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. 


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.



One thought on “What Trail-Braking Looks Like

  1. Pingback: Teaching Points For Novice Autocrossers | Saving Time – An Autocrosser's Blog

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