kb58

I've been to a half dozen HPDEs and a lot of autocrosses. At the last HPDE, the Streets of Willow track was hard on everyone's brakes, and I ended up with a soft pedal (boiled brake fluid.) An experienced driver later said that's a sure sign of my "noviceness."

Okay, I'm willing to learn. I want to know how two drivers can take the same car around the same track, and the fast driver doesn't overheat the brakes. Can someone please explain the different driving style?

Let's say the same car is coming into the same corner. The fast driver approaches with a speed of, oh, 100mph, while the novice is going maybe 90mph. Okay, what is the fast driver doing different that keeps his brakes from not getting as hot? After all, he is going faster, so he has to get rid of more heat, not less. The only thing I can think of is that the fast driver slows to maybe 50mph, to make it around the turn, while the novice slows to 40mph. Both cars have some energy to get rid of, so how is the novice dumping more energy into his brakes?



Modified by kb58 at 7:46 AM 12/8/2006

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CRX Lee

Could be a number of things. The faster driver may be slowing less as you mention, the faster driver may be using the brakes in fewer locations on the track than the other guy and giving them more chance to breathe between uses. The faster lap time guy may actually not be faster at the ends of individual straights as well, he may be the same vehicle speed or possibly even less max speed. Being a tick faster in speed but using 100% of the brakes and maybe not as smooth will be more taxing on brakes than a smoother guy down a few peak mph who can use 90% of the brakes. If the brake temps are possibly the weakest link in the chain, you need to strengthen the link (ducting, compounds, etc.) and make sure that the drivers are working best within those strengths and weaknesses.

I think the main thing is that the faster guy is probably smoother all the way around and using the the car or at least the braking parts less at their limits. Put a data logger like the DL1 in the car for both guys and look at segment times and vehicle speeds all over the place. I know when we took four drivers to VIR for the 13 hour, on the test day they were spread across about 6+ seconds per lap. With using the DL1, we have everyone faster and within about the same second and the guy with the max straight speed was often not the fasteset lapper.

Come to think of it, I did the Derek Daly School at the Las Vegas Motor Speedway infield course about five years ago and we had data logging. We had a number of people there and I was the second fastest driver in our group. The fastest was an early World Challenge racer with a ton more experience than I and he was always faster than I and I was always faster than the next guy. This faster guy was also about 140 lbs lighter than I and we were driving essentially hard compound tired Formula Continentals. I was happy to see that my peak mph was often higher than his on the short straights (I took to the dog ring gearbox quicker and loved shifting clutchless and he didn't take to it as well). I equated that my disadvantage was much due to the weight difference. He was still faster in lap time but with less peak speed so maybe the weight was not as big an effect. the real answer was that there was a mix of things but he was generally smoother overall. Even if I had just one bad corner that he beat me in time through, that would show on the lap time.


Modified by CRX Lee at 6:50 AM 12/8/2006

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kb58

Huh... good stuff, thanks. I know about improving brake cooling, but even so, a faster driver won't overheat the brakes, while a poor one will. I'm just looking for how that occurs.

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solo-x

i think lee hit the nail on the head. it's not who has the fastest top speed during a lap, it's who has the fastest average speed for the lap that will set the fastest lap time. i see it all the time autocrossing, and even when the only data logging i did was with a memory tach there was always a trend that as the peak rpm's came down, so did time on course.

there is a catch in this "brake less" theory though. i see it all the time with experienced HPDE guys that come to an autocross. they've gotten it so engrained in their minds that they should use the brakes as little as possible and to scrub speed with their tires that it's actually hurting their lap times. i tell all my students to think of the brake pedal as another tool to reduce their lap times/time on course instead of just a way to slow the car down.

braking, it's the last thing any racer learns to do properly.

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WWDTrackRacer

Wow, I hope the experienced guy was at least riding around with you before making a judgement like that. Seemed kinda harsh.

Some cars have poorly engineered (maintained) brake systems. I have been in some cars @ Streets where the pedal will get soft and the pads smell just after three aggressive laps. I think any well engineered car should last a ~10 lap session out there.

Give me your specs of your car. Tire size, rotor size, weight, power. It should be fairly easy to judge if your brake system is adequate.

Here's what I usually see:

* braking too early (which leads into #2)
* braking too long
* braking too hard (sometimes caused by braking too late)

Most novices have a hard time correctly judging entry speed, and will wind up being all over the map with braking problems.

