If we are thinking seriously about the brake design then perhaps we could start by asking why the two shoes have individual pivots? A single pivot would allow longer brake linings. Then if that pivot was allowed to 'float' slightly, and/or the pivots on the shoes to slide slightly on the pivot, it should allow some self centring, hopefully overcoming some of the alignment and centring problems. Any thoughts on this?
Norman,
I think that approach has a lot of benefits. One of the benefits of Burt's mod is that it links those two posts together, which aids them in resisting the high compressive forces. It would not be difficult to make a single post, except for getting the dimensions correct on the new single post shoes. It could probably look stock from the outside, also. Floating the shoes would not be a problem either. The post would need a flange or washer at the top and the shoes would need an additional spring at the post.
I don't have much against long linings, but given all the circumstances I don't think they give a lot of bang for the effort. They work fine and can even increase the servo action slightly. Long linings are used on the 8" brakes and they don't seem to present many problems. If I relined the shoes I would shorten the linings considerably.
Long linings do not effectively increase the braking power of the brake, except for the slight increase in servo action, which would be a noticeable improvement. On the other side, you need to increase the clearance of the shoes to the drum, which means greater brake lever movement. I think that those riders, including me, who use brakes with long linings fudge the clearance by running tighter than recommended clearances based on the fact that the lining material is sufficiently low in friction that it won't be grabby with the lining ends running too close to the drum.
The other problem is that the shoes pressure on the drum is greatest near the middle of the shoe and weakest at the heel and toe of the shoe. The reason the leading shoe is so effective is that the toe of the leading shoe grabs the drum with very little pressure (this is what servo action is.) If you want the middle of the shoe to do its work correctly, you need to keep the ends of the shoe out of the way because they risk touching the drum before the hard working middle of the shoe.
I think you get the most bang out of brakes by increasing the coefficient of friction of the linings rather than the area of the linings. If we compare a brake lining to a tire, when a rider says the front end is sliding in a corner we would not suggest a bigger tire with a larger contact area, we would suggest a softer tire compound with a higher coefficient of friction.
David