Experimenting with linings is always interesting. Many brake linings contain petroleum products such as resins. All of the resins I have been exposed to seem to degrade over time. I suspect that this happens to the resins used in brake lining too. So, old age might be dangerous for linings!
The list of ingredients in brake linings is always interesting and many contain cashew nut dust. I assume it is a good abrasive, but it does make me wonder about the "use by" date. Metals are often used in brake linings because the coefficient of friction is usually good, but the addition of metal particles also conducts heat well and evens out the temperature of the lining over the entire shoe.
Modern high-performance linings for drum brakes contain ingredients that coat the drum surface with a film. The benefit is that the lining has a very high coefficient of friction with itself, that is, the film that has been transferred to the cast iron drum. This means that a good brake tuner is concerned about the layer of film on the drum surface. They usually want the drum to be skimmed so the traces of the last brake lining are removed because it may not be compatible with the new lining. I opt for a good cleaning instead.
There are usually two problems that are difficult to fix. First, the H48s have to be perfectly flat and smooth. any damage and the brake will not work properly. The H48 plates usually have a slight indentation from repeated use.
This H48 was on the Comet Racer I got from John Renwick. It has indents on both sides. Once the H48 gets to this stage, the brakes will not work well. The brake cam will fall into the indentation and no matter how hard you pull you will not slow down anymore.
Second, I don't hear much talk about the break-in procedure for new linings. If the linings are not broken in properly, they will never achieve maximum bite.
This is the graph of a Ferotec lining 3921, which I believe are the linings used in many of Godet's bikes. Dick Hunt uses this lining for front and rear, but it is mostly used as a rear lining on race bikes.
The first graph on the left shows the number of applications of the brake. Note that the more the brake is used, the higher the friction becomes. I have often heard it said that you can't get max braking until you have applied the brakes at least 75 times.
Just as a guide, AM4 lining has a coefficient of 0.44 on the vertical scale. I don't have a materials test graph for AM4.
This is the graph for Vintage Brake 2540V, which is generally the best race lining:
The flatness of the lines are very impressive. The friction is high with little or no fading. Also, the number of applications before the maximum bite is achieved is quite small as shown on the baseline curve.
David
