Monday, November 23, 2009

Tandem Climbing Analysis from Brian and Janet

It's often suggested that tandems are slower climbing than single bikes. Yet it's rare that one gets to see solid data supporting this.

One nice thing about the Low-Key Hillclimbs is they generate a lot of interesting data. One nice thing about this year is we have Brian and Janet. They each did a climb solo, and did another climb as a tandem pair. Brian further ran another climb, but that's another matter.

While each climb is different, as long as a climb is continuous and doesn't provide much opportunity for drafting, the ratio of a rider's speed to the median male speed, using single bikes only (I specifically refer to male speed as the number of males tend to be a lot more than the number of females, so there's less random variation in the median male speed), tends to be fairly constant one climb to the next. So I'll use the ratio of climbing speed to median male climbing speed as a measure of general climbing speed.

So here's a comparison:
  1. Brian and Janet on Mt Diablo in the Low-Key Hillclimbs: 110.3% of the male single median speed.
  2. Janet on Alba in the Low-Key Hillclimbs: 81.3% of male single median speed.
  3. Brian on Old La Honda at Low-Key Hillclimbs: 131.8% of median single male speed.
I'll assume the ratio of masses of Brian to Janet is 4:3. Then I should weight their speeds (roughly proportional to power:mass) proportionately. Doing this, using their individual speeds normalized to the median male single speeds, I get 110.2% for the predicted tandem speed. As a tandem they were 110.3%.

Not bad, all things considered. Of course, there's a ton of flaws with this analyis. But it's interesting how close the result is. Certainly there's no indication that the tandem is particularly slower than what would be predicted from the individual riders.

I was interested to see how out of phase their cranks were: 90°. It's more conventional for cranks to be in-phase, or sometimes 45° out of phase. Some people think out-of-phase cranks smooths out the pedal stroke. Brian and Janet make a strong case for this. It certainly doesn't seem to slow them down much.

Brian and Janet on Diablo


specialist said...

We ride out of phase too, not so easy for standing climbing, but the power is smoother and my wife prefers it which is enough. I didn't see if you took account of the difference in bike weights. Tandems don't tend to be as weenie as the same people's solos, our tandem is well more than twice the weight of the singles. I reckon mostly people thing tandems climb slow because they go fast on the flat. We can tow a fast group on the flat, but will get dropped on the climbs.

djconnel said...
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djconnel said...

I've devoted considerable attention to modeling climbing power, and I agree there's a lot of factors.

I suspect their tandem, which looks like it's very much a road tandem as opposed to a touring or mountain tandem, is probably less than their single bikes combined. Additionally wind resistance is a non-trivial factor, and that's obviously an advantage to the tandem. I'll ask Brian about the weight, however.

Motivation may be a different factor, either in favor of the tandem or to the singles, depending on goals.

Overall, I think from a wind and weight perspective, the tandem probably has a bit of an advantage. Rolling resistance probably goes to the single bikes, I'd guess. Drivetrain efficiency probably also goes to the single bikes, I'd think. So there's a lot of uncertainty.

Still, it's interesting the brain-dead analysis I did worked out so well. There's no glaring deficiency in their tandem climbing, that's clear. So it's something to aspire to if if Cara and I discover we're tandem compatible.

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