Rear Disc Brake Noise

Newby MkIII owner here again. Near the end of a mostly stop and go city errand running trip, I started getting a rubbing noise from the rear disc brake. The weather was much warmer than it has been so far - mid 80s. It has been my experience that brakes and rubbing noise are never a good thing. However, the brakes seem to stop OK with minimal pedal travel, and the rear disc is very smooth with no signs of abnormal wear. My first inclination is to replace the pads, but this being a Norton, I wanted to check first and make sure this noise isn't another "normal" things that Nortons do. Any thoughts?

I have done some further investigating. The disc doesn't get hot if not used so it doesn't seem like I have a dragging brake. It appears to me that the caliper is not positioned properly over the disc. There is at least an 1/8 inch gap on the outer side of the caliper while on the inner side there is almost no gap between the caliper and the pad. When depressing the brake the outer pad clearly moves but doesn't make contact with the disc while the inner pad doesn't move, either because it is tight to the disc or maybe the inner piston is seized. My first thought is to slightly bend the mounting plate off the swingarm so that the caliper is centered over the disc - that's how the front caliper looks. Does this seem reasonable or am I going to make things worse? Any thoughts would be appreciated.

spartanman said:

It appears to me that the caliper is not positioned properly over the disc. There is at least an 1/8 inch gap on the outer side of the caliper while on the inner side there is almost no gap between the caliper and the pad.

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Perhaps some further investigation is necessary, as under normal circumstances, the gap should be approximately equal on both sides.

spartanman said:

When depressing the brake the outer pad clearly moves but doesn't make contact with the disc while the inner pad doesn't move, either because it is tight to the disc or maybe the inner piston is seized. My first thought is to slightly bend the mounting plate off the swingarm so that the caliper is centered over the disc - that's how the front caliper looks. Does this seem reasonable or am I going to make things worse?

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The brake pistons should advance through the seals to compensate for pad wear or any minor misalignment, so if one pad isn't touching the disc then it sounds like the caliper (or the complete brake system) is in need of an overhaul. The brake caliper plate is basically flat and the disc should be at a set distance so there shouldn't be any need to bend the plate.

One thing I suggest you check is that the inner end of the lower caliper mounting bolt isn't touching the disc as there's very little clearance between them, and a wrong length bolt or even lack of a washer can result in the bolt touching the disc face.

Thanks so much for the info and photos. Further investigation has been conducted. The lower bolt is not touching the disc. I removed the caliper and the pads appear to have worn equally and I believe have little wear - there is about 1/4 inch of material remaining on each. Once the caliper is loose it can be moved so it sets plumb on the disc as shown in your photo, but when tightening the bolts it gets pulled askew. Now apparently the PO bought the bike and it had a 16 inch rear wheel, which he replaced with an 18 inch rear wheel. During all this work it would seem to me that even the slightest alignment issue with the wheel could move the disc off center enough to cause a problem. Now I don't have the special tool to remove the cover cap and check the seals and pistons, but the outer piston obviously works and just from looking in, there is no corrosion, gunk etc.. so I'm assuming the inner piston is OK as well. So...back to the fix. Bending the mounting plate does not seem to be a good idea, it is a very heavy piece and would likely result in putting undue stress on the swingarm. However putting a shim on the backside of the plate, just a thin washer - should move the caliper over. Does this seem reasonable?

spartanman said:

So...back to the fix. Bending the mounting plate does not seem to be a good idea, it is a very heavy piece and would likely result in putting undue stress on the swingarm. However putting a shim on the backside of the plate, just a thin washer - should move the caliper over. Does this seem reasonable?

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It would seem to be the most logical solution, as long as you are confident the brake is functioning correctly.

Success at last! A thin washer behind the mounting plate moved the caliper over so it is now perfectly centered. This required filing a small section off the mounting plate so the lower washer would clear. Getting the lower washer in place requires skill, patience, and small hands, none of which I possess. But after a half hour of profanity laced futility, things finally lined up. Boths pistons work fine and the rubbing sound is gone. Thanks again for your advice.