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Old 09-09-11, 11:59 AM   #1
chucky
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calling eccentric/chain-tugless users: help tensioning chain

I've always preferred horizontal dropouts with chain tugs for tensioning my chains,

because I've found that in order to get the most life out of a chain it's essential to be able to get the chain very tight and to be able to do it easily on the road (in case the chain starts getting thrown).

However, horizontal dropouts also create well-known problems with rear brake and shift cable (for IGH) adjustment. So I built a new bike with vertical/upward-facing dropouts:

and a telescoping bottom-bracket/crank-bracket to tension the chain:

And the best technique I've come up with so far for tensioning the chain with this configuration is to hold the front wheel with my knees, pull the BB with my right arm, and tighten the pinch clamps with my left arm. But although this seems to work as well as horizontal dropouts without chain tugs, I can't seem to tension the chain as tight or as precisely as I can with chain tugs.

Does anyone have any suggestions on possible techniques or methods I might be able to use to better tension the chain with the telescoping bottom-bracket/crank-bracket? What techniques do users of eccentric bottom brackets or eccentric hubs or other chain-tugless setups (who seem to face a similar problem) use?

The following are not solutions:
1. Idler: can't provide sufficient chain tension. Also the whole reason I ride SS/IGH bikes is because I can't stand the other compromises involved in using idlers.
2. Use body weight to press the rear wheel into a tensioned position: Not possible due to upward facing dropouts...gravity is working against me.

Thanks for your consideration. I'm cross posting this over in the SS/FG forum.
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Old 09-09-11, 12:48 PM   #2
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I am an eccentric user, but you seem to be in quite the mind-boggling predicament. I would take a design cue from conventional chain-tugs: they utilize a threaded nut or bolt to both move the axle and to hold it stationary whilst tightening the axle nuts/bolts. If I were you, I would try to devise a threaded or lever system that could accomplish both of those tasks (move the bottom bracket to the desired position and hold it there).

I would probably build a lever similar to the Wipperman chain wear indicator with one end resting on the headtube and the other resting against the the upright tube protruding from the bottom bracket. The hinge would be pressed toward the telescoping tube and held at the desired tension with a toestrap or similar to allow the binder bolts to be tightened. Of course, this may only be practical for at-home use as the tool itself would likely be rather bulky. An idea to consider, at least.

Cheers
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Old 09-09-11, 01:27 PM   #3
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chucky: It would not be difficult to make a small jack with a piece of all-thread, some metal tubing slipped over it and a coupling nut - http://images1.mcmaster.com/Contents...g?ver=12220722 - to tension the assembly and push the bottom bracket to the desired tension. If you size things properly you should be able to reverse the tube over the all-thread and use another nut to hold the assembly together and clip or bungee it to the bike frame for transport.
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Old 09-09-11, 01:55 PM   #4
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Quote:
Originally Posted by lverhagen View Post
I would probably build a lever similar to the Wipperman chain wear indicator with one end resting on the headtube and the other resting against the the upright tube protruding from the bottom bracket. The hinge would be pressed toward the telescoping tube and held at the desired tension with a toestrap or similar to allow the binder bolts to be tightened. Of course, this may only be practical for at-home use as the tool itself would likely be rather bulky. An idea to consider, at least.
Thanks, that's a great idea. In fact, I think your idea has inspired me for an even better idea:
Since the front derailleur tube angles backwards it seems I might be able to achieve the necessary tension by simply wedging a simple stick/dowel/tube between the FD tube and the head tube because the higher the stick hits the front derailleur tube the tighter it will be wedged and, thus, push out the BB. This could also be rather light if I use, for example, a balsa wood dowel (cupped at the ends so it cradles the FD tube and the head tube).

It's amazing how many problems you can solve with simple objects. For example, I tensioned the headset on this bike with only a shoe string (and a seat clamp to maintain the tension)! Far simpler, lighter, and easier than a star fangled nut and top cap!

Thanks everyone!

Quote:
Originally Posted by lverhagen View Post
I am an eccentric user, but you seem to be in quite the mind-boggling predicament.
Even though it seems like we've found some good solutions, I'm curious, how do you get proper tension with your eccentric? I mean, yes it's true that the eccentric provides enough positions to tension in theory, but how do you overcome the difficulty of actually forcing it into the tensioned position?
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Old 09-09-11, 02:48 PM   #5
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chucky: I am not sure I understand your statement: "in order to get the most life out of a chain it's essential to be able to get the chain very tight". It would seem to me that the more tension you apply to a chain the faster it will wear. Of course you don't want it flopping around or skipping, either. Would you elaborate?
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Old 09-10-11, 12:13 AM   #6
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Originally Posted by chucky View Post
because I've found that in order to get the most life out of a chain it's essential to be able to get the chain very tight and to be able to do it easily on the road (in case the chain starts getting thrown)..
You're at odds with 100 years of experience in bicycle chain drive. During those 100 years it has been standard practice has been not tension chains. A correctly tensioned chain isn't tensioned at all but has a bit of slack left in the return (lower) loop. Some non-bike applications such as timing chains or machine drive chains need chains to be tensioned to avoid backlash or chain play, but this doesn't apply to bikes.

Your design should work fine, and allow you to pull out all but a bit if vestigial slack with no gymnastics. If you want to work hard at it you can tension the chain, but it'll rapidly wear to where there's a bit of running slack as there should be.
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Old 09-10-11, 04:23 AM   #7
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Your upward facing dropout seems to be a safety compromise. On normal dropouts, the nut prevents the axle from slipping but it doesnt support the weight of the bike/rider. On your design, the only thing preventing the frame from dropping away from the axle is the tension of the nut.
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