Bicycle Mechanics - Glazed Rotor?

I´m constantly battling disc brake overheating because of a very steep down-hill on my commuting route.Bought a brand new pair of XT brakes and xt 160 rotors 2 weeks ago, they performed great up until yesterday when the rear brake started lacking power and
making an awful noise when used.The rear rotor has a sort of rainbow look to it, clearly different in color than the front one.Brake has bled just moments ago with no result.

Is this a glazed rotor?If so, can I do anything about it?Some internet articles suggested using sand paper to renew it.Any ideas?

You can scuff them with 280/320 sandpaper.Maybe some more front brake action or the rear caliper is dragging.The front brake should be colored also if the hill is that big,most of your braking power is in front.

Heated until blue and too hot. What condition are the pads in? Are they sintered? Are you pumping the brakes or just dragging them the whole way? Disc brakes work really well for mountain biking with long steep hills etc. You should not be maxing them out on a commute. How long and how steep is your hill? I run BB7's on 3 bikes with no problems. They are cable brakes that work well in all conditions.

In my experience, the pads are much more likely to glaze and cause brake problems.

Also, as mentioned above, perhaps you should be using your front brake more. Rear brake is so innefective that you have to drag the brake a lot more to slow down the bike. You should certainly be relying on the front brake for most (~75%) of the stopping duties.

Heated until blue and too hot. What condition are the pads in? Are they sintered? Are you pumping the brakes or just dragging them the whole way? Disc brakes work really well for mountain biking with long steep hills etc. You should not be maxing them out on a commute. How long and how steep is your hill? I run BB7's on 3 bikes with no problems. They are cable brakes that work well in all conditions.

Pads are stock sintered, like I said, brakes are 2 weeks old so almost no wear at all.I must have dragged the rear brake on my last descent because over night stopping power halved.I´m sure it´s the rotor cos it looks like burned.If I can´t manage to get it fixed and have to get a new one, will probably need new pads as well. Do organic pads offer better stopping power than sintered?

Have you tried larger rotors, as this will improve your braking performance. Also, you say that the brakes are 2 weeks old, have they (the pads) been bedded in? If not, this could be the cause of the overheating

bedded in? what's that mean?

All you need to know from Shimano themselves

http://bike.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50rparsys-0013-downloadFile.html/07%29%20Brakes%20and%20Heat.pdf

http://bike.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50rparsys-0012-downloadFile.html/06%29%20Brake%20Burn%20In.pdf

If you have glazed your current rotors, would consider them scrap, and replace

most interesting - thank you.

I disagree with the Shimano info, pads glaze, not rotors. To restore a glazed pad, sand with 600 grit paper until the surface striations are gone; when changing pad compounds, sand the swept area of the rotor. To the OP, bleed the caliper, place sand paper on a flat surface, place the pad friction surface down, sand while applying light pressure to the backing plate, reinstall the pad without touching the pad surface, follow bed-in procedure, followed by a cool down for at least 10 minutes without applying brake. The bed-in does not need to be aggressive, and 10 near stops at a jogging pace is enough for most sintered pads.

Organic compounds generally provide better initial bite, but have less heat tolerance than semi-metallic & sintered pad compounds. A near exception would be a hard ceramic compound, however wet performance is compromised somewhat. I enjoyed a soft ceramic compound for its silent operation and good cold bite, but this particular compound could not handle dragging the brake on descents.

Pads are stock sintered, like I said, brakes are 2 weeks old so almost no wear at all.I must have dragged the rear brake on my last descent because over night stopping power halved.I´m sure it´s the rotor cos it looks like burned.If I can´t manage to get it fixed and have to get a new one, will probably need new pads as well. Do organic pads offer better stopping power than sintered?

It's very unlikely your rotor is burned. You left burned rubber on the disk.

It's very unlikely your rotor is burned. You left burned rubber on the disk.

How? there is no rubber anywhere near a disc rotor, there nearest is on the tires, please explain.

Metal takes on oxidation colors when it gets up around 500 to 600F. If your rotor is blue and yellow'y straw colored then that implies that you used your rear brake more aggresively than your front.

Depending on the pads you may have overheated the pads and produced a glazed hard surface on the pads which, depending on the materials they are made from, could transfer to the rotor's surface. But a transfer of that sort is fairly unlikely.

Because disc brakes work by friction if you deglaze the pads with some sanding as mentioned and re-install the friction of the pads will soon wear away the colouring on the rotors. So you won't gain much by sanding them as well.

