Warped brake discs - is it me??!!
Discussion
Mave said:
The subtle difference is where the heat goes. The conductivity of the pads is lower than the discs- in a short hard stop more of the heat goes into the discs. In a longer more gradual stop there's more time for the heat to conduct through the pads and pistons to the fluid, so although the total energy is the same, you're more likely to boil the fluid.
Plus the harder you brake the more time you are off the brakes giving them a chance to cool again.Mave said:
blueg33 said:
DocJock said:
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You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.
Not convinced.You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.
Brakes dissipate kinetic energy as heat (that's how they make a car stop). Whether you brake for a protracted period or a short period of harder braking, you are still converting the same amount of kinetic energy into heat energy. The only difference is that in hard braking you are converting kinetic energy to heat in less time, ie a higher temperature.
So I think the converse of the statement above is true, unless I am missing something?
lostkiwi said:
Think about holding matches one at a time to a 1kg mass of metal. Hold it on for a month and there will be little change in the temperature of the metallic mass.
Now put an oxy torch on for 10 minutes. The metal will heat up significantly even though the total heat input is probably about the same.
Sorry, I stand by my original analogy. Heat absorption in a braking system is time critical over much shorter periods than those you cite. The system you describe is too simplistic; the differences in power are too large, the long time periods flatten out differences in absorption rates and magnify hugely the heat loss of the slower system. It's just not the same as a braking system where the difference in stop time is a few seconds and the difference in power 2 or three times. A match burns at c. 100 watts/cm^2 and a torch at nearer 1000 watts/cm2... 100 times more powerful. Comparing the two is nonsensical. The analogy may hold if you took 2 mins to break to a stop. But that's not what we're talking about.Now put an oxy torch on for 10 minutes. The metal will heat up significantly even though the total heat input is probably about the same.
Lower temps over a longer period are absorbed to a greater extent in the wider braking system. A sudden hard stop heats the immediate disc and calliper to a greater extent but dissipates more quickly- lots more heat is lost/wasted to the environment. A firm stop over a longer period releases heat energy more slowly allowing smaller temp differences and therefore lower rates of radiation... so more absorption. A sudden hard stop will see a higher peak temp in the disc but a lower system temp t seconds after the brakes are released when compared to our gentler (but still firm) stop.
This is one of a few reasons why F1 drivers perform firm decelerations to get heat in to the tyres/brakes during the procession and not max decelerations.
Edited by Freddy88FM on Wednesday 5th August 13:09
kambites said:
bobtail4x4 said:
after heavy braking do you sit with your foot on the brake?
that causes warping.
Ah that would make sense I suppose - if you sit stationary with your foot on the brake, the discs will cool down unevenly which might cause them to warp. that causes warping.
But that is after extreme speeds, so Im surprised the OP has problems with a road car unless he is a getaway driver??
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