The Last 944 – Part 2

in line at Bristol

At the end of Part 1 I mentioned doing well at this year’s first regional autocross event at Milton Frank stadium in Huntsville. After Alan McCrispen drove the car to 2nd in ES and 3rd overall and I took 3rd in ES and 5th overall, another driver, a national champion, came up to me and said, “I think you may have something with this car, especially on big national courses.” This was encouraging since the next week I’d be taking the car to it’s first national event, Dixie Tour (near Valdosta) and then to another one, the Charlotte Tour, the very next weekend.

But all was not perfect. For one, the car was difficult to drive, at least for me. I had an idea as to why but there was no time to make big changes. For another, the winner of ES (who was also 1st overall) was a Miata driven by Goofy Gomer #1 who I knew I would probably have to beat to ever win a national event trophy. 

The Goofy Gomers are two brothers that co-drive a Miata named Scarlett (painted red, of course) in ES. We often attend the same national events. They’re good friends and named themselves by calling their enterprise Goofy Gomer Racing. Friends refer to them collectively as the Gomers. Both have PhDs, by the way. They’re very smart and, yes, perhaps a little goofy, especially when they get together with brother number 3.

Winning a trophy at a national event was The Goal I set for myself and the Last 944. As PedalFaster, an accomplished autocrosser who’s run both a 968 and a Boxster, wrote to me on Rennlist, “If you win, or even place well, the sound of minds being blown across the country will be deafening.” I thought that if I could get the car competitive and learn how to drive it then I stood a chance at snagging a trophy. Coming from a high-power class I knew I had much to learn about racing a low-power car. The driver and the car would both have to get faster to claim a trophy at a national event.

Dixie Tour on March 16th and 17th was the first SCCA national autocross event of the year. I ran into one of the top drivers at breakfast the morning of the first day and he asked me, “What are you driving now, something weird I saw…?” 

“I’m driving a 944 in E-Street so I can learn momentum-maintenance,” I told him.

“Well, you sure picked the right car!” he said, looking away and shaking his head.

The event was a disaster. ES had 20 racers, including the Goofy Gomers, with six trophies to be awarded. The others didn’t know me and were probably a little suspicious of my odd car. I promptly allayed any concern by spinning on two of three runs the first day, landing myself in 14th and long out of the trophies time-wise.

I mentioned that the car was difficult to drive. One of the spins, right before the finish, was such a spectacular fail that it received an ovation from the crowd. The best I could hope for on the 2nd day, when we would run the same course in reverse, would be to climb a couple of places. That didn’t happen. It rained. I didn’t have rain tires. I dropped to 16th.

The necessity to have two sets of tires in Street classes, one set for dry and damp conditions the other set for heavy rain, is a new thing. What’s happened is that the tire makers have created specialized tires that have huge grip on dry pavement. You must use one of these specific types to be competitive. They’re about as good in the dry as race slicks of 20 years ago yet they have treadwear ratings of 200 and are perfectly legal for use on public streets. Unfortunately, they gave up some wet traction and hydroplaning resistance when they optimized these tires for warm (non-freezing) and dry conditions. Then they created different tires that work really well in standing water. I don’t have a set of those or a second set of rims to mount them on.

I watched the car in front of me in grid switch from dry to rain tires as it really began pouring. The driver went from last place, after three spins in a single run, to first. The Gomers had a set of rains and placed 7th (one place out of the trophies) and 10th. 

On to Charlotte, but only after a week of licking my wounds on St. Simon’s Island.

Charlotte was hot and dry. I finished lower mid-pack in 10th place out of 15. The Gomers were close behind in 11th and 12th. I managed to not spin and not hit cones but was discouraged by the result. The course should have been good for the 944 but I was one second per run behind 5th, the last trophy position. In autocross a second is a looooong time. I thought I’d driven well but it wasn’t nearly fast enough.

The 5th place trophy winner told me he was impressed by my times in the 944, not far off some good Miata and MR2 Spyder drivers. (Racers from Pennsylvania to Florida were there.) He had autocrossed a 924S back when they were competitive against the first generation Miata, but no one had ever been competitive in the heavier 944, much less later against the 2nd generation Miata. This was nice to hear but it was clear that being able to compete for a trophy, which was my definition of competitive, was not yet in the cards. I’d come to a tentative conclusion that the car was lacking steady-state front grip but I didn’t know why. 

