Mountain Biking - disc brake temp

does anyone have any idea how hot disc brakes can get, and if you ever suffered from brake failure? i 've seen ppl actually pour water on the brakes and massive amount of steam coming off but never know how hot it gets, any ideas?

wow you said steam did you well id say it would depend on the speed (of the ride)and tempreture of the day.

Next time ask him if you can touch it jks.

does anyone have any idea how hot disc brakes can get, and if you ever suffered from brake failure? i 've seen ppl actually pour water on the brakes and massive amount of steam coming off but never know how hot it gets, any ideas?

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It wouldnt be the brake fluid boiling that causes the failure it would be the brake pads melting on the rotor. I have heard of that happening but have never personally seen it.

does anyone have any idea how hot disc brakes can get, and if you ever suffered from brake failure? i 've seen ppl actually pour water on the brakes and massive amount of steam coming off but never know how hot it gets, any ideas?

Not the temp. But I have seen burns into the skin like branding. Extended use of brakes causes overheating and then brake fade.

It wouldnt be the brake fluid boiling that causes the failure it would be the brake pads melting on the rotor. I have heard of that happening but have never personally seen it.

If brake fluid boils it causes condensation in the lines which contaminates the fluid, causes lines to rust from the inside out, and rubber hoses to wear out from the inside.

I seen a picture online (can't remember where now) of a rear 8" rotor glowing red hot. It's on a guys personnal web sight. No idea if the brakes had failed, but I'd guess they would surly be hot enough to boil the hydraulic fluid at those temps.


Not only does it boil hydraulic fluid, it causes air to go in, which could kill your brakes as you're going down a downhill descent. That can be dangerous. (No really) And if you're extreme, I would think getting these would help
1) Mech Discs
2) Bigger rotors.

I find this "glowing" disc brake thing hard to believe.

Not only does it boil hydraulic fluid, it causes air to go in, which could kill your brakes as you're going down a downhill descent. That can be dangerous. (No really) And if you're extreme, I would think getting these would help
1) Mech Discs
2) Bigger rotors.

No offense hooligan but thats the dumbest thing I have read. Yes bigger rotors would help but mech brakes are worse. You loose signifigant power and increase strain on the hands (which means a couple of runs and your hands are fried) with cable actuated. You have just said the opposite of every dhiller and freeriders on the planet (no really...every one of them)

If you are loosing power because you have overheated a pre-bled system (where the bleed is fine)...here is a good option...don't use your brakes so much and learn how to modulate to allow them to cool. I promise you are overusing them if you suffer brake fade without REALLY long steep descents. (I am thinking like oasis in rossland which is excruciatingly long and very steep in section where it warrants laying on the brakes for extended periods).

I have never had my brakes overheat, disc or v. Even going down a 14mi descent riding a bike with v's I have never overheated my brakes.


I haven't seen anything like what you describe. Next time you see this could you try to get a pic.

[QUOTE=Maelstrom]Yes bigger rotors would help but mech brakes are worse.


Speaking of bigger rotors, could I adapt my shimano XT's to run 8" rotors, or would I need beefier hubs for that?

Cooling discs by sloshing water on them?! Surely the disc would warp and become unusable?

[QUOTE=Maelstrom]Yes bigger rotors would help but mech brakes are worse.


Speaking of bigger rotors, could I adapt my shimano XT's to run 8" rotors, or would I need beefier hubs for that?

Should be fine.

I've seen discs glowing red on cars and quads. Both had hydraulic systems.

I've wouldn't think on a steel rotor with not even 100kg of weight at 30mph tops you could cause a rotor to glow. F1 brakes glow as there carbon/ceramic?? they run hotter temps and the actual materials boiling point is lower. FIA GT cars run iron discs and never glow even though they can reach hundreds of degrees. As far as I know anyway...

I read too that to bed in the brakes properly you should do some hard stops then pour water over them to cool, then repeat. Supposed to do something to the surface?

I saw an Open University video from the '70s where they drove a Ford Escort upto only 40mph and braked it hard and the disc went orange hot...

Pouring water on really hot brakes is not a good idea. Im not sure why but in the motorcycle world if your brakes are hot and you go through a creek you lose your brakes for a good 5-10 minutes. I can find out hwy if someone really wants to know.

Where I live there are plenty of large hills that give my discs a nice workout. No problem with them fading from heat. Even in the rain the hold up well, better in fact. I can't say to date that I've seen the rotors glow red, orange or any other color. They are cheap Hayes mechanical disc brakes but they work great for me and never have to worry about over-heating anything like V brakes did with the rim and tube.

I've looked around and I read somewhere that disc brakes get somewhere around 300+ degrees. Don't touch it... Yes bigger rotors help as well as better ventilated calipers. (Hayes G2 come to mind) But on the longer steep runs, its not odd to see a glowing rotor.

"my friend went to panorama and everytime he went down the hill on crazy train the brakes woudl heat up so much that they wouldent work u have to spray down with water... he touched his brakes he burned his fingers lol .. they get pretty hot.. he has hayes hfx 9 and in all his biking he has never seen brakes glow red or orange i think ur kinda lieing there"

I didn't type that up, my friend did, thats why its in quotation marks. But I was suppose to say that my friend etc.. you get the idea.

From you last post I kinda figured you were a bc guy (or at least near enough) Still gotta hit up panorama sometime. Welcome to the forums.

Thanks, That was a friend of mine though that had wrote that, i just copied and pasted it from our msn convo, he went up there during the summer for biking, he met byron grey.

