The BSA A7-A10 Forum

Technical => Frame => Topic started by: Tone on 11.04. 2009 16:27

Title: Brakes
Post by: Tone on 11.04. 2009 16:27
Guys, I had the rear hub skimmed and bought some new shoes on my 58' A10. on putting it together, when you pull on the lever it has to move so far to be in contact with the hub that it's almost on the end travel of the cam! What am I doing wrong, HELP.
Title: Re: Brakes
Post by: dpaddock on 11.04. 2009 17:15
Check that the ends of the new shoes have steel clips where they contact the cam.
(Do the old shoes have such clips?)

Are the new shoes repros and, if so, how do they compare dimensionally with the old ones?

How much was "skimmed"? The drum's new ID should be no more than 7-3/16" or thereabouts.

Title: Re: Brakes
Post by: Tone on 11.04. 2009 19:12
Hi David, Thanks for your reply, there are no steel shims and never was, I have looked in Dragonfly's catalogue and other manuals I have and none of them show the steel shims that I can see, any idea where I can get them? regards, Tony.
Title: Re: Brakes
Post by: LJ. on 11.04. 2009 19:54
Tony... Your answer might be to get some fatter cams like seen in the picture... I dont know if they are available from Draganfly or C&D etc... I never knew these were available myself untill only recently.

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Usually with having the drum skimmed, oversized brake linings are applied to shoes.
Title: Re: Brakes
Post by: dpaddock on 11.04. 2009 19:59
OK. How about my second and third questions?
Title: Re: Brakes
Post by: Tone on 11.04. 2009 20:59
David, the hub is 7 1/8 dia. I can't compare them to the old ones as I had to exchange them, but they look like new, I got them from dragonfly which as usually good. Tony.
Title: Re: Brakes
Post by: a101960 on 11.04. 2009 21:38
Tone, I would venture to suggest that there is nothing wrong with your brake shoes as supplied by Draganfly. But In all likely hood the shoes are to thin if they are original specification shoes. If you have had the drum skimmed normal practice is to have thicker shoe material fitted which is then machined to fit the new drum diameter. Depending on who did the skimming I would have thought that the company carrying out the task would have advised you about this, and that they would have supplied compatible shoes to match the machined drum.
Title: Re: Brakes
Post by: MikeN on 11.04. 2009 23:28
Tony.You say your brake dia is now 7 1/8".
 Is that on a brake that started out at 7"?
  1/8" is a great deal of material to remove from a brake drum.
  Regarding the shims you mentiond I have made many of these over the years.They only take a few minutes. Find a bit of sheet metal say 1/16" thick. hack saw and file 2 strips slightly narrower than the width of the operating cam.Put the strip in your vice vertically and tap the end 1/4" over with a hammer to 90 deg.Offer this up to the end of the brake shoe and mark where the other bend goes .Bend the second end on the shoe end and tap both ends a bit further over until they are a nice snug fit.this is your shim.You need 1 for each shoe. Assemble the shoes onto the brake plate and try it in the hub. You may have to remake them again with thicker or thinner metal to obtain the desired fit.
 If your brake has been bored 1/8" oversize It sounds an awful lot to have to shim out. Unless you have thicker linings then they will only have a very narrow point of contact and not be effective because the shoes will be a different radius to the drum .Whenever i have bored out brakes I start to get concerned if I have to take out more than .020" (.5mm).
  If you have access to , or know someone with a bigish lathe then the following may be of interest.
  I get the shoes relined with thicker material so that they are just too big to fit the drum.Although this isnt always necessary if the drum hasnt been bored too big and you can get away with fitting shims described above. I Assemble them on the brake plate with a piece of temporary packing (a piece of hacksaw blade would do) under the shoe/cam. I make a mandrel for the lathe, to fit the brake plate so it runs true and machine the linings to fit the drum.I use a T/C tip tool as the friction material rapidly wears an HSS tool . Removal of the temporary packing gives you the working clearance.
 Excess cam movement that you describe, if left ,can eventually cause the cam to go over-center. If this happens to you,it will spoil your day.
Title: Re: Brakes
Post by: RichardL on 12.04. 2009 01:16

I can envision that I will be using your information in the not distant future, but I am a bit confused. It seems, from your discussion, that an 1/8" more diameter (1/16" radius) would be the perfect amount when fitting 1/16"-thick shims on the ends of the shoes. What am I missing here (which is probabaly right in front of my face).

Richard L.
Title: Re: Brakes
Post by: MikeN on 12.04. 2009 10:06
  Its not quite that simple.If you add 2  x  1/16" shims to the feet of the shoes you wont move the shoes apart 1/8" because you are adding that material to the  circumference of the brake shoes drum not the diameter.
  Theoretically,to gain an increase of 1/8"  on the dia. you would have to multiply 7 1/8" x  pi (3.142), less the thickness of the cam less the  effective diameter of the pivot and divide by 2 .Actually thats not true you would have to take into the account the PCD (pitch circle diameter) of the cam and pivot plus the 1/8" increase x pi.

 The actual mathematics and geometry involved are way beyond me, and quite unnecessary.

Which is why I said ,if its not quite right, try again with something a bit thicker (or thinner).
Title: Re: Brakes
Post by: Tone on 12.04. 2009 11:24
I had a guy build up my wheels and he had a mate who skimmed the hubs for me, I have just had a remeasure in better light and the hub is barely 7 1/6"dia.When you measure the shoes fitted on the backplate with cam straight they measure a bare 6 3/4". Where can I get the shoes relined with thicker stuff?
Title: Re: Brakes
Post by: MikeN on 12.04. 2009 18:34
Tone ,
  I use Supreme motorcycles in Leicester . Tell them you have had the drum bored and you require oversize linings.They will reline them in what they call their "high friction material"which is good stuff.You send your shoes of in a jiffy bag and they come back within a week. I think I paid about £35 last time.I think they will also machine your shoes to fit your drum exactly if you send the brake plate assembly complete and tell them the exact dia of the drum.
Title: Re: Brakes
Post by: Tone on 12.04. 2009 21:01
cheers Mike, I'll have a search see if I can find them, thanks again you lot, regards Tony. Don't know what I'd do without you all!.
Title: Re: Brakes
Post by: A10Boy on 16.04. 2009 10:42
Apologies if this has already been said, but its nothing to do with cam thickness or shims.

In basic terms, if you have the hub skimmed that changes the radius of the curve of the drum, so STD shoes [even if they are shimmed out] will not contact the drum over the full length of the lining. Fitting thicker linings obviously changes the radius of the curve of the shoes which corrects this. As has been said, the optimum is to have the drum and shoes matched thereby the linings will contact the drum over the full length of the friction material.

To check the match, hold the shoes inside the drum by hand and make sure the lining is contacting the drum over its full length without daylight or rocking.