Back in Huntsville I got together with the Gomers to compare data and it showed what was happening. I had a slight acceleration advantage over the Miata, but this was being equalized by a need to shift to 3rd gear when they could stay in 2nd. Shifting to 3rd usually means a downshift back to 2nd, sometimes twice per run, and extra shifts cost time even when done perfectly. Downshifting is especially tricky. Autocross requires ten times the number of driver inputs per time interval as compared to racing on a track and we don’t get to downshift while braking at the end of a straight. There are no straights, only curves where you are either accelerating or braking while turning. (If you try to “create” straights with a low-power car You. Will. Beeeee. Slooooooow.) 

Typically the downshift must happen while simultaneously trail-braking into a corner that’s either an increasing or decreasing radius but almost never a constant. It requires a high level of car control and coordination to a) not lose the rear end of the car when letting out the clutch in the middle of the corner, properly rev matching while continuing to trail-brake perfectly, b) keep the car within a foot or two of the correct line and hit your apex at just the right angle, and c) be back to full power at the apex not a moment late. A PDK transmission would make things so much simpler!

In transitions, like in a slalom, the data showed that the best I can hope for is to limit the time lost to my narrower, lighter, more nimble competition by driving the heck out of the car. When Goofy Gomer #1 remarked, after driving an autocross run in the Last 944, “You should never be able to beat me in this car” I think he was mostly reacting to a lack of agility. When I explained to him that Porsche had intentionally put the engine at the front and the transaxle at the rear to increase the polar moment of inertia and force the car to react more slowly than, say, a mid-engined car like the 914 or the Boxster/Cayman or every purpose-built Porsche race car in history he just looked at me like I was, uh, goofy.

“Why the heck would they do that?” he asked.

“I think it was so magazine testers who couldn’t drive could go fast without crashing and then they would write about how great it handled,” I said. “Chevy did the same thing with the C5.”

It was in the sweepers where I thought the car should be a match for the others or perhaps even have an advantage. Unfortunately, the data showed the Miata walking away from me in every long corner. It was simply generating more lateral grip. 

This was really bad. It meant that even if I was a better driver than many I was unlikely to ever beat enough Miatas and MR2 Spyders to climb into the trophies.

Now I did something I should have done long before: I calculated the stiffness of the front and rear suspension of the car with the steel springs, torsion bars and bump stops all acting together. The results were surprising.

You may remember from Part 1 that a key advantage for the Last 944 is the ability to use the bump stops to stiffen the car in roll. How stiff should the suspension be for an autocross car that competes on surfaces that, unlike a real race track, are often various levels of bumpy even at relatively low speeds? And how stiff was my car, anyway? The answers will require me to get a little bit technical, so brace yourself.

Turns out that a Canadian autocrosser and engineer named Dennis Grant (see his website Autocross To Win) did some testing and decided that the front end of an autocross car works best with a natural frequency of 2.2Hz and with the rear end a little stiffer at 2.5Hz. (People like to argue about these numbers, but Grant became an SCCA national champion in the car he developed.) Natural frequency is a measure of the stiffness of a spring-mass system, like the mass of one corner of a car sitting on top of a suspension spring. These numbers are lower than a typical road-race car, which might be 3Hz, and many autocrossers were then using and continue even now to use setups even stiffer. For reference, a modern sports car might be as much as 1.5Hz as delivered new. The stock springs on the non-modern 944 give a stiffness of only about 1Hz.

Elastomeric bump stops don’t really work like coiled steel springs (or torsion bars) but I was able to squint a little and derive an estimate for their effective spring rate over the distance they were probably being compressed. I made a spreadsheet and put these values in parallel with the steel, working through the correct motion ratios, along with estimated weights of the sprung mass at each corner.

The calculated rear stiffness was 2.6Hz. Almost perfect. Perfectly dumb luck.

The calculated front stiffness was 3.5Hz. OMG! as the kids text these days.

I had originally purchased bump stops simply based on the front stack of three being the same stiffness as the rear stack of two. I had utterly failed to take motion ratio into account. The bump stops on the front struts are very efficient with a motion ratio of 0.91. The bump stops on the rear shocks, due to their position, work through a motion ratio of only 0.63, a huge difference since it’s the square of these numbers that’s actually used in the equation.

