Tandem Cycling - Eccentric Question
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Is it possible to always get the BB axle in the forward part of the shell,using a half link is needed? Or will some chainring sizes result in it being in the rear half? Can you change chainring sizes and end up with it in the forward half?
I'm getting ready to get a new frame which has a longer BB-BB distance. The 28.1 workrd for this, and on paper the new 29.1 should be 4 half links longer with the same eccentric setting.
Thanks
If I understand your question correctly, it really shouldn't matter where in the BB the eccentric is situated (e.g. either front or back half) as long as the length of chain is fitted properly.
If you're concerned with leg fitment over the small amount that the eccentric moves, then you should be able to adjust your seat on the new frame to bring it back to your preference.
The only possible concern might be if the chain stretched to the point where the eccentric being in the "front" half of the BB isn't enough to take out the slack. If that happens, then you would only need to remove a link or two to get it back in position.
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dvs cycles
11-09-09, 03:40 PM
Assuming it's not a deal breaker for you I'd just deal with it when the bike comes.
On my Santana even number teeth are optimal for excentric position and if I were to go to a 39 instead of a 40 the positon would change unless a halflink is used from what I've read on this forum.
bikeriderdave
11-11-09, 12:45 PM
Doing some mental math (er, geometry)...
If your bike has BBs separated at an even 1/2" increment (e.g., 28, 28.5, 29") and has even-tooth timing rings (38, 40, 42), the front eccentric should be in the middle of its fore-aft adjustment when the chain and 'rings are new. Because new timing chains typically need adjustment after very few miles as everything wears in, it makes sense to build a tandem frame with the boom tube just a touch longer than "nominal" (e.g., 29.1" instead of 29"). You'll still end up with the "BB axle in the forward part of the shell," but it might take a few rides to get there.