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Drum brake binding on

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 Topic moved: from The Workshop to The Cycling Forum by stinkwheel (30 May 2017 - 11:15)
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techathy
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PostPosted: 13:21 - 28 May 2017    Post subject: Drum brake binding on Reply with quote

This is on a pushbike rather than motorbike issue but I think you guys & gals will have more knowledge than people on pushbike forums.

My sister's pushbike has a binding front drum brake & I'm a little stumped as to what's happening. The symptoms are:
* The brake has become very fierce, kind of switch like.
* The brake releases fine when moving at walking pace or faster
* The brake binds on, locking almost solid if you release the brake when stationary or nearly stationary (below walking pace).
* If you apply the brake when stationary the brake I progressive, will not bind on, however if you you start to move forward the brake will start to grab on.
*When the brake is bound on the cable is completely free. If you release the handle bar brake lever the other end of the cable and drops away from the activation lever on the drum.

I've taken the offending brake apart, cleaned out all the dust and greased all the pivots, the only served to increase the switch like nature off the brake. Inside everything moves freely & smoothly. The return spring is still very stiff and will return the pads from about 70 degrees of actuation lever movement, the actual movement of the lever is about 25-30 degrees with the shoes in the hub.

Anyone have any suggestions as to why this is happening? I thought about replacing Part 12 in the parts list but have a feeling this would deliver the same results.
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Suntan Sid
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PostPosted: 16:25 - 28 May 2017    Post subject: Reply with quote

Cable needs adjusting, oiling or replacing if it's not moving freely!
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techathy
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PostPosted: 18:31 - 28 May 2017    Post subject: Reply with quote

Definitely something in the hub as when jammed the cable the cable is lose and not under any tension what so ever. The actuation lever on the hub is also free it seems the stickion of grease holds it up, as the lightest touch lets it fall onto the cable end but does not release the pads. The only thing that seems to release them is several shocks to the wheel (eg. kicking the tyre or bouncing the front wheel on the ground).

I just simply do not understand what the symptoms are telling me about the hub.
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Teflon-Mike
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PostPosted: 18:57 - 28 May 2017    Post subject: Reply with quote

If the cable falls free when the brake binds, then what else moves?
Part 12 is the brake front plate. Err... why you would think to replace that is a bit of a mystery to me. Its a non moving part, its not likely to sieze or bind! Camshaft that passes through t might though.. have you greased that?

Best bet, though, is that the brake cam is over-centering; shoes are so far worn that the cam is rotating to the peak of its travel where when you release, the cable pressure, it stays, because the shoes dont have the agle it to twit t home.

If you need new bits, I would lay odds ts new shoes.

Meawhile, Setting up Drums is a lost art; that cam gives a lot of shoe travel at the rest position, that reduces as it rotates, providing less travel and more pressure. Dums are ot 'sharp'; they don't give the 'istant' brake reaction of a disc, stirrup or V-Brake, and they take a lot of travel to get the brake pressure they offer. Means they dont feel like they are as effectve, as they dont give the instant bite people expect these days, and they take a lot of lever travel folk are also unused to; but they can, if properly set up, offer an awful lot of power for very little lever force, and a lot of feel through that range of travel. Trouble is, used to more modern, more reactive brakes, people tend to adjust them to withing an ince of the bite point to get the immediate effect they expect.. and loose the feel and progression, and the power, at the long end having to make up for the advantage offered by the cam action with shear lever pressure, and at the other, having well knackered shoes adjusted n the cable to well past thier service travel, to get that fast-bite on the cable.

Prod ad poke some more; make sure that the cam is free in the back-plate; check for index marks for the cable lever, and if you want to splash cash o new bits, start with the ones most likely to be at fault the ones designed to wear out, the shoes! Then pay attension to proper adjustment, and DONT adjust them for instant' bite; remember they are progressive, they dont bite suddenly and they are supposed to take a good 2/3 of the handle-bar lever travel to get max power from them so you have that progressiveness and feel as well as the power without having to have so much ham-fisted force on the lever to get it.
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MCN
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PostPosted: 19:20 - 28 May 2017    Post subject: Reply with quote

As Tef McTeferson has intimated, worn shoes will allow the cam to rotate too much to a 'locked' position.

New shoes OGTF..

Smile

And who uses brakes? Confused
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techathy
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PostPosted: 20:10 - 28 May 2017    Post subject: Reply with quote

MCN wrote:
As Tef McTeferson has intimated, worn shoes will allow the cam to rotate too much to a 'locked' position.

New shoes OGTF..

Smile

And who uses brakes? Confused

Actually the backing material puts a huge gouge in the drum face, writing off the hub shell before the pads over-cam, along with massively ineffectual braking. Not that I found this out around 30k miles after buying my veolomobile Laughing Embarassed. Plus the pads are barely broken in, if this was my velomobile it's have around 20k miles left of 25k miles & my velo' is seriously hard on brakes.

Tef, part 12 includes the lower pivot pin, which while not a moving part in its own right does effect how the pads move. My thought being that if this is too small due to wear/manufacturing defect the pads could skew in the hub and jam on.

I'm also well aware of how a this hub brake should feel. It's the reason I mentioned it; It's wrong, very, very wrong.
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techathy
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PostPosted: 22:16 - 01 Jun 2017    Post subject: Reply with quote

A bent axel was the culprit.
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