What config were you were running, and identify the corners where you applied brakes on + the amount of braking?

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thawley

If you look at it from a pure physics point of view, you only have X amount of traction. What your instructor is saying is that you are using more of it in braking than is necessary. A more experienced driver will be using less of the available traction for braking and more for cornering. Carrying more momentum thru the turns will allow you to use less brake.

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bad-monkey

kb's car is like nothing you've ever seen

i'm guessing the brake setup is mechanically adequate, though it's hard to say-- summertime de's at SOWS will often result in boiled fluid for anything south of the best fluid.

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kb58

tires: 185/60-13 front, 215/50-13 rear
rotors: 10"
weight as run: 1800lbs
power: 200hp

Track configuration was the full SOW course, run CCW. The two hard braking corneres were coming into the skidpad, and going into the "bowl."

I am currently replacing my front calipers with Wilwoods, in an effort to eliminate a soft pedal that has plagued the car since it was built. It is this soft pedal that is causing me to brake early. Self-preservation causes this, and I know I'm losing time because of it. My question was one of curiosity; how can a faster car have cooler brakes, and the above comments are very helpful.

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chrisw85

kb, your mini has drums in the rear? sometimes the small gap of the shoes can create a 'laggy' pedal, not a really direct feel, try one click on the parking brake to bring the shoes closer. Although it could just be inherent flex in the calipers up front.

WWDTrackRacer

I would have to say the 10" rotor is probably not enough heatsink for heavy lapping considering your car is 9 lbs / hp.

BTW I'm @ 12.6 lbs / hp on a 10.2" rotor, and I'm okay. But I forsee a rotor upgrade once I get a nice bump in power.

I find CCW @ Streets easier on the brakes than CW. Those two braking areas are a deceptive. I'm factoring in a bit of tire scrub to my braking input for the entry into the skidpad sweeper. Going into the bowl, you're on uphill banking so you're getting some speed scrub through there as well.

Maybe you should get a DAQ on there and we can see what kind of g's are going on under braking.

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Chris F

Would a 10" rotor be big enough for a motorcycle with 9 lbs/hp?


But, I tend to agree with the statement, 10" rotors are on the small side for a 200 HP or 200 WHP car. You'll cook any street pad with that. One should be able to solve it with brake pads and fluid, but that gets a little expensive over time.

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WWDTrackRacer

yes!

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kb58

No, 10" disc brakes all around, and 40/60 f/r weight distribution. Carbotech Bobcat pads all around, and Carbotech AX-9 [edit: AX-6] pads at the front after the brake upgrade.

With 13" wheels, it's impossible to use brakes larger than 10" discs.

If you haven't checked out my site, the only "Mini" parts on the car are the doors and windshield -- nothing else.



Modified by kb58 at 4:45 PM 12/8/2006

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Chris F

What's a Carbotech AX-9? Do you mean XP9? Those suck. Suck is the wrong word, but I've had some odd behavior with them (glazed). Try XP10 or 12.

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kb58

Crap, I meant to write AX-6.

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6spdKEG

238hp = 10.4lbs per hp @ 2480lb with driver here. 209whp dyno tuned. GSR brakes all around on my 95 EG cpe with K20. I have been using AP racing DOT 5.1 synthetic fluid.

4HPDEs / 6days and 3 autocrosses worth of experience on K20 cpe

I cook the Carbotech AX6s on the track.
I have no problem with carbotech XP8s front and rear. Could prolly go with AX6s in rear if needed.

Late into hpde's the Ax6s were always getting weak.

WWDTrackRacer

Man that's a mean rear weight distribution! and you only have a 30mm tire stagger? Do you have a wider track in the rear? How do you tame that sucker down?

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coot_er

i read about you car in GRM pretty impressive!

anyhow i have autoX for a year now. and i still consider myself a newbie. the first time i autoX'ed i boiled my brakes, i changed my fluid and have not done it since. there is no way you are boiling your brakes because of inexperience. there are always newbies at Autox and i haven't heard of too many problems with thier brakes

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Main thing I see with less experienced drivers is that they stay on the brake pedal too long, and enter the corner too slow, and they don't brake hard enough for each corner. They even drag the brake at corner entry. You have to commit and get off the brake as early as possible.

