Update February 2006

The mid-drive has been regeared to shift the entire range downward.  This had the added effect of ensuring that the top side of the front chain never drags against the idler or the bottom side of the chain.  I made a spreadsheet to calculate the new gears:

    Gear No. Chainring Mid Front Mid Rear Rear Wheel Wheel Dia. Gear-Inches
    1 39 34 32 32 19 21.8
    2 39 34 32 24 19 29.1
    3 39 34 32 21 19 33.2
Chainring 39   4 39 34 32 18 19 38.7
    5 39 34 32 16 19 43.6
Mid Rear 32   6 39 34 32 14 19 49.8
    7 39 34 32 12 19 58.1
Mid Front 1 34   8 39 34 32 11 19 63.4
Mid Front 2 26   9 39 26 32 32 19 28.5
Mid Front 3 16   10 39 26 32 24 19 38.0
    11 39 26 32 21 19 43.4
Rear Wheel 1 32   12 39 26 32 18 19 50.7
Rear Wheel 2 24   13 39 26 32 16 19 57.0
Rear Wheel 3 21   14 39 26 32 14 19 65.1
Rear Wheel 4 18   15 39 26 32 12 19 76.0
Rear Wheel 5 16   16 39 26 32 11 19 82.9
Rear Wheel 6 14   17 39 16 32 32 19 46.3
Rear Wheel 7 12   18 39 16 32 24 19 61.8
Rear Wheel 8 11   19 39 16 32 21 19 70.6
    20 39 16 32 18 19 82.3
Wheel Dia. 19   21 39 16 32 16 19 92.6
    22 39 16 32 14 19 105.9
Range 6.18   23 39 16 32 12 19 123.5
    24 39 16 32 11 19 134.7

The entire range is covered by just the 34 and 16 tooth mid-drive gears, but the derailleur can't step more than about 10 teeth without having problems, so the 26 tooth gear was included to make the shifting more reliable.  In daily riding the 34 tooth mid-drive gear is only rarely used.

The noise from that plastic disc in the mid-drive was driving me nuts!  I restacked the gears so that the bike would not autoshift to higher gears when pedaling backwards.  The plastic disc that was putting side pressure against the chain and making all the noise was unceremoniously retired.  The whole thing is VERY smooth and VERY quiet - the only sound it makes is the normal clicking of the chain in the derailleur jockeys.  Now the gear that drives the rear wheel is on the outside of the three gears that are driven by the pedals.  It took a little messing around with spacers to get everything to fit but it works well. 

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Update October 2006 - Disaster Strikes!

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