New sturmey brake shoe rubbing

[hoogerbooger]

It's a NOS brake shoe in a used Sturmey AT5 hub.

The top pad looks a bit thicker and extends over the baseplate recess for the labyrinth seal.

It's a tight-ish fit into the brake shell and it seems the top pad being thicker means the plate is pushed down wards so the clearances on the labyrinth seal at the bottom are compressed...and this may be the rub.

looking at the pads there are scuff marks on one edge ( as annotated) and these where there when I got it. Perhaps the previous owner tried it in a shell then took it out as it rubbed?

The strange thing is it doesn't rub all the time. Initially I thought it may be that it rubbed in one location every turn and this was possibly due to a slightly oval brake plate or labyrinth seal rim. BUT it doesn't happen every turn...which is puzzling me ?? Can any one work this out?

So I guess now I have to sand the top pad a bit ?? to reduce it's width a bit ?? anyone had this sort of problem.




(I also tried the brake plate in a NOS AT5 shell....but it's too big to go in)

[rjb]

cant see your anotated marks. You may be correct in dressing the offending pad may stop the rub. I remember having to scuff over the brake pads in my dad's car ( a 1960's Austin A35) back in the day. A coarse wire brush deglazed the pad surface and restored normal service. We didn't worry about asbestos in those far off times.

[531colin]

Does it make any difference if you swop the top/bottom pads?

[hoogerbooger]

Does it make any difference if you swop the top/bottom pads?

Ooh ! I't hadn't occurred to me that they are reversible. Can't try that straight away as the star washer will probably die, but can get some spares...in due course.

having said that.....if it did make a difference that would suggest a post(s): cam & and fulcrum, are not correctly positioned ??... which seems less likely than misshaped pads.

[cycle tramp]

I wonder if there's something up with the interface between the cam and brake shoe.
Perhaps the cam not fully returning home after the lever has been released or part of the brake shoe resting on the cam when it shouldn't be...

[Brucey]

normally after they have been bonded, the brake shoes are mounted in a special fixture and ground to the correct radius. Yours look like they have not been done, for whatever reason.

I think it should be possible to grind the shoes to size using the SG method. I suggest that a return spring is fitted, the brake actuating arm is temporarily removed entirely, and the brake plate is mounted in a fixture that exactly counterbalances the weight of the reaction arm thus making an assembly that is dynamically balanced. You should then be able to SG away merrily, as you see fit.

FWIW I think it should be well worth grinding some angled grooves into the brake linings, once they have the correct radius. These should be right-handed ie. so that any debris in the grooves is likely to be ejected to the left. Finally you may need to grind a lead-in on the leading shoe; with the cam contra-rotating in the rear brake plate, the leading shoe sees a near-radial push, which can make the brake a bit grabby unless steps are taken. I plan to install a pulley (with the mountings welded to the reaction arm) so the brake cable can be given a 270 degree bend, allowing it to pull down on a modified (non-contra-rotating) actuating arm/cam.

I almost forgot to say, I think the current SA part ought to work here.

[531colin]

Does it make any difference if you swop the top/bottom pads?

Ooh ! I't hadn't occurred to me that they are reversible. Can't try that straight away as the star washer will probably die, but can get some spares...in due course.

having said that.....if it did make a difference that would suggest a post(s): cam & and fulcrum, are not correctly positioned ??... which seems less likely than misshaped pads.

To be fair, I have no idea of the practicalities in this particular case, it’s just something I habitually try when stuff doesn’t fit together properly.
The shoes do look “iffy” but grinding them down sounds like a pain.

[hoogerbooger]

I think it should be possible to grind the shoes to size using the SG method. I suggest that a return spring is fitted, the brake actuating arm is temporarily removed entirely, and the brake plate is mounted in a fixture that exactly counterbalances the weight of the reaction arm thus making an assembly that is dynamically balanced. You should then be able to SG away merrily, as you see fit.

Am in no great rush....so I will consider a SG method(Spin Grinding, I presume) ....but will also keep my eye out for another brake plate.

Constructing an exact counterbalance for the reaction arm is probably beyond my maths....but I can probably bodge something in the ballpark......I could bolt it to the cam arm mounting (or is there no point trying SG unless it's very well balanced?) [getting a second plate and bolting back to back might be a way?]

I could wire the existing spring together so that the shoes don't spin outwards( there is already a return spring fitted, or am I confusing what you mean) .......then mount the plate on the angle-grinder......

There is a high risk of loosing fingers though without a guard to keep the the reaction arm & counter-balance away from contact. Plus dust control, partic if asbestos, which I don't know.

If I just file a crude approximation with a metal file, what's the risk ? presumably a long time with poor braking until it properly beds, but I'm wondering if it is the whole circumference that is the problem ....or a particular sector. I guess I can tryuse pencil, paper and compass to compare the radius of the brakes pads, as in situ, and the drum and see how they compare....incase I can spot a local issue?

[PT1029]

Sometimes securing the back plate on the axle results in it being a bit off centre.
Have you tried mounting the back plate, but before tightning the axle lock nut, putting the brake on hard to center the back plate in the drum?
If the drum is quite out of round, or the shoe too thick (as suggested), then this might not fix the issue.

[hoogerbooger]

Sometimes securing the back plate on the axle results in it being a bit off centre.
Have you tried mounting the back plate, but before tightning the axle lock nut, putting the brake on hard to center the back plate in the drum?
If the drum is quite out of round, or the shoe too thick (as suggested), then this might not fix the issue.

Thanks. Have had a go at this, but no luck, unfortunately.