chucky

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:
telescoping_CB.jpg
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 Mechanics forum.

prooftheory

Could you put a jack between the lip of the telescoping arm and the bottom bracket itself? This seems like a hard problem.

chucky

I could, but I don't want to carry a 5lb jack on the road for changing flats, etc. I find that, if ridden to disintegration, chains require frequent tensioning at the end of their lives.

Another possibility is to tie the back of the frame to a post with a shoe string to allow me to pull the telescoping arm with my body weight while tightening the pinch bolts, but I suspect the best solution will simply be in good technique. After all, don't the eccentric BB and eccentric hub users face a similar problem?

Scrodzilla

I don't even understand what I'm looking at here:

homebrewk

srsly. Is that an Orbea frok?

ddeadserious

I am so confused.

cbresciani

Glad I'm not the only one.

prooftheory

I'm pretty sure its a recumbent, guys. Don't panic.

hairnet

fixie recumbent? DUE EEEET

jdgesus 

tando recumbo i hope

chucky

Yeah, don't panic. It's a recumbent (non-tandem):
carbon frame (built upside down for better geometry and chain clearance)
carbon fork
carbon seat
rear internal gear hub
front internal brake hub
niobium rims

Every detail is designed to my preference, except the chain tensioning method which I'm still not entirely pleased with.

rustybrown

half-link

hairnet

Kilo TT

Scrodzilla

Yeah, would you be able to use a half link to take out some of the slack?

prooftheory

The telescoping bottom bracket arm is more than sufficient to cover what a half-link would do. He is trying to get extra tension on the chain rather than simply trying to get a good fit.

Maybe, if you had a long bolt going through some bracket at the lip of the telescoping arm you could use it the same way as a jack but be lighter. If you did it right it could just stay in place.

I'm a little suspicious of putting a lot of tension on a really long chain though. Don't your chains wear out really quickly?

Scrodzilla

How about something like the Blackspire stinger?

WoundedKnee

"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"

I am pretty sure that is the opposite of correct. Also, won't sliding your bb back and forth change your leg extension?

chucky

Nope, they last forever. With good tensioning you don't have to get rid of a chain until it snaps in half regardless of how stretched it becomes. The chain tugs typically break before the chains do:


Hmmm....that's pretty clever, if only because it might be used to eliminate the possibility of chain/tire interference in order to use an even bigger/more-efficient chainring/sprocket combo.

I'd rather not get into that argument again. Let is suffice to say that it's the conventional wisdom on the matter which is "the opposite of correct". If you've experimented with it and found differently then I'd like to hear it, but until then I'm going by what I've observed in the real world.

The leg extension changes just like it does with an eccentric bottom bracket (8-10mm)...in other words not enough to make a difference in fit.

WoundedKnee

I was just testing you.

chucky

Ok, the guys over at the mechanics forum helped me figure out a good solution:

I'm just going to wedge a piece of balsa wood/dowel between the front derailleur tube (which I'm not using) and the front most pinch clamp. Notice that the higher up I position it on the FD tube the more it will push out the BB.

prooftheory

It seems like what you are saying is that you are okay with the chain stretching. But for most of us a stretched out chain is a worn out chain because it will either wear the cog/freewheel and chainring out or not fit them any more. A chain being worn out to the breaking point almost never occurs with with fixed-gear non-recumbents, both because the chain is relatively short and we tend to use 1/8 inch chains.

At any rate the wood wedge solution seems like a good one as long as you can get enough tension for your needs that way.

chucky

Yes that's what I'm saying. IMO one of the primary advantages of running SS is that it mostly doesn't matter when the chain stretches. If you have a sturdy chain-tug what happens is when the chain stops fitting the sprockets then you can crank up the tension to keep it from jumping until it starts to fit better again (ie until the tight spots wear down). That is, assuming you don't snap the chain-tug first like I did with the NJS one pictured above (now I use chain-tugs with thicker bolts).

Recumbent chains last even longer because the wear is spread out over more links, but most of my bikes have regular/short length 1/8" wide chains and I always try to wear them to the breaking point because I have a surplus of chain/cog longevity and a dearth of time to perform maintenance (so I trade what I have for what I need).

Every minute spent on maintenance is like a minute lost to slow riding. Lots of fast bikes aren't so fast once you subtract time spent in the shop, but I try to make my bikes as fast as possible overall.

I think it should. More than that and I'd worry about damaging the wheel bearings or the frame.

Scrodzilla

Huh? I know I'm opening a can of worms here but a "stretched" chain will cause a lot more wear on drivetrain components than a newer, properly tensioned chain and there really isn't any disagreeing with this. "Cranking up the tension until the pieces fit better again" is only creating even more wear.

I wash my hands of this thread.

hairnet

money well spent

it takes 5 minutes to install a new chain. A few minutes to prevent replacing a whole drive train.

also, lolwut?

FastJake

Let me see if I'm understanding this correctly. You run the chain super tight until everything is totally shot then replace the chain, cog, and chainring all at once? How long does your stuff last? I don't see how this is better than running it at the proper tension and just replacing the chain as necessary.

Also, the types of forces you're talking about certainly can't be good for your IGH, or the efficiency of your drivetrain.