group. Those folks will probably stop reading, thinking, “Well, he finally just pushed it too hard, found either the car’s or his own personal (talent) limit, and went off. Serves him right. Don’t invite him back.”

While I’m not really prepared to argue this point, I will say that I have no data readouts in the car to be a distraction or goad and, in fact, I’d be in favor of ruling out any sort of in-car timer that is visible to the driver, especially the ubiquitous lap-time predictors. On the other hand, ruling out the use of data entirely would be SCCA PDX suicide. New sports cars come with GPS lap data standard, for Heaven’s sake.

I always use point-bys, I always lift when being passed, I never pass without a point-by and generally try to be as polite and safe as possible on track, especially in the vicinity of others. This day we had at most six cars on track, which gave us lots of room. If the full complement of 12 were on the track simultaneously I probably couldn’t have ever put a full fast lap together given the speed disparity from fastest to slowest. (This is an issue almost unique to the PDX/TT format where all the PDX cars run together no matter the skill or speed.)

On the particular lap when I went off my goal was to drive one fast lap using all the “standard” cornering methods as a comparison to all the “non-standard” techniques I’d been using to get faster. Well, I never got that comparison lap.

So, what happened? Afterwards when asked, I said a couple of things: “I think I braked too late and not hard enough.” and “My brake pads had been tapering more and more as the day went on and when I tried to take that corner on a fast lap I didn’t have the braking power I expected on entry.” Let’s see if the data bears out these statements.

Here is a (very poor) overlay of the off and another earlier lap:

Corner 1 Off vs. Non-Off

Corner 1 Off vs. Non-Off

It doesn’t show up very well, but the off is in red and the non-off in purple. I think you can tell which is which in any case! The inner radius of the corner is marked by a series of cones. If you know what you’re doing you always take the corner right on the cones as there’s just a little bit of banking down there to help out. Outside of the inner one-car width the track is flat.

At the entry, the little green dot, I’m generally at 103 to 104 mph if I’d done the previous corner at max and used all the revs in the Pit straight. At F, above, the car velocity became zero. You can see this in the next graph, which has three parameters shown: speed at the top, lateral acceleration in the middle and longitudinal acceleration on the bottom.

TGPR Off Graph

TGPR Off Graph

After the car stopped, I could see the black flag waving and that no other cars were approaching. I re-entered the track and proceeding around to Pit-In. I’m very sorry that I caused the others to lose some track time.

The first thing I notice in the graph data is that at A I starting rolling off the throttle at essentially the same spot on track on both runs. So, this proves that I didn’t actually brake late.

However, the roll-off seems a bit lazy and looking at the slopes down to B, we can see that the purple (non-off) lap is much steeper. For whatever reason I did not or was not able to slow the car nearly as fast as previously. The purple lap reaches -.93G at B, while the red lap reaches only -.73G. This may have been a careless error or maybe the tapered pads.

By the time I reached C in the top two figures the car was a full 10 mph faster than normal and I knew I couldn’t make the corner. In response, I threw the car hard left into the corner, initiating a slide and depressed the clutch. Who knows? Maybe a miracle will occur!

Upon reflection, I don’t think I’d have done that (turn even harder into the corner) when I was a novice. Most novices will sort of freeze up and just run straight off, arms locked, while pushing the brake pedal down. This is something we may need to train folks not to do, though I’m not quite sure how to teach it. Others, I’m fairly certain, will disagree.

The lateral-Gs peak at D and then descend as the car transitions into the slide. From D to almost E the longitudinal G value is almost a constant negative as the tires are scrubbing off speed and making a lot of noise for what seemed like a long time. The car has yawed a full 90 degrees left. Corner workers are looking up to see what mistake the fool has made.

Not wanting to spin and either stop in the center of the track, where I could be hit by a following car, or to go off backwards, I counter-steer into the slide. The car very slowly (it seemed to take forever) turns back to the right and straightens just before reaching the edge of the track. Lateral Gs drop and Longitudinal Gs increase at E as the tires bite the pavement just before hitting the grass. The cars leaves the track surface at around 45 mph.[updated]

Once on the grass the car rotated to the left as it slid. I was just a passenger at this point, as the saying goes. You can see that the car arcs left even on the grass, but I really don’t know why. Wasn’t anything I consciously remember causing. The ground is soft and the tires are biting into the soft surface. The car is fully sliding broadside as it hits a bump at F which brings it to an abrupt halt. I was lucky that no tires de-beaded.

My conclusions:

  1. I definitely drove that corner too hard for the state of my brakes, spurred on by the desire to get some data.
  2. I drove that corner too hard, too late in the day, in the last session, when I was a little bit tired, mentally if not physically.
  3. I did not give due consideration to the loss of track camber if you enter too fast and can’t remain on the cones on the inner radius.

It all comes down to not leaving sufficient margin for the conditions, especially at the end of a long day. No one was gonna kiss me when I finished. No one was gonna pop any champagne. No matter what, I was going home poorer than when I arrived. I was lucky it wasn’t worse.

Wilmington Data Crunch Part 3: Killer Oversteer

After the big error discussed in Part 2 of this series, I got mad, got fast and made up some time. Then, it happened again!

Recall that this is what the course looks like from the GPS data, with sectors designated by dots. Run 1 is in blue, Run 3, overall slightly faster, is in red.

