disc caliper mounting/adjustment problem
 

I'm new into disc brakes having added two bikes with them into the household about a year and a half ago. So four calipers.

Three of the four calipers are dead simple to center if they ever need it. Generally don't need it, but if wheels are changed, sometimes they do. Three of the four take about 2 minutes to re-center using the common technique of loosening the mounting bolts and tightening the bolts while squeezing the lever.

The fourth caliper, which is the front caliper of my bike is just much more fiddly than that. The simple squeeze and tighten doesn't work. I've even got one of the feeler-gauge disc caliper spacer things that provides a thin gap between the pad and the rotor on both sides when the lever is squeezed. It doesn't help.

I can get the caliper centered by loosening the bolts, spinning the wheel and very carefully manipulating the caliper until it's silent.... then very carefully tightening the bolts by a couple increments, alternating the tightening until it's snug at which point I can get it to proper torque. I'm getting the hang of this so it only takes a few minutes but there's always the risk of bunging up my fingers, and it's hard to get the wrench in place when I'm holding the caliper in the position it needs to be.

Any ideas of why this one caliper is finicky? Do the mounts need facing maybe? is this something a normal bike shop will do? If that is likely to be the issue, I can call around and see if any of them are set up to do it. I'm not confident I can get it done locally.

What happens with the odd-ball caliper if you just try to center it normally by loosening the bolts, squeezing the brake lever and re-torquing the bolts? Does it shift as the bolts are tightened?

I’ve one where the keep squeezed while tighten, results in it rubbing a pad.
I slip a piece of a business card in the side that rubs, then keep squeezed while tighten.

Barry

I'll try to see that, but when the lever is squeezed, both pads are tight against the rotor, so I don't think I could see if they shifted when the bolts are tightened. But I'll look closely next time

I have tried that, but it didn't work. I'll try again, and also look more closely as to what is actually happening when I try to go through the routine as mentioned above.

You don’t mention if they are mechanical or hydraulic brakes, but I’m assuming they’re hydraulic. You may have a dirty brake seal that is preventing one piston from retracting far enough. I ran into this recently on a Promax hydraulic brake and it was a pain in the head. Park Tool has a good video here explaining how to clean brake piston seals:

I use a feeler gauge on the side that rubs before squeeze and tighten. Start with 0.010" and change to thinner or thicker if it doesn't work first try. The problem I have is the caliper does move as fasteners are tightened. I experimented, and using less torque helps with that. Too bad caliper doesn't float on mounts like car calipers...

I just put my 1st disc brake bike together today. The front pad was rubbing. A Google search lead me to this video. The first part was loosen the screws with the brake depressed. Which I did. Doing it a few times worked. If that didn't work, the second part of the video showed what to do. I purchased a disc brake bike to learn about disc brakes. Similar to a cars disc brakes, I remember pushing back the pistons while working on my old Toyota truck.
I hope this helps.

Thanks for the replies

I was hoping to avoid dealing with a sticky piston, but that could very well be the problem. I have tried using a feeler on one side but didn't have luck with that. I haven't changed wheels since my OP so haven't messed with it. I'll re-try the one-sided feeler and if that doesn't work, I'll take another look at the Park sticky piston video and do that.

Still wondering if this could be a symptom of a mounting surface that isn't flat and square enough?

You may have to grab the loose caliper and move it to the center while the wheel is spinning and the rubbing noise stops.Then tighten the bolts a little in each until they are tight. If it doesn’t work, the mounting surface needs to be re-surfaced. According to Park Tool.

I had this issue on my bike. No matter what I tried, the caliper always shifted when tightened. I called several shops in my area. Only one shop even knew what I was talking about in regards to having the brake mounting surfaces faced, but didn't have the tools. My bike uses the old IS brake mount standard, and I was able to find the old Park brake mount facing tool that was only for IS mounts. I think I found it for around $75.00 After removing the brake adapters and facing the mounts, all was good and I have no problems centering my calipers. It even came in handy when I had the same problem on my Ice Cream Truck, which also uses IS mounts.

I’ve never had much luck with the squeeze-and-center method, to the point I don’t even try it anymore.
My preferred method is to loosen the bolts just enough so that tapping the caliper with something like a screwdriver handle will shift it around enough to allow for adjustment. Then tighten and ride.
I too have suspected surface imperfections as a reason for why some calipers are so eager to creep a little when tightened.