The overly stiff front bump stops and the heavy shock damping they require for control are almost certainly what’s been impairing the grip.  If the spring rate and shock damping are too high then when the car hits a bump the impact energy is not well absorbed and it accelerates the car upward. This actually pulls the weight of the car off the tires (until it comes back down) so the tires lose grip. This not only happens on big bumps but also smaller irregularities where the pressure at the contact patch will vary so much that the grip is uneven. This lowers the effective grip and makes the car hard to drive. In a sweeper you only have as much grip as the weakest end.

To fix this I calculated what spring rates/durometers the bump stops needed to be to get 2.2Hz in the front and ordered new ones. At the next event with the new bump stops, TAC/TVR #2 on April 6th at Milton Frank, I won ES. I beat both Gomers by a second. The car had more grip and it was easier to drive. One event, though, especially at Milton Frank which has a weird surface, doesn’t mean much. I missed TAC/TVR #3. At TAC/TVR #4 I lost to both Gomers by a second, which muddied the waters.

What you need to realize is that I’m making constant changes to alignment, shock absorber damping and front sway bar stiffness in between and even during the events. Changing the front bump stops necessitated a cascade of other changes. To get to the top in autocross we treat all events, especially local events, as tests. If you’re not testing, making changes and evaluating the effects you can’t develop the car. (The same goes for the driver.) So, results with a new car are often inconsistent. Add to this the natural inconsistency of human performance and the varying character of different courses and surface conditions and… well, you get the point. I’m making excuses.

I made another major change before what might be my last and best chance at a trophy, the Bristol Match tour at Bristol Motor Speedway upcoming on July 5th and 6th. (In the parking lot, of course!) I mounted new front tires in 245mm width, up from 225mm. This is way “over-tired” for a 7” wide wheel and the same tire I have on the 8” rear wheels, but it worked. With all the other changes I now had so much more grip from the front than the rear that when I ran at our local Test and Tune event the week before Bristol the car was seriously unbalanced. Though I’d increased overall grip by improving the weak end of the car I was forcing the poor driver to adapt quickly or die.

Bristol was really hot. This was good for me because I’m using a tire that likes heat, but takes a run to warm up. Without a co-driver to warm the tires you know going in that your first run better not be your best. The driver needs to be consistent over the span of three runs, recognize and fix the inevitable mistakes and rarely throw runs away by coning. You must be confident that you can lay down your fastest run on the last one.

bristol start

Lining Up For The Start At Bristol

At a Match tour you get three runs in the morning then three more that same afternoon. The fastest times from each set of three are added together for your total time and that decides the class results. After the morning set I was 6th of 9 drivers. Trophies would be awarded only for 1st, 2nd and 3rd. 

I was way behind 1st (a Miata) and 2nd (an MR2 Spyder), but from 3rd on down was very tight so I had a realistic chance at the last trophy. I had an equally good shot at last.

My first run in the afternoon is faaaast, baby, and I vault into 2nd place. The MR2 that started out in 2nd cones his first run but I know he’ll probably clean it up, so to be honest I’m still racing for 3rd place. My real competition for 3rd is now an older guy (like me) in a Miata. We’re gridded with our cars right next to each other. He’s only 0.1 seconds slower so far. It’s clear that the one who improves more on the subsequent runs will likely snag the 3rd place trophy. We wish each other luck as we adjust tire pressures.

bristol showcase

In The Showcase Turn At Bristol

My second run is almost a half-second faster. I’m very pleased. Once all second runs are complete I’m still in 2nd place! The Miata next to me in grid spins and knocks a wall of four cones into the next county. The MR2 that started the afternoon in 2nd lays down a clean but slow run. He needs to get a lot faster or he won’t even trophy. Each of these two guys has one more chance to take 2nd and 3rd and drop me to 4th. The pressure is really on them to find some speed and finish off this pesky 944. Meanwhile, the Gomers are down in 8th and 9th but giving me encouragement.

I make a significant driving mistake on my third run (while downshifting) and go slightly slower, so I must stand on my second run. The MR2 finds his speed and goes 0.004 seconds faster than me to take back 2nd place. My competition for 3rd place in the Miata gets faster and runs clean but is still a tenth off my 2nd run. I win 3rd place by 0.054 seconds and take the last trophy. 

Goal achieved!

trophy2

With The 3rd Place Trophy

I think I’m finally getting the hang of this momentum-maintenance driving. It’s a complex equation, trying to figure out when going a few extra feet here will save time through there, or not, within the complex set of connected corners that constitute a modern autocross course. It’s a mental skill necessary to reach the highest levels of the sport. With a more stable car configuration I hope to focus harder on the driving going forward. That’s probably where the real time is to be found in the future.