Here is my theory on braking and a few other things:

1. Every time you brake, you brake as hard as possible, every corner. You go to max deceleration g's as soon as possible and keep as close to max g's as you can or the corner will allow. You stay at max g's for as long as possible. Don't half brake for a longer time. If you use a logger, you will see this as every corner having close to maximum braking g's. But every corner is different depending on banking, slope, track grip, bumps, etc. Have a faster driver than you drive your car and log it. Then compare your braking g's and duration and brake points to the other driver to learn.

2. You get on the brakes smoothly to not upset the car, but you do this fast. Being smooth does not mean being slow to build up the braking g's. You get off the brakes smoothly to allow the chassis and tires to adapt to the new attitude. But you also do this fast. "Smooth and fast", not "abrupt or jerky and fast" or "smooth and slow". Anyone not experienced to riding around a track who gets a ride with a very fast driver will not think that it is smooth driving. It is hard and aggressive, but smooth in the race driving sense. It is on the limit. The more experienced and/or skilled you are, the more time you can be on the limit. It is easy to master the limit on slow corners, and harder and riskier to master the limit on fast corners. Get the slow ones down, then the medium speed corners, and finally master the high speed corners. then do it all again.

3. Brake at the last possible instant, and get off the brake as soon as possible. Coming off the corner fast after the apex is easy. Entering the corner fast is where skill comes in. If you get off the brake as soon as possible, then you barely make it around the corner. It is a fine balance on the edge of traction, and you have to work your way up to it slowly. You have to brake later an later and get off the brake pedal sooner and sooner.

4. Learn to trail brake. But do it carefully, as you may swap ends. This uses the most grip from the tire at all times. You need to link the brake pedal with the steering wheel in you mind. Trail braking is done in different amounts in different corners. A data logger will show you how much of the traction envelope of the tire you are using. It is much easier to use the tire envelope at corner exit, as you just press on the gas more as you un-wind the wheel and track out. And when it "feels-a-good", mash the throttle. Braking is harder to learn because you fear not having braked enough to make it around the corner, which of course you want to avoid. So novice drivers will probe the limit of traction much more easily on corner exit than corner entry. This is because if you didn't brake enough and you transfer to maximum g's but you cannot get a radius tight enough to make it around the corner because your velocity is too high, you might be in trouble. On corner exit, if you are going too fast, you can often just reduce the throttle a bit and still make it around.

5. When probing braking limits, work on one corner at a time, and pay attention to where you are on track at each point in the corner. Where do you get on the brake, and where do you get off? On corner exit, where do you get to full throttle? You want that point as soon as possible. That is also a quick way to evaluate different cornering lines. You also want to check your revs coming off each corner at a given point on track (I was at 7400 RPM last time coming off this corner and this time I am at 7500 RPM at the same point around the track out point, so whatever I did, it was better)

6. Try to keep on the gas for as long as possible before braking, as this is when you are covering the most distance at high speed for each tenth of second you are not braking. You will of course not overcome the laws of physics, and if your car can brake at say 1.15 g's, then you will need to brake for x amount of time to scrub off y amount of energy, depending on "brake on" speed and "brake off" speed. But if you do not brake at maximum as soon as possible (i.e. you pussy-foot the brakes early to be safe), then you are braking early and longer. This means that for every tenth of a second you were not at maximum braking g's, you lost time by not being the high speed still accelerating towards the latest possible braking point.

7. Feel your brakes. If they are getting too hot and fading, don't go so deep into the high speed parts, as this is what heats the brakes up the most, since energy=mv^2, and that kinetic energy is what you need to turn into heat with the brakes. Try going little slower into some corners and not braking at all to give your brakes time to recover and cool down. Then test them slowly by braking a little later than last time at the fast parts of the circuit and feel for fade.

8. You need cooling ducts if you drive really hard. You need disks that have enough mass to absorb the heat and not rise in temperature too much. This depends on cooling efficiency and many other factors.

9. Get some brake temperature measuring paint or strips and know what temperature your brake parts are running at. This is cheap, but hardly anyone does it. All pro teams do it, and they do it for a reason. Match the pads you buy to the temperature you measure on your rotors from on track readings you get from the paint or strips.

Slow-in, fast-out is slow. Fast-in, fast-out is the way to go!

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