[Yak] Tandem Traveller XL Gearing-Success

[John S. Allen]

At 04:00 AM 7/19/2010, Paul Webb wrote:
>Good to hear!
>
>To state the obvious, you'd probably need a different rear
>crankset to use a chainring below 39T - and then you'd probably
>need to buy three new chainrings.  ...and it may well be that 39T
>rings are not so readily available for chainsets other than 130
>BCD, to match the timing ring at the front...

There are adapter chainrings available. The smaller inner chainring 
bolts directly to the adapter chainring of a 130 mm  or maybe 110 mm 
BCD (bolt circle diameter) crankset. Try Harris Cyclery and Peter 
White Cycles. On a BF there will be no need for a different bottom 
bracket, because the chainstays are well out of the way.

The Hulls appear to have a 110 mm BCD crankset, and its 34T minimum 
chainring is as small as usually ever needed on a BF, especially with 
the 3 x 9.

>I have heard questions raised about the strength of the Dual
>Drive hubs to withstand heavy loads and very low gears.  There
>are two reasons to consider it anyway: (1) the torque on the hub
>is less with smaller wheels than large ones (for a given
>speed/gradient) (2) if the need for really lower gearing is to
>allow the riders to keep their effort down, then you're probably
>not going to blow the hub.

How often 3 x 9s fail on BF tandems, I don't know. BF would have a 
better idea. I think that the 3 x 9 is usually OK on a BF tandem, but 
I have seen a 3 x 9 hub from a BF tandem which stripped out the 
axle-gear teeth and seized up. That's the bad news. The good news is 
that after cleaning out the metal chips and relubricating (so the hub 
will turn and to avoid damage to bearing races in the hub shell and 
the need to rebuild or replace the wheel) , it is still possible to 
ride the bicycle (say, in a remote touring location) with the hub in 
the middle gear.

The worst loading is an impact loading if the top run of the chain 
gets slack due, say, to backpedaling after not completing a shift 
with the rear derailleur, and then the riders pedal forward and the 
inertia of the two chains, cranksets and rider's legs is transmitted 
to the hub with a "Wham!": This is more likely on a tandem because 
both riders pedal but only one is fully aware of shifting.

Lower gearing does not reduce stress on the gears inside the hub. 
Assuming smooth pedaling, that depends only on the steepness of the 
climb. And if the hub is in low gear, power is being transmitted 
through the gears inside the hub.

I'd think that failure of the chain due to shifting under power is 
also more likely on a tandem, but the rider who is shifting does have 
control over that, by pushing back hard enough to negate the other 
rider's forward pedaling (but not hard enough to turn the pedals 
backwards and get the top run of chain slack).

Pedals out of phase do reduce stress on the hub somewhat, by making 
the power flow more even -- but at a serious cost in ability to 
coordinate mounting and dismounting, starting and stopping.

John S. Allen

jsallen *at* bikexprt.com

http://bikexprt.com