But I’ve continued to make changes to the car. At Bristol the grip was unbalanced with a tendency for wicked lift-throttle oversteer that could be disconcerting. Pictures also show that it rolls too much thanks to the softer front bump stops. I’ve reduced the rear shock damping and added more front roll bar stiffness. These changes get tested tomorrow at Milton Frank.

I have one more national event, the Peru Tour in Indiana, before Nationals themselves the first week of September. The Gomers won’t be at Peru but many other racers in Miatas and MR2 Spyders are registered who represent the best in E-Street from the Midwest. 

The National Championships will be held in Lincoln, Nebraska. I’m registered in the 944 and the Goofy Gomers and my wife and I will be sharing an Airbnb rental. If anything interesting happens maybe there will be a Part 3 to this story.

Edit: I just became aware that Chuck Matthews is using bump stops in a similar fashion to tune his ES Miata. Rats! Seriously, go see his blog at mathews racing.com

The Last 944 – Part 1

 

bristol showcase

This year I’m autocrossing the last base-model 944 that will ever run in Street class in national competition with the Sports Car Club of America (SCCA).

I know this because the SCCA has a 30-year sunset rule in Street. 1989 was thirty years ago and was also the last year of 8-valve-engine 944 production. So all the other years of the base 944 have sunsetted except the 1989 model.

Street category is the lowest SCCA autocross preparation level, meaning the car has to be almost entirely configured as it left the factory. The sunset rule was implemented because after 30 years memories fade and documentation gets scarce so it can be difficult to agree on what is or is not a legal configuration.

In SCCA autocross the cars are separated into classes within preparation levels. The base 944 is classed by the SCCA in E-Street. E-Street is dominated by two cars: the 1999 Sport Edition Miata and the 2003 Toyota MR2 Spyder. No one seriously runs much of anything else in national competition. Both are 2200lb-ish cars. The 1989 Porsche 944 is, well, nowhere near that weight figure what with a heavy, balance-shafted 2.7 liter motor, a back seat, a huge glass hatch, a/c, cruise control, and power everything. It doesn’t have enough additional wheel and tire width to make up for the heft so no-one thinks it can be even remotely competitive. None have been run seriously in national competition in the 10 years since I started in the sport.

In my feverishly foolish imagination I hypothesized that the 944 has two quirks that might possibly allow it to be competitive in E-Street today where handling and cornering ability are king. The first is big negative tire camber made possible by the adjustability of the M030 sport suspension option. The second was Porsche’s use of tall bumpstops. While the M030 sport suspension option is well-known, none ever took advantage of the tall bumpstops as far as I can tell. Please allow me to explain, but I’ll have to get a little technical.

Camber is the angle of the tires as compared to the road. If the tires are perfectly vertical the camber angle is zero. If the top of the tire leans in toward the center of the car then that’s a negative camber angle. Most cars come from the factory with a slight amount of negative camber, like ½ degree or maybe as much as one degree. This allows the tires to wear evenly as long as miles spent in hard cornering are limited. Generally, more negative camber, up to as much as 4 degrees, is better for hard cornering, which is about all we do when autocrossing.

The rear of the 944 is sprung by torsion bars, almost exactly the same design as on the 911, and always allowed for infinite adjustment of rear ride height. Not an easy adjustment, but it can be done. The M030 option adds ride height adjustability to the front struts. With that you can lower the front and rear evenly while still using the stock springs required by the rules. Significant lowering of the car is what allows a big negative camber angle at the tires.

So, I gathered up the various parts that make up the M030 option and installed them. Some parts were still available new but many were not and I had to find them used. Now the car has three degrees of negative camber all around and is about 3/4” below the standard M030 ride height which, in turn, is somewhere below the stock suspension ride height. The car is now quite low, as in I’m getting to old for this low, especially with thin modern tires on 15” diameter wheels.

Three degrees is a serious camber number for a Street-class car. You can’t get three degrees in any standard Boxster, Cayman or Carerra. Only the GT cars now allow that much adjustability. Tires like that much camber in the corners. It makes them happy. When they’re happy they deliver more grip and the car corners faster. 

