Disc brake caliper centering
 

Hi, newbie to disc brakes. Just got a 2003 bike with Shimano XTR discs (BR-M965). The front wheel calipers are not centered and rub very slightly on the inside caliper. It's consistently off-center through the entire rotation. The rotor seems straight and tight and the wheel is true. These brakes have the "post" mounting thing, where the mounting bolts are perpendicular to the rotor (see pictures). So it doesn't seem possible to adjust in/out with those bolts? I found a manual online here, but it's way over my head:

https://hollandbikeshop.com/images/SI-8CM0G-000-ENG.pdf

Any advice would be much appreciated, thanks.

https://cimg0.ibsrv.net/gimg/bikefor...de6409943b.jpg
https://cimg2.ibsrv.net/gimg/bikefor...f3a183f1b6.jpg

Maybe retract the pistons, slip a business card in the side that rubs, squeeze brakes several times, remove business card.
It's cheap to try.

Barry

that mounting will be hard(er) to center

best I can suggest is take the pads out and either replace them or re-use the old pads but

RE-ARC the retraction springs !

I have found that the Swiss Stop retraction springs are wimpy and so use the Shimanos which are better but I still "re-arc" them to get a bit more spring action

you might also try bleeding the line. if there's a bubble inside the caliper that could account for the behavior noted.

/markp

you might be able to adjust the axle to shift it over, or if that is not an option, add an axle spacer in between the hub & fork leg.

Is there a model number on the caliper itself (a Shimano model number)? Perhaps with that, you could find some service info from Shimano that might offer a tip or trick on where to start.

On that mount, it's possible to shim the caliper inward, but not outward. Before doing so, try the steps suggested above, especially bleeding, since any air will limit pad retraction.

Then if the action is good, and you still have issues, you can make shims of paper, cloth or plastic. My personal favorite is paper soaked in oil because it's easy and durable. Also the hole punch reinforcements sold for ring binders are spot on perfect if the thickness is right. A caveat is thickness. I prefer to limit paper or cardboard shims to under 1mm. For thicker shims, I prefer plastic shim stock, or stock metal washers.

If you opt to shim, regardless of thickness or material, make sure that the shim fits on the flat mounting area ONLY, any contact outside that area will cause the brake to be tilted.

Soda Can makes great & durable shims.

Barry

Most definitely. But IME people have problems working with them when things like holes are needed. Since they're thin enough to be in my paper range, I opt for the convenience of going with paper in these applications.

FWIW soda can cuts are easy to get, and tend to be worse than paper cuts. That said, if you have a decent hole punch, soda cans are a great option for this.

there are no pictures showing the description of the issue imo. cannot understand which way the tolerance issue is in question. ymmv

That manual shows that the calipers are centered using .5mm and .2mm shims under the mounting bolt.

However, this is not a new set up, so it is more likely you are having a problem of the pistons no longer moving symmetrically. Grab the lever and watch the pistons - do they move at the same time? Do they move the same distance?

If not, the pistons should be cleaned and lubed. If that isn't sufficient, I have had great success getting the slow piston to move by holding the more active piston while I pull the lever. A couple of those in and out gets the late moving piston freed up.


Don't just shim the pads. You'll find that the off center condition just gets worse as the one side gets stickier.

You can also buy/make shims for the rotor

I tend to agree with Kontact for the most part. You should check to make sure the gunk is cleaned off around the piston seals. Though it's not really necessary for both to move the same amount. All that is needed is for both to move some but not necessarily equal amount. And even if the pad doesn't visually move a perceptible amount, then it still isn't going to be causing you to pedal harder. Any warpage or out of true on your rotor will move the piston far enough to be out of the way. And even if there doesn't appear to be a gap, it's still okay, It doesn't need a perceptible gap if the rotors are true enough.

Thanks very much for the advice. I ended up using a thin metal washer on the inside of each fork mount. They are pretty crappy washers, but I suppose they're as good as soda cans. It centered the rotor anyway, now it spins clean w no rubbing.

Pics by chance?