Greg,
I have reviewed all of the brake articles written in MPH through the wonderful work of Graham and others on the Database. Thank you to those involved!
I asked my local brake specialist about the possibility of the shoes jamming or self energizing too much, but he said "No that never happens"
Unfortunately, it does happen! It used to happen quite a lot at road races in the 1960's. A rider would grab the front brake and the bike would go down due to the drum brake locking. Brake engineers call this "spragging" and often the only way to release the brake is to roll the wheel backwards. This is the ultimate good news/bad news story. The good news is you could make a 7" brake do a stoppie, the bad news is it might do this anytime you grabbed the brake! As the engineers say, the the twin leading shoe is less stable than the single leading shoe or two trailing shoes (which is the most stable drum brake of all) The duo servo shoe brake is the most powerful drum brake, more powerful than 4 leading shoes, but is the most unstable. Patrick Godet uses the duo servo or dual servo brake on his new Flashes based on the old G 250 single leading shoe brake used by Seely.
What would make the brakes lock up easily? Two things, the geometry of the brake and the coefficient of friction of the lining. It is usually the lining we get to fool around with although we can mess with the geometry a bit. Drum brakes are rated by something called "brake factor." It is essentially the ratio of the drag of the shoes on the drum to the amount of squeeze on the brake lever. Each shoe has a "shoe factor", like the leading shoe has a factor of 1.75 and the trailing shoe has a factor of .5. The brake factor is 2.25 when both shoes are added together. This number gives you an idea about how little squeeze you have to put in to get a lot of braking. A brake factor of 2 is not memorable, but a brake factor of 6 might be grabby (or it might sprag).
For example, a Vincent front brake with Ferodo MS3 (the green non-metalic linings) has a coefficient of friction of .38 and will create a brake factor around 2. If you switch to the AM4, which has a coefficient of .44 the brake factor jumps to 2.75. That is quite a bit more self energizing. I think that is why it is so well-liked.
I have just had some linings put on that are .61 coefficient of friction. This runs the brake factor up to 9.0. I suspect that I will find in testing that this lining may be too unstable to use and will grab or sprag, but I have to start somewhere.
I don't believe there is any value in relining stock Vincent brake shoes. I think Trevor is correct and these shoes are junk. They are not hardened correctly. Assuming the replacement shoes are the correct material and hardened properly, that should be the first course of action.
There are lots of linings that are as good or better than AM4. It is just difficult to find this info. I would not get a lining without knowing its coefficient of friction. The Ferodo MZ41 that I believe Martyn is using has a coefficient of friction of .43 cold and .45 hot, which is essentially better than AM4, which goes down to .40 when hot. I am not sure if they have asbestos. I would pass if they did and choose a similar non-asbestos lining.
...and yes, I was trying to improve the brakes this way, much like the Girdraulics. Drum brakes, however, are wildly complex, unlike the Girdraulic!
David