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Proper Braking on the Track

Just read this article over on NASA, written by Joshua Allan of RaceMentor:

From the article....
Many drivers have difficulty in the braking zone. A typical driver approaches a red light or stop sign with light initial braking and increasing pressure as the intersection approaches. This technique is also common in new and intermediate level drivers on the racetrack. While the street driver makes the experience uncomfortable for passengers, the track driver is artificially reducing the capacity of the tires.

Drivers often try to shave time in the braking zone without reaching the corning limits of the car. Braking technique is the most critical factor to carry momentum into a corner and it has more to do with brake application and release than it does with late braking and heavy brake pressure. Some believe it’s all about timing: brake at this point, with this pressure, and release at the turn-in point to be at the right speed. This ignores how track and tire conditions vary throughout a race. More important is the benefit of managing weight distribution between front and rear to aid turn-in. Drivers who consistently feel they could carry more speed into corners should evaluate their braking technique and get back to basics:

  1. Apply the brake rapidly, firmly and just so. If it’s too fast then the wheels lock or the suspension bounces. Too slow and the braking distance is increased. Also, don’t worry about threshold braking initially — that comes with experience. It is more important for drivers to be consistent with incremental improvements.
  2. Maintain steady brake pressure for a period to scrub speed as the corner approaches. Cars with aerodynamic aids must gradually release the brake pressure throughout the braking zone to compensate for the loss of downforce.
  3. Gradually release the brake approaching and through the turn-in point. This is key to carrying momentum into a corner. A rapid brake release causes the nose to lift along with a momentary reduction in grip and turning capacity. A gradual release allows the driver to gauge his entry speed and adjust the braking force accordingly. This also allows the suspension to maintain load on the front tires, which are doing most of the work. Carrying a little bit of brake past the turn-in point is called trail braking, which provides a bit of extra grip for the direction change.

BrakeCurve.jpg
The middle plot of this AiM data graph, longitudinal g’s show the red line dips steeply and rises sharply, indicating two things when compared with the green and blue lines: over-slowing and a sudden release of the brake pedal. It is better to release the brake gradually approaching and through the turn-in point.

There are exceptions of course but these are advanced concepts to be explored later. Finding the braking threshold can be difficult and takes practice. Proper braking technique on the other hand is fundamental to maximizing speed into and through corners. It’s better to brake with slightly less force and find a consistent, optimal entry speed than to dive in deep with tires at the limit.
 

Dave_W

Cones - not just for ice cream
356
343
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
A typical driver approaches a red light or stop sign with light initial braking and increasing pressure as the intersection approaches.
And that's opposite of how I do it - ease off the brake as the car slows to a stop, having done most of the braking early. Muscle memory (or is this inner-ear memory because it's g-force?) learned on the street can be applied on the track.

I autocross, so my inputs are probably more aggressive than folks would use on a road course. I'll stab the brakes hard to maybe 1/2-2/3 of what I'm planning on needing, to get the weight transfer initiated quickly without shocking the contact patch too much (and locking a tire), then quickly ease in the rest of the braking as the front tires increase grip from weight transfer. No aero so I don't have to worry about downforce bleeding off with mph. I'll get most of the braking done in a straight line before turn-in. I start my turn-in with a very rapid movement of the wheel by about 20-40 degrees -- this seems to quickly get the tires a good starting slip angle (30 wheel degrees and 16:1 rack means 1.875 tire degrees, which seems about right for 200TW competition tires). Probably has to do with loading up the suspension bushings as well, as I seem to use a bit less for my car with urethane bushings as I do for the cars with rubber bushings. Tires need slip angle to produce lateral force; note that "slip angle" is not a skidding tire. As I crank that initial steering in, I reduce brake pressure by about 50% from wherever I was. As soon as you ask the front tires for some lateral grip, you need to give up some longitudinal grip. Then as I smoothly increase steering angle to turn in to the apex, I'm letting off the brake at the same time (trying to ride the edge of the friction circle). Ideally, I should be completely off the brake a bit before apex, "float" the car through the apex, and start both unwinding the wheel and feeding in throttle on the way out. I should be able to steadily increase throttle and unwind the wheel so that I'm at full throttle with the steering wheel straight right at corner exit. If done right, I should not have to reduce throttle once I start applying it.
  1. Smooth is fast.
  2. Be aggressive with your inputs, remembering rule #1.
  3. If the steering wheel is straight, you should either be full on the gas or full on the brakes.
  4. There are only 4 little patches of rubber on the asphalt that are keeping you and your car on the desired path. Treat them nicely - don't surprise them (see #1).
In my view, being fast is all about managing the load on each tire (through weight transfer) as you ask it to for grip in some direction, and generally not asking the tire for more grip than it has at any one time. Tires will increase grip with more load, but not in a linear relationship (rate of grip increase is less than rate of load increase). So the car in total has the most grip with all 4 tires evenly loaded. As soon as you move that weight around with g-load (braking, accelerating, turning), you're actually reducing total grip. The trick is to reduce it the least amount possible for the tires you want to do the most work.

