Slalom vs. Slalom

I’ve always assumed that there are parallels to be found between downhill ski competition and autocross. I never bothered to look into it, until now.

Full disclosure: I never became anything more than an intermediate recreational skier. So, you ski-racers out there can comment and tell me where I’ve got it all wrong.

Four main types of ski-racing events seem to exist at the world-championship level. The event called Giant Slalom (GS) most closely parallels autocross, I think. It runs about 1 second between gates, basically the same as an autocross slalom. The distance between gates varies a bit and sometimes there are two gates to pass on one side, but there are ranges for proper course design based on vertical drop. From Wikipedia, based on a formula: “… a course with a vertical drop of 300m would have 33 to 45 direction changes for an adult race.”

Maybe we could learn something about standardized course design for national autocross events from ski-racing.

The second type of event, Slalom, is shorter in length than GS with faster transitions and only about .82 seconds (I saw this number somewhere) between gates. I think this was the first type of ski-racing of the four to be established. Slalom transitions are faster than most, but not all, cars can achieve.

Of the two so-called speed events, Super Giant Slalom (Super-G) I’d say is most similar to Solo Trials and the Downhill is more comparable to road racing. The speed events have gates set farther apart, thus allowing higher velocities, are longer, have big radius turns and not much in the way of quick transitions.

From my brief reconnaissance of the sport, ski-racing technique has evolved rapidly over the last 20 years and maybe much longer. Much of this change has been driven by the evolution of equipment, in particular the advent of scalloped (side-cut) skis. As equipment got better humans have had to adapt their technique to take full advantage.

The ski-racing authorities have imposed limits on the equipment to keep things from getting out of hand, to keep things (somewhat) safe. They’ve had to impose minimum ski length and side-cut radii for each of the four types, for instance. They’ve had to change these requirements more than once. We call those “take-backs” in autocross, of course.

Something vaguely similar to equipment limits is happening in autocross. As power-to-weight ratios go up along with lateral-g’s in the lower prep classes, the courses at most venues have to necessarily get tighter to keep the maximum speeds in the safe range.

In general,  however, I’m not sure we can say that autocross evolution has been much driven by equipment evolution. While street tires have increased rapidly in performance, and street cars certainly handle better straight off the showroom floor than ever before, there have always been race tires and race cars at a much higher level of performance. That’s where most evolution due to technology has occurred. Someone with more knowledge than me will have to discuss whether better race tires have caused evolution in the classes where pure race tires are used.

Not that I think autocross is static. Far from it. I think it has been evolving fairly rapidly over the last 15 years or so. I think the best drivers of today are faster than those of 25 years ago. The reason? One word: data.

It’s almost comical what some people considered gospel 25 years ago. Data has cleared away a lot of the rubbish, the old wives tales, and the many ideas borrowed from the more mature sport of road-racing that just don’t apply to autocross. I think data is still driving autocross (and, for that matter, road racing) evolution today.

So, most of the talk/forums/instruction in ski-racing deals with human technique, and rightly so, especially since there are significant physical dangers in the sport, but there is some thinking about line and course strategy.

For instance, “high and early” vs. “low and late” discussions about apexing gates are common, especially with respect to the best line for beginner vs intermediate vs expert. Most teachers seem to recommend completing about 2/3rds of the turn prior to the gate, except for an expert skier who may do more of the turn after the gate. I’m sure the reader will recognize the direct parallel to autocross. I’ve done quite a bit of related data analysis elsewhere in this blog. Turning high and early is equivalent to late apexing in most peoples’ minds (not mine… I think that phrase should be banned from autocross as it is almost invariably misused) or, more properly, “back-siding the cones.”

I was particularly struck by the thoughts of Bob Harwood in an essay entitled The Road Not Taken- A philosophical approach to line and tactics, published on-line at modernskiracing.com.

Mr. Harwood writes “…what Bode [Miller] has taught us is that the old myth of one right line, the high line, is simply not true. Bode has learned that if he rocks his weight back a bit at the apex of his turn, he can ski a lower, tighter turn and still carve. Bode is able to bend the tail of the ski with more arc to carve a small radius turn with a high degree of confidence. Bode also has an amazing ability to shift his weight forward at the end of a turn so he can initiate the next turn smoothly and not get caught on the back of his skis at the start of the next turn. The end result: Bode’s balance and skills let him ski a lower, straighter line with less chance of DNF-ing than a more tradition skier…”

Sound eerily familiar? Race car driving has often been boiled down to the aphorism “The driver is simply a manager of shifting weight.”