A fly just flew into the ointment, however. The low ride height has a negative side effect on the 944: it makes the front of the car roll more easily while making the rear roll less. Why it does this is very technical so I won’t bore you with it. (If you really want to know why, I explain it in this post.) The upshot is that the more the car is lowered the more the famous handling balance that Porsche gave the 944 gets upset. This roll balance problem can, in theory, be overcome by tuning the tall bump stops.

Bump stops are generally pieces of rubber that keep the shock absorbers from internally crashing metal into metal when fully compressed during a big bump, say, when you hit a deep pot hole on the interstate at 70mph like I did once. They are often only about 1/2” thick. 

What many people don’t know is that by the 1980’s Porsche was doing an innovative thing with bump stops: they were using tall elastic bumpstops as auxiliary suspension springs. The stock front bump stop, shown below, is 3-5/8” tall. It occupies almost all of the free shaft length at normal ride height. Notice the complex shape.

The idea was that while cruising down the highway the soft primary springs give a comfortable ride. When you crank the car over into a corner the bump stops come into play and progressively stiffen the suspension for responsive handling. This design philosophy was still in use when the 986 Boxster and 996 Carrera began production in the late 1990’s. 

The tall bump stop design method is important in Street-class because one of the few allowances is that bump stops are FREE!!! along with the shock absorbers. Some have called this a loop-hole in the rules but it’s a well-recognized and legal loop-hole. Miatas, for instance, also have relatively tall bump stops that need to be in good condition for best handling because they come into play when cornering.

Well, bump stops are almost free. You can make them from anything you want, including jello (not recommended) or solid steel (also not recommended) or get them in various rubber stiffnesses which is what I’ve done. What you cannot do is make them any taller than they were before. 

So you must be very careful because if you take a trophy from a Miata driver at a National Tour event while driving a 30-year old Porsche that everyone knows is slow you’d better be able to prove that you’re legal. I carry a notebook with relevant information for my competitors (or a protest committee) to peruse at their leisure.

This means that I’m free to install stiffer bump stops to stop the car from rolling over like a drunken sailor, which is the stock behavior of most 1980’s sports cars, including the 944 and especially a 944 that’s been lowered, as previous explained. Bump stops allow the roll stiffness of the front and rear to be tuned separately and keep the negative camber we obtained from totally disappearing in the corners due to body roll. The stiffness of the bump stops, if chosen to be stiffer than the stock springs, should also make the car transition from turning one way to turning the other way much faster. Maybe almost Miata fast. Transitioning quickly (nimbleness) is another thing that’s very important in autocross.

The picture below shows what the new bump stops look like on one of the front struts. The three yellow donut-like things are the bump stops sitting on top of the shock body and riding on the strut shaft. In a corner they will slide up the shaft and get squeezed against the top hat which is out of the picture.

frontbumps

The next picture shows the rear shocks with their new bump stops:

rearbumps

 

All is not quite so simple, however. I found out that some autocrossers have tried to use stiff bump stops before, if not on a Porsche. Apparently, they all gave up. At least the ones that are talking gave up. (Edit: I recently became aware of Chuck Mathews and his efforts in this regard.)

The problem they encountered is the abrupt transition from the stock soft suspension spring to the stiffer bump stop. Porsche solved this problem by using a tall, tapered bump stop shape that very gradually adds stiffness. I can’t do that. I need to stop the car from rolling right now, or at least almost right now. To explain this problem I have to tell you a super-secret engineering secret not normally revealed to those not initiated into the secret engineering lore known only by those who actually read and understand the textbooks: load is preferentially and proportionally attracted to the stiffer path.

Let’s imagine we brake the car and turn left. The right front spring and shock absorber will compress the most. The load into the tire increases gradually and proportionately as the spring compresses. The back of the car is following along, but later. When the stiff front bump stop is encountered there will be an immediate increase in the tire load. It’s as if weight from all around the car suddenly jumps into the front right tire. If this increase happens too fast the tire contact patch becomes overloaded and gives up grip. The right front tire begins to slide. Massive understeer is the result and the driver has to make a big correction which costs time.

In other situations it may be a rear tire that gets shock-loaded and calls in sick. The result in that case is oversteer. A car that oversteers one second and understeers the next is not fun. It is not fast. It can be, essentially, undriveable. This is the big fear, the big problem others have run into. I have only a few weeks and a handful of events to work the bugs out.

So how has it been working, you ask? Find out in Part 2. Here’s a hint: multi-time Nats champion Alan McCrispen remarked, after watching me take a run in the car at its first local autocross, “I’ve never seen a 944 corner so flat or change direction in a slalom so fast.” That was the good news. 

