Tandem drivetrain experiment

[100Tours]

 

So we've been riding tandem for about 2 years now and I've been experimenting with a number of drivetrain changes.

First off the frame is a Tank Matrix, which is awesome, but the designers didn't put a whole lot of thought into the back end - which is straight off a single bike and doesn't accommodate anything bigger than a 25mm wheel, and that's only with the slimmest front derailleur clamp I can find. This also means that you can't run the 'normal' tandem f/d clamp that you would need for a triple front ring, more on this later. But anyway, bike runs well on those 25's, and I'm not sure I want to go over 100kph with a 140kg payload on 23mm tyres - we can do 100 on cedar road, nevermind the hills around Sabie.

 

 

f/d clamp - a bit tight in there

Next step was the timing chain and the eccentric (what's a timing chain I hear the single riders ask..), which links the two cranks. Rather than go for a carbon drive we borrowed a little new mtb tech and installed two wide/narrow chainrings. This instantly resolved all of our timing chain issues - the wide/narrow design means you need less tension in the timing chain to keep it on, and the chain runs silently. That's not impossible with the original design, but it's not easy to get right either when you're trying to achieve mm adjustments with technology that precedes the derailleur..

 

wide narrow chainings on the timing chain

Then crank lengths - the bike was originally running 172.5 cranks front and back, but we have quite different leg lengths (thanks Sheldon Brown for this one) and we were finding that we couldn't manage higher cadence. We dropped the rear crank to a 165(!) - again Sheldon, who claims that you can adjust all the way down to 140 quite happily, and that basically shorter legs can manage faster cadence on shorter cranks. This also allowed us to raise the rear saddle by 7.5mm for more hip-to-ankle space over the top of the pedal stroke. Happy stoker.

Tandem drivetrains are generally sloppy, and the typical triple front derailleur and the long cage mtb r/d don't do anything to improve this, so we have gone 2x10 and the improvement to shifting crispness is remarkable. Current set up is 53/39 on the front and 11/36 on the rear with a medium cage ultegra derailleur. World of a difference.

Finally after watching what the tandem teams are doing in the hillier parts of the country we've switched to elliptical rings - these open up a whole new level of climbing for us. As a comparison our gear ratio at 39-36 (granny gear) is now lower than the 30-27 we used to ride with the triple. Gear ratio of 2.3 in both cases, and an 'equivalent' 2.1 now if I adjust for the ovality of the rings. Similarly on the top end we previously had a max gear ratio of 9.1 and this is increased to an ovality-adjusted 10.5 on the new ring, and the Strava KOMs in our area are coming under some increased pressure as a result.

 

Elliptical rings and short cranks

 

2 goldfish in a tank, one turns to the other and says "do you know how to drive this thing?"