- Wed May 23, 2012 10:21 pm
#12853
Hi Riggers. I've heard of the method of machining the shoes to the drum, but can't really see how this is any different to bedding in shoes conventionally. It is true they would be more likely to perform at nearly 100% efficiency right from the start, but that would not make them perform any better if the linings were of conventional thickness and had bedded in to fit the drum by normal wear from normal application. I have never removed old brake shoes and found any part of them not in contact with the drum, which means that 100% was in contact for them to wear down normally, so as you cannot get more than 100% contact, no machining of them will increase that figure no matter what tricks you do to them. With 2LS brakes there is a very fine line from having them too grabby or having too little braking force. This is because both shoes work on a self servo mechanism....i.e.: the more they grip the harder they force them selves on. So with just a fraction too little friction, the self servo action is reduced dramatically along with the braking force. Whereas with just a little more friction, the braking force snowballs or multiplies and the brake is quite powerful, sometimes even too powerful. I can generally get any brake to work better by stripping the whole unit down and cleaning everything thoroughly. The cam spindles and the cam pressure faces need to be perfectly clean, smooth, polished and greased. Likewise the pressure pads on the ends of the brake shoes should be finely polished along with the pivot ends too. The brake cable should be well oiled. Sometimes cables can be too heavy for their own good and have too much inner friction. The brake surface should not be roughened up as it should be unmarked and very smooth and completely clean, this developes the heat that is required with most linings to operate well. Obviously getting the right friction coefficient on the linings is very important, and one would hope the ones you buy had the best properties but there are friction lining specialists around. I sometimes consider that new brake shoes are often too eased off (chamfered) on the start of the leading edge. This is a very critical area for the self servo action to work correctly. The more you take off this area, the less servo action is produced whereas if you leave too much on, the brake can grab. The trouble is that you are stuck with what you are sold as you can’t put it back unfortunately. Perhaps if you have the time you should strip down the brake again and you will probably find that the shoes still have not fully bedded in, they really can take a lot of time to get that 100% contact. While you are looking, strip them down again and polish and lube the parts I described earlier, assuming you have not done that already. If you have, then it will be down to the other reasons I have mentioned. Another thing that can reduce performance is ovality of the drum; as a drum brake can only work well if the drum is perfectly round and the shoes can apply pressure and get the force multiplied by the self servo action. If the brake drum surface is moving in and out relative to the shoes, the action is spoilt. Longer brake arms can help but too long an arm will create too much movement at the lever. The same effect of increasing leverage can be obtained by using levers that have ¾†pivots rather than 1†pivots. The brake is very similar to a Velo Venom size wise and design wise which is known to be an excellent stopper. So there is potential there.
