Sachs Spectro hub: low gearing limit?

[Mark]

I'm in the process of building a utility cargo trike along the lines of
http://www.organicengines.com/SUV/index.htm (with Dan K.'s blessing). I'd like to get by with a
7-speed internal hub for the front wheel drive, for reasons of drivetrain simplicity (no derailers
to get mucked up in the winter). I plan to gear it so that the top two gears would be for cruising,
and the remaining five for getting up hills or starting up with a heavy load.

I'm aware that the Rohloff 14-speed hub has a lower limit for the recommended tooth size of the
front chainring and rear cog to avoid over-stressing the hub, (though it is apparently tandem
rated). Does anyone know if the Sachs 7-speed has recommended limits like this? Gear inch-wise I'm
looking at 65 for the top and down from there to about 20, using a 20" drive wheel, 39-tooth
chainring, and 20-tooth cog.

Thanks, Mark Rehder

 

[Chalo]

[email protected] (Mark) wrote:

> I'm in the process of building a utility cargo trike along the lines of
> http://www.organicengines.com/SUV/index.htm
...
> Does anyone know if the Sachs 7-speed has recommended limits like this? Gear inch-wise I'm looking
> at 65 for the top and down from there to about 20, using a 20" drive wheel, 39-tooth chainring,
> and 20-tooth cog.

SRAM and Sachs have not published gearing limits for that hub, but even if they had you would still
be in the clear. You will need something like a 44/24 primary drive (delivering 21 to 64 gear
inches) to get the range you're after, which is completely normal for that hub.

In your case, the reduction in gear inches comes mostly via the smaller-than-usual wheel diameter.
That way the hub doesn't see additional forces to deliver more force to the contact patch. For
instance, with a 700c rim and a very fat 700c tire, the gear inch range would be 30 to 92 with that
same primary ratio. Yet the force on the hub's gears would be the same for any given pedal load.

I weigh 360 lbs, and I use a Sachs 7-speed hub with a 44/24 primary on one of my bikes. If it works
for me (and it does, without issues), it will certainly work for you.

Chalo Colina