Brake linings contain many odd materials to make them work better and I don't think that they all age at the same rate. I tend to look at linings like tires: fresh ones seem to work better. Additionally, adhesive technology has improved considerably over the decades.
When the friction of a lining is increased it makes the brake slightly less stable. Squeal is one of those problems that arises from improving the mu. Squeal, being a result of vibration, can cause mechanical destruction of the springs or the hooks. The lining itself tends to wiggle around and vibrate, which is why we chamfer the leading edge of a new, tall lining. You don't have to chamfer worn linings because they have lost enough mass to be much more stable.
Manufacturers tend to "detune" the rear brake to prevent the rear wheel from steering the bike by stepping out sideways during heavy rear braking. Vincent detuned the rear brake by activating the trailing shoe most. On the front brakes, it is the other way around. the leading shoes are activated most.
With the detuned rear brake, the AM4 linings cannot get the bite they need to cause squealing. Almost all brake linings are hygroscopic, so they will absorb water if they are left idle for a period. This will also increase the friction of the lining which results in that early morning squeal or harshness that disappears quickly with a little heat.
David
When the friction of a lining is increased it makes the brake slightly less stable. Squeal is one of those problems that arises from improving the mu. Squeal, being a result of vibration, can cause mechanical destruction of the springs or the hooks. The lining itself tends to wiggle around and vibrate, which is why we chamfer the leading edge of a new, tall lining. You don't have to chamfer worn linings because they have lost enough mass to be much more stable.
Manufacturers tend to "detune" the rear brake to prevent the rear wheel from steering the bike by stepping out sideways during heavy rear braking. Vincent detuned the rear brake by activating the trailing shoe most. On the front brakes, it is the other way around. the leading shoes are activated most.
With the detuned rear brake, the AM4 linings cannot get the bite they need to cause squealing. Almost all brake linings are hygroscopic, so they will absorb water if they are left idle for a period. This will also increase the friction of the lining which results in that early morning squeal or harshness that disappears quickly with a little heat.
David