Short Cranks for Tandems
Here’s my theory why some tandem teams don't perform as well as they should
Many stokers do OK with 170s on their single bikes. But often pedaling
at a slower cadence than their captain's. Perhaps because they
ride less and/or have shorter legs. When you put them on the same bike,
with similar crank lengths, the only cadence where both riders are
effective is a narrow range that includes the high end of the stokers
useful RPM range and the low end of the captains. (Assuming they
overlap at all.) Lots of shifting is the usual result.
Example; Stoker 60-80 rpm, Captain 75-95 rpm = Team 75-80 rpm.
Shorties can raise a rider's comfortable RPM range. And in
most cases the range is wider, losing little, if any, off the bottom
and gaining quite a bit on the top end. (Great for single speeds.
You can gear lower and still have a decent top speed.)
In a perfect world, lengths would be such that the stoker's and
captain's efficient RPM ranges would be matched. But lacking
perfection, getting the overlap wider will get the bike down the road
better.
When one rider’s comfortable cadence is 70 and the other’s is 90, going
from 170 to 165mm, about 3%, isn’t going to make a noticeable
difference when trying to increase the stoker’s RPM by 28%.
When a person is uncomfortable at higher RPM it isn't due to the
muscles switching from extension to contraction more often. It is
because their muscles are extending and contracting at a speed that is
too fast for them. This recruits more fast twitch muscles, which
produce more heat and lactic acid. Shorties reduce this speed by moving
the
muscles a shorter distance per revolution. Allowing
more use of slow twitch fibers for a higher comfortable cadence.
Most of my tandem stoker sets have been 150mm or 145mm with a few even
shorter. Tandem stoker sets are made from the right arms of two
cranksets. Short captain's sets are made from a single set and I
just use the LH tap in the arm with the chainring and vice-versa.
For MTB tandem cranks
I use SRAM S600 arms. (shown)
If you want 110/74mm BCD
(Bolt Circle Diameter) I start with Andel crankarms
For 130/74mm BCD "Road" cranks I have FSA Vero.
Unfortunately these can only be shortened to 150 or 153.5mm.
I can also supply 10 speed 172.5mm FSA Gossamer Tandem sets shortened to 151mm for $309, MegaExo BB included.
BTW Before Terry Osell retired from framebuilding, he would usually put
shorties on the back without even telling the buyers. He says no
one ever noticed.
For left side timing rings I use "Single Speed" rings as they have taller teeth.
I can also provide 170 or 175mm "Captains" sets.
A couple of comments from satisfied customers;
“I got the cranks two days ago, and about ten
minutes later, they were on our tandem. The next day, we went for
a ride, and boy, was she happy to be able to pedal those little
circles!”
"Mark, we are totally on the same page! I
share the same theory. Especially the part about the captain usually
being the more experienced rider. So the captain's natural cadence is
likely higher, as in your example. That is the case here, and was also
the case with my previous partner. In hindsight, I greatly erred in not
getting her shorter cranks. Our ranges had no overlap, and
my wishing that her range would come up to mine didn't happen.
I've grown up a bit since then.
Back to the present, now that I have the short cranks on the back of my
present tandem (thanks to you), I no longer over-rev my partner. In
fact, I've really let the RPMs come up, and have asked her if she is
still OK, and to my surprise (and delight), she is.”