The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Frame => Topic started by: mikeb on 10.04. 2020 06:14
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I'm following this great post to arc my new rear brake linings:
https://www.a7a10.net/forum/index.php?topic=7252.msg50596#msg50596 (https://www.a7a10.net/forum/index.php?topic=7252.msg50596#msg50596)
for my question, the important bits are:
Assemble OS shoes onto backing plate, but replace the shoe springs with turnbuckles; I make my own (refer to pics below). Put 0.010" - 0.015" shim packing between each shoe and the actuating cam....
Do up the turnbuckles tight; this locks the shoes square against the cam, and the fixed pivot. The fixed pivot should be adjusted out most of the way, so plenty of adjustment remains.
Make a dummy mandrell same size as axle, and some sort of custom nut, and install (do up tight) the backing plate. When re-using the mandrell, set up in 4 jaw chuck, to no more than 0.001" run-out.
OK, so i've made the turnbuckles, shimmed the cam end and got it spinning in the lathe on the rear axle with spacers and a live centre at the tailstock. currently about 2 thou runout on a 3-jaw check, (i'll try for better). but 2 questions:
1. the brake plate flails around quite in both dimensions. i guess its not flat and also out of round by ?20 thou at its circumference. should i be worried?
2. I'm confused about the bit above that i've made bold underlined, re adjusting the pivot. That sounds like move the pivot to the outer of its oversized hole (ie away from the hub centre), but I would have thought I should move it to the inner side of its hole (towards the hub's centre) to ensure it can move out later in life when the linings are worn. Or, being less clever, just centre it so the shoes/linings are also most centred. then the whole set-up may spin more true in the lathe.
Any advice on this from anyone? Orabanda?
BTW a pic of test fitting my turnbuckles below. for an experiment i zinc plated them. came out quite well.
thanks!
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Hey Mike; great work; the turnbuckles put mine to shame!
Can't remember why I would have adjusted the fixed pivot out! Here & now I would simply lock it up centered as you have worked out.
Ultimately, the set of shoes need to spin concentric with the axle; then after machining they will be concentric with the drum.
Brake plate wobble should be OK, providing it doesn't touch anything.
Richard.
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thanks Richard!
i'll centre the pivot and maybe setup the four-jaw and give it a spin
cheers
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I usually replace the springs with welding wire, twisted tight with pliers. After all, we're not taking huge cuts into hard material.
As for mounting backplate, I use a stub mandrel mounted in the chuck and a nut to tighten backplate against it. Yes, the backplate can be untrue radially, but as orabanda says, that's basically irrelevant. Just make sure the shoes and backplate don't wobble.
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An update and notes for the next amateur having a go at this.
I ended up making a mandrel for the job – see below. I wish I’d make a bigger step for the brake plate to sit against – I used some 26mm round but should have used some 30. I got this spinning with less than 1 thou runout – better than mucking around with an old axle. I was surprised at how untrue the brake plate was, tho I’m confident the linings were concentric with the axle. The other surprise was how tight I needed to adjust the turnbuckles to hold the shoes firmly. If I hadn’t have already made turnbuckles I’d have tried RD’s wire approach first (didn’t think of that).
I also cleaned up the surfaces of the shoes facing the cam and swapped their position to avoid the existing wear marks on the shoes from years of the cam grinding against them. This resulted in holding the shoes much more parallel to the plate. There’s also a pic below of the final setup – I had to run the small lathe backwards to accommodate the boring bar set upside down as the cross slide was too small. (wish I had a better lathe!)
Result – I had a short ride today and the rear brake is SO MUCH BETTER than previously. Quite a bit of set up tho definitely worth doing. I’m tempted to re-do the front tho probably won’t just yet.
Also, a pic below of the zinc plating process – I used household chemicals and zinc from old batteries for some science fair fun. Recommended!
Thanks for the help RD and Richard.
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Very impressive Mike, you're making good use of the lockdown. re the plate wobble, with vincents the stock plates are quite flimsy as are the shoes; whereas the factories racing brake plates are far better braced, so much stiffer and these days stiffer brake shoes are also made. I guess the BSA shoe was fit for purpose in it's day. The later BSA group TLS plates are more substantial too, so not just large gains from the TLS but an improved plate. Having said that the Chris Vincent '62 TT winning A7 had a heavily cut away brake plate
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Thanks mike, one day I may give it a go, albeit I’m not sure my old southbend lathe is big enough (9” dia max from memory).
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The simpler 'old method' didn't need those lovely turnbuckles, (cool though they are!)
We just used to wedge the brakeplate lever slightly open - just to a point where we want the shoes to contact the drum, then with a suitable mandrel, spin it up in the lathe to machine the linings circular at the point where they contact the drum.
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Jools - last year I arced some front linings with a threaded rod holding the brake lever at the ‘correct’ angle – a similar action to a wedge? The shoes were held in place only by the usual springs. I now wonder if this could have caused some sideways rotational movement in the shoe when arcing with a cutting tool moving across the face of the lining. Ie maybe causing a slightly convex shape across the lining – resulting in less contact between the outers of the linings and the drum. I don’t think this would be a problem if one had a proper brake arcing machine with even contact across the whole face of the lining.
So with a lathe and wedging the brake arm, I’d at least want to try RD’s method of holding the shoes tight and square to the pivot/cam. Tho obviously this is speculation on my part. Maybe I’m really trying to convince everyone to make turnbuckles and try zinc plating.
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Absolutely Mike, however we do it, it's all got to be held nice and secure and square eh.
There's one advantage to the other method... of gluing coarse carborundum or sandpaper in the drum 'brake tracks' then assembling and going for a spin 'round the block (don't try this at home ;) !)
Nice thing about this is that the carborundum 'sands' the shoes IN SITU with everything aligned as the spindle etc bolts it all up together, -not guaranteed when you do it in a lathe.
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I wouldn't recommend machining the shoes just using the springs for location. Tried it once and the tool ripped a shoe off the backplate - as the tool digs in, the spring lets go a little and it then digs in deeper .....