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The photo the tandem I built in 2005. The bike is made out of two old mountain bike frames, together with some cut out tubes from other bicycle frames.

To have enough place for myself at the rear end, I prolonged the tubes connecting the rear to the front. These tubes are bicycle tubes welded end to end, so they get the desired length.

The head angle and trail are just taken over from what the starting bikes provided, but still.. the tandem rides very smooth and easy. There is no need to excert large tortional forces on handlebars to master this bicycle.

wheel base	173cm
headset angle	x
trail	x
height front bottom bracket (BB)	x
height rear BB	x
distance BBs	70.5cm

The Timing Chain

In hind sight, I should have determined the optimal distance between the bottom bracket (BB) before building the tandem. But that is too late now. And I have to live with the distance of 70.5cm, which is 0.5cm off the optimal distance :-\

The pitch of a chain (=distance between the rollers of a chain) is always 1/2" (see site about chains.

chain	pitch	roller diameter	roller width
with derailleur	1/2"	5/16"	1/8"
hub geared	1/2"	5/16"	3/32"

The formula for the diameter of the chainring (the circle going through the rollers): pitch / sin(180/N), where N is number of teeth. For an even number of theeths on the two chainrings, then the ideal distance is a little less then 1/2" * n, where n is a whole number.

The distance between the BBs is 70.5cm, and the chainrings 44T. If the distance would have been 71.05cm then the idler would not have needed to lift the chain high up, and thus the force and wear on the idler would have been small. But now I am stuck with 70.5 which is .5cm off that ideal. And so, the idler needs to lift the chain 4cms. (sqrt((71.05/2)^2 - (70.5/2)^2)).

I first tried an old derailleur to tension the chain. But that had the problem that the tension was not adjustable. In my case the tension was to high, making the chain run hard.

A more serious problem of this spring tensioned idler occurs when the stoker continues to pedal, while the captain coasts. The derailleur is then pushed downwards, and the upper chain slacks. This can lead to the chain falling off.

In my case the derailleur broke off, because of that problem: while we were off road, the slackened chain actually jumped onto the derailleurs housing, and the next moment we ripped it off!

Plan B works much better. The idler is simply adjustable with a M8 bolt in height. The chain tension can be set just so it is perfect, and it is strong enough to resist backward pedalling. The idler is taken from a derailleur and is of the kind that has bearings inside. I am interested to learn how long it takes for the idler to be worn out.

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