Modernizing a Family Tandem
James Moore
John Thurston
Having been at this for a few decades, modernizing a Friday
drivetrain for the sake of modernizing is putting the cart before
the horse. IMnsHO, you should determine what gear range you need,
and then search for the technology which will let you achieve it.
That 3x8 hybrid hub (you don't specify the brand) . . if it is a 'real' 8-speed freehub will handle everything up to 10-speed cassettes (and 11 if the large cog is large enough to be dished). If it is 8-of-9-on-7, then you have a 7-speed freehub and you'll never be able to get 11 cogs on there.
The power of the hybrid hub is providing you a high top-gear without requiring a crazy-large chainwheel on your small-wheeled bike. On a tandem, an extra tall gear can be nice to utilize the extra power available. On a tandem, the hybrid hub can also be nice to let you drop a 'range' while stopped. I know that proper tandem practice is to perform a controlled downshift (to facilitate startup) while coming to a stop. But there have been lots of times (particularly with a Family Tandem with young stokers) when that wasn't possible. In those cases, being able to address the critical problem in the stoker compartment without having to wonder if we'd have enough torque to startup was nice.
Another nice thing about the hybrid hub, is you get a wide gear
range without having to carry around a long-cage derailleur. The
cage only needs to be long enough to absorb the chain from the
cassette. And with the small wheels on a Friday, a cage long
enough to absorb the cassette slack and the chainring slack can
leave you very little ground clearance.
Before you commit to discarding the hybrid hub and going 2x,
study some gear
calculators to see how large the chainring will need to be
to get you the gearing you want. If you want something ~100
gear-inches, and you're using a 11t small cog, you're going to
need a 58t chainring. They exist, but they aren't cheap, and
you're going to have to find a front derailleur which will work
with it. The hybrid hub, on the other hand, will let you reach
that 100 gear-inches with only a 42t chainring.
John Thurston Juneau, Alaska
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James Moore
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John Thurston
I COMPLETELY misunderstood your setup. I thought you meant it was a 3x8 hybrid hub (like the Sachs/Sram/Sturmey). Not that it was 3-chainring / 8-cog setup with two derailleurs. So sorry!
Re: the 11-50 cassette
Ground clearance is the snake that will bite you.
I think you will find that the size of the wheels will prevent you from using a 50t cog (while still having the derailleur clear the road). In my experience, 40t is as large as I'm comfortable using, and that's as a 1x with a short-cage derailleur. If I wanted to rig that up as a 2x, I'd need at least a medium-cage derailleur, and I'd be dragging the cage in the dirt :(
On my 1x, I use a 50t chainring with an 11-40t cassette. This only gets me 24-88 gear-inches, but my 1x is a half-bike and doesn't need +100 gear-inches. That same cassette/derailleur could be used with a 53t ring to get 27-100 gear-inches with your 60mm tires.
John Thurston Juneau, Alaska
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James Moore
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