Example: I recently co-drove a BS S2000 at the last two regionals of the year. I’d never autocrossed an S2000 before. The first event I wasn’t particularly fast. I focused on an efficient line and not spinning. (I spun anyway. But only once.)

Several places I took too slow of a line and dropped out of VTEC, which you really don’t want to do in an S2000 if you can help it. By the end of the day I had managed to get a feel for how much I could slither the rear of the car and yet not spin. I managed to beat the owner by a small amount but we were both down in the standings… I PAXed 17th of 112 and 8th of 9 in Pro, down from my average position.

In between events I watched a particular video (here) over and over again. This video shows a split-screen comparison of the best runs of Geoff Walker and co-driver Matthew Braun in an STR S2000 at the 2014 Wilmington champ tour.

Geoff  Walker is one of the guys from an adjoining region that I’ve always considered quite fast. I’ve been trying to match him for years. He trophied in STR at Nationals in 2013, one of the very toughest of classes.

Matthew Braun is simply one of the fastest autocrossers alive with multiple Solo National Championships and podium trophy positions. Lately he’s been 3rd in SSR in both 2015 and 2016, having been the SS National Champion in 2006, 2010 and 2012 and the A-Stock champion in 2003, just to name of few of his accomplishments. I’ve been lucky enough to meet him on occasion.

Walker was driving great, by my standards. As far as I can tell he only makes one slight mistake in the entire run, getting a little bit late in the first slalom. That’s it. Everything else is just perfect. Perfect, until you see what Braun does on the same course.

I see a consistent difference between the two. Braun takes a slightly smaller radius at each offset cone. He then rotates the car while going past and is able to get on the throttle earlier as a result of the car being pointed in the new direction sooner. He walks away from Walker with a higher average speed (and possibly a shorter distance traveled) at every point in the course.

Braun and Miller: Both take a tighter radius by controlling weight shift. Braun manipulates weight over, and lateral forces at, the rear tires and gets them to slide at just the right spot and rotate the car during a tighter radius turn. According to Harwood, Miller manipulates his body weight, bending the rear of his skis more, allowing him to carve tighter at the apex and take a more direct, and thus faster, downhill line. The parallelism between these techniques is striking to me.

Cause and effect are often difficult to sort out in autocross, but the 2nd event in the S2000 I beat the owner by a much larger margin, took 2nd of 6 in Pro and PAXed 6th out of 74. This is about normal for me, maybe even better than normal.

axsm033

 

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.

Tapered Brake Pads

Here’s a shot of a pair of front pads (Hawk DT-10s) used on Sunday at TGPR.

Tapered Brake Pads

Tapered Brake Pads

The pair are clamped together toward one end and you can see the gap at the other. They still have a good amount of material left, about 50%, and you can see that one is significantly thicker than the other. Anybody know what causes that?

Investigation with a micrometer shows that they are thickest in the center and worn down toward each leading and trailing end.. about .030″ max difference. They’re also tapered from top to bottom in the middle… about .015″

They weren’t working too well.

Never Late-Apex!

 

Well, almost never.

While the late-apex cornering technique is a staple of road-racing and track-day driving, it has almost zero applicability to autocross. Why? Autocross almost never has an acceleration zone long enough to make up for what you give up in the corner for the late apex. Occasionally, yes, you will want to late apex, but not very often. Autocross is mostly made up of connected curves of varying radius. Usually it’s best to simply take the shortest path from one to the next, as Piero Taruffi stated in the first-ever scientific book on race-car driving in 1959. He was right then, he’s still right.

Late-apexing is done on track for various good reasons, but the only one related to saving time is to increase exit speed off the corner by “lengthening” the straight. The increased exit speed is carried down the ensuing straight whose average speed is now increased, reducing lap time. This is the only occasion to use a late-apex: when the length of the ensuing straight is long enough to save more time than lost in the corner.

Be careful not to confuse late-apexing with the technique of “back-siding” the cones. Back-siding a cone is not late-apexing. Back-siding a cone is a result of the racer deciding where the beginning and end of the corner are and on what radius. When he tells you he plans to backside a particular cone, it means he has decided that that cone is on the minimum radius but not at the apex of the “corner” he has imagined out there among the orange cones. When he passes it he plans to try to run over the base of the cone with his rear tire, meaning he is wrapping around it in order to be going in the best direction toward the next feature. By definition, therefore, the backsided cone is not an apex in the road-racing sense because the car continues to turn hard well past it.

