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?
H: Hubs, Wheels and Tyres Eight inch brake ?
- Thread starter Julian R.
- Start date
David,
I suspect you might well be right, especially as those who have done the mod on the otherwise std system seem to be very pleased with the result.
However, as much as I understand the apparent logical benefit of a "floating self-centring system", I do wonder if there might be an unintended consequence elsewhere - especially on the trailing shoe? After all, there are whole text-books written on the theory of 'brake drum dynamics' and I don't profess to be an expert on the subject, nor have I the time to sit down and read them!
Nevertheless, there are some excellent ideas coming out of this thread. But I would suggest that any testing should initially be limited to a minimum number of variables in the first instance - exactly as you are advocating!
Peter B
I have a good customer who has a 68 BSA Lightning that has been in his family since new, and I have done quite a lot of work on it for him as he has no mechanical knowledge. This has the twin leader 8 inch brake standard to most like it from that era and the last of drum brakes before early 70's Triumph's and Norton's went to disc's. Even though it does not have the weight of a Vincent it pulls up extremely well especially if the rear brake is used as well. No wonder the early racers used two of them back to back which would work quite well except for the heat build up in the drums. To me this is the kind of braking design to look at, and especially if you went up to the 8 inch brake instead of the 7's. To me no matter what you do, a 7 inch brake on a large bike like a Vincent is too small. I know Vincent Speet's brakes are excellent, but the same in an 8 inch brake especially if alloy drums with iron insert would be fabulous brakes for a Vincent.
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
Greg,
Phil Irving agreed with you about the 7" size. It was too small. Particularly if you use large rims. The larger the rims the less effective the brakes will be no matter what they are. If you want the best mechanical advantage for braking, you need big brakes and small rims.
David