_adrien_

hi fellow cyclers,

I'm intending to put back on the road an old bike with shimano moutain LX (nowadays Deore LX family of components) parts and I'm having an issue with the rear U-brake. Basically, there is a spring system inside the brake housing but it doesn't seem to be effective at all as the brakes are not pushed appart when releasing the lever. Any idea why?

Also, when I unmount the brakes and look at the return spring, it looks just fine. Note that the springs are self-contained inside each arm (pivot?) of the cantilever, with no attachment to the studs/frame fittings.

Here's a link to a picture and description of this brake model.

thanks for any help!

EDIT: I found a post on the same brakes model linking to Sheldon's take on the U-brakes. got my answer on setting the springs but it still doesn't work, there's no elasticity in the mechanism... Might the brake be dead?

Bianchigirll 

Hi Adrien, Welcome to the forums. Sorry to hear about your brake troubles. U brakes were nice and strong stoppers but that under the chainstay placement made them hard to work one. Also a down fall of the shimano U is that it really lack a way to preload the spring.

I looked in my old shimano parts book and the Mount LX shows something on the left arm (mounted on the left stay) has something called an "Eccentric Adjuster" but just looking at the diagram I don't see how to adjust it. I am not even sure if it is for the brake spring or just for the booster.

rhenning

As you said it is an old bike so there may be nothing wrong with the brake itself. It may just be the the brake cables are dirty/rusty and dragging and not allowing the brake to return. I would check for free movement of the cables first. Roger

IthaDan 

Sounds gummed up. WD40 is a cheap and ubiquitous solvent to get things moving again. Just be sure to use a proper lube to finish it off once it's moving properly.

Check the cable first.

_adrien_

Hi guys and thanks for your replies.

I don't think it's a matter of the parts being stuck together. Also, the Eccentric Adjuster gave me the hint of what was possibly wrong. The fact is this bike was last serviced more than 15 years ago - and although by professionals - I think they missed something because I feel like the system is lacking two parts, probably some kind of washers/spacers that should fit right inbetween the plate holding the spring (fig b. in the image below) and the Eccentric Adjuster. It seems to be missing on the other side as well because the metal parts are pressed together by the bolts they don't move anymore and although on the other side there is no Adjuster. I figured this out because the Adjuster was mounted upside-down when I took the bike of out the cellar.

Bianchigirl, any chance you could upload an exploded view of the system because I couldnt find the shimano techdocs online.
thanks a mil!

mechBgon

I can help with the eccentric part. It actually has no role in brake adjustment at all. Its purpose was to allow the installation of the brake booster, and the eccentric feature was to accomodate the stud-to-stud spacing, which varied from bike to bike.

If all the parts are there, then you would simply grease the frame studs, put the arms onto the studs, (and next the brake booster, since yours has one), and rotate the arms so the pads are as far from the rims as possible. Then tighten the bolts, and you should have spring tension. The metal bowtie plates will be pinned against the end of the frame stud, holding one end of the spring stationary so the other end actuates the arm. If the spring response seems weird, check to confirm the springs are in the correct arms, since they're directional.

Again, the key is to have the arms swung away from the rim when you fasten the bolts.

_adrien_

But in a case where the springs would be the wrong way around, there would still be some kind of elasticity when pulling the arm either way?

And about those "metal bowtie plates", the problem is that once the bolt is tightened, it presses against the black plastic washer/cap that sits on the top of the canti and the arm is stuck on the stud.

Also, which side of the Eccentric Adjuster should show upward? Like in b.) or like in the image left from it on the picture?
I'm gonna check if those springs where not switched!
thanks for your help!

mechBgon

For the brake to work as designed, the bolt needs to pin the bowtie against the frame stud without clamping the arm down, so you'll need a bolt or a spacer that achieves that.

One side of the eccentric fits the brake booster plate, so aim it like that (photo A).

Airburst

If the brake is dead, there are plenty of U-brakes still available for the BMX/freestyle market which will fit your frame, from cheap budget units to expensive lightweight ones.

cs1

Dia-Compe are nice and very inexpensive. Personally, I like the look of the CNC U-Brakes the BMX guys use.

LarDasse74

For the brakes to work the force of friction from pivoting the brake arms must be less than the force from the spring.

I think that type of brake has the arm pivoting around an internal part of the brake, not around the pivot on the frame.

Airburst

Some of the higher-end BMX ones do, but a most of them just have a plain brass bushing which pivots around the thing on the frame. If you haven't done it already, OP, grease the brake bosses. You might not think it would help much, I didn't the first time I tried to work on one, but it does.

_adrien_

Thanks all for your suggestions and help. I'm now convinced that I am missing a few parts, at least on one of the arms. I'll have to look for a couple of cantis.

Airburst

https://www.winstanleysbmx.com/catego...Brake_Calipers

More of them there than you can shake a stick at. Obviously that's a UK site, but it ought to give you some idea of what you can get.

One suggestion: Make sure you get a brake which has a socket on the end of one of the arms for a standard straddle cable, some of the cheaper ones have a pinch bolt on either side, and they're a pain in the behind to disconnect when you want to get the wheel off.