cyccommute 

The back nut should be tight enough for the brake not to move. No part of the brake should rotate. The fact that it does says to me that you haven’t got something tight. I suspect the screw the green arrow is pointing to is not tight enough




Be careful when tightening it, however. The brake has to rotate around the screw so it can’t be too tight or too loose.

The Park OBW-4 will help but if you don’t have one, loosening the back nut and rotating the caliper will do the job. The OBW-4 will provide a finer adjustment but that only works if you have one. Order one for the future but loosen the bolt and get the job done now...after you tighten up the front part of the brake.

You are correct that the caliper shouldn’t be able to rotate. If it is loose enough to rotate, the rear nut isn’t tight enough. If the rear nut is tight and the caliper still rotates, there is something amiss with the brake and you need to find where you went wrong with reassembling the brake. The first place you went wrong was with disassembling the brake. Seldom do brakes need disassembly. You will need to tighten the pivots...probably the central one...but they need to be just tight enough to hold in place without being so tight that the arms won’t pivot.

“Just grab[bing] a side and pull[lng] it over” won’t fix the problem. If you can grab a side and pull it over the brake is far too loose to stay there. Dual pivot brakes (nor any brake for that matter) should not be adjusted by grabbing and pulling.

The angle of the photo isn’t such that you can tell if the wheel is centered in the fork or not. If the brake is off-center, centering the wheel is something I always check before doing any brake adjustments. It’s best to start with simple adjustment

No, the screw adjusts the caliper in both directions. There is no need to adjust the cable at all.

The angle of the photo isn’t such that you can tell if the wheel is centered in the fork or not. If the brake is off-center, centering the wheel is something I always check before doing any brake adjustments. It’s best to start with simple adjustment

RubeRad

Thx for more detail. The wheel is centered, I disengaged the QR with the bike vertical, nothing moved, and tightened it again. Also visually the rim is centered in the fork. I think that photo shows that the location of the tire is in line with the head tube, but perspective can be ambiguous.

When I had the front arm off (and that's the only disassembly I did, other than detaching the cable), I checked that bolt at the green arrow (5mm allen), and it was tight. The 5mm behind the fork is also tight. Yet the whole caliper is loose. Or maybe green-arrow bolt wasn't really tight, it was just frozen?

I haven't tried loosening the behind-the-fork nut, only tightening. I guess I could loosen it and see if it creates some different kind of brake looseness than what I currently have, as a diagnostic.

Iride01 

Depends on how you look at it. If you are looking only at the one side it opens or closes, then I'd agree with you. But if the side that moves is not the side that is rubbing and needing adjustment, then moving that side won't do anything for brake clearance. The barrel adjuster or changing the cable in the holdfast will have to be fiddled with. That will open up both calipers and then the little adjust screw will do it's thing, but not if previous futzing with it put it near the end of it's adjustment range.

I'll agree, I may well be wrong for these brakes, but every Shimano dual pivot brake I've had worked like that. And I haven't had many. So there.............. <grin>

cxwrench

No, this is not true. Mechanics all over the world center dual pivot calipers all day every day by hand and they don't fall off or end up being too loose. The bolt that needs to be tightened is the one that no one has talked about yet. It's the pivot on left (as we look at it) side of the brake. That is the only one that would allow that outer arm to be that far from where it should be. That pivot bolt needs to be tightened and then locked down w/ the nut on the back.

cyccommute 

I guess I’ll just have to keep throwing stuff at the wall Looking at your photo again, I noticed the silver circle the blue arrow is pointing to




The bottom of the adjustment screw should be sitting in that recess. You really shouldn’t be able to see that recess at all if it is in the right place. You are currently pushing on a part of the arm that is too high and that will push the right arm towards the wheel and the left arm away from the wheel. Try unscrewing the centering screw (not all the way) and see if that causes the brake to move. The left arm should just clear the right arm if the caliper is properly assembled. Yours is cocked forward in your picture.

RubeRad

Incorrect, EVERYBODY has talked about that.

