This is really a question for Roger, but I thought others might be interested. In the Scott newsletter, Roger is advocating removing 25% of the trailing end of the trailing shoe lining to improve efficiency of the leading shoe, now this is something that I have been doing for many years, ever since Titch Allen recommended it, probablly in Motor Cycle Sport back in the 60,s. Now my understanding is that the trailing edge of the trailing shoe is on the cam opposite the leading shoe, and this is the portion that I have been removing, and gaining an improvement, but Roger, you are saying that the remaining portion of lining is over the cam.Have I been getting it half wrong all these years, or am I just getting senile.
Hi Dave I have experienced a moment of temporary mental aberration (foolish boy) I was wrong and you are correct. The area to remove will be nearest to the cam on a SLS brake. Perhaps you will tell us to what degree you have found it improves braking. Pass the sackcloth and ashes! Roger
Well its déjà vu all over again then! It may seem counterintuitive but the area of contact has nothing to do with braking efficiency, it is a function of coefficient of friction and pressure only. Were such magical materials available, in theory at least, a single point with enough pressure behind it would do the job. Of course there are other considerations, area has a bearing on the useful life of the shoes and then there is the small matter of heat dissipation and fade but the basic premise still holds.
The effect on the brake assembly when pressure is applied is for the wheel to try and rotate the whole brake plate. As the assembly is anchored this is impossible but the shoes themselves do have a degree of freedom and the forces applied attempt to rotate the shoes about their pivots, in the case of the leading shoe some of the energy stored in the moving bike gets used in a wedging action driving the shoe harder against the drum increasing the braking force. The effect of these forces on the trailing shoe are opposite, the friction tending to force the shoe off the drum as evidenced in the different wear rates usually seen.
So have I missed something? I can’t help but wonder if the reported improvements are more in the heart than the head. If not then it is hard to see what benefit removing material from the trailing shoe can have, certainly it’ll increasing the pressure per unit area on that shoe so maybe that’s it. But then as the brakes are the sum of their parts I’m having a hard time getting my head around what exactly is going on.
If I were to be really picky and if I was concerned about the efficacy of the brakes I’d think about having the drum lightly skimmed with the wheel fully rebuilt as it is entirely possible that the spoke tensioning could cause some distortion of the drum. I’d certainly check its concentricity before I tried anything else.
As usual, there is no one magic bullet that will transform a poor brake to an effective one. Within the limitations of the design, everything has to be correct. Roundness after spokeing, Correct brake shoe type, Brake shoes turned to correct diameter and concentric with bearings. Many brakes were bought by manufacturers on cost and pressed steel drums polish and have a lower frictional value than iron. On many makes, the pre war malleable single sided brake drums were superior in efficiency to post war pressed steel drums, even if they looked full width. The Dominator used an iron lining cast into an aluminium drum, but the process was faulty, as in use the aluminium grows away from the iron lining. There is no heat path, the lining bakes the brake linings and if used hard, the unsupported lining breaks.
It is a big subject and my home made race brakes are amongst the best on the track, but are a Dural body for high heat dissipation and strength and a thin iron lining shrunk in w3ith about a 0.030″ interference. Iron has a very good frictional value, but a poor heat transmission ability, so the answer is to have the iron liner thin so heat will pass though to the alum body as quickly as possible. With 0.030″ interference, the whole job will not get hot enough for the alloy to grow away from the iron liner and so loose the heat path. Folks are also fond of putting a nice thick powder paint coating on brakes which certainly does not aid the heat to escape. As regards the suggestion to reduce the trailing shoe lining. Titch Allen advocated this years ago, but recently I saw several folks who’se opinion and judgement I respect, confirm the effectiveness of the ploy. I thought it worth mentioning, as anything that improves the efficiency of brakes on todays crowded roads without major surgery is to be welcomed. I have not tried the method, but it just made sense to me.
What I have done in the past that makes a good brake, is to use different grades of linings on leading and trailing shoes on my MM and these brakes are beyond reproach Roger
An interesting topic. It is one of those I would like to see run, with full input and then print in Yowl for the benefit of all. So go to it folks.
Putting in my own thoughts. I would agree with erf215 about the pressure and contact area and I would think this is probably the nub of the matter. As I see it with new linings well set up then the pressure on the leading and trailing shoes will be equal, however, as the extra bite taken by the leading shoe causes it to wear more quickly than the trailing shoe there comes a point where the trailing shoe is taking all the pressure from the cam and the cam pivot is hard up against its bush with the leading shoe only biting because of the wedge action. The only way round this would be to have 1. a softer lining on the trailing shoe. 2, Less trailing shoe to wear. 3, A floating cam bush. Have all three of these been tried and is there any knowledge of attempts to quantify the relative merits of any system.