Okay, got off on a tangent there; sorry about that. Interested to hear what other people think, though. Have I been doing it all wrong? Is my theory all wet?
 

Norm Peterson

Corner Barstool Sitter
930
702
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Just read this article over on NASA, written by Joshua Allan of RaceMentor:

From the article....
So what are the blue and green traces depicting, and what are the top and bottom plots trying to show?


Norm
 
I think that @Dave_W and the author of the article are saying substantially the same thing, although the training I had was more along the lines of Dave's description. The perfect longitudinal acceleration plot into and through a corner should look like a check mark - a rapid but not sharp application of the brakes that doesn't trigger ABS followed by a smooth and relatively gentle ramp back through zero as you go from brake to throttle through the turn. The instructors I had, back in the day, were emphatic about getting it right and I'm glad. The first of two key messages was that you're using the brakes and power to balance the car and shift grip from front to back through a corner. The goal is to carry more speed through a corner by using trail braking to increase front grip on turn-in, passing through neutral at or near the apex, then using the power to shift grip rearward gradually as you open up the steering and accelerate out of the corner. The second message, for those of us driving street cars, was that if you got your heaviest braking done when you were still moving relatively fast, easing out of the brakes as you slow down, you wouldn't cook your brakes. As the author of the article says, many drivers maintain high brake pressure all the way into a corner. Not only does it slow them down, it creates a lot of heat when they're moving the slowest and there's little airflow to remove it. They tend to burn up pads and rotors at a shocking rate.
 
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The way we were taught at Track Attack almost nine years ago was to imagine a string between your brake pedal and steering wheel. As you go from the limits of tire adhesion during braking to the limits during cornering, you maintain essentially the limit of the tire to do either. While you are braking heavily, you don't have the ability to also steer the car. As you let up on the brake, the "string" rises and allows you to steer more and more to take over the limits of adhesion. Maybe that makes sense. It did when Joe Foster explained it much more clearly at the time.
 
This is 70% of what I work on with new drivers when I'm instructing. The "racing line" is a big part of it, but learning how to translate car feedback and keep the car neutral/smooth is more important, IMHO. Safe, smooth braking and teaching the concept of slow in, fast out translates to much safer intermediate drivers from what I've experienced. I've had some debates with other instructors on focusing on shift points, brake points, lines, etc. But with different lines working for different cars, I just try to keep them relatively on track (pun intended) and instead focus on braking and car balance. This way they're more focused on the cars feedback instead of forcing the car to apexes when they start to push it.

One thing I've found in my own experiences, I have to massively slow down my own laps to illustrate a braking method to my students. When I do it anywhere near my normal speed and say "see how smooth that was?" they agree and them immediately go dive into a brake zone like a ballistic missile on their first lap. Haha. Slowing it down quite a bit helps with comprehension. Who knew? :)
 
The way we were taught at Track Attack almost nine years ago was to imagine a string between your brake pedal and steering wheel. As you go from the limits of tire adhesion during braking to the limits during cornering, you maintain essentially the limit of the tire to do either. While you are braking heavily, you don't have the ability to also steer the car. As you let up on the brake, the "string" rises and allows you to steer more and more to take over the limits of adhesion. Maybe that makes sense. It did when Joe Foster explained it much more clearly at the time.