One more time for the record, I already took care of that problem. That problem was created by me trying to fix the centering, the brake rubbing was there before I caused that problem, and the brake rubbing is still there after I fixed that problem.

If it will help, I will take a new picture tonight and post it in the OP so everybody will stop talking about it

cxwrench

Right, I missed that post from earlier. So you now have the brake assembled correctly (visually) and still have one side loose/not staying in the proper position?

cyccommute 

The only brake you can center by “pulling” on it is a cantilever with a straddle cable...and even that is a poor way of centering the brakes. If the dual pivot caliper is loose enough to move by hand...i.e. the rear bolt is loose..., it won’t stay centered. If the caliper is tight enough for the arms to stay in place when moved by hand, it likely won’t open properly when the lever is released because the pivots are binding.

Now if you are talking about moving the brake to the center position by loosening the crown bolt and then retightening it, that’s not what I call “centering it by hand”. That’s just A coarse centering of the brake.

RubeRad

Yes

Not one side, the whole caliper can be rotated by hand with not too much friction, and no matter where you put it, it drifts so that the right pad (from photo&rider perspective) rubs on the rim.

In terms of operation and safety, there's no problem, the brake opens and closes with the lever as it should, and braking power is fine. The rubbing is light and probably doesn't even slow the bike down that much, but the constant scraping sound is annoying.

(Certainly it doesn't slow the bike down nearly as much as that one time on a different bike with cantilever brakes I discovered one arm's spring had come detached, so the other arm's spring was pulling it against the rim -- THAT slowed me down for sure)

cxwrench

Remember what happened last time we got into a disagreement?

cxwrench

Ok, I think you have a problem w/ the adjustment of the nut on the back of the brake assembly. The pivot bolt runs all the way through the brake and is stepped down for the nut that goes through the fork. It looks like you need to tighten the pivot bolt up a bit. You'll need to take off that front arm and adjust then put it back on.

RubeRad

I don't, but I can imagine. Please let's don't

RubeRad

When I had the front arm off, I tried that bolt (5mm allen) and it was tight. (Or maybe I was deceived and it was just frozen?) The bolt behind the fork is also very tight. The only thing left I can think of is the nut between the caliper and the fork. What is that nut's purpose?

Russ Roth

Actually you should undo the straddle cable in order to adjust spring tension by pulling one side around but don't do this if it has the plastic covers with adjustment screws, then you should use the adjustment screw.
If you have the brake pivot bolt so tight that you can't center it by hand without a tool of any form you're risking damage to the carbon fork. By hand literally means grab one side and pull over to where you need, the bolt should be tight enough to keep it there.

So the nut in the middle is what adjusts the tension on the center pivot, if this pivot isn't moving things may have gone out of adjustment or you need to drip some lube into the pivot. When a brake pivot bolt is properly adjusted the recessed nut that holds it to the frame or fork should have no impact on the brake's ability to pivot. With a lot of these brakes the nut just bottoms out on the bolt and a tiny amount of play is built into the system, that bushing in the middle can be bound up due to grit, and what I've seen most often is salt water, which isn't allowing the two pieces of the caliper the spring is connected to actually move freely. Just drip your thinnest wet lube in there, again remove the wheel, and squeeze the pads towards each other to have the movement help move the lube through. Taking it apart to clean and grease is also a possibility if you're inclined.

cyccommute 

I seem to recall someone else saying exactly the same thing.

cyccommute 

The nut (yellow arrow) is connected to the stud that the back nut threads onto. It isn’t threaded so it can’t be “tightened”. It is tightened via the back nut.


I took my brake off to illustrate. The yellow arrow points to the equivalent of your nut. Shimano uses a different kind of “nut” but it serves the same purpose. Sometimes that nut is just machined onto a the stud.




The central pivot may thread into an that has been attached to the “nut”. It is attached some way so that the pivot can “pivot”. The yellow arrow points to the central pivot. The second pivot is off to the right.



