A few questions:

1. What brand, model fork? 20mm TA, 15mm TA, or 9mm QR? What size rotor on the front? Post mount, right?

I usually recommend a particular manufacturers adapters and rotors when switching from Avids to another brand. You can get away with not doing it but you have to make sure the edge of the rotor comes to the outer edge of the pad or a bit more.

Without seeing it, its hard to tell exactly what's wrong. If you're in the area I'd be glad to take a look at it for ya.

Okay you really had me stumped (no pun intended) on with this one.  I believed though I know what is going on.   

For disc brakes there are two mounting styles I.S. or Post Mount (PM).  Your bike has PM front and rear.  The rear PM however can come in PM 140 or PM 160, your bike has PM 140 mounts in the rear.  All this means is that if you have a 140mm (the 140 in PM 140) rotor on the rear you can mount the caliper directly to the frame with no adapter.   

Anyway back to your problem.  Avid's method for caliper alignment is CPS, it uses spherical washers on both sides of the caliper (see the attached picture). 

Shimano brakes DO NOT used spherical washers on the mount side for 160mm rotors.  If I am correct you are reusing the Avid CPS hardware and it is moving the calipers out too far and thus why the calipers are only clamping down on half the rotor width.

Since you have PM 140 rear mounts you will need to use the spacers that were used with the Avid brakes.  The spacers should move the caliper out 10mm (the difference between 160 and 140 is 20mm but the caliper only has to move half that distance).   

I would be skeptical (or careful) about trying to use the Avid spherical washers on the bolt side of the caliper.  My concern would be the length of the caliper bolts, if they are too long the caliper will not be secured properly or worst case you could damage the fork or frame.  If the caliper bolts are too short the bolts can rip out the threads by not being engaged enough.   

The XTs should have come with mounting bolts and plastic caps (which is cool little safety item).   

Also, remember to tighten the caliper bolts to the correct torqued, 53-69 inch-pounds I beleive.

I hope that helps you out. 

better check your brake pads for wear. you may be able to remove all the spacers and conical washers and use the avid adapters but your best bet is get some shimano's or even hayes adapters

I have a final update for you.  As I said before you should have no issues with the front brake, it mounts directly to the fork (no washers, shims, etc. should be used).   

As far as the rear goes I believe this is the issue.

Assumptions: You are using Avid CPS 20S spacers and 160mm rotors.   

The Avid spacers you have are approximately 17.5mm (upper) and 12.5mm (lower).  If you install the XT brakes with just these the brake will be 5mm to far away from the rotor.  If you install the XT brake with the spacers and the spherical washers the brake will be even further away from the rotor.   

What I said yesterday will work.  If you install the XT brake with the Avid CPS 10S adapters (12.5mm and 7.5mm) it will work.   

The preferred solution would be to use a Shimano adapter but you will not find one that specifically states it is for REAR PM140 to PM160 (at least I can't find one).   

However, you can use a Shimano front 160mm to 180mm post mount adapter. Shimano part number SM-MA-F180P/P2.  Available online for sure but you may be able to find one at a LBS.

It will work properly don't worry. Here is why. 

The PM 140 mounts on your bike as I said before allow you to use a 140mm rotor and mount a brake directly to the frame with no adapters.  To mount larger rotors you need to move the brake out and why you need spacers or an adpater.   

Using a 160mm rotor with a PM 140 mounts, the change is 20mm.  That is no different than going from a 160mm to 180mmm rotor on the front (mountain fork post mounts use 160mm rotors as the base).  The increase in either case is 20mm.  The spacers or adapter in either case will be the same.   

Also, I checked (physically) and the spacer blocks on the Shimano adapter are aproximately 12.5mm and 7.5mm.   

I hope this helps you out and that I didn't confuse you.  I'm an engineer and things like this drive me nuts.   

The Cycle Path wrote on 05/08/14 at 3:16pm:


HEY!  TAKE THAT BACK!   

QCB had one of the Shimano adapters in stock yesterday so I grabbed one there.

For the front brake I removed all spacers and it contacts the pads perfectly.  For the rear I installed the Shimano 160 to 180 spacer and mix-and-matched some bolts to get enough thread contact and it contacts the pads perfectly as well.

Power feels like it increased a lot, although I can't get the brakes to lock up as well as I'd expect considering how hard they are grabbing.  I just cleaned the rotors with alcohol on a fresh t-shirt rag to remedy some squealing, so I'm hoping the lack of ability to lock up is simply the slightly uneven pad wear from the first 100 miles or so of only half the pad being used and that'll it'll work itself out on my next ride.  

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