Gearing

The Greenspeed tandem was once again my main reference for gearing range. I read a few other web sites and looked at the Segal "SameSpeed" tandem bike (reviewed in HUFF) as well. This plus my days of MTB touring gave me a strong desire for a wide range of gears. The Greenspeed has about a 20-80" range, which is not even close. Low gear is 50/28 * 0.66 * 18" = 22" or so. The high gear is only 63/13 * 1.0 * 18 = 87". That gives Kelly and I a speed range of about 8kph to about 60kph before we spin out. On my TriSled I have 42/28*18=27" to 87" again, but I can spin faster solo so that's not so bad.

The most useful site for this that I've found is Sheldon Brown's one, because he lists the available clusters and chainrings in detail, and that lets me decide what I'd like and some idea of what alternatives there are if the one I want isn't easily available. That page is http://www.sheldonbrown.com then Harris Cycles, Parts, Cassettes.

Initially I was planning on a standard triple chainset, with something like 24/50/73 chainrings. But the more observant members will have noted that rear derailleurs with a capacity of 50+ tooth changes are rare, and a front derailleur to do likewise might also be hard to arrange. The rear derailleur is always going to be the limiting factor here, because you can play with the front one to get it to shift even very large jumps.

SameSpeed shifts about 24/44/63 without too much trauma, so I could go with that plan. Trouble is, that gives me a best range of about 24/34*18" = 13" to 63/11*18"= 103" with about a quarter of the gears unusable because of the chain length change. And the average step size is pushing 15% between gears, which is not really what I'm after. Brian currently runs a 73T single ring on his Mountain Drive, with an 8x3 rear hub, so his top gear is roughly 73/12*1.2*18=130" and his lowest gear roughly 73/28/2.5*0.8*18=15". That's more the range I was thinking about.

And lets face it, mostly I think a mountain drive is a cool toy and I want one. I emailed Florian Schumpf and asked what he thought of the idea of running two chainrings on a tandem, and he said "use a steel spider", but otherwise seemed quite happy. So at least if it does break it should be covered by warranty :)

An 80T/50T combo on the front plus a Type I Mountain Drive gives me four effective chainrings, the smaller ones being 2.5 times less or 32T and 20T. A 12-34 cassette would give a range from 11-120", and playing with the different cassettes in that range mostly affects the top gear which doesn't matter so much. What does attract me is a "racing" cassette, 13-27 or similar, which makes the range 13-120" but it makes the step size a lot smaller, from an average of about 13% to three steps of 8% and five of 12%. I had one of these on my bike, and I like it a lot for longer rides, because I can twiddle gears to get comfortable. If you have Excel, here's a spreadsheet. In summary though, here's what I'm most likely going to use initially.