Forum

[Wheaters]

Some owners recommend loosening the pivot screws juust loose enough to allow the shoes to centralise just the once then nip them up tight again. I’ve not had to do this so far because my bike’s rear brake is very good but it makes good sense to me.

[Chris]

I would guess that the cause of my lock-ups is that the leading shoe gets bedded in perfectly and on a damp day, when the shoes bite better, that it is locked on hard, somehow expending itself as it pushes against the fixed pin. I can't see how any setting of that shoe would change that possibility, because it would always bed itself perfectly after a short period. Hence my amazement that I appear to be the only one who has this every year for the past 20.

[Wheaters]

Chris, what size are your bike boots?

[papasmurf]

Chris, what size are your bike boots?

[Chris]

Chris, what size are your bike boots?

. I see the joke, but what happens is that with a light touch it will snap and lock the wheel, with the shoes tightening themselves.

[Count Johnny]

Chris, what size are your bike boots?

. I see the joke, but what happens is that with a light touch it will snap and lock the wheel, with the shoes tightening themselves.

Exactly that. On every occasion, these failures happened when I was just touching the brake at relatively low speeds (like when moving in town traffic).

[black fingernail]

A similar thing happens on early Chinese 125's with a drum rear brake, ( they tend to fit discs now, with linked braking!).
It locks up quite dramatically when used first time on a morning, worse in damp weather, but goes back to normal after it has been used a bit
I tried chamfering the shoes leading edges, fitting stronger pull off springs, all you can imagine.
I found the best results by fitting some hard compound linings.
I think the problem is bad quality metal used to make the drum, (iron, steel, chinesium?), I think it's surface is very porous and gets rusty very quickly causing the initial grab, then cleaning it's self off quickly.
My Enfield does it a bit, but only after it has been unused for a while.

[Cranky]

As Vince said ---self servo. This is a phenomenon used on many brake shoes but can cause trouble.

Self servo is when the leading shoe pulls itself into the rotation of the drum. The trailing show does not do this, as this shoe is pushed away from the drum rotation.
Also the cam should not have to move very far if so it can lock. If the drum has been skimmed true then you need to have thicker shoe material to take up that gap. It is not sufficient to make up that slack with adjustment.

Disc brakes cures all of this crap.

[p]

I wonder if the self servo effect may be reduced by cutting back the leading edge of friction material on leading shoe? I have noticed that on some systems the friction material does not extend right to the end of shoes..... maybe this is the reason??

[Cranky]

I wonder if the self servo effect may be reduced by cutting back the leading edge of friction material on leading shoe? I have noticed that on some systems the friction material does not extend right to the end of shoes..... maybe this is the reason??

No --cutting back the leading edge will cause self servo, you need a full shoe to reduce self servo.