Really hot rear brake disc

I just had a new rear tyre fitted, and at the same time the mechanic noticed I was almost out on my rear pads. So he prised apart the pistons, removed the pads and stuck in some new Bendix (?) ones witha load of coppergrease. When the wheel was back on, he spun in and there was a faint metal on metal sound on the odd turn of the wheel, like a chef sharpening a knife. Wheel turned ok though. So I rode home. Now I have ridden to work, normal commute, using front brakes to do the slowing, just dragging the odd back brake round slow roundabouts and parking etc, and the front discs are lukewarm as you'd expect, but the rear is very hot. Like you can tap it but not hold it hot. There is also a rainbow discolouration of the pads, like when you heat metal with a blow torch. This would suggest to me that the brakes were binding, but when I have it up on my paddock stand, the wheel spins very well. Better than my front did before I cleaned the calipers, it would hardly turn if mounted wrong. So my questions are these:

1) is it normal for new brake pads to go all rainbow colour where they are touching the disc?

2) why is my rear disc so hot if the wheel seems to spin fine??

3) is it damaging i.e. going to warp my disc or set my ar$e on fire??

Hi Nick, I would say the pads are binding except that don't appear to be when it's up on the stand. Maybe they've cooled down and withdrawn slightly by then.

I'd take out the pads and pump the piston out a bit and clean them up. Damage may already be done to the seals though. Sometimes the dust seals get jammed too.

Could definately warp your disc IMO.

[the_lone_wolf]

could the disc be bent? when he took the wheel off did he lean it on the disc side?

if it's bent it'll rub against the pads at certain points of the revolutioin, and make that knife sharpening noise

try gently using it, if the pedal feels like it's pulsing it could be that perhaps?

Originally Posted by the_lone_wolf

could the disc be bent? when he took the wheel off did he lean it on the disc side?

if it's bent it'll rub against the pads at certain points of the revolutioin, and make that knife sharpening noise

try gently using it, if the pedal feels like it's pulsing it could be that perhaps?

Good point TLW or the disc could've been warped before the pad change.

Didn't feel noticably juddery, and now the metal scraping noise is gone, it was just the pads bedding in. Basically you can see the pads working with the lever, they break well, wheel spins fine, everything looks good, just it gets mega hot . If you spin the wheel, break, then release, they spring back fine and the wheel can turn again. Perplexing...

[G]

Could your foot have been resting on the brake lever by mistake?

[the_lone_wolf]

Originally Posted by Warthog

Didn't feel noticably juddery

it wouldn't feel like a judder, the pedal would pulse slightly, tbh you may well not notice it through a boot. i'd be tempted to fire it up on the paddock stand, bung it in first and let the wheel spin* and then lightly apply the rear brake by hand, as you gently increase the pressure you may or may not feel the pedal resistance changing, but as it's not making the knife sharpening noise i'd be inclined to think it was the pads settling down

*keep pets / kids / loose clothes / treasured anatomical bits away from the moving chain...

Originally Posted by the_lone_wolf

it wouldn't feel like a judder, the pedal would pulse slightly, tbh you may well not notice it through a boot. i'd be tempted to fire it up on the paddock stand, bung it in first and let the wheel spin* and then lightly apply the rear brake by hand, as you gently increase the pressure you may or may not feel the pedal resistance changing, but as it's not making the knife sharpening noise i'd be inclined to think it was the pads settling down

*keep pets / kids / loose clothes / treasured anatomical bits away from the moving chain...

That sounds like a good test for the juddering. I could see when they were fitted that they pressed unevenly against the disc (as the disc and previous pads had been so worn down together and the pistons had to be completely pushed back again). Now they seem to sit fine and evenly. When the mechanic pushed the old pads apart, some brake fluid squirted out of the reservoir. Don't think that has anything to do with it, as the brakes are too good, not too bad! Maybe loosen the lid in the reservoir to releive some pressure if there is any? TBH, all this would be something to do if the wheel was clamped tight, but it seems fine .

Originally Posted by Warthog

That sounds like a good test for the juddering. ......

It ain't "Juddering" it's "gently pulsating" (ooer) & believe me - I am now fully qualified in how to tell the difference.....

Originally Posted by Warthog

When the mechanic pushed the old pads apart, some brake fluid squirted out of the reservoir. Don't think that has anything to do with it, as the brakes are too good, not too bad! Maybe loosen the lid in the reservoir to releive some pressure if there is any? TBH, all this would be something to do if the wheel was clamped tight, but it seems fine .

Wider pads mean pistons pushed in further - which means less space for brake fluid - worth checking resevoir not overfilled and wapping lidd off.