And it would appear that on the long descent that you're favouring the rear brake. Try to even it up. On clean dry surfaces you're in far more danger of using enough front to actually lift and flip the front end on a descent than of locking up the front, skidding and going down. So don't be afraid of the front. And if it's really long and steep get off the seat and get your weight back more.

If road and traffic conditions allows you to do it safely letting the speed come up so the air friction holds you back more will take some of the speed limiting duties off your brakes. Sit upright as possible for this to act as much like a parachute as you can. It means you'll have to use the brakes harder at the bottom of the hill but you'll do so for a shorter time so you actually dissapate less energy in the brakes than if you use them all the way down the hill to hold your speed to a low value. You'll still use your brakes during the descent but in more of an occasional and short way then allow the speed to come up between applications. But if your route has lots of side streets and driveways this likely isn't a great idea. Too often drivers stuff the nose of their cars and trucks out into the curb lane area before even stopping to look at what traffic is doing.

Thank you guys, lots of helpful info here.The shimano article that Jimc101 posted seems to apply to my situation.Since the brakes were performing at their best(yes properly bedded-in) and then like crap over night in dry conditions, it let me to believe that overheating might have occurred.Rainbow appearance and all.


But if your route has lots of side streets and driveways this likely isn't a great idea. Too often drivers stuff the nose of their cars and trucks out into the curb lane area before even stopping to look at what traffic is doing.
Yep, this is exactly it.
I´ll go ahead with sanding the pads, see if that helps.Hope I won´t have to get a new rotor.Will post results here later.

hi,
Took a new 2012 Specialized Epic Comp 29 up and down my training hill a few times. At the time was unaware of bedding in pads. Didn't have to do it with my Orbea with Avid mechanical discs, which I thought worked very well. After a few runs up and down my hill, 5.25 mile up 2,500' gain, my rear Avid Elixir started screaming. Rotor was dark blue. Maybe went up, then down, about 6 - 8 times. Caliper aligned well, pads not rubbing on rotor, rear wheel spins freely.
Front Avid Elixir 160mm was working just fine. Both fr and r were stopping very well, much better than mechanical cable Avids.
Bought new aluminum backed pads and new 160 rear rotor. Read the many articles on bedding and took my time trying to do it carefully.
Rotor turned blue same amount of time and howled so people could hear me a mile away, just like first set.
Front still just fine. Fr and r still have great stopping power.
People, mechanics, said to feather rear brake, use more front, pump rear ...
Sorry, but I'm used to off road motorcycles and have ridden mountain bikes for a living so can use brakes to their limits, to where the front wheel starts to lock up. Am using the front brake up to the point of locking up, then lighten up for corners. Using rear as much as I can to stop on sustained steep downhills. If I need to feather or pump rear I feel the brake is not working up to what I'm demanding. If I bought larger front rotor will still be using rear just as much; to the point just short of wheel lockup.
Bought rear 2012 XT 160mm entire brake setup, w. ICE technology. Brake levers all the way back to caliper, pads w. cooling fins, rotor(3 layer, steel/al/steel). Bike is a month old at this point. All I'm riding is up and down training hill. Getting ready for event.
Rear XT brakes were great. Took special effort to bed in pads. Worked very very well. Went up and down my training hill about 8 times. All good.
Just did the Vision Quest. Has some long ups and long downhills. After about 50 miles and 11,000' gain my rear XT started talking. Loudly. But not as bad as Avid Elixir got to. Rotor is discolored but not as dark blue as the 2 Avid rear.
So it took longer for the XT's to get to the point of hot/noise.
May try XT 180mm rear, more for heat dissipation rather than stopping power, but if I don't need to pull the lever as much, because the 180 will be a bit more powerful, then they will run cooler. I know it is unusual to have larger rear than fr rotor but possibly sometimes you need to think outside of the box. Thinking the 29er, slightly longer wheel base, more weight over rear tire, is simply able to use and demand more rear brake.
Thanks
Steve

Sfenn99- really the wrong way to look at it-
the rear of anything should not demand bigger or stronger brakes than the front-

you should look into 40 years of vehicle testing- don't know if there was ever a case where rear braking was more important than front

OP- think of it this way- get on your hands and knees and slowly decent a steep hill- you should find that the control is from your front end (hands on the ground) and not your knees sliding-

BMX and motorcross racing is all about front end control- as should be most everything- think about it a second

also- sure you can sand the pads and rotors- won't make a bit of difference if you are still going to count on the rear to control your decent

think 70% front- 30% rear

Maybe a 210 rotor will be better than the 160, put a 180 in the back.