The one area that I felt like was missed in Track Attack was that you can't just let off the brakes and immediately crank the wheel. Going from full braking to full release and a wheel turn is really upsetting the car. I work to undo that behavior on most of my novice students.

Other than that, the string analogy is excellent.
 

Grant 302

basic and well known psychic
The one area that I felt like was missed in Track Attack was that you can't just let off the brakes and immediately crank the wheel. Going from full braking to full release and a wheel turn is really upsetting the car. I work to undo that behavior on most of my novice students.

Other than that, the string analogy is excellent.
I don’t think it was missed when I was there. I think it was Charlie who covered the topic. I think it’s the whole point of the string example. Gradual release with or literally tied to steering input.
 
I don’t think it was missed when I was there. I think it was Charlie who covered the topic. I think it’s the whole point of the string example. Gradual release with or literally tied to steering input.

In my experience, most novice drivers take away that you can't brake and turn at the same time. I asked Ronnie about it when I was there and he mentioned that they are cautious about discussing trail braking due to reasons. I understood. Does bum me out, tho. The string analogy is great as an analogy, but there's a ton of room for interpretation. Whereas trail-braking is fairly straightforward.
 

Dave_W

Cones - not just for ice cream
356
343
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
For those who are SCCA members, Randy Pobst has a regular column in SportsCar (the SCCA magazine) and he's mentioned the importance of using trail-braking to keep weight transfer on the front wheels down to near the apex several times in the past year or two. I find his columns are always a good read, no matter what the topic.
 
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Well, I'm ruined for life, as a kart racer, I left foot brake, even in the supers, you have a sequential box, I sorta figured us left foot brakers are the reason God gave transmissions synchros.
 

FLGator

2013 Boss 302 LS #196 - Shadow
121
40
Exp. Type
HPDE
Exp. Level
20+ Years
Florida
I have been teaching the "Check Mark" analogy for many of the reasons mentioned. Slow in, fast out; weight transfer; tire grip (and tire life); and keeping it smooth. With these basics, a driver can learn to asses track conditions, driving lines, and car response. I think that this puts the driver thinking ahead of the car/track as opposed to reacting and being behind the car/track.

As a note, I began my racing endeavors on two wheels. I think this helped me considerably. Keeping the inputs smooth and understanding how throttle and brakes affect what the tires, suspension, and bike are doing. And how this all ties together for the perfect lap. As a trackrat, I realize that I might never get the perfect lap, I'll settle for a good corner or a good sector.

Good article and good comments!
 
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Forgive me if I’m being a little dense here...Isn’t that what the article is pointing out not to do? As in the red trace on the middle graph?
Here's a set of check marks from six superimposed laps at Area 27, turns 2 through 11. They're not all perfect, but if you look at the track map or watch a video, you can see that it's a pretty complicated track with in-corner elevation changes, particularly turns 4, 5, 7 and 11. The horizontal red line is the "0 G" crossover and the "tail" of the check mark should continue smoothly as you move from brake to accelerator. The oddest corner trace is turn 5, where the track falls away from you as you turn in, so you have to get off the brakes more quickly than normal. Similarly, turn 11 has a flat spot in the release because you go over a rise at an angle to the corner and you have to keep the weight shifted to the front or risk understeer. I drive this whole section in 3rd gear.

1597512838221.png
 

FLGator

2013 Boss 302 LS #196 - Shadow
121
40
Exp. Type
HPDE
Exp. Level
20+ Years
Florida
Grant 302,
Guess I was speaking more to the word picture than to the graph.
" Apply the brake rapidly, firmly and just so"
" Maintain steady brake pressure for a period"
" Gradually release the brake approaching and through the turn-in point."

The graph does look more like a backwards check-mark. I see your point. I think the relative dementions of the check-mark are dependent on so many variables that I think of the symbol less than I do the description and how to apply these actions. As a driver, we need to make the adjustments to make these principles work, taking into account the constantly changing conditions. As instructors, we need to add to this, the students capabilities and understanding, providing them the best possibility to progress and enjoy this sport safely.

JAJ,
Thanks for the graph. This shows how you apply these principles to each corner.
 
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