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.

## Gallery

nearly finished, out of two mountain bikes. The tubes in
the middle are prolonged

bad luck with the fork: I needed to build my own
headset for an uncommon inner diameter of the fork shaft

bad luck with the BB: needed to shift out a BB shell
because it was some italian make

the rear steer flexes a little

Marianne poses in front of the bicycle