Why is my front brake still useless?

[BSA_54A10]

Next is incorrect radiusing of the shoes.
To work as intended the shoe must have a LARGER radius than the drum so the initial contact will be at the end only. ***
The leading end digs into the drum which locks onto the shoe and forces it back against the pivot end which can not move so the shoe flexes into the drum.
Thus the rotational energy in the drum is supplying the breaking energy , not your grip.

Do you mean like this?

A trailing shoe marked 68-5525 which is 197mm across and only touches at the extreme edges as seen in the second image, the gap is about 1.5mm. Next to it is a leading shoe which is 191mm across and has good overall contact as shown by the blue ink. Obviously not a matched pair so which size is the best fit?

Yes rather like that
The idea of the steel caps on the brake shoes is so you can shim the  pivot end of the shoe to account for wear in the drum.
just the same as the oval pivots on the Ariel type hub
The pivot end of the shoe should be as close to the drum as you can get it.
The closer the better

[philwhitelaw]

Thanks Trevor, getting hold of those end caps is a bugger tho. Might have to make something myself.

[Peter in Aus]

Decided to have another go at my 58 A10 full width rear cross over (near useless) brake. Ground off about 35mm of the top of the leading shoe (cam end) lining and filed the cam on the trailing side of the cam so as the leading shoe came on a bit first.
The result was a 50% increase in braking efficiency, but still not as good as it should be, but much better than it was.
Peter

This worked for a while but gradually went back to being next to useless, so start again, having brake drum skimmed and seeing if I can get some different linings. would like to get some with some nice asbestos in them
Peter 

[BagONails]

Tacking today's conundrum onto an old but very informative thread.

The PO had the drum skimmed and the shoes relined or bought new shoes maybe , I'm not sure but either way it is all like new in there.
That said, I've never been happy with my front brake (who is?)  Previously I found the PO had got the linings in the wrong way around (for leading and trailing shoes) and the steel shims at the wrong end (cam end of the shoes) so I happily reassembled by swapping the shoes around and lo it was somewhat better but still not great.

At this point I will say I made an error because I failed to realise what I've found now which is that the shoes are offset to the cam and pivot centreline and therefore can only go in one way around.  This meant that I'd been happily tooling around with the shoes too far into the drum, and about a 6 mm gap to the upstands on the backplate although blissfully unaware all the while, it's been like this for two years!

So now with the shoes reassembled in the right way, contacting the upstands, it seems to me that the linings are actually bonded at the wrong end of the shoes? I just thought I'd run it past the brains trust here before I rush off tomorrow have these relatively unworn linings taken off and a new set bonded in what I believe is the correct orientation.  I've done some measuring and there is room due to the drum width being greater than the shoes so I can fit them this way and they will still run correctly in the drum without running over the spoke heads...

I'd like to confirm I have the correct shoes?
The shoes are now correctly assembled i.e. butting up to the upstands on the brake back plate?
The linings are indeed incorrectly bonded at the wrong end of the shoes?
The shims should be put at the fixed pivot end?

Thanks in advance lads, so helpful for us with less knowledge of these old machines. 

[Triton Thrasher]

That looks like the shoes have been lined the sensible way, with an unlined section at the trailing end of the trailing shoe.

[chaterlea25]

Hi BON and All,
Those shoes look like the commonly available pattern items ?
I tried a set of them and no matter what I did could not make them work !!
I went back to the original shoes and got them relined with oversize linings (soft) and turned them to fit the drum.
Now i have a brake

John

[BagONails]

Thanks TT and John. I was going by the info put up earlier in this thread and in others on the forum showing the linings in relation to leading and trailing shoe position. I've attached another photo of what I've got and Roger SB's graphic for reference. With my shoes assembled what I thought was correctly i.e. against the brake back plate with no gap the linings seem to be bonded at the wrong ends of the shoes. My yellow arrow denotes wheel rotation. I can reverse the shoes but then they are sitting about 10mm off the brake plate upstands which must be there for a reason?

One of the shoes has a part number 68-5524 and is marked with 'L' I assume for leading the other I can't make out but is the opposite orientation. With this L shoe fitted as the leading one the linings are then correctly positioned per Roger's drawing but they are way off the back plate as described above.   

John your comment about material certainly rings true but our best chance here in Adelaide is 'Power Brakes' and they say the material looks the same as they would fit and is generally well liked by the old bike community here. It's a soft black material with some sintered metal in it.
My drum measures pretty much spot on 8" and the shoes are only sitting about 1mm off. I guess I could try spacing them out a little, maybe a touch more on the trailing shoe and arcing them in the lathe.

As TT has alluded trimming some material off the trailing end of the trailing shoe may help the leading shoe bite first so I might have a go at that before I do anything else.

[Klaus]

Hello Ian,
The leading shoe will make 80 % from the brakeforce. The geometrical design of this brakes isnt realy good. The brakecam is symetrical that will cause that the trailingshoe will come first to the drum. A good improvement is to grind down the camside that feeds the trailing shoe. Make the cam asimetical to be shure the leading shoe comes first.

regards Klaus

[Triton Thrasher]

OK, so according to that, they were concerned about too much self-servo action locking the wheel up and throwing you off the bike, to your doom. 

A timely reminder that we experiment with brakes at our peril.

