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 along with the trace from an 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 car 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.

TGPR Part 1

Spent the day in Munford, Alabama at the Talladega Grand Prix Raceway. This is a little 1.4 mile technical track originally designed for motorcycles. It was the first track I ever drove back in 2006 with the Heart O’ Dixie PCA region and years before I ever autocrossed. I like it because it’s got some corners not easily deciphered and there’s very little to hit if you go off.

TGPR Early Morning

TGPR Early Morning


I will never forget Alan McCrispin spin-proofing me on the skid-pad that January day. After going around in circles forever, intentionally inducing spin after spin and learning to anticipate and stop them, Alan finally had me head to the pits. I thought we were done (everyone else was) but noooooo. Alan says, “Now we swap the tires.” So, we swapped all four of the wheels left to right and did the same exercise all over again while everyone else ate lunch. Thanks, Alan! It came in handy again today, though I was sorely pressed to avoid a wicked spin on one memorable occasion. At these speeds my normally pushy autocross car wants to oversteer just a bit at the limit.

This event was the Fall Time Trial put on jointly by the Tennessee Valley and Alabama Regions of the SCCA. They traditionally do two a year, Spring and Fall, and I’ve been to several. Not being a time-trialer, I was running my B-Street autocross Corvette on last-years’ Dunlop Star-Specs (which pulled a steady 1.2g’s lateral) in the PDX group (Performance Driving Experience) and I took data all day with the Vbox Sport, reviewing it between each session. The next post will probably be a data-rich analysis of what I learned, or at least confirmed or disconfirmed today, but for now I’ll just mention the best lap for each of the six sessions, starting in the cold morning: 109.9, 110.4, 109.4, 108.9, 108.7 and, um, well, there seems to be a problem with the data for the sixth session. Oh yes, now I remember: some sort of agricultural excursion occurred that will require in-depth explanation at another time!

I think that a certain amount of track-driving is necessary for the ambitious autocrosser. Unless your local site happens to be very big, there just isn’t any other way to get experience at higher speeds. We autocrossers get really good in the 35mph to 55mph speed range. In National competition we run into courses from time to time that are quite a bit faster than what most see regionally. While we may hit the same top speeds briefly locally, we rarely have to maneuver violently at 55mph+ at a regional competition.  Where else are we going to get the car-control skills we need at the higher velocities that will allow us to do well at Nationals? Most of us can’t follow the National Tour to every event around the country.

One of the nicest race-cars in attendance belonged to Bill Coffey Sr., up from Florida, who bought the car brand-new in 1971:

Bill Coffey's 240Z

Bill Coffey’s 240Z

This beauty puts down 315 hp, weighs 2200 lbs, (which is 300 lbs over the class limit) and sports 11″ wide wheels at the rear. (Not sure about the front, except they are not narrow!) Bill says he won’t take any more weight out of it… he doesn’t want to cut on it further. One of the best-looking Z’s I’ve ever seen.

By the way, in the background of that picture, left side, is a 1976 TVR 2500M. I know this only because I had to ask the owner what the heck it was. All I could tell was something British and that it’s a really well put-together machine.

We had a great group of PDXers today, some of them auto-crossers from the area that I’ve met before. Everyone was enthusiastic, safe and polite on-track, no doubt due to the instructor group led by Chief Instructor Brett Whisenant and the safety talks given by Safety Steward Andy Tow.

As usual at TGPR, my day gradually ground down and was finally finished due to severe front brake-pad tapering. This track can be tough on brakes (I hit 104 mph and brake below 50 mph three times each 70 seconds) and while I use good racing pads which do not overheat and good, fresh fluid which does not boil no pad can prevent the tapering that results from the stock Corvette calipers not being quite stiff enough, especially when they get really hot. The modulus (E) of aluminum drops with rising temperature, about 10% by 300F, and these calipers don’t seem particularly stiff to begin with.

Here’s the track map. The running direction is counter-clockwise.

TGPR Layout

TGPR Layout

Cars & Coffee October

Under overcast skies the C&C faithful met at Books A’ Million this morning. Amid discussion of the upcoming TT & PDX at TGPR next weekend, some interesting old-school hardware, including Der Panzervagon in for some upgrades with Brett at Fusion Works:

2015-10-03 08.20.55

and a Mikuni-carbed, headered, Panasported Datsun 510:

2015-10-03 08.18.42 2015-10-03 08.18.33

A Volvo 1800ES from back when they had quirky style:

2015-10-03 08.19.57 2015-10-03 08.20.33

and a very nice Boss 302, among other more modern machinery:

2015-10-03 08.19.38

Cars & Classes In The Silly Season

The latest news from Fastrack and SoloMatters has been more about what’s not changing than what is. My car, the standard C5 (that’s Corvette-5th Generation for those not in the trade) is not being moved up and buried in A-Street. So, I’m not being forced to sell my car and buy a C5Z06. Rats! Why did S2000’s have to place 1st & 2nd at Nats in B-Street?

The Acura RXS-S will also not move to STX and the 997 GT3 will not be removed from SSR. Whew! Dodged a couple of bullets there.

On the other hand, it seems that the SEB will put some P-cars (Por-schees, as my wife pronounces it, again, for those not in the trade) into STU, But, we will have to wait until the next Fastrack in December to find out which ones. I wrote a letter with some recommendations. Can’t wait to see if they followed my excellent advice!

Actually, STU is facing other changes as well, like increasing the allowable width of tires for the 4WD boost-buggies to 265mm after much moaning on the internet about how they needed it and, I assume, at least one letter to the same effect. Any new mid-engined or rear-engined (read P-cars) added to the class would be restricted to 255mm. Hmmm. Someone that’s got a 911 with 67% of it’s weight on the rear tires is going to downsize the rears to 255? Not bloody likely.

Actually, I just realized that the notice in Solomatters mentions only mid-engined cars when the original request for comment stated both mid and rear. Maybe just an oversight, but I bet when the list comes out no form of 911 will be on it. Bummer for some people over on the Rennlist autocross forum.

Most assume the 986S Boxster will be in. I also assume the 987 non-S cars as well. The only P-car I think that might get in that can actually hang with the C5 and 350Z if only given 255s is the 987 CS (Cayman S, for those…) but I expect it’s a long-shot. Personally, I would never own that car (don’t trust the engine) but a 981 (base or S) I could talk myself into.

Here’s the results of my rating system for a selection of current and possible future STU cars with mid-engined limited to 255mm. The C5 is set to 100 points and 4WD cars are not listed because I’m clueless about how to rate 4WD. (Some say I’m just clueless all around!)

Update: 350Z added

STU Cars Rated

STU Cars Rated