The bad news was that I found it a difficult beast to drive and it didn’t seem to have all that much cornering grip. Alan, a much more accomplished driver, was having fun with it and we both placed highly. But we were beaten by a well-driven Miata that took first place overall. Not a good omen since I know that at a national event the drivers will be even faster.

I think I’ve figured out the big issue and have a plan to fix it. Stay tuned.

Below is what the car looked like ready to compete at Dixie Tour, the first national event of the season.

dixie

Line Theory: Perfect Corner 1 & 2

In Perfect Corner 1 and 2, Adam Brouillard has made the most important contribution to racing line theory since the first technical book on the subject, Taruffi’s 1959 The Technique of Motor Racing.

A few months ago I started reading, studying and applying knowledge gained from these two books. They are an exposition of line theory, the theory of what driving lines are most efficient, i.e. the fastest, around a race track. Brouillard takes a physics-based approach.

With Perfect Corner 1 and 2 we can look at all the various permutations of line theory since 1959 and clearly understand what is right about each one and why and what is wrong about each one and why. When a new theory explains all the previous ones, that makes for a powerful theory.

Brouillard teaches that there are only three types of corners in all of road racing and autocross. (He tells me he began as an autocrosser.) The three are the standard corner, the chicane and the double-apex. That’s it.

A standard corner has enough space before and after to stand alone so that the entry and exit can be optimized without regard to the previous or following element. A chicane is defined as two corners in opposite directions that are so close to each other that they must be optimized together. (The autocrosser’s slalom is two or more chicanes end to end.) A double-apex is two corners in the same direction that are so close to each other they must be optimized together.

The books give rules for how to classify each corner you will encounter and rules for how to determine the most efficient line through each type.

My approach to autocross is now forever changed because of Brouillard. What I’ve realized in doing autocross events this year with his concepts and rules in mind is that many autocross courses are more complicated than most road-race tracks. Some autocross courses essentially have no stand-alone elements. Everything is connected in a series of chicanes and double-apexes with only the rare standard corner. So, applying the methods and rules he gives is not easy. Some of it is immediately applicable, some is not. He tells me he’s thinking of writing an autocross book. I hope he does. Soon!

In the meantime, get these books and start a new journey into autocross.

What Trail-Braking Looks Like

TAC3 180

Big Sweeper At TAC/TVR #3

 

One of the difficulties in learning anything is working through the trash. The ‘trash’ is what I call all the myths, supposed common-knowledge and just plain wrong stuff people tell you that can send you down less than optimal learning paths. If you’re someone like me that has to first get it in his head intellectually before the body can do it, you may be particularly susceptible to well-meaning but wrong advice or supposed facts that aren’t so factual. Even correct information delivered at the wrong stage of development can cause learning to go off track. We can’t start at the top. We have to start at the beginning. There’s always a progression.

I guess if you begin with a world-class instructor in a well-developed field (one where effective teaching techniques have been developed over time and are widely known, like music or golf) then the trash problem is minimized. I don’t think autocross is quite there yet, but the data revolution is changing that. If you’re Dad or Mom happens to be a great autocrosser, knows why she’s fast and can teach it, then you’re in the soup. Very few get so lucky.

In my case my Dad was a multi-sport athlete and tremendous competitor who could never understand this nutty autocross thing. He always wanted to come watch the event if I was racing in his city but he never, ever rode with me.  Not once. He just wouldn’t do it and I never understood the reluctance. He would say, “I don’t want to encourage you” and smile as if it was a joke.

Trash example: the purpose of trail-braking is to help get the car turned in a long corner, like the one shown above.

Maybe I heard it wrong (multiple times?) but this is what I remember people would say when discussing track driving and the difference between braking for a corner in a straight line then turning in for a late apex (Slow-in, fast-out) vs. the more advanced (and, OMG! dangerous!?!) technique of trail-braking into a corner. My problem was that I’d believe stuff like that and think it was the real reason for trail-braking when maybe it was just an easy thing to say, or it was being said by someone who didn’t really know why trail-braking was a technique for Saving Time. (Yes, I’m kinda slow like that.) I carried that idea into autocross.

It was in my head and wouldn’t come out without great difficulty, i.e. progress in learning that can replace the simple idea that trail-braking is for rotating the car with a more sophisticated idea.

