Well, there may be a glimmer of light at the end of the tunnel......
Sheldon's MA of 1.31 means 1.31:1 (thanks Jon)
Snibgo....I've tried to read that stuff...really, I have tried...but
sorry, its beyond me.
Chris, I've read your explanation (many times!) and I can now follow it...just. It gives the MA of the whole canti, taking into account the various things that influence the MA...all the important lengths and angles.
On the other hand I think Sheldon (and the stuff snibgo linked ?) is considering the MA in sections, rather than dealing with the whole brake in one go.
I think this "1/sin yoke angle" is supposed to be a component of the MA which derives solely from the straddle wire itself, and I am interested in finding out the effects of yoke angle (straddle height) on the brakes performance....it is something we can control, the other stuff is pretty well set by the manufacturers, all we can change is to vary the spacers behind the blocks.
I have taken some of Sheldon's yoke angles ; 5, 10, 20, 30, & 40 deg, and I have drawn them for this canti. with the straddle anchor points 140mm apart.
Then I have "raised" the centre of the straddle yoke 10mm, and using compasses, have drawn where the straddle anchor would move to. (moving it in a straight line, ignoring that it actually moves in an arc)..This is a list of the total movement of the straddle anchors (ie left plus right) for a 10mm raise of the yoke for each yoke angle, and Sheldon's MA figure;
5deg....10:3......Sheldon 11.47
10deg...10:6.....Sheldon 5.76
20deg...10:8.....Sheldon 2.92
30deg...10:14....Sheldon 2
40deg...10:20....Sheldon 1.55
I may be completely wrong, but it seems to me you can get an MA out of just the straddle wire, if the yoke angle is kept very small; somewhere less than 10deg will give an MA of 2:1. (main wire moves 10mm, sum of straddle anchor movement less than 6mm.)**
Somewhere between 20 and 30 deg yoke angle, you are down to unity, the combined movement of both straddle anchors is the same as the main wire.
At 40deg yoke angle the straddle anchors move
twice as far as the main cable, halving the MA.
And yet Sheldon gives an MA between 2 and 3 : 1 for 20 & 30 deg yoke angle.....whats going on? (Probably I've got it wrong!)
**Probably can't actually be used in a working brake, even with the barrel adjuster in the straddle rather than the main cable?