...there is maybe a hairline crack and a welding seam in the front brake drum

[blethermaskite]

Jordan, what's the difference between how much each shoe moves in a mechanical single leading shoe design with a symmetrical operating cam centred on a fixed pivot point? maybe I am just being stupid but I don't understand
Cheers,
George

[Jordan]

With a symmetrical cam like the Grimeca double sided SLS brake has, rotation of it will push on two shoes, but at different places with respect to the pivot point at the other ends of the shoes (on the same brake plate).
One shoe is effectively "shorter" than the other, and this one will be pushed farther for a given amount of cam rotation, compared to the other shoe.
That's OK, so long as they both make contact with drum at the same time.
But if it's assumed they both get pushed the same amount, and that is simulated by using the same sized spacing shims for grinding, only one shoe will make contact after these shims are removed.
Eventually, wear will bring both shoes into simultaneous contact. In the meantime, you have "half a brake".
With a twin leading shoe brake, this phenomenon is theoretically absent. However there's always a possibility of dimensional error so the "set lever" method I think is still preferred.

[blethermaskite]

Thank you Jordan......very clearly explained......I see exactly what you mean
Cheers,
George

[DBDBrian]

The SLS brake with a symmetrical cam, is not geometrically perfect in it's design, but works in practice. There are different schools of thought as to which is the best arrangement for the rotation of the cam, in relation to the rotation of the wheel.
BSA used the outer face of the cam for the leading shoe for the standard production brake, but used the inner face on the works Daytona race machine.
I suppose the introduction of the TLS brake answers the question, which is a far better design, and in operation a better brake. The downside being the extra unsprung weight.

Re, mounting the plate on centre in the lathe, I have various mandrels which I centre in a independent four jaw, but as Jordan comments a self centring three jaw can not be relied upon to be on centre. In which case a mandrel can be turned in situ for a one off job.

P1040149.JPG

BSA Goldstar Daytona.jpg

[Jordan]

There are different schools of thought as to which is the best arrangement for the rotation of the cam, in relation to the rotation of the wheel.

On a bike I had with an offset front axle - forward of the tube's centre line - I swapped over the left and right bottom sliders to have them behind the centre line. I was experimenting with fork trail.
An interesting side effect was that the SLS brake, which had the brake cam rotating in the same direction as the wheel, when the brake plate was on the other side of the bike, it rotated opposite to wheel rotation. There was a noticeable improvement in braking effect.

[DBDBrian]

That's interesting Jordan, an improvement which was not the primary reason of your experiment. Thinking about it, changing the rotation of the wheel, also made the trailing shoe, the new leading shoe, as the trailing shoe it had done the least work, and usually less wear, my thoughts being, this may also be another reason for the marked improvement.
BSA presumably came to the conclusion, reversing the rotation of the cam on the race machine improved the brake, but to the best of my knowledge, it never followed through into production.

[blethermaskite]

So if you change the direction of rotation of the wheel relative to the brake plate ......then what was the trailing shoe becomes the leading shoe and due to (as Jordan explained) the cam acting at a different point on each the the shoe pads.... therefore effecting the leverage..... so the leverage on the former trailing shoe (because because the cam is now lifting the shoe nearer the fixed pivot) will be stronger than was with the former leading shoe thus giving a stronger braking effort........I think
Cheers,
George

[Bevel bob]

I regularly swap shoes over to equalise wear and as long as its done regularly it seems to give better braking . Have to be carefull with brake experiments as a sudden lock up can be dangerous. My best improvement to the Ducati SLS was a shim between fork leg and plate stop to load up and correct some plate distorsion that then allowed much closer adjustment.

[DBDBrian]

Yes and no George. By changing the rotation of the wheel the leading and trailing shoes are reversed in their operation. The cam face nearest the pivot/fulcrum point, gives more movement of the shoe but less force. The cam face furthest from the fulcrum, gives less movement of the shoe but more force.

Changing round the shoes on a SLS brake is not a bad idea Bob, especially if the cam spindle is a good fit in the plate / bush. Yes one has to be careful, altering cam profiles, or the distance of the leading edge of the lining from the cam pad, this can effect the self servo action of the shoe, and indeed cause a lock up situation.
Rigidity of the assembly is also key to a good brake. That being the reason for the stiffener plate I installed in the TLS brake plate I made. It also ties in the cable anchor, where the is also a considerable load.

[Wolf]

...ok, the front brake issue should be solved. I've machined the brake shoes to the inner diameter of the brake drums. Everthing went smoothly - thank you a lot for your valuable input.

Cheers

Wolf