Fig 1 Overall Course Day 1

Fig. 1 Overall Course Day 1

I want to focus now on the revised seventh segment from the start (green dot), which is the one preceding the cross-hairs, i.e. the last one before the finish segment.

A closeup of segment 7 looks like this. It contains three sweeping corners.

Fig. 2 Segment 7 Close-up

Fig. 2 Segment 7 Close-up

Right off we can see significant differences between the two tracks. There even appears to be a kink in the red path, Run 3. Uh-oh! That’s probably not good.

Continue reading

Wilmington Data Crunch – Part 1: The Big Sweeper

This is part 1 of a series that will look at basic autocross data crunching, using data from my car at the Wilmington Champ Tour last weekend. First up: the big sweeper on Day 1.

The Mile-Long Wilmington Day 1 Course

The Mile-Long Wilmington Day 1 Course

The picture above is the whole track from start (lower left) to finish as recorded by a Vbox Sport with 20Hz GPS. Two runs are shown, Run 1 in blue and Run 3 in red. Run 3 was only 0.135 seconds faster than Run 1 overall, but that was enough to move me from 2nd to 1st for the day. What I found by looking at the data was that there were huge swings in time between the two runs. In other words, mistakes in Run 1 were being offset by better driving in Run 3, but then a mistake in Run 3 would knock it back down again. (Run 2 was mostly a running disaster. I don’t want to talk about it!)

This post will be fairly technical. That’s just the way it is. I’m convinced that most folks can’t get fast in autocross fast without learning from data. It’s really not that hard. It just takes some effort and the money for the electronics, of course. In my case, being Apple people at home, I had to purchase a PC because the V-box software, even though free, only runs on Windows. (I got the cheapest laptop money could buy.)

Continue reading

Deceptive Cones & Late-Apex Revisited

Many course designers will include a sucker cone. Reader DeWitt had some really good comments on an earlier post and submitted a photo of the finish of an autocross he did (last year with ETRSCCA?) that’s a perfect example.

The figure below shows the section leading to the finish. The orange dots represent the approximate position of the cones and the white line is DeWitt’s path from his data. (Remember that everything in the following figures is approximate… don’t get hung up on preciseness.) I added the black lines to show how I think of the “shape” of the course boundaries. I try not to reduce autocross courses to road-courses, but we will cover that in a minute. The inside corner cone conveniently circled in orange near the bottom is the “sucker” cone.

basic course.001

Dewitt, in my estimation, did well to ignore the “sucker” cone. Folks that took the bait drove a line more like the red one in the figure below. By doing that, some got into a problem at the end of the straight line of cones where they had to slow severely to negotiate what turned out to be a severely decreasing radius. Reader Dewitt thinks the red path was slower than the one he took and he is probably right, especially if they took too fast and direct a route along-side the straight line of four cones on the inside of the corner.

tight path.001Now, if you look at the way I drew the red line, ask yourself if the minimum radius is really that much tighter than the white line? It is tighter, but not by that much. Notice also that on the approach the red line is shorter. My guess is that the red line, properly executed, would not have been slower than the white line.

What do I think I would have done, if I had been as smart as I am sitting at this desk, all cloaked in warm hindsight? Something like the figure below, I think, which has two major differences from the white path actually taken by DeWitt.

best path.001

I would have departed from the white line at A, continuing to accelerate longer, taking a more direct and faster route to B. (This is almost, but not quite the same as the red path in the earlier figure. It also depends on there not being other limiting cones, not shown.) Before B I would have braked very hard to get down to the speed necessary to rejoin the white path from B onward. So, I think I get to B in less time and then go around at the same radius and speed thereafter.

This is actually a really good example of open vs. closed course design. Many clubs would have mandated the white path with more cones, forcing everyone to take the same path. By leaving it open, this club encouraged the competitors to think about what they wanted to do and what they thought would be the faster line.

The other difference is that I would have been tight on the cone at C, which means my arc past C is bigger and therefore faster, on my way to an accelerating finish at the same spot as the white path, but at a different angle. This assumes my car accelerates strongly. If my car is very low-power, I might take a tighter arc from C, ending right of center at the finish, cutting the distance to the finish as much as possible.

One other point: reader DeWitt characterizes the white path as a “late-apex” path. I disagreed. This is a good opportunity to explore this question.

To figure this out, I have drawn a more-or-less constant width track through the cones. I then draw in what I think would be a road-race late-apex path through the course, as shown in green.

late apex?.001

If I were doing a track day, faced with the track boundaries shown in black, and wanted to late-apex this corner, I would actually double-apex it, first making an apex at A and then another at B. Beyond A the car is still going very fast, at some point trail-braking into C. To form a very late-apex, one drives a path that goes down to C, which allows an early acceleration point and increases the length of the “straight” beyond B. Now, no one should do this because there is no significant straight beyond B, just another connected curve, so it would waste time to take the path at C. If there were a long straight beyond B, this is exactly the path to take as it will decrease lap time overall.

So, I leave it to you guys to decide whether the white path is a late-apex path or not, or whether both are late-apex, with the green path just very late, having made the apex (the closest spot to the inside of the track) beyond the last cone in the wall of four cones. I call the white path the “momentum maintenance” path. It makes an apex before the last cone of four in the wall. I think it is basically correct except for the revisions I mentioned.