On my brakes there are two set screws that hold the stud in place and hold the short arm for the second pivot in place. You may want look under the brake and see if there are similar set screws. If they aren’t tight, this would allow one or both arms to rotate. I would suspect that there is a flat on the stud that the set screw is tightened against. I didn’t check when I had the brake off but I think I can make out a flat side on the mounting bolt.

cyccommute 

Only in very old brakes and that’s a very rough way to center even those old brakes. The cable hanger on the straddle cable can slide from side-to-side to do a little bit of centering on the cantilever. It’s better to have some kind of centering mechanism on those.

Shimano suggests 8 to 10 Nm torque on the back brake bolt of their dual pivot brakes. My Ultegra brakes are on a carbon fork at 6 Nm of torque and they don’t move by hand at all. Add 2 to 4 Nm more torque and the brake won’t be any easier to move by hand. In other words, properly installed, brakes should stay in place under significant force.

Nor should the brakes move by hand. If you can “grab one side and pull over to where you need”, the brake will rotate under braking. Even when using the Park centering tool should take significant effort to get the brake to move.

RubeRad

Hidden set screws! That would make sense. I'll pop the wheel out and see if I can find those underneath.

Thx for all those pictures, and taking the trouble to pull your brake apart, super helpful!

cxwrench

What do you do for a living?

If you have a dual pivot brake tight...tight enough...and it's not centered, how did it get that way? If it will 'rotate under braking' how does it get off center? If you center a caliper by hand (or with a wrench) it won't want to move off center...

cyccommute 

I’m a retired chemist. But, since I know where you will take that, I have also done all my own bicycle mechanical work since the early 90s. I’ve owned 39 personal bicycles as well as my wife’s bicycles (about 20), and my daughters bicycles (10 to 15 more). Of those approximately 80 bicycles, not one ever went back to a shop after I bought it. Further, I’ve spent the last 12 years working 6 hours shifts at my local co-op every Saturday at Fix-your-bike hours. We have 6 stands that are filled every minute of every shift of every week. And I put hands on every bike that comes in the shop, often as the only volunteer. We put 90 minute limits on the amount of stand time. That means I see 20 to 30 people per shift. Over the 12 years, I’ve worked on 15,000 to 18,000 bicycles.

I also teach 5 week long mechanic classes that cover nearly all aspects of bicycle mechanics except hydraulic brakes and suspension systems. I also teach wheel building.

Bottom line, I may not be a professional mechanic but I do a very good job of working like one.

You aren’t saying anything that I haven’t already said. If the brake is tight enough it shouldn’t move. If it can be moved by hand without tools, it’s not tight enough and won’t stay centered. Vibration will move it off-center or even pulling the cable will torque the brake to one side. Again, Shimano says 8 to 10 Nm of torque to hold it in place. 6 Nm is what I have currently use...set by a Park Tool ATC-1.2 torque wrench...and I can’t move it off center by hand.

Gresp15C

I'd like to ask the more experienced eyes to check my impression...

Doesn't it look like the whole brake assembly is out of whack? I've got what looks like the same or similar brake, and there's no gap where I've drawn the red arrow. It looks like the arms are spread apart.

Gresp15C

Both my parents are retired chemists, and I expect to be married to a retired chemist someday.

cxwrench

You do have some experience. I've been in shops since '95 and worked for a bunch of pro teams starting in '04. Nationals, World Cups (track and cx), basically the entire domestic road race season, and the entire pro mtb season. Tour of CA, Tour of CO, all the East Coast races.

I'm not going to back down on the fact that it is indeed possible to adjust a properly tightened brake by hand. I don't do it that way every time but it absolutely possible. If it's tight enough by your standards there is no way vibration or cable tension will move it off center. What you're saying doesn't make sense. If it's tight and centered nothing short of a crash or a goat kicking it is going to make it move. If it's tight and not centered it will usually push the rim and stay off center. But I'll still be able to grab it and center it...no problem.

RubeRad

lol asked and answered many times, that is a separate problem and has already been dealt with

RubeRad

That's exactly what's happening