I’ll tell you The Non-Trash Truth: no one can Save a lot of Time in autocross without trail braking the heck out of any long corner, like the one shown at the top. We see a lot of those in autocross and many tracks have something similar. The feature shown above didn’t even have an apex cone. Just an entry and an exit and you figure out how to get from one to the other as fast as you can.

Trail-braking has little to do with turning the car by putting weight on the nose and freeing up the back tires to slip more. Sure, you can use it for that and may need to, depending on the type and setup of the car, but it’s not the most important reason why you should trail the brakes entering most long turns.

The real reason is because of the physics of tire performance. Unlike me when I started out, tires can do two things at once. They can both brake and turn at the same time, just like they can accelerate the car forward and turn it at the same time, but that doesn’t seem as hard to understand. The two capabilities added together are more powerful than used separately. Proper use of trail-braking allows you to brake later into the corner, thus extending the time spent at a higher speed (extending the length of the previous straight for you track drivers), to take a shorter, elliptical path to the apex, and to take that path at a higher average speed. Those three things sound like they’d Save some serious Time, don’t they?

So, go learn how to trail-brake.

This isn’t a how-to article on trail-braking, but I will show you what it looks like in data. If you’re like me, you need some convincing first so you can really commit to learn it later. Read this article then go read some books on racing. I like Krumm’s Driving On The Edge. He’s a professional racer that figured out how to drive long corners by trail-braking into a double-apex by analyzing data of the same corner driven over and over again various ways by various drivers.

Then, go practice. Where? At the autocross event, of course, where a spin only costs you a little tire rubber.

The data below is for the turn shown at the top of this post. The top trace is speed, the middle trace is how hard the car is turning (lateral force) and the bottom trace is how hard the car is braking (negative) or accelerating forward (positive).

From the point marked ‘Lift’ the LongAcc goes steeply negative. This is hard braking. Notice that just above the LatAcc is turning positive. That means I started turning left at exactly the same time as I was braking. (This is a little unusual, but I was in a bit of a hurry.) And I keep it up.

In the section marked ‘Trail braking’ the negative acceleration is gradually trending up to zero, i.e. I’m gradually coming off the brakes. Meantime, the LatAcc continues to build up to well over 1g. The tires are providing the ability to brake and turn simultaneously. This is the data signature of trail-braking.

TAC#3 180data

TAC/TVR #3 First 180

The other thing to notice is the shape of the path. It’s an almost perfect portion of an ellipse. The physics of the situation dictate that it be this way if you do it correctly.

A Real World Comparison

At the Blytheville Pro-solo a few weeks ago I put my data device into another car and got data for three different drivers: Ryan, Tom and me.

Ryan and Tom were in a BSP Miata on race compound Hoosiers; I was in my BS Corvette on Bridgestone street tires. The course contained an almost perfect, more-than-180 degree sweeper, entered from a slalom just like in TAC/TVR #3, above, marked by an entry cone, a center “apex” cone and an exit cone. Each of us did this corner in his own way. You can see the path differences in the right of the figure and the data on the left. 

BPS180 data

2018 Blytheville Pro-Solo Turnaround (Left Side)

 

Looking at both the LongAcc (longitudinal force) and the LatAcc (lateral force) we see the trail-braking signature in the data. After braking hard, Ryan’s red line only very gradually heads back to zero, that is, he’s staying on the brakes as he turns in more and more, only very gradually releasing the brake pedal, taking best advantage of the tires’ ability to multi-task. This allowed him to maintain the highest entry speed and yet not overshoot.

The major difference as I see it was that Ryan, clearly the highest level driver of us three, did a much faster straight-in approach and a perfect trail-brake entry. His minimum corner speed was 38.7mph. I (green) did a slightly wider approach and a less than perfect trail-brake, attempting to agressively go shallow and accelerate to the apex.  (It’s a big corner, much larger than the corner from TAC/TVR #3, so big and with such a fast and difficult entry that everyone was accelerating to what would normally be called the apex. Effectively, we all double-apexed this monster.) My minimum corner speed was 35.8mph, almost 3mph slower than Ryan, not too surprising given the car/tire difference and the different strategy. Tom (blue) went widest for a classically best entry angle, did not trail-brake, but was able to accelerate to the apex sooner than I and catch back up to me. His minimum speed was 38.3mph, just slightly less than Ryan.

