brakes losing power when heated
Have a set of Shimano Hone/LX calipers with what I believe to be metallic brake pads and Avid G2 rotors. I've noticed that if I do extended braking (e.g. maintaining the mandated speed limit on long downhills) that the brakes get weaker and make a scraping sound, like getting sand on your rim brakes. After a while, the power eventually returns, and the scraping sound goes away. Is this normal?
Brakes work in two ways, by squeezing the disk between the two pads causing friction and the transfer of the kinetic energy to heat energy (energy cannot be created or destroyed - only changed to a different type of energy) the heat energy comes from the friction.
As you are going down a hill, there is heat build up as you slow down, and then when you are rolling along again, the disk and pads cool down ready for the next time.
on long hills, with the brakes on lots, the disk and the pads get a point where they can not transfer any more kinetic energy to heat energy, they are heat 'saturated'.
This is what you are experiencing.
As the pad and disk cannot dissipate any more heat, it then transfers it to the brake fluid and the caliper. when this happens the brake fluid 'goes off' and looses effectiveness, causing the need to change the brake fluid. you can squeeze the lever more and more but there is no improvement.
if you should get to the bottom and deicide to stop, don't stand there with the brakes on, this will cause a hot spot on the disk and likely cause disk warp / bending, it will be hotter whilst the rest of the disk cools. it would be best to keep rolling, the airflow will also help with the cooling
There are some little devices out there to assist the brake cooling which is what I am looking for when I ended up in here. A2Z have one but it doesn't look like it works for Avid brakes.
what can you do about this? me, i brake hard, and then roll up to speed again and then brake hard. some times I alternate the brakes as well when I feel that this is happening.
in my ride yesterday this happened, I shall be pulling brakes apart tomorrow for a caliper repair and new fluid.
Last edited by Hay Ewe; 09-19-2010 at 04:34 AM.
get a bigger rotor, larger rotor dissipates heat faster, I like a 180mm in front ( where most of the braking power is) and 160mm in rear
so what is the second way?
Originally Posted by Hay Ewe
brakes convert rotational motion into heat, get a bigger rotor and you'll generate less heat over a larger area
Yes it is normal and is called heat fade specifically.
Originally Posted by beanbag
Hay Ewe gave an apt description of why it occurs. The only caveat that I have is that there is no need to change the brake fluid should you heat up your brakes to the fade point. Now if you were to get your brakes hot enough to boil the fluid, then yeah. But most disc brakes will start to fade long before you hit the boiling point of the fluid. Most brake fluids don't boil until you hit temperatures in excess of 400 degrees. If you get em that hot you'll know it, they'll be smoking, literally. While fluid temperature does affect brake fade, it isn't the primary reason for it. If it were then a mechanical disc brake wouldn't fade hardly at all. However, I can tell you that mechanicals certainly do fade.
And dan0 gave the best method of helping reduce the chances of heat fade, larger rotors. I will take exception to the "less heat" statement though. The kinetic energy of a bike moving and 20mph will require the same transfer of that energy to heat in order to reduce the speed to 10mph. So the amount of heat generated will be the same. The difference comes in the amount of rotor surface that you are heating up. The larger rotor has more surface and takes more energy to heat up. Thus it takes longer to get to the fade point.
As far as devices to help with cooling, I've never seen one devised specifically for bicycle applications that actually works. Most cooling devices rely on directing air flow, and require 2 things a way to direct the flow and speed to create the flow. With the way bicycle brakes are mounted and the speeds that are normal, it's pretty tough to create something that will actually enhance cooling to the point that it would make a significant difference. The A2Z Cool Bomb is effective at keeping brake fluid cool, but it still won't stop fade.
Anyway, bottom line is, if you are using 160mm rotors, go bigger in the front. If you are already using a 185/160 combo, then (if your fork and frame will handle it) try a 203/185 set up. Your call.
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