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edscoble
@Brommers
The problems with the gearing being too low were easy to solve with a 13t sprocket, the smallest that will fit on the Powertap hub. Combined with a 52 tooth chainring, and given the 16" wheels, that gives a 64" gear which seems fine so far. I did get a 14t sprocket as well, but I think I'll stick with the 13t one for now.
The main problem was with the chain tensioner. If you tried to resist through the pedals, the chain tensioner would swing upwards, ramming the jockey wheel furthest away from the sprocket into a welded boss on the titanium rear triangle. The jockey wheel, being made out of relatively soft aluminium, was losing this ongoing battle. Also, because the slack in the system introduced by the chain tensioner, the chain would skip if you tried using the drivetrain to slow down.
The problem, in a nutshell, was that the bottom of the chain, running from the rear sprocket to the chainring, wasn't straight. Leaving the sprocket it first headed upwards over the first jockey wheel, then down over the second.
However, the chain tensioner doesn't actually do anything on a Brompton, not when you're underway anyway. The chain tensioner is really only there to take up the slack which arises when the bike's folded. The top jockey wheel doesn't need to be there at all, and while the bottom one needs to be in contact with the chain, so that it can swing into action (quite literally) when the bike is folded, it doesn't need to do anything else provided the chain's reasonably taught.
So, a wee bit of re-engineering and we have this:
The chain tensioner is a modified 2/6 speed one, as this puts the lower jockey wheel further way from the rear sprocket. The lower jockey wheel is fixed in place (I cut down the chain tensioner mount and machined up a small aluminium bobbin to mount the jockey wheel on), and the upper jockey wheel no longer exists. It's been replaced by a bracket, some penny washers, and a machined aluminium spacer.
It's not the finished product - more of a proof of concept - but after a quick test ride it seems to work reasonably well. If it does, then I'll make a rather more polished version later.