Once at the apex cone all three cars had speeds contained within a 1mph band. From entry to apex cone took a bit more than 4 seconds during which time Tom and I lost 0.25s to Ryan. This can be seen in the bottom trace, where Tom and I (blue and green, respectively) are compared to Ryan, the horizontal red line. The more the blue and green lines are above the red, the more time they’ve lost to red.

For my part, I think trail-braking is what allowed me to match another car to the apex that had greater grip but whose driver didn’t trail-brake. 

Addendum

I’ve become aware that Brouillard claims that the shape of the trail-brake curve is an Euler spiral, not part of an ellipse as I stated above. I’ve now ordered all his books and will study on it. I don’t see how Brouillard can be correct (if this is actually his claim… I read it in Wikipedia) when the radius of curvature of such a spiral varies linearly. That’s the definition of an Euler spiral.

Euler spirals were first used in the railroad industry to transition from a straight to a curve without literally jerking the passengers around. They also reduce loads on the tracks. In autocross, of course, we’re not too worried about a little jerking, which is literally the time derivative of acceleration. Lots of little jerks in autocross.

The trail-braking curve seems a non-linear situation, even if we assume a perfect circle for the tire traction “circle” and a linear release of the brakes, since the radius varies with the square of the velocity. I think the the curve shape is more complicated, more like an ellipse with a non-linear variation of the radius of curvature. My assumption of an ellipse, based on what the path actually looks like in the data, may be an approximation that’s not mathematically correct. So far, I’ve not found a mathematical description of the trail-braking curve geometry. Maybe I’ll find it in Brouillard’s books. If so, I’ll come back and tell you about it.

 

 

Extra Twist?

Someone asked this question in an on-line critique of various run videos from our latest event: Not being the expert you guys are, I enjoy the critiques. What I notice is that I and others will start a turn, hold it for a while and then just as we [get] to the cone we give the wheel an extra twist to get around the cone and on the line we want. Or am I just seeing good technique?

While we all make mistakes, and we all have to make corrections (for instance, the level of grip is not necessarily constant in even a single turning element) Steve Brollier (multi-time national champ) taught in our autocross school last year that we should strive to turn once for each slalom cone, for instance, and once for each offset cone. I think this applies, in general, to all turns.

As someone pointed out in my video (which can be seen here TAC/TVR#3 Run video) at 1:07 in the final turn to the finish I make a preliminary turn and then the “real” turn. As a result, I have to turn sharper, which means slower, and I lost time there.

The “extra twist” being talked about may be a valid technique in certain situations. I’ve always called it taking advantage of the ability to dynamically shock the tires and get a little extra out of them. There aren’t many places where you can use it, however. If you’re doing it at every corner, it’s probably covering up a basic fault. You’re probably cornering too much under the limit over a large portion of the turn and only at (or above) the limit in the final phase. You may be turning too early and too slow, rather than turning later but with greater steering wheel speed.

I remember doing a lot of the “extra twist” technique when I was new. I think it may be caused by the lack of confidence in turning hard at higher speeds. We get comfortable with turning hard a low speeds first, so that’s where we do it. As our level increases we get more comfortable with quickly getting to the cornering limit at higher and higher speeds. Turning the steering wheel as fast as conditions allow reduces the transition time from one turn to the next, or from going straight to turning in, which has a direct effect on the speed you can carry, how late you can brake and ultimately elapsed time on course.

I think the process of “getting fast” is 1) learning how to evaluate the proper line to take, for your particular car and driving style, 2) developing the car control skills necessary to make certain maneuvers and be able drive the line you’ve decided to take, which again is highly dependent upon the type of car, and 3) gradually reducing the number and severity of mistakes, which implies that you have gained the knowledge of what constitutes a mistake. Making multiple inputs in what should be a single, smooth arc is definitely a mistake, but doesn’t by itself mean you won’t be “fast” in relation to someone else just because you’re not perfect. A lot depends upon the magnitude of the “mistake.” It does mean you have room for improvement. (I’m discounting the often-rapid corrections you have to make to keep a car on the limit of adhesion.)

Earlier this year I got to sit in on a video critique session with a group of accomplished autocrossers. One of the top drivers on the national circuit (another multi-time national champion) was watching his own video from the course we’d all run that day. The level and completeness of the critique he gave himself was impressive. “Oh, I got late there,” he says at one point, and I’m looking at it thinking the error was so incredibly slight that I would have never noticed it. Upon first view I would have said it was a flawless run. Only after repeated viewings could I see what he saw.

There are levels and levels.