I am rather confused by many of the comments about twin plate forks and their relative merits or demerits. I found Harvey's post to be clear and logical.
I also feel that Richard is very correct, that one must first define what a twin-plate fork crown is. Without this the discussion is moot. Personally, I believe there are two crown types that I would consider true twin-plates: a) the ones made of two plates alone, b) the ones where the crown is made of a center 'sleeve' attached by almost any means to a top and bottom plate (without any joining between the top and bottom at the point where the fork blades are inserted). As an aside, I also remember that there was somebody at the dinner at le cirque last year who had brought an NOS triple-plate fork crown as a discussion piece (this is just one of the great things about le cirque)
I find that it is very misleading to generalize that twin-plate fork crowns are lighter. In case a) above, they could definitely be lighter, but in most applications where case b) is used the weight is not appreciably different. I checked the two twin-plate forks that I have in my garage and neither is any lighter than other similar vintage 'traditional' ones.
It would also seem to me to be a canard to link brake type to function with regards to fork crown type. Isn't the lower headset race the equivalent of the fulcrum and the fork blade the lever? If this is the case, given that the force required to stop any bike remains constant no matter what brake type used, and that the angle of the force to axis is close to 90°, the torque is quite easy to calculate. It varies to the comparative distance from the fulcrum. This means that drum brakes that were popular on 1940's French tandems, that mounted almost at the end of the fork blades would have applied more than 10 times the torque of traditional brakes. This fact did not however stop the builders of these tandems from using twin-plate fork crowns. Besides, if any decisions were made based upon torque applied, it would seem unlikely that teh constructeurs would have used low-rider racks that also apply hugely greater torque on the fork crown than traditional. It doesn't seem to add up.
I suppose that Harvey comes closest to the truth when he says it is all linked to financial and marketing logic.
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Steven Maasland
Moorestown, NJ