Rear disc brake rubs when pedalling hard.

[arallsopp]

SOLVED: Rear disc brake rubs when pedalling hard.

Howdo.

This week's odd noise coming from the bent originates in the rear disc rotor, rubbing on the pads when I'm pedalling hard. Hard as in high power, low cadence.
It was pretty minor originally, but now develops in every gear, as soon as I hit an incline.
On a DF. I'd say you'd trigger it as soon as you get out of the saddle.

Rear wheel looks tight in the stays, so a bit stumped as to how I might fix it. I tried loosening the brake off (it looks aligned L/R correctly) but it didn't fix it even when I had pretty much no brakes!
There's a lot of flex somewhere in the system, but I'm not sure where to start...

Cheers in advance for anything you can offer.

 

[Steve Austin]

whats a DF?

 

[RedBike]

Diamond frame (I assume the OP is riding a bent).

It's worth checking the cone nuts are tight. Grab the wheel and see if there's any wobble.

On MTBs it's normal for large rotors to rub slightly due to the frames / forks flexing.

 

[arallsopp]

whats a DF?

Yep. Diamond Frame. I figure the issue isn't bent specific, but then used bent specific language to describe it. Oops. Apologies.

 

[Steve Austin]

Never heard it called a df like that before.

It'll be one of 3 things
1 frame flex
2 warped rotors
3 wheel flex

1 and 3 make 2 worse.
2 would happen all the time, so if it only happens under pressure my money would be on a combination of 1 and 3.
a quick retighten of the spokes, and a hub service, and making sure cones are tight can help. if its frame flex, then a stiffer stronger hub is about the only solution.

 

[arallsopp]

Thanks all.

I took the wheel off this morning, and checked it over. Cones seem right. Little loose off the bike, but the QR axle adds some pressure back when the wheel is installed. No play on the axle that I can detect, rotor looks good.

Wheel does flex a little, but as the rotor is connected to the hub, I wouldn't expect this to give too much of an issue. Gave spokes an experimental tweak as they all seemed a little low pitch.

Infact, full of confidence I reinstalled the wheel and pitched it up a mile and a half of 12% this morning. No problems at all as yet, so am concluding this one solved.

Cheers!

 

[jimboalee]

If the axle itself has a crack in it, you might not detect it even with the wheel out. Cracks usually happen level with the inside edge of the driveside cone.
Only when some pull is applied will the crack widen with bend on the axle.

Strip and rebuild would be my way forward.

 

[GrumpyGregry]

BB7's on a DF tourer did the same. I took the wheel out, looked at it, put the wheel back in, et voila, problem solved by a simple hard stare. Can only conclude the wheel was originally not squarely seated in the dropputs.

 

[arallsopp]

Problem solved by a simple hard stare.

Strip and rebuild would be my way forward.

Hmmm... With long rides now hitting +200miles, I'm tempted to be thorough. Given my technical skills, thorough might have to be a thoroughly hard stare.

Will cross my fingers and hope Greg wins over Jimbo. If it starts up again, will be time to sort it for real. Rear cassette needs a bit of work anyway, so will factor a proper strip down in the next evening's service.

 

[jimboalee]

Put your weight on the pedal with the front brake full on.

Does the rim move toward the chainstay?

 

[arallsopp]

Interesting question, and one which will have to wait for an assistant and spotter.

One thing for sure: On my own, the wheel will be obscured from vision, and unless on the turbo, the whole rig will move toward the floor pretty rapidly.

My bike: