Disc caliper moving durning final tightening. GAH!

avalonracing · #1 02-15-2023, 07:18 PM

The rear disc on my GF's newish Litespeed Ultimate has a bit of disc rub and if there is something I hate it's a noisy bike. I did all the usual stuff to adjust it: pressing out the pad pistons for more space, loosening the bolts and using a spacer on the disc and tightening, eyeballing the caliper and tightening, etc.
The issue is that when I give the caliper the final tighten to recommended spec the caliper creeps over no matter how much I hold it in place. Even if I push it to the max beyond where I want it to be it creeps back over when I fully tighten

As it moves too far back away from the wheel I'm thinking about adding a spacer to the Spline hub to get the disc further out. I just need a fraction of a millimeter. The other option is bending the whole disc out but they seem to have a memory when they heat up plus the freaking clearance is so small it would be fairly impossible evenly to do.

That's my issue with road discs... that they can't design them with more than .5mm clearance. If they could just design one with 1.5mm clearance on each side there would probably be world peace.

Any thoughts other than the disc spacer?

Blown Reek · #2 02-15-2023, 07:24 PM

Welcome to the world of adjusting discs. Keep at it and you'll get it. Sometimes you have to really crank down on one of the bolts to tighten the other, and sometimes you need to tighten just a little. Maybe crank down the one that's moving and tighten the other one.

Remember how it was almost impossible to adjust the gyro/rotors of BMX bikes in the 80's? This is the new version of that. Sometimes it's easy and perfect, sometimes you need to keep on doing it. And there's nothing worse than when someone comes in at 4:30 with a brake emergency "because they're racing tomorrow" and it takes two fcuking hours to get it, and that's the last thing you want to do.

prototoast · #3 02-15-2023, 07:35 PM

My guess is you need to face the brake mounts. The "creep" is probably a slight rotation as it's pressed flat against the slightly-off brake mount faces.

charliedid · #4 02-15-2023, 07:35 PM

Quote:

Originally Posted by prototoast

My guess is you need to face the brake mounts. The "creep" is probably a slight rotation as it's pressed flat against the slightly-off brake mount faces.

Beat me to it.

batman1425 · #5 02-15-2023, 07:38 PM

Edit^^^ They both beat me to it.

When I run into this, I find it can have more to do with vertical alignment - that is keeping the caliper vertically square to the plane of the rotor. On a post mount caliper, that was easy to fix with some conical washers. Harder to fix on a flat mount.

Sounds like your mounting surface isn't flat and as you tighten it, it's sliding down until it bottoms out on the bolts. I would check the mounting surface for any high spots. Even a bit of overspray around a corner can be too much. If that doesn't do it, might consider having a shop face the mounts to make sure they are flat and square to the rotor.

John H. · #6 02-15-2023, 07:45 PM

I agree with the facing- At the same time, Boyd wheels sells spacers for exactly what you mention. I have used them mostly to match different wheel sets to one bike so I don't have to re-adjust. But I have had to use them in situations where it seems like the disc caliper mount was not brazed on in the perfect spot.

vespasianus · #7 02-15-2023, 07:49 PM

Yeah, this sounds like a classic case of a brake mount that needs to be faced. That should do it.

Blown Reek · #8 02-15-2023, 07:53 PM

Everybody talks about how every time there's an issue the mount needs facing, but every quality bike is faced from the factory (or builder). Sometimes manufacturing tolerances are at opposite ends of the compatibility spectrum, and that's the way things work. Chances are you're doing the same thing over and over and need to do something different.

Much like Sol lecturing Max about Archimedes in the movie "Pi", you need a break.

https://www.youtube.com/watch?v=OGKPmBtBpBo

avalonracing · #9 02-15-2023, 07:55 PM

Well, it isn't paint that is causing the facing issue as it is an unpainted titanium mount. Having it faced could be a real hassle for a couple of reasons. She'd rather deal with hearing Tsss Tsss Tsss 974,373,726 times a year than send the bike back to Litespeed and have them say, "Nope looks fine to us". Also, I'm sure she won't trust a local shop to do something that for all intents and purposes could make it much worse (I can't say I blame her).

I think the spacer might be the first route unless you guys have any other ideas.

And oh, damn Shimano for requiring different tools for an Ultegra center lock rotor vs the tool I already have for an XT centerlock rotor.

EB · #10 02-15-2023, 07:57 PM

Facing sounds like a good thing to look at, but just asking a dumb question- you’re applying pressure to the brake lever with the rotor in place while tightening the caliper bolts, right?

avalonracing · #11 02-15-2023, 08:10 PM

Quote:

Originally Posted by Eli Bingham

Facing sounds like a good thing to look at, but just asking a dumb question- you’re applying pressure to the brake lever with the rotor in place while tightening the caliper bolts, right?

Yes, I am. But when I tighten the bolts to spec it just bends the disc to where the caliper wants to be. It's not a constant rub (and it doesn't touch in the bike stand) but when riding but it is so ridiculously close that the weight of pedaling under any load gives you the tssss tssss tssss. And it is a stiff bike with a T47 BB and decently-sized chainstays.
Shimano GIVE US SOME PAD CLEARANCE! We know you can make it happen.

dolface · #12 02-15-2023, 08:15 PM

I've dealt w/ the same issue by getting the caliper as tight as I can without it moving, and then doing the final tightening in annoyingly small increments.

Tiny turns (~3-4° or less) to each bolt. No idea if it'll fix your issue (or if you've tried that already) but might be worth a shot.

EB · #13 02-15-2023, 08:21 PM

Quote:

Originally Posted by avalonracing

Yes, I am. But when I tighten the bolts to spec it just bends the disc to where the caliper wants to be. It's not a constant rub (and it doesn't touch in the bike stand) but when riding but it is so ridiculously close that the weight of pedaling under any load gives you the tssss tssss tssss. And it is a stiff bike with a T47 BB and decently-sized chainstays.
Shimano GIVE US SOME PAD CLEARANCE! We know you can make it happen.

Sorry to hear this. I would definitely check out the facing issue. Flat mount is the devils work - I live in fear that Shimano finally manages to get it adopted in the mountain bike world.

avalonracing · #14 02-15-2023, 08:23 PM

Quote:

Originally Posted by dolface

I've dealt w/ the same issue by getting the caliper as tight as I can without it moving, and then doing the final tightening in annoyingly small increments.

Tiny turns (~3-4° or less) to each bolt. No idea if it'll fix your issue (or if you've tried that already) but might be worth a shot.

Interesting thought. Maybe I could try that. When we got the bike there didn't seem to be any rub and it started later (possibly after the rotors got heated from enough riding and there was a bit of warp). When I went to loosen the calipers they were tightened to around 72,000 Nm. Perhaps whoever was initially building the bike had some issues and overtightened it to hold it in place.

avalonracing · #15 02-15-2023, 08:25 PM

Quote:

Originally Posted by Eli Bingham

Flat mount is the devils work -

Ha. That is a cleaned-up version of what I was saying last night. Initially, she was going to go with a rim brake frame and I talked her out of it as rim brake wheels are getting hard to find. So I was cursing myself last night for telling her to get the disc model.