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      07-04-2019, 02:00 AM   #1
xingqianli
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Front brake calipers changed color

Obviously I overcooked some street pads with the re71r tires in my last track day. I noticed the color of my front brake calipers changed from blue to greenish (see the comparison pics below, they are front and rear)

The question is that is this normal and should I worry about anything?
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      07-04-2019, 12:35 PM   #2
dradernh
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It's normal when you're over-heating the brakes. IIRC, there have been a number of threads about discoloration of the calipers due to heat.

The PFC08s you're going to get may help with that. That's because brake pads act as a heat sink, thereby slowing the transfer of heat into the caliper and the brake fluid.

Nevertheless, our cars are fairly heavy and powerful, so if you're on a heavy-braking track and aren't yet skilled at running quickly while minimizing brake use, the brakes may well get too hot.

I believe the solution to our cars' issue with getting the brakes too hot is to get more air into the center of the rotor. To that end, I'm having some brake duct work done early next month and will post photos and the on-track results in the track section.

I use these two products to collect brake system temperature data: https://www.amazon.com/Brake-Tempera...dp/B075MR5LBW/.

To find out how hot your rotors have gotten, you match the color that the brake temp paint takes on to a color card that comes with the paint. You can use that temp to approximate how hot your pads are getting. Then compare that temp to the pads' published heat range to see where you stand.

I use 350 as the maximum temp I want my calipers to get - that's to (hopefully) preserve the pistons' dust boots. Ideally, the caliper temp will remain well inside the maximum temp of the brake fluid - if it doesn't, you'll boil the fluid.

Below are examples of my results after a recent event (both are hotter than I'd like them to be):



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Last edited by dradernh; 07-04-2019 at 12:43 PM..
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      07-04-2019, 01:09 PM   #3
xingqianli
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Quote:
Originally Posted by dradernh View Post
It's normal when you're over-heating the brakes. IIRC, there have been a number of threads about discoloration of the calipers due to heat.

The PFC08s you're going to get may help with that. That's because brake pads act as a heat sink, thereby slowing the transfer of heat into the caliper and the brake fluid.

Nevertheless, our cars are fairly heavy and powerful, so if you're on a heavy-braking track and aren't yet skilled at running quickly while minimizing brake use, the brakes may well get too hot.

I believe the solution to our cars' issue with getting the brakes too hot is to get more air into the center of the rotor. To that end, I'm having some brake duct work done early next month and will post photos and the on-track results in the track section.

I use these two products to collect brake system temperature data: https://www.amazon.com/Brake-Tempera...dp/B075MR5LBW/.

To find out how hot your rotors have gotten, you match the color that the brake temp paint takes on to a color card that comes with the paint. You can use that temp to approximate how hot your pads are getting. Then compare that temp to the pads' published heat range to see where you stand.

I use 350 as the maximum temp I want my calipers to get - that's to (hopefully) preserve the pistons' dust boots. Ideally, the caliper temp will remain well inside the maximum temp of the brake fluid - if it doesn't, you'll boil the fluid.

Below are examples of my results after a recent event (both are hotter than I'd like them to be):



Wow, very helpful post, thanks a lot!

Yes I guess the temperature is the key and different pads have quite different working temperature ranges. The EBC yellow stuff seems to be not very good at high temperature area and I think overheat made it wear even faster. PFC08 will definitely help in this regard.

I used to have StopTech sport pads on my car and I drove in a similar style using the brakes. They were not as bad as this one in terms of overheat and wear. The changed variables since then are:
1) I replaced the old PS4S tires with RE71R. Stickier tires will for sure increase the workload of the brakes
2) I changed the wheel size from 19'' to 17''. The free space around the rotors and calipers is smaller than before thus I may get less air flow and worse cooling. However, I'm not sure how big role this factor plays. So I'm very interested in your brake duct work. Please keep us updated!

Now I'm using RBF 600 brake fluid. The dry boiling point is 312C. I did get mushy feeling on the brake pedal late in the day. But how can I tell if the fluid is boiled or not?
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      07-04-2019, 04:53 PM   #4
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Quote:
Originally Posted by xingqianli View Post
2) I changed the wheel size from 19'' to 17''. The free space around the rotors and calipers is smaller than before thus I may get less air flow and worse cooling. However, I'm not sure how big role this factor plays.

Now I'm using RBF 600 brake fluid. The dry boiling point is 312C. I did get mushy feeling on the brake pedal late in the day. But how can I tell if the fluid is boiled or not?
Given the physics of the problem, cooling airflow is a big factor. Typically, to ensure optimal braking performance on cars like ours you'd run a 3" brake duct hose directly into the centers of the rotors. Those provide a LOT of cooling air; so much so that on some tracks you'd block the ducts off if they were over-cooling the brakes.

From what I saw when the car was up on a lift, though, we're not going to be able to push a lot of air into the center of the rotors. IIRC, it's the steering knuckle that's blocking off most of the inside opening into the rotor - there was just a small eyebrow-shaped opening available. But as the race shop owner said when we were looking at it: "Something is better than nothing."

----------------------------

My experience is that if you get a soft pedal and it stays soft, you've likely boiled the fluid, and the calipers need to be bled. However, if you're getting an intermittent soft pedal (e.g., it goes soft but doesn't stay soft), you're likely experiencing pad knockback.

That issue is explained better than I can about halfway down this lengthy (and excellent) page telling you everything you need to know about braking performance: https://www.essexparts.com/essex-des...87M2f80M3f82M4. I've used one of Essex Parts' AP Racing-based brake kits on a race car I ran, and they know their stuff, inside and out. If they made a kit for our car, I'd already have one installed on mine.

