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Old 07-13-18, 09:20 AM
  #64  
Alan_F
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Join Date: Jul 2017
Location: Maryland
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Bikes: DaVinci Joint Venture Ti S&S, DaVinci Symbiosis 27.5", DaVinci Symbiosis XC 29", Specialized Roubaix SL4 disk, Diamondback Haanjo gravel (commuter), and more

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Originally Posted by jethro00 View Post
Leisesturm, I agree with you that da Vinci confirms your assessment, and also adds a factor.
So, now I have some questions for you and others about IPS versus traditional tandems.
Let's start with my da Vinci.
If captain (C) and stoker (S) pedal at different rates, then the total power = 100% of the rider spinning faster, right?
If C and S spin in sync, then is the total power 100% C + 100% S?
Do the C & S have to be perfectly in sync to combine power or just close to the same rate?

Now, let's go to a traditional tandem, like the Trek T900 we both have.
If C and stoker pedal at different rates (with C faster), then the total power = 100% C - some factor for C pulling Ss pedals + 100% S?
If C and S spin at equal rates/power, then is the total power 100% C + 100% S?

I am finding this thread educational.

Having four DaVinci's in the basement at the moment (but trying to reduce that to 3... check tandemclassifieds.com if you want one!) I have some experience with the DaVinci drivetrain including using various crank lengths on different bikes. On our new road tandem we tried 160mm rear cranks - not for cadence reasons but for fit reasons. At 5'00" my wife has short legs and the difference between 165mm and 160mm was very apparent to her. It reduces the amount her leg has to bend at the top when the saddle height is set correctly for the leg extension at the bottom. The shorter the legs, the more a small change in the diameter of the circle being pedaled matters. I'm sure a 5mm change would not make nearly as big a difference to me with much longer legs and 175mm cranks.

But as for the spin and power. With a few exceptions already noted (Counterpoint, etc) the riders can't spin at different rates on a traditional tandem and can't deliver power to the drivetrain at different rates on a DaVinci. So on most tandems either the stoker is able to put power down at the current cadence (contributing), just able to keep up with the motion but not able to put any power down (neutral), or actually being pulled along by the captain (negative input). But in all cases the riders are turning the same cadence.

On a DaVinci only the third scenario changes. You can have the stoker providing power (contributing), just able to spin but not putting any power in (neutral), or not able to keep up (still neutral, but the phase of the pedals will constantly change and you'll hear the left freewheel clicking). I'm a bit surprised how rare it is for one of us to pull the pedals even a single click ahead. When it does happen it usually means we're spinning above 100 rpm and one of us is tired.

I love the DaVinci drivetrain, but i don't think it does any more to solve this issue than a traditional drivetrain set out of phase. It is easier to experiment with different out-of-phase options, but on the other hand it is probably pretty tricky to reliably get the exact same out-of-phase position of the cranks. I suppose it is something the team could learn with practice, but if you prefer to be out of phase all the time a regular drivetrain (or a locked-down DaVinci) would work better.
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