Single-pivot brakes will not stay centered
 

A lot of people attribute this to the inherent design of single-pivot brakes, but am I truly cursed to lopsided-brake hell? :twitchy:

The mounting bolt is super tight, the housing doesn't appear to be too short... But when I squeeze and release the lever after centering, one arm pivots away from the rim more than the other. This leaves the other pad closer to the rim, screwing up the centering.

Why, oh why?

Single pivot, side pull brake were ALWAYS fussy to setup. The difference now is that with their rarity the knowledge of how to adjust them is disappearing.

Now the thing is I'm struggling to remember exactly what I did myself. Here's some random points. You need to make subtle adjustments. You can think you have it right but when you squeeze the caliper and release them its still wrong so you adjust them again. Thats NORMAL. You need to treat the front and rear bolts differently as they do different jobs. The front bolt (close to the brake arms) adjusts the friction between the arms. Too tight and they don't move. Too loose and they flop everywhere. They need to be just right. The rear bolt holds them onto the frame AND sets where they centre. Its a tricky thing to get right, it always was and when you get it right you leave it alone. Anyway one of the things you need to do it to hold the caliper OFF centre to compensate for the torque placed on it when doing it up so try that first. The other thing I remember doing was to tap on the springs with a punch to adjust the centering although I'm not sure exactly how I did this anymore.

Anyway keep trying. They ARE difficult to get right.

Anthony

This outlines the use of a simple Offset Brake-Wrench that Park Tool carries. Actually there are 3 of them in different sizes - mm's. And a cone-wrench will do the same thing in most situations.

http://www.parktool.com/repair/readhowto.asp?id=22

Most semi-modern single-pivots have a flat on the side of the bolt just ahead of where it comes through the fork/stay to attach the body to the bike.

Put away the hammer and screwdriver.

The easy way to adjust this is with a flat head screw driver and a hammer. All it takes is one tap on the spring (right where it loops) on the side that is away from the rim. No special tool and works like a charm. Learned that trick down at the bike coop.

Oh, and the last bike I used this on did not have enough clearance, due to the reflector and general mixte rear brake set up, to get the Park tool to work. I went for that first -- but it was a no go.

If you're not already using one, try a serrated or star washer against the fork or frame. All of my single-pivots use one (and so do my dual-pivots).

Are you saying that on the squeeze, they both contact the rim at the same time? But on the release, one moves more than the other?

my trick is not tightening the bolt too much. so long as it is snug that's all you need. not sure why that works.

http://sprinterdellacasa.blogspot.co...ot-brakes.html

"Centering" the brake only moves it temporarily. The center bolt itself has to be turned to center the rest of the brake.

cdr

Ah Ha! Now I want to find my small spanners and try re-centering all my side-pull caliper bikes' brakes!

Side pulls never center perfectly, because there is too much friction between the spring and the arms. The spring needs to slide a little in the stops on the arms, and there is generally no way to control that friction. The friction is always a little greater on one side or the other, so they never retract symmetrically. AS long as both arms retract enough to avoid contact with the rim, the brake is working perfectly well.
Double pivot brakes can be centered perfectly because the arms retract until they reach an adjustable stop. That allows them to be centered exactly, and to retract to the exact spot every time. The advantage of the is that double pivots can be adjusted to keep the pad closer to the rim. The reduced pad travel is what allows the higher mechanical advantage possible with double pivots. You could easily design more mechanical advantage into single pulls, but you would need to adjust the pads closer to the rim, and the poor centering of single pivots would cause them to drag all the time.

em

maybe that's why not over tightening the side pulls helps a little

What model brakes? Every Shimano brake-caliper since 1991 has had plastic inserts on the arm to allow the spring to slide smoothly. If friction from the spring sliding on the arms was the culprit, it would only be a problem if the friction was not equal on each side. If the friction was the same, the arms would move the same.

The problem in this case is uneven arm-movement when the brakes are centered. This is caused by an imbalance in spring preload. This is caused by someone in the past using a punch on one side of the spring to bend it and change its preload to "centre" the brakes. So you've got a centrebolt that's twisted by 5-10 degrees and rather than spinning the centrebolt, they bend the spring instead. So you've got a spring that has 0 preload on one side and one with 5-10 degrees of preload on the other. Guess what happens to the movement of the arms?

Here's a good thread on brake adjustments with photos: http://www.bikeforums.net/showthread.php?t=566357

On one of my old bikes, this was caused by a $0.03 part:

Depending on the model, there may be a little metal pin on the front caliper that slides along in a metal groove on the back caliper. The pin is smaller than the groove, and there's supposed to be a little plastic/rubber/teflon? cap that goes over the pin. Without it (as I found out the hard way), no amount of tightening will keep the thing centered.

I eventually found a rubber cap made to go over pointy fender-mounting gear, cut the tip off, and crammed it on. No more slipping off center.

Campagnolo pioneered this feature in 1969 with their "Record" sidepull brake.

Using the screwdriver & hammer trick used to be 'the way' to handle this. But nowadays it's the last resort as it can cause damage if not done right. Or you miss and gouge something un-intended. Look for that flat-spot on the bolt-mechanism with the spring. If it's a truly old - or cheap - single-pivot side-pull, then go ahead. I just did this on an old Dia Compe on a 3-spd. Worked fine - for a last resort.

I like the spring adjustment, but mine is opposite: pull the spring out on the side that's not returning properly and bend it further out. Replace. So instead of weakening return on the strong side, you're strengthening it on the weak side. Especially useful in systems with pre-aero levers without return springs in the lever.

Panthers007, I use the side of a wrench to make sure I don't do damage.

Single pivot brakes are a real thing of elegance. I'm sorry to see they've gone out of fashion.

I found these in Norway after scouring the eBay-version of the planet. I have 3 sets of the Speedy - promo gold, black, and these. From the rear:

http://i424.photobucket.com/albums/p...g/IMG_0385.jpg