18-10-2024, 03:49 PM
Dave, thanks for your input yesterday. I found this really helpful and caused me to to have flash of understanding! What this was saying to me is that a wear free system will have the ball ends on the cam levers correctly under the kingpins and on the kingpin axis. So with the brake shoes correctly set up with shims so that the cam face is in full contact, ie parallel to the end faces of the shoes, the pivot pin of correct diameter, then a brake lining of suitable thickness will be in near contact with the drum at both ends of the shoes. This then is the optimum set up, preventing the inner wheel from being braked on full lock, and can be kept in this state by adjusting the shims at both ends of the shoes to take up wear of the linings. The brake cable is not used to adjust brake shoe clearance - its length being set to minimise cable sag without affecting shoe clearance.
However because the pivot pin is fixed the shoes are not self centralising within the drum so unequal wear will occur along the length of the shoe. This can be overcome in principle by mounting the pivot pin in a separate block on the back of the backplate with the hole in the backplate being 1/8" larger in diameter and the fixing holes on the block also being larger by the same amount compared to its two fixing studs in the back plate. The block is held by the nuts, being fitted with Thackery washers so that the assembly is just free to move under load. When the brakes are applied heavily whilst stationary this will cause the brake shoes to be centralised. I had this system on one of my cars and it worked well for me. You could make the bearing surface of the pivot pin tapered and the shoe ends tapered to match so by screwing in the taper the end of the shoe could be adjusted to take up play.
If the sag of the front cable is a problem then by using a pivoting balance bar with two rods in place of the cable would work and the compensation would likely be improved due to lower friction.
However because the pivot pin is fixed the shoes are not self centralising within the drum so unequal wear will occur along the length of the shoe. This can be overcome in principle by mounting the pivot pin in a separate block on the back of the backplate with the hole in the backplate being 1/8" larger in diameter and the fixing holes on the block also being larger by the same amount compared to its two fixing studs in the back plate. The block is held by the nuts, being fitted with Thackery washers so that the assembly is just free to move under load. When the brakes are applied heavily whilst stationary this will cause the brake shoes to be centralised. I had this system on one of my cars and it worked well for me. You could make the bearing surface of the pivot pin tapered and the shoe ends tapered to match so by screwing in the taper the end of the shoe could be adjusted to take up play.
If the sag of the front cable is a problem then by using a pivoting balance bar with two rods in place of the cable would work and the compensation would likely be improved due to lower friction.