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MG MGA - Disc brake temperature
|I have recently replaced the front pads, after having great difficulty in getting the pistons back far enough to insert the new pads (I used a jemmy bar in the end). However, after a short run (normal speed, but with several good stabs of the brakes), the discs are very hot (82degC using a laser reader). What should I expect the temperatures to be? The wheels rotate relatively freely, with only noise of pads on discs. Is it normal to have such hot discs after replacing the pads?|
|82 degC seems to be normal, considering that the hidraulic liquid boils to temperatures closed to 200 degC or above in racing applications. |
Calipers may be hotter than engines in some cases if not in all!
|DOT 4 dry boiling point is 230 deg C , DOT 5 dry boiling point is 260 deg C. - "dry" means no water contamination, with water contamination this comes down markedly.|
If you have trouble getting the pistons back unbolt the caliper and use a G clamp - works a treat.
|Rotor running abnormally hot with normal driving means the pads are dragging on the rotor. This can be caused by a brake hose deteriorated and swelled internally constricting to resist return flow of fluid. If the drag goes away when you open the bleed nipple to relieve pressure, think about how old the hose might be.|
Replacing pads is not the same as a caliper rebuild. In normal operation the square cross section O-ring seal around the piston it resilient, meaning it deforms and springs back similar to a spring. This actually pulls the piston back a short distance allowing an air film to flow between rotor and pads for zero contact during normal driving.
With heat cycling of older brake parts the O-ring seal may be hard and much less resilient, therefore not pulling the piston back properly, allowing the pads to drag on the rotor. Solution for this problem is to replace the U-ring seals around the pistons in the calipers (rebuild the calipers).
|82C/<200F is not very hot as a disc temp, if you had recently used the brakes as you seem to be saying. In fact it is barely warm, such that high performance pads don't even work right. Get them cold and drive it, stopping preferably on a (up) hill or by very light application for final stop, They should then be quite cold, if it's not dragging excessively.|
The "several good stabs" are sufficient to get the metal up to this temp, and the nubblies on new pads are enough to maintain such a temperature until they wear off. If "The wheels rotate relatively freely, with only noise of pads on discs." is true, it will get better as the pads wear in, which won't take long. If the brakes continue to run noticeably above ambient when they haven't been used for ten minutes or so in normal driving,then you may have a problem.
Disc brakes can run very hot, 600F is sort of a baseline for standard pads at the disc under heavy use, 1200F (red hot) for high performance stuff, beyond red hot into melted metal for top end racecar bits. These temps have to be dissipated or insulated from the wet bits; top end fluid has boiling points around 550-600F. It takes a good while for hot parts to cool off, that's part of why race cars run cooldown laps. I have "blued" shiny new discs in quick road work (Subaru WRX, all 4 corners, after pad breakin)- that would represent c660F at the disc. When you do this you do not park the car until things have cooled off, or you will boil the fluid and warp the discs!
This thread was discussed on 13/12/2009
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