Kurt Sperry wrote:
> Someone mentioned earlier in this topic dual sided
> drivetrains for vintage racing bikes and their being
> subsequently outlawed I think. If so what were the
> alleged advantages?
I heard of a kilo timetrial or pursuit bike with fixed gears of two different gear ratios. Obviously it's an advantage (however small) to be able to accelerate briefly in a lower gear, then switch to a higher gear - automatically without letting go of the bars.
How they achieved this was by threading the smaller rear cog on the right side, but not all the way (maybe just a turn or to onto the hub), with no lockring. The chain on the left, the side with the larger cog, had a purposefully-weak link, a piece of light gauge wire instead of a chain link IIRC.
The weak link was placed on the chainring at the start of the race, forward of those 3 or 4 teeth at the top of the ring that do all the pulling. So when the rider started, the weak link traveled back along the unloaded under-side of the chain run, where its weakness didn't hurt anything.
Meanwhile the smaller cog on the right is turning faster than the hub is turning, because of the difference in the gear ratios. It is threading on to the hub due to the right-hand thread.
The amount the cog was backed off to begin with was carefully calculated so that it would fully tighten against the hub shoulder just at the very moment that the weak link on the left was coming around to the top of the rear cog, where it would finally have to bear some load. The weak link pops, the chain falls to the track, and the rider continues accelerating, now in a higher gear, and with less weight to boot! His holder or starter would have to get the chain off the track before the next lap, but due to the constraints of the length of chain, it wouldn't be very far down the track.
I don't remember who came up with this (some guy named Rube I think, I forget the last name), nor do I remember if it was ever allowed in competition. I doubt it though - seems like cheating to me. Officials have leeway to disallow something that seems unsporting even if it isn't specifically forbidden in the rules. I'd bet (if I were a betting man) they didn't allow it in any important competition.
Don't I wish I'd thought of it first though!
Mark Bulgier
Seattle WA USA