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Eccentric BB

Old 03-01-17, 10:12 AM
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Tandem Tom
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Eccentric BB

Okay, this is a excersize in curiosity.
I see that Phill Wood make a product call Philcentric. Apparently it is a outboard BB that is also an eccentric. It provides 1/4" of adjustment.
How realistic is it to assume that this amount of adjustment will be enough to take up the slack on a chain in say a Rohloff hub converted bike?
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Old 03-01-17, 10:25 AM
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Well, when the chain is new the 1/4" of take up will be enough to tension a chain that has a 1/2" pitch. But as the chain wears this ability can become too little take up over time.


I have no experience with the BB in question but simple math and years of both tandem and shop experience suggests things wear and initial adjustments need redoing as wear builds up.


In theory a bike could be made with a chain run length (chainstay length) that is designed for a set cog/ring teeth count set up and thus the Bb would only be slightly wound forward initially. As the chain wore the Bb could be rotated further forward bit by bit as needed. But if the actual chain stay length and cog/ring combo has the BB already run out a bunch initially then further adjustments might get lost.


BTW are you familiar with half links? Do you know if your chain can accept one? There are BMX chains with each link a half ling design but I don't know if this type (pintle?) of chain will work on the cog/ring you have.


There is a reason that Rohloff bikes usually have a traditional eccentric BB, sliding drop outs or swinging drop outs. Andy
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Old 03-01-17, 10:46 AM
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Given that a little bit of sag is acceptable, desirable even on a non derailleur drive line, there is a very high probability that if the 1/4" of adjustment doesn't get you quite there, combining that with a half link connector would.
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Old 03-01-17, 01:50 PM
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Thanks!
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Old 03-01-17, 02:22 PM
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The worthwhile eccentrics are such as those on the front of a tandem.. Original Frame builder, not retrofits,

Your R'off retrofit, wife's bike.. will need a Chain-tensioner, My R'off Pocket Llama uses one...
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Old 03-01-17, 11:02 PM
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Originally Posted by fietsbob View Post
The worthwhile eccentrics are such as those on the front of a tandem.. Original Frame builder, not retrofits,

Your R'off retrofit, wife's bike.. will need a Chain-tensioner, My R'off Pocket Llama uses one...
Bob, you need to read up on the Philcentric before you comment:
Phil Wood & Co.
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Old 03-01-17, 11:18 PM
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To clarify
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Old 03-01-17, 11:29 PM
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Originally Posted by Andrew R Stewart View Post
Well, when the chain is new the 1/4" of take up will be enough to tension a chain that has a 1/2" pitch. But as the chain wears this ability can become too little take up over time. .....
To clarify, and disagree a bit.
While Andrew mentioned 1/2" pitch, I suspect he meant chains with a half link, since bike chains are 1/2" pitch.

That means that you can shorten chain by 1/2" instead of the normal 1", divide that between the upper an lower loops and you get the 1/4" travel needed.

While Andrew is concerned about stretch, it's manageable because should you start out with the BB all the way forward and the chain stretched, you could cut out 1/2" and adjust the BB to just slightly forward of the rearmost position.

BTW- On a well designed tandem, the distance between the BBs is set such that the eccentrics would be in mid-range.
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Old 03-02-17, 06:32 AM
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Thanks for the responses.
The idea came about as I am building up my wife's new touring bike with a Rohloff. I ride a Surly LHT and started wondering if there was a way to convert it with out a chain tensioner.
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Old 03-02-17, 06:41 AM
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Originally Posted by Tandem Tom View Post
Thanks for the responses.
The idea came about as I am building up my wife's new touring bike with a Rohloff. I ride a Surly LHT and started wondering if there was a way to convert it with out a chain tensioner.
Sometimes, you can get lucky without an eccentric. In the many gearhub conversions I've done, I have had a few with vertical dropouts where a workable chain tension was achieved without a tensioner.
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Old 03-02-17, 11:41 AM
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Originally Posted by Jeff Wills View Post
Bob, you need to read up on the Philcentric before you comment:
Phil Wood & Co.
Whole threaded BB shell is at most what? 1.36" across inside.. 1/4" of eccentric difference is not significant, considering this is just avoiding using a chain tensioner,

on a vertical dropout frame.... which is simpler and probably cheaper..

R off chain tensioner is low cost when bought in a package with the hub.



Paul and QBP and several others also make a chain tensioner..

a Frame builder can change the Dropouts. of course..




...

Last edited by fietsbob; 03-02-17 at 11:47 AM.
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