Now, I suppose one can be perverse and “decide where the corner is” and decide to late apex it and decide that the cone marks the late apex or is at least within shouting distance of it. In that case I admit that you have sort of backsided the cone and pulled off a late apex simultaneously. Good luck with that. Please go back and read the title of this post again.

Be aware as well that designating an offset cone as your corner exit and deciding you want to be accelerating at that point toward the next offset cone is also not late-apexing. (I plan a later post on that subject, complete with diagrams and spreadsheet calculations. I know: You. Just. Can’t. Wait.) How you performed that corner, what path you took, etc. determines whether you late-apexed it or not, not what you were doing as you passed the cone marking the exit. Did you take an extra-long, small-radius, time consuming path that allowed you to increase the length of the straight you created toward the next cone, rather than get to the cone on the shortest, fastest geometric radius? Then you did, indeed, late-apex that baby. You were probably wrong. And slow. 2019 Update: Adam Brouillard in The Perfect Corner has shown that the car that takes the “extra-long, time consuming path” never catches up to the car that didn’t, no matter how long the straight that follows.

Remember that in good autocross course design, the exact location of the corner and even it’s radius is to a large extent at the discretion of the racer to decide. Yes, most of us have seen courses completely lined on both sides with a zillion cones, all marking a path 12 to 20 feet wide. Unfortunately, some organizations still do courses like that, but the top levels of the sport have moved beyond such drudgery. People who always race on such courses will never get FAST at high-level autocross. If they should attend an event where the path is not dictated to them, or even if it has only one or two sections not dictated to them, they become lost, dazed and confused. It’s not the heat, it’s the course.

This is one reason why autocross is so challenging and so rewarding. Every course is different and full of “corners” that the racer has to look ahead and “find” before he can even analyze and then drive them! This may also be why road-racers typically have a hard time adapting to autocross if they didn’t start with autocross. On a track the “corner” is more of a fixed, known quantity. There is great skill in figuring out how to take it at ever greater speeds, how to pass someone in it, and how to not get passed in it. Plus, the same corner is different in different conditions. However, the skill of “deciding where the corner is and what it looks like” doesn’t get developed.

All Those Books On Cornering Are Wrong

Most books present racing cornering in three stages: the approach, where (after braking is completed) the car transitions from an infinite radius (straight) down to the minimum radius for the corner, the middle stage on a constant, minimum radius that (mercifully) ends at the apex, and the exit on an increasing radius, accelerating and tracking out until straight again. This is a nice, neat theory, perhaps first worked out by Piero Taruffi in his book “The Technique of Motor Racing” in 1959. If you think this concept has relevance to autocross racing, best get it out of your head.

As Taruffi goes on to say (after spending many pages working out the geometry of the three stages of every corner) the three-stage method is not the cornering method used by the fast drivers, not even in the 1920’s when he was coming up, much less the 1950’s when he wrote the book and certainly not now in the 2010s. So, put this concept into your head instead: the FAST way through a section of smaller radius has only two stages: Stage 1 is a different type of approach, which ends at the apex, followed by the exit stage as before. The Stage 1 approach is done by turning in later and while traveling faster, then transitioning into a slide (trail-braking as necessary) to scrub off speed, with the path radius constantly getting smaller (and the car slower) until a minimum radius is finally reached at the apex. The second stage, the exit, is done by accelerating on a line of increasing radius just as in the three-stage description. This method allows you to delay your braking and, in fact, to use less braking. Your average speed from turn-in to the apex is higher and the car is pointed better for the exit. It requires a very high degree of car control skill to execute. It requires you to not care about the life of your tires.

I know you want to ask, “Why don’t road-racers corner this way all the time if it’s so fast, Fool?” Probably for two main reasons. Two-stage cornering 1) causes rapid tire wear, and 2) is more prone to error, which is often positively dangerous on a track because an error can easily send you directly off the corner or off on the exit. For road-racers in qualifying, tire wear is no issue, so the fast ones do it. (Somewhat modified this technique is also used to execute passes at corner entry. Fernando Alonso is particularly good at it.) For Time-Trialers going for a fast lap or two per session, tire wear is no issue, so the fast ones do it. For autocrossers, tire wear is a given, there is zero danger, so the fast ones do it.