A:, Because They Can!
Having spoke to a couple of people regarding setting up a drum brake on a set of springers, I came away with a bad impression, they both said that the suspension locks up when the brake is applied! That can't be right, leastways not if the geometry is correct to start with, it is only the most simple of four bar linkage arrangements after all. As long as the relationship between the rocker, the brake plate and the length of the stay form an accurate parallelogram I can't see any reason at all why the suspension shouldn't continue to function......................Bet it does on standard H-D's!!!
Possibly thinking too deeply about it, I started to convince myself that the brake plate itself also needs bushing to allow it to rotate/float on the spindle to allow for the change in angle between the spindle and the anchor point. Laid out the brake stay arrangement on CAD to figure out how much movement was involved, and whaddya know, in a perfect scenario there is no change in the dimension. The bottom drawing in the horizontal position shows the gap between the stay pivot point and the spindle is 31.68 mm, and at 15 degrees of rise, shown in the picture above that dimension is constant. I did a further check at 30 degrees of movement and the number remained the same.
Of course that's all well and good in the virtual world of CAD where everything is perfect and lines and holes are in exactly the right place. Bush the plate anyway,.........maybe!
A fully functioning CAD system can be downloaded HERE