Vincent,
I should have been more specific about the source of this material. This is not my work. It is the work of Newcomb & Spurr, who were the top engineers at Ferodo. It is in their book: Braking of Road Vehicles.
The pressure lines are a relatively accurate drawing of the pressure on a fully lined leading shoe in motion. If there were a corresponding trailing shoe in motion opposite the leading show it would look the same, but the force lines would be only about 1/3 the length of the leading shoes. The drawing is done to demonstrate the relative difference between the center of pressure and the areas of lower pressure on the same shoe. It does not show the difference in pressure between a leading and trailing shoe.
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This is a good example of the difference between leading and trailing shoe forces. on the left is a pivoted system. I think it is a good example of showing that when you trim the lining back you do not lose a lot of force, but you certainly start to "square it off" in terms of the graph. Volvo did not like the wear in the leading shoe, based on their scheduled service intervals. By tuning the lining (cropping it back at the leading edge) they decreased the servo action of the leading shoe and made the wear on the shoes a bit better. This is also a rear brake and manufacturers tend to detune the rear brakes, on motorcycles particularly, to keep the rear braking at a minimum.
There is nothing wrong with wanting the lining as long as possible at the cam end even though the maximum force is closer to the pivot end. Instead, at the cam end, of a leading shoe you have some control of the servo effect. It is difficult to get too much servo effect, but when you do the front brake will be
dubbed "grabby". The compromise is usually in the lining material. It is difficult to make low friction lining grabby.
Chris,
The cam design of the Vincent is difficult to work with because it is flat. Because it is flat, it lifts each of the shoes to different heights and pressures. On the front, the lift favors the leading shoe. On the rear, the brake arm is turned so the lift favors the trailing shoe. This is part of the detuning the rear brake that I mentioned.
The shape of the cam can be changed to an equal lift and pressure using an "S" involute cam.
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I do think you could fool around with the shape of the stock Vincent cam and get some beneficial effects. Many years ago an owner from Chicago named Wozney, if I remember correctly, came up with a design.
David