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Let's say I take my car track racing, and during it, the hard braking leads my brakes to massively losing stopping power due to them overheating (a likely outcome on stock brakes). Or likewise, I take my car down a gigantic hill and don't use engine braking.

Two questions:

  1. Can I let them cool down, then safely drive off to go home?
  2. Will I need to replace them immediately afterwards, or just when they get thin as usual?

I suppose the core of this question is "After overheating, do brakes cool down to their original state, or do they substantially lose effectiveness?".

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4 Answers 4

up vote 5 down vote accepted

My experience, limited to a few very specific configurations is:

  • I've never had rotor warping issues even after a LOT of heat in the brakes (both from track days and also stuck brakes while driving on the express way). I've used generic NAPA rotors, OEM rotors, and fancy heat/cryo treated ones. I can't tell any difference between any of them. They all function like big slabs of metal heatsink. :-)

  • The brake pads themselves (including dirt cheap ones, Carbotechs, and Porterfields) have always worked the same after being very hot as before. However, the cheap ones and the Carbotechs (Panthers, a fine brake pad overall, but just not up to stopping a car as heavy as mine with such small brakes and so much power) went in the trash the day after the events I used them as they performed so poorly while hot.

  • If you boil the brake fluid, you should bleed the system as it does seem to lose a little effectiveness (but was nowhere near as bad as I had expected). Wet boiling point seems to be more important than dry. I've boiled 2 week old Motul 600, but the off the shelf at the auto store synthetic (with much lower dry boiling point, but higher wet boiling point) is fine in my cars on the track for years.

  • A bigger concern is the condition of the caliper seals. I've melted the seals out of the caliper without hurting the pads, rotors, or boiling the fluid before... That happened only with cheap pads, better quality pads have backing plates that provide at least some insulation value. :-)

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Awesome, I'll give you the tick because you were so comprehensive with talking about each part. Is there anyway to know if you've boiled brake fluids, or check the caliper seals? –  andrewb Oct 6 '13 at 11:34
1  
In my experience: If the brake pedal goes to the floor and you don't slow down much, you're currently boiling the fluid. After the fluid cools, the pedal just feels a little mushier than normal. When I melted the caliper seals out, the pistons in the caliper seized up so I had a real hard pedal, but very little braking force available (I melted out the seals on the fronts and only had rear brakes working). –  Brian Knoblauch Oct 7 '13 at 11:23

For the majority of brakes, they should be fine after a track day, but the real risks come from significantly overheating the brake fluid, or from stopping with hot brakes and having them cool while parked.

This is why at track days it is always recommended that you stop after ten or so laps, if you have standard brake systems, to allow your brake fluid and brakes to cool.

While stopped, you don't use your handbrake. Instead, use chocks or stones to hold the car stationary - having the handbrake on holds the brake disc during cooling, and this can easily cause warping.

As long as you follow this guidance you should be fine. Obviously, a track hammers your brakes far worse than general driving, so you do run the risk of wearing your pads down to an unsafe level, so get them checked before driving home.

I also find that my tyres only last one track day, so I need to replace them all before heading home - so I tend to only go to track days with a tyre provider on site.

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Wow you and Brian have both given great answers - I'm in a bit of a predicament in picking one! To me, Brian's answer is more comprehensive, but I see you've got a lot more up votes. Can someone else explain why this is so far the preferred answer? –  andrewb Oct 5 '13 at 23:27
    
People up vote answers that they think deserve an upvote, but you should accept the one that you fell answers the question best. These don't have to be the same answer:-) –  Rory Alsop Oct 6 '13 at 7:41

All of the other answers are great, but I just want to add that rotors are very, very hard to "warp" from 'normal' use (including track day abuse). Any variance in thickness is because pad material has transferred from the pad to the rotor. Turning the rotor gets rid of this.

See "Warped Brake Disc And Other Myths".

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great info after the link –  mac Oct 4 '13 at 16:56

In absolutely no expert on the subject of physical properties of metal. However, in relation to this specific question my main concerns would be:

  • Will the overheated discs warp during cool-down? This will lead to varying degrees of shaking during braking afterwards (and a replacement.)
  • The brake fluid on the other hand should probably be flushed and replaced due to the extreme temperatures.

I wouldn't expect the pads to lose effectiveness1.

EDIT:

1) Other resources suggest that pads can be glazed.

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Interesting, so you reckon pads will actually return to a good state after cooling but the discs may not. Hopefully we can get some more input on this. –  andrewb Oct 4 '13 at 7:15
    
@andrewb I'm far from certain :) I guess what I'm trying to say is; my main concern would be warped discs (which would probably need replacement) and air bubbles in the (closed) fluid system. –  jensgram Oct 4 '13 at 7:57

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