If my worn XT brake pads …

… have an untouched arc at the top, say varying from 1mm to about 3.5mm, where presumably the rotor isn’t overlapping the pad, is that OK or does it mean the build has the wrong mount adapters between the calliper and the frame/fork?

(Not a bike I built myself).

there’s some variation between rotor manufacturers I think..are the Shimano rotors and shimano brackets?

The rotors are Avid. Not sure what the mounts are but I suspect not Shimano.

The brakes work fine in use, I was just a bit surprised when changing the rear to see a not-miniscule area (say 10-15%) of the pad untouched. Maybe I’m loosing 15% of braking power!

My XT pads wear down evenly across the pad. I’d say it’s misaligned somewhere and quite probably in the adapters.

Ah, just seen that, rotors are avid, isn’t their 180mm rotor a 183 etc?

Yes, I think you are losing that much contact area and possibly pressure where the pads try to touch around the rim

I would say it was 180 rotors and 185 adaptors myself. I had something similar a while back, and the brakes bound eventually leading to much fretting (and an abandoned ride) until I worked out the cause!

This is rear, 160mm rotor, IS frame mount, post mount calliper.

With a little more thought – this is plainly not right. Because a) it’s hard to tell visually from above how much the pad is worn b) on my night ride last night the rear brake more or less ceased to function without notice (possibly because the top unworn parts of the pads finally made contact over the rotor, which was then running more or less freely between the lower worn parts of the pads), rather than the slower degradation and clunking that usually happens with worn out pads.

(Second hand bike).

100% certain, that’s not right.
That to me, looks like a 165 adapter and a 160mm rotor.

Total cost, about £5 + new pads.

also that pad ^^ has actually got a fair bit of wear left in the worn bit, but presumably the unworn bits contacting above the rotor are stopping further progress

Agreed, I put new pads in but I’ll get proper Shimano adapters, until then that bikes’s off the dirt. There’s a similar wear patten on the front (post to post, 180mm) so that’s probably a wrong adapter too.

My rear brake failed (well maybe there was 10% left) as I was JRA last night, rolling back home on the flat. Had this happened at other times of the ride the outcome may have been a little less pleasant.

My XT rear does that on my singlespeed but only leaves a arc of around 1mm and that’s with a 160mm shimano rotor and shimano adapter so I can only assume that the disc mount on the frame is very slightly out. Also used to have similar problem on some old Pace sus forks, I used to remove the pads when they were half worn and file the ridge off so the brakes continued working to the end of the pad life

im running xt brakes on avid rotors.i had to loose the spacers on the adaptors and it worked fine,do you still have the wee spacers on the adapters??that would maybe explain why your pads are not contacting the whole rotor…

give it a couple more rides and pull the brake… 😈

pad material will contact and wont touch the disc. not worth the risk. get the mount/disc size sorted asap

@hainman there aren’t any spacers pushing the calliper further from the axle/frame than it could be, so that’s not it

it’s either (in order of most to least likely IMO):
* incompatible adapter for the rotor, or the calliper, or both
* compatible adapter mounted wrongly (backwards mostly)
* incompatible rotor, for the calliper
* frame issue

anyhow now I understand why my rear brake stopped working last night, I’m gonna buy a Shimano adapter and try that.

@greeble for sure

Update: I took off the installed rear adapter, it says on it “185 F ->” with the arrow and F presumably indicating the orientation. There are no other branding or marking on it at all.

I put on a Hope mount F (Post to IS 160mm) for the interim (a Shimano is en route), pad to rotor overlap is perfect as far as I can see.

Oddly the removed mount arms are only a few mm longer than the Hope mount arms, that matches up with the pad wear pattern too … but if the mount is intended for a 185mm rotor I’d expect to see a 12.5mm difference with the 160mm rotor mount.

Presumably the difference must be made up in calliper designs, and the fact this adapter almost works with XT callipers on a 160mm rotor explains why the previous owner thought it was an acceptable setup.

Yeah the removed adapter is Avid, like this, http://www.chainreactioncycles.com/Models.aspx?ModelID=13721 “Post to Front IS 185mm”

ah cool,i know when i 1st tried to put my XT calipers onto my avid brackets they were a few mm out,lost the wee silver shims and they were spot on.how the other owner never spotted this unless he took of the 185 rotor and put a 160 on before selling you the bike??

Don’t know to be honest, I suspect he put the bike together from bits but didn’t ride it much. At a cursory glance it looked like it was ‘OK’, it’s not until it comes to a pad change/(brake failure!) the issue becomes obvious.

The setup’s pretty dangerous actually, the rear went from 100% to 10% instantaneously, without any warning or feedback you’d usually get from wearing pads (because they weren’t worn out). So these things are worth checking on second hand bikes!

And from now on I’ll always match calliper and mount brands.

Just changed from old IS mount hopes to post mount Hayes on my bike and bought a used shimano mount for the rear. Fitted the rear calliper loosely and all good, did the final tighten and the calliper was touching the edge of the Hayes rotor and the pads were fractionally too far ‘into’ the rotor.. Used a pair of IS mount shims in between the adapter and the calliper and its now spot on.
The shimano mount must have been out by less than 1mm butt it made all the difference.