He wanted me to basically re-write it but I'm too lazy to do that.

Hahaha...no worries. Byron Grey is a pretty cool guy. Most of the bc riders arethoguh :)

The stinky is autographed by Byron Grey, same with his brothers Kona Stuff. :)

Well, then why won't dhers use 6'' rotors? Because they overheat! :S

Some people have to use a 6 inch rotor in the back.

Well an lbs mechan. said taht the larger the rotors, the harder it is to brake, but it will have a smaller chance of boiling the liquid. He also went off a tangeant of how xtr deraillers are real smooooth.

Well that lbs mech must not be a downhiller, downhillers don't rant and rave about the smoothness of a deraillieur.

Hahaha...good point kona. My deraileur barely does its job :)...and it is XT :)

All any braking system does is convert the mechanical energy of the bike's motion into heat. In a disc system, the more massive the rotors (e.g. larger diameter), the more surface area there is to allow the heat created by the friction of braking to dissipate into the air.

I've looked around and I read somewhere that disc brakes get somewhere around 300+ degrees. Don't touch it...

:eek: :eek: obviously fahrenheit right?, in celcius that would be as hot as a Car, :rolleyes:

I have 8" f & r, and they get damn hot sometimes, just ask my fingers....

What makes you touch something that you know is hot?

Stupidity?

I have 8" f & r, and they get damn hot sometimes, just ask my fingers....

What makes you touch something that you know is hot?

Stupidity?

What makes little kids stick paper clips in power outlets? What makes them lick a 9-volt battery?

Yeah its farenheit. I forgot about our friends from diff countries. Sorry.

I use a 6" in the rear. And I DH. On Occasion. Yes an 8" would be ideal for DH. But it won't fit on my frame. And honestly (quoted from a WC DH racer)

"If you really want to win, you shouldn't even use your brakes."

So I ask why does the rotor size matter. My 6" rotor stops me perfectly fine. Plus most of the braking in DH should be front brake. :)

So I ask why does the rotor size matter. My 6" rotor stops me perfectly fine.

Imagine if the rotor was only 2 inches in diameter. It would get very hot then, for the reasons I described above. Your six inch rotor may work for you, and that's great, but heavier, faster riders with more accomodating frames will see better performance from a larger rotor.

Imagine if the rotor was only 2 inches in diameter. It would get very hot then, for the reasons I described above. Your six inch rotor may work for you, and that's great, but heavier, faster riders with more accomodating frames will see better performance from a larger rotor.

Thats very true. But If someone downhills with a 2 inch rotor. I don't even want to look at them. 4" rotors are SMALL for XC. To see a 2" rotor, thats rediculous. (I know you said Imagine, but eh) Yes heavier, fastier riders prefer to use bigger rotors. But a 2" rotor should never be put to the extreme where it would be glowing.

F1 cars typically stop over a thousand pounds of metal from hundreds of mph. Remember ke=1/2mv^2? Mass doubles, ke doubles. Velocity doubles, ke goes up by 4. Their brakes heat up to over 1000F and are glowing. I'm not sure you can have a guy on a bike putting that much energy in their brake discs to make em glow. And if you poured water on em, I'd be surprised if the disc ever worked again.

8 inch rotors, in my opinion, have better modulation.
This is probably because the rotor moves faster when it is further from the centre of the wheel, therefore you can change the speed of this more gradually than when the grip spot is closer to the centre of the wheel.

Glowing Rotors? I wont believe that until i see it. So post a pic!

8" is better than 6" obviously because otherwise the pros would be using 6" rotors.

Glowing Rotors? I wont believe that until i see it. So post a pic!

I am gonna go with that I wanna see it

does anyone have any idea how hot disc brakes can get, and if you ever suffered from brake failure? i 've seen ppl actually pour water on the brakes and massive amount of steam coming off but never know how hot it gets, any ideas?

Good question - I've also wondered about the disc temp during a descent. One way to check is with a remote thermometer that reads the infrared signature without physically contacting the rotor. Radio Shack sells one for $50, but it only reads up to 400F.

I think you can get temperature sensitive stickers for this task. Maybe a slice of one on the rotor edge would suffice? An infrared pyrometer is ideal though... Summit Racing sells a 500 F model for about $100 and a 700 F model for about $170.

Glowing Rotors? I wont believe that until i see it. So post a pic!

8" is better than 6" obviously because otherwise the pros would be using 6" rotors.


30 years ago I used to be involved with Karts (Go-Karts to the unitiated) and was involved with the manufacture of Frames and Brake units for them. These are fast machines, and the 100cc class only has one brake on the rear axle. I did not believe how hot the disc's got until a meeting overran its time, and we raced just about on dusk. Depends on the material, but the stainless steel disks were not showing any glow, but those made out of Gauge Plate, definitely glowed Orange. Why Gauge plate by the way, which is a softer material? Because it worked better, even though it wore fairly quickly and got covered in rust between meetings. If any of you do think your Discs are Glowing, Do a fast hard descent in the Dark and then look at your discs.

Incidentally, My bike runs Hope Mono M4's on 200mm discs, is driven aggressively and I have never had the fluid boil, never had brake fade, never had any problems of any sort, other than at times they are a bit too effective. This is on a heavy weight machine, but for Lightweight XC machines, where weight is a priority, 6" up front and a smaller disc on the rear is quite sufficient, unless you are prone to finding the big long hills wherre speed and hence braking efficiency is required