Rotor Warpage?

[Peter P.]

I have no experience with disc brakes, but reading THIS THREAD made me wonder:

Some discs have steel spiders attached to aluminum discs. Could this be done for heat absorption to mitigate any disc warpage from high heat braking?

Anyone experience disc warpage from high heat where the discs straighten out once they cool off?

[Mark McM]

I think you mean steel discs attached to aluminum spiders (aluminum is too soft and would get worn through quickly). Yes, many rotors have spiders and discs whose shapes and joinery (such as floating rivets) are intended to minimize warpage. And yes, I have many times experienced disc rub due warping after hard braking, with the rub disappearing after the discs have cooled.

[bigbill]

I experienced temporary warpage the worst with SRAM Red CLD discs. I replaced them with Shimano XT/Ultegra discs.

[JMT3]

I rode disc brakes when they just started on mtb’s. Road did not come until 2018 for me. Never had a warped disc and me being a weight weenie I always use the lightest. There are Ashima 140mm discs on my Litespeed T3 with almost 18,000 miles on them and the only time I get any noise from them is once in a while when something from the road gets on them and once in a while in the wet. If it’s road grim a quick wipe down with isopropyl alcohol on the rotor and pads and all is quiet again.

[marciero]

This effect is most pronounced on our tandem with 220mm discs. Sometimes the ting-ting or shhp-shhp takes a little while to go away.

[benb]

I’ve never had an issue on MTB. IIRC i have a 180 rotor in the front and a 160 in the back. But I also don’t just drag the brakes.

I completely don’t get why they are weight weenie on rotor size on road. Steep winding descents in the mountains on road are taxing on the brakes and it seems like more people are likely to drag brakes now and not use good braking technique since the discs tolerate it longer.

[Mark McM]

Originally Posted by benb

I’ve never had an issue on MTB. IIRC i have a 180 rotor in the front and a 160 in the back. But I also don’t just drag the brakes.

I've noticed brake warpage on my road but, but I've never noticed it on my MTB - but that doesn't mean it doesn't happen on the MTB. There is so much more rolling drag tire noises with low pressure knobby tires that it may well be happening, but it isn't as noticeable.

[QUOTE=benb;3435670I completely don’t get why they are weight weenie on rotor size on road. Steep winding descents in the mountains on road are taxing on the brakes and it seems like more people are likely to drag brakes now and not use good braking technique since the discs tolerate it longer.[/QUOTE]

Because road bikes are lighter than MTBs, there is an expectation in the mind of the riders that the brakes should be lighter, too. But as you note, braking heat can actually larger on road bikes due to the higher speeds; remember, at any given braking rate, heat increases with speed, so braking from 40 mph to 30 mph generates more than twice as much heat as braking from 20 mph to 10 mph. Even though disc brakes had been used on MTBs for years, many disc brake makers were cautious about adapting their brakes to road bikes, until it coudl be determined whether they were up to handling the heat.

Also note that the flat mount brake standard was initially developed to allow either 140mm or 160mm rotors front and rear, but some bike makers are starting to redesign their fork caliper mounts to either allow 160mm or 180mm rotors instead, or only allow 160mm and not 140mm calipers on the front.

[MikeD]

Originally Posted by benb

I’ve never had an issue on MTB. IIRC i have a 180 rotor in the front and a 160 in the back. But I also don’t just drag the brakes.

I completely don’t get why they are weight weenie on rotor size on road. Steep winding descents in the mountains on road are taxing on the brakes and it seems like more people are likely to drag brakes now and not use good braking technique since the discs tolerate it longer.

Discs tolerate it longer? No way, although you can't blow a tire.

[MikeD]

Originally Posted by Mark McM

I've noticed brake warpage on my road but, but I've never noticed it on my MTB - but that doesn't mean it doesn't happen on the MTB. There is so much more rolling drag tire noises with low pressure knobby tires that it may well be happening, but it isn't as noticeable.



Because road bikes are lighter than MTBs, there is an expectation in the mind of the riders that the brakes should be lighter, too. But as you note, braking heat can actually larger on road bikes due to the higher speeds; remember, at any given braking rate, heat increases with speed, so braking from 40 mph to 30 mph generates more than twice as much heat as braking from 20 mph to 10 mph. Even though disc brakes had been used on MTBs for years, many disc brake makers were cautious about adapting their brakes to road bikes, until it coudl be determined whether they were up to handling the heat.

But, off-road descents are a lot steeper, you are always on the brakes, and because of the low speeds, there is less convective cooling.

[Mark McM]

Originally Posted by MikeD

But, off-road descents are a lot steeper, you are always on the brakes, and because of the low speeds, there is less convective cooling.

There are many factors that affect the generation of brake heat - mass, slope, speed, cooling, etc. But let's look at a simple example A cyclist with a 100 kg mass (bike + rider) is descending a constant 10% slope, and drags their brakes to maintain a speed of 14 m/2 (31.4 mph). Neglecting air and rolling resistance, they would have have brake with a force of 97.6 N, dissipating 1366 W of brake heat. If they went half as fast (7 m/s = 15.7 mph), they would dissipate half the heat. How steep would the slope have to be to dissipate the same heat at 7 m/s as they do at 14 m/s? Working the number backward, the slope would have to be 20.3% So even with steeper grades on MTBs, the higher speeds on a road bike allow it to generate as much or more brake heat. Add to this that road descents can often be longer than MTB descents. MTBs intended for long steep downhills often come with large rotors (180 - 220mm), but road bikes typically can't even take rotors this size.