[muskrat]

G'day Fellas.
 *lol*I keep saying "brakes only slows you down". Learn how to dodge & weave 
Now to dodge and weave.
Cheers

[Swarfcut]

 Think along the lines of a giant hand moving the linings along the shoe in the direction of wheel rotation until the lining reaches the end of the shoe. It's the same for all drum brakes with leading and trailing shoes and is an easy way to correctly orientate otherwise identical shoes.

 Shown to good effect in the above line drawing.  Removing lining surface from the trailing shoe means the leading shoe contacts first.  Reverse lever arrangement and unequal cam mods are all other things to try, but as mentioned, playing with brakes carries a certain unease....

 A better quality lining material suited to riding style could be what is needed, rather than one size for all processed cardboard variety. Glazed linings can be improved by removing the glaze with a belt sander, light tickle, eye protection, face mask, outside in the wind.....Greasy or otherwise contaminated shoes could be revived by boiling in HARPIC toilet cleaner, reckon that to be a myth, but it did appear in print back in the day.

 Maximum leverage is applied as the operating arm is at 90 degrees to the cable, you want the arm to approach 90 in normal use, over 90 is less leverage and the sign of a well worn brake. Some brakes have a floating or adjustable pivot position. Getting this adjusted correctly will centre the shoes, sadly not a facility available in this case. Floating shoe design, see post at the beginning of this topic, goes some way to achieving this with a fixed pivot.

 Swarfy.

[Rex]

OK, so according to that, they were concerned about too much self-servo action locking the wheel up and throwing you off the bike, to your doom

I guess no-one told this Velo owner-
https://www.ebay.co.uk/itm/186991206194

[BagONails]

I reassembled everything with the correct orientation (as it was when it came apart) and will live with the gap between the shoes and the back plate up stand features. The more I thought about it the more I think this is how it should be. I think they are just an insurance back stop to prevent displacing the shoes when refitting into the drum and when fitting the springs.  While it was apart I took the opportunity to file away about 25% of the trailing end of the trailing shoe which should encourage the leading shoe to bite first. I'm not keen on grinding the cam down Klaus, well not yet!

I'll see how this goes. There are signs the leading shoe is starting to bed in, it probably needs another 1000km or so. Trouble is I don't use it enough...

John, I like your idea of fitting oversize linings but with these floating shoes I can't see a way to hold them on the back plate adequately to machine them on the lathe without making a very elaborate jig! Are yours the same? How did you hold them? 

[chaterlea25]

Hi BON,
I went back to the standard shoes for the oversize linings.
Just wondering if one could make a hollow mould with blue tak or similar and epoxy the floating shoes to the pivot then some heat will soften the epoxy to clean them up after machining .
I use soft wire to hold the shoes together on the backplate

John

[PrewarMG]

I came upon this thread when looking for information on adapting some 8” Triumph/BSA sliding brake shoes to fit my 1933 M.G. which, believe it or not, uses essentially the same design of brakes. As you can see, the installation has been successful although I’ve put very few miles on them so far so the jury is still out on how effective they are.

However, the amount of misinformation that has been posted on this thread has prompted me to post some of the FACTS that I have uncovered during the research I did before embarking on my brake adaptations. These are not my personal opinions and they can be confirmed by reading any brake theory textbook that covers fixed cam operated drum brakes. So here goes!

1. With fixed cam brakes both shoes are equally displaced by the cam (subject to the slight variation caused by where the cam makes contact with each shoe) and so must wear equally, and thus must do equal work. This is sometimes referred to as the equal work condition. The oft quoted claim that the leading shoe does 60-70% of the work is, quite simply, wrong. Once bedded in, both shoes do equal work.

2. The self-servo effect that is present on the leading shoe is not caused by the lining 'wedging' against the drum - it is caused by the moment about the pivot end of the shoe of the friction force of the lining against the drum which tries to increase the force with which the lining contacts the drum.

3. The length of a lining is generally in the range of an included angle of 90° to 110° with the Triumph (and I assume the BSA) shoes being 110° (the same as my M.G.). The length of the lining is not a factor in the amount of self servo generated.

4. On pivoted shoes the lining is equally displaced about the centre of the shoe as this is, more or less, where the centre of pressure lies. In the “off” position, with the cam flat, new linings should be concentric with the drum and with a radius slightly less than the drum. (On my M.G., 'slightly less' means 1/64” (15 thou) and I imagine the figure would be the same for Triumph/BSA shoes.) Because of this concentricity, pivoted linings make initial contact with the drum more or less in the centre of the lining and bed-in outwards from there.

5. On sliding shoes, the linings are displaced in the direction of rotation of the drum towards the trailing end of the shoe. This is because the centre of pressure on a sliding shoe is not in the middle, as it is on a pivoted shoes, but moves towards the trailing end. The higher the coefficient of the friction of the lining, the more the centre of pressure moves towards the trailing end. Hence the offset location of the linings. Because the shoes are not constrained by the pivot they are free to move slightly and thus self centre in the drum. Linings on sliding shoes, therefore, have the same radius as the drum and make contact more or less along their full length. In consequence, they need much less bedding-in than pivoted ones.

6. On the question of shortening the trailing lining, you won’t find a direct explanation of the benefits of this in the textbooks but it’s something I have discussed at some length with a university academic who teaches a course on brake theory. The logic of the modification is that by shortening the lining, for the same force applied to the shoe, there will be greater pressure on the lining (because of its smaller area) which will therefore wear more quickly and thus, because of the equal displacement of the fixed cam, the leading shoe lining will also wear more quickly and thus do more work, i.e. provide more braking power. The work done by the shortened trailing lining remains the same.