The solution for pad knockback is to tap the brake pedal two or three times with your left foot after you've gone through turns or over curbs that have knocked the pistons back. Those are the turns/curbs immediately preceding the braking zone in which you experience the soft pedal. That soft pedal then goes away because your use of the brakes has solved the pad knockback problem. This assumes it's not a brake fluid boiling problem that you're experiencing. (I'm not sure I'm articulating all this very well - let me know if you have any questions, and I'll do my best to answer them.)

I've experienced a soft pedal in my M240i once (and only once) during each event I've attended, except for the last event. Not once every lap, or every session, but just once per event. I've discussed that circumstance with two race shop owners with many decades of high-level racing experience between them, and both immediately said..."pad knockback". So I'm now practicing making light, left-foot tapping of the brake pedal second nature.
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Last edited by dradernh; 07-04-2019 at 05:08 PM..
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      07-04-2019, 10:20 PM   #5
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Quote:
Originally Posted by dradernh View Post
Given the physics of the problem, cooling airflow is a big factor. Typically, to ensure optimal braking performance on cars like ours you'd run a 3" brake duct hose directly into the centers of the rotors. Those provide a LOT of cooling air; so much so that on some tracks you'd block the ducts off if they were over-cooling the brakes.

From what I saw when the car was up on a lift, though, we're not going to be able to push a lot of air into the center of the rotors. IIRC, it's the steering knuckle that's blocking off most of the inside opening into the rotor - there was just a small eyebrow-shaped opening available. But as the race shop owner said when we were looking at it: "Something is better than nothing."
There's indeed not enough room in the front axle of our cars to work with. I noticed that when deciding the right size of front wheels with enough suspension clearance. The space is very tight for adding anything additional for sure. I hope it can work out for you. Please keep us updated!

Quote:
Originally Posted by dradernh View Post
My experience is that if you get a soft pedal and it stays soft, you've likely boiled the fluid, and the calipers need to be bled. However, if you're getting an intermittent soft pedal (e.g., it goes soft but doesn't stay soft), you're likely experiencing pad knockback.

That issue is explained better than I can about halfway down this lengthy (and excellent) page telling you everything you need to know about braking performance: https://www.essexparts.com/essex-des...87M2f80M3f82M4. I've used one of Essex Parts' AP Racing-based brake kits on a race car I ran, and they know their stuff, inside and out. If they made a kit for our car, I'd already have one installed on mine.

The solution for pad knockback is to tap the brake pedal two or three times with your left foot after you've gone through turns or over curbs that have knocked the pistons back. Those are the turns/curbs immediately preceding the braking zone in which you experience the soft pedal. That soft pedal then goes away because your use of the brakes has solved the pad knockback problem. This assumes it's not a brake fluid boiling problem that you're experiencing. (I'm not sure I'm articulating all this very well - let me know if you have any questions, and I'll do my best to answer them.)

I've experienced a soft pedal in my M240i once (and only once) during each event I've attended, except for the last event. Not once every lap, or every session, but just once per event. I've discussed that circumstance with two race shop owners with many decades of high-level racing experience between them, and both immediately said..."pad knockback". So I'm now practicing making light, left-foot tapping of the brake pedal second nature.
Very well explained and good reference article! It's my first time hearing the term "pad knockback". Good to know! Given that I had consistent soft pedal after a couple of sessions and even on the way driving home (almost failed to stop before the first stop line with normal brake pedal pressure after I left the track..) It's likely that the fluid was boiled... Now it still needs longer travel and higher pressure to have the same brake force as before. But when I press the pedal hard enough, the brake force is still there so I can still drive on the street. Anyway, I think I'll just flush the fluid when changing to the new pads.
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      07-10-2019, 07:35 PM   #6
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This happened to mine within 10 laps of installing PFC08s. I have titanium backing plates to try and keep heat out of the calipers, but no ducting.

The fluid (Endless RF650) didn't boil and the pedal and braking performance was consistent throughout every session.



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      07-10-2019, 11:52 PM   #7
xingqianli
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Quote:
Originally Posted by Nugget View Post
This happened to mine within 10 laps of installing PFC08s. I have titanium backing plates to try and keep heat out of the calipers, but no ducting.

The fluid (Endless RF650) didn't boil and the pedal and braking performance was consistent throughout every session.



Wow that looks even darker than mine seems we do need better cooling. anyway I won't care too much if it's only about the look.
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      07-11-2019, 10:07 AM   #8
dradernh
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Quote:
Originally Posted by xingqianli View Post
Wow that looks even darker than mine seems we do need better cooling. anyway I won't care too much if it's only about the look.
It's not likely to be restricted to the look. The caliper pistons' dust seals become hard and brittle once high-enough temperatures are reached. If my calipers become discolored, the first thing I'm going to do is have the shop pop the pistons out to look at the seals.

Another member here didn't just discolor his calipers, he destroyed them. His piston seals were ruined, and the pistons distorted such that they no longer moved back and forth in their bores. He had to buy new calipers.

Part of this issue is down to how the driver is using his or her brakes. Some drivers, novices especially, but some others as well, are not efficient brakers, and they end up putting much more heat into the system than is necessary. That's not going to turn out well on heavy, powerful cars like ours when they have no brake ducting to the center of the rotors.

Tracks make a difference, too. Heavy-braking tracks, for example, exacerbate this problem. I run on a track with what I consider to be only moderate-braking demands, and I'm getting my components much hotter than I'd like to see.
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      07-11-2019, 08:34 PM   #9
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Yeah my local track has three big braking points in a ~67 second lap. So it's pretty tough on them.
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