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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 02-16-11, 07:07 PM   #1
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Eccentric high or low?

I have started re-assembling the '82 Peugeot TH8 for which I've been gathering parts through the winter. One seeming option is the vertical position of the captain's BB eccentric. It could be mounted with the BB centerline above or below the shell's centerline. The effect is to place the BB higher or lower, exactly how much depending on the longitudinal position.

The '82 and '83 catalogs online seem to have placed the BB high. Two of the bikes pictures in PeugeotTandem.com seem that way too, but another seems to have placed the BB low.

I'm just wondering what the effect is, other than the obvious of placing the captain higher or lower. Lower means more likelihood of ground strike by a pedal.

I'm about 5'9", 162 lbs, the stoker is about 5'2" and maybe 110 lbs.
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Old 02-16-11, 07:11 PM   #2
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I put ours higher for the pedal strike reason, which actually matters when you race it. Otherwise I don't think it matters a lot. Lower would give you a bit lower center of gravity, but with the pedal strike trade off.
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Old 02-16-11, 07:31 PM   #3
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Putting it higher allows the stokers bars to be raised higher if that matters.
It also changes the captains saddle height relative to the bars.
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Old 02-16-11, 10:34 PM   #4
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The purpose of the eccentric is to keep the crossover chain snug; whether high/low or level front/back is up to you.
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Old 02-17-11, 07:44 AM   #5
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FWIW, all of our tandems, both dirt and road run the eccentric with the BB up. As mentioned in post #2 & #3 are why, offroad for strikes, all around on account of stumpy captain legs...and while I would like to be a pirate, oh wait those are peg legs.

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Old 02-19-11, 11:09 AM   #6
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If you did the below centerline , your downward force could loosen your chain if it moves . Hi setting wood make it tighter if it moves. Did not see it mentioned.
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Old 02-19-11, 11:20 AM   #7
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If you did the below centerline , your downward force could loosen your chain if it moves .
Wouldn't precession (the reason left pedals have left-hand threads) make it tend to rotate the opposite way? Not that it really matters: if the pinch bolts are tight, it shouldn't move at all.
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Old 02-19-11, 01:16 PM   #8
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Interesting point but I would consider downward force as the ruling factor here & the pedals are 2 seperate rotational forces . I do understand what your saying. There are about 7 tandems in our group now . Only one is below center & thats the one with the droopy chain.

? Coincidence ?
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Old 02-19-11, 05:02 PM   #9
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Yes, coincidence.
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Old 02-20-11, 10:03 AM   #10
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Over the last 27 years riding my Santana, I've found the timing chain loosens quicker if the BB is low in the eccentric.
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