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Old 10-03-05, 11:29 AM   #1
robo
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This is a Sram PC-1 chain, a Chris King aluminum sprocket, and a generic chain tensioner.

I am wondering if there is enough chain wrap here. If i take another two links off the chain, the wrap will be better, but the tensioner will be very wound up, and i'm wondering if that will be a bad thing (long term damage? Excessive friction?)

Incidentally, it seems to me that this chain tensioner design is kind of bass-ackwards.. Wouldn't it make more sense for a chain tensioner to be tensioned the _other_ way, so that it would push the chain up. This would _increase_ chain wrap, and at the same time allow a shorter chain! (true, derailleur hangers have that little ledge for the spring to catch on to in only one direction, but i don' think that's a big problem. I can think of a couple easy ways to get around that..)

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Old 10-03-05, 11:47 AM   #2
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You whole setup doesn't look right aside from definitely not having enough chain wrap.
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Old 10-03-05, 11:59 AM   #3
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Quote:
Originally Posted by The Fixer
You whole setup doesn't look right aside from definitely not having enough chain wrap.
Why thank you! How extremely helpful!

What doesn't look right? There are no spacers or lockring because i haven't put those on yet, and the loop on the pulley is flipped around so i can put the chain on and off easily while i'm working on it. Other than that, i don't see anything out of the ordinary, except for the question about chain wrap.
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Old 10-03-05, 12:10 PM   #4
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Originally Posted by robo
Why thank you! How extremely helpful!

What doesn't look right? There are no spacers or lockring because i haven't put those on yet, and the loop on the pulley is flipped around so i can put the chain on and off easily while i'm working on it. Other than that, i don't see anything out of the ordinary, except for the question about chain wrap.
OK.......not enough chain wrap.
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Old 10-03-05, 12:22 PM   #5
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I just shortened the chain by two links (or do you call it one link? An 'inside' and an 'outside' anyway). The chain wrap is a bit better, but the chain is under so much tension that i can easily feel that turning the cranks backwards has a lot more friction.

Hmm... I wonder if i can somehow modify this tensioner to work the other way?
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Old 10-03-05, 12:22 PM   #6
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Does the 2 extra links rule not apply for single speed? Looks like too much chain to me.
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Old 10-03-05, 12:26 PM   #7
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you could run a bigger cog to have more teeth engaged.

if you put a 22 cog on there you would have the same number of teeth engaged as a 16 tooth cog in a traditional non tensioner setup.

Or get a melvin

that setup looks like definate skipsville when standing up and hammering.
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Old 10-03-05, 01:19 PM   #8
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Wire-tie the tensioner to the chainstay to make a push-up tensioner. Oh, and shorten the chain as much as possible.

You may have to turn that little guide dealie on the pulley of the tensioner taround also.
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Old 10-03-05, 03:22 PM   #9
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I'm not familiar with this kind of setup, but it looks to me like there's a loop (cage) on the top side of the pulley wheel - shouldn't that be on the bottom? I guess what I'm saying is it looks like you have it installed backwards or something, the chain should make kind of an S around the pulley. That'd give you more chain wrap and the cage would also be useful whereas it's not now.
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Old 10-03-05, 03:58 PM   #10
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If you can't figure out a way to get it to push up on the chain, you can either replace it or use the zip tie method already mentioned. I'm not a fan of that myself, but it will work. At the very least, you should shorten the chain until the tensioner will barely fit between the rear cog and chain.

Is that tensioner sprung like a derailleur or is it fixed in position after you bolt it on? Just curious, mainly. At any rate, shorten the chain and set it up so that the tensioner isn't pushing as far down as it is. Or get a Surly singleator with a spring in it to push up on the chain. Just remember that you may have to play with your gear combos to get it to work properly. But, it'll push the chain up instead of pulling it down so it engages on fewer teeth.
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Old 10-03-05, 06:17 PM   #11
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Yes, i know the loop should be down and around the chain. It can be rotated around to any angle, and it's where it is so i can take the chain on and off easily while i fiddle with the tensioner.

This tensioner is indeed sprung like a derailleur, in the sense that it pulls the chain down.

Thanks for the zip tie suggestion - i was thinking of something similar while on the train this afternoon:
I was thinking of trying to rig up something with a small spring between the QR skewer nut and one of the holes in the tensioner arm, so that it would pull up. I could easily remove the internal spring in the tensioner (i already took it apart to see if it could be tensioned backwards, but that doesn't seem to be an option).
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Old 10-03-05, 06:37 PM   #12
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Something like this:

(sorry about my crappy drawing skills)
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Old 10-03-05, 08:50 PM   #13
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Quote:
Originally Posted by robo
Something like this:
Bingo!

You might get better results if you attach the spring to the chainstay, directly above where it is mounted on the tensioner.

That is a very good drawing.

EDIT: You have also found out the differences between the cheap tensioners, and a more expensive like the $50 surly tensioner. The surly can be run in push-up mode with just a simple spring adjustment, where your tensioner you have to jerry-rig a little dealie.

Last edited by BostonFixed; 10-03-05 at 08:57 PM.
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Old 10-03-05, 11:55 PM   #14
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Working on the bike at 1:30 am again tonight, and i think i have the answer: It's working like in my drawing, but using the tensioner's internal spring. It was quite simple really.. the tensioner only wants to 'tension' in a clockwise direction because of the design of the spring. The solution? Mount it backwards!

It works beautifully. The derailleur mounting bolt keeps the non-moving part of the spring housing in place quite solidly. The clearance with the chainstay is tight, but OK, although it looks too close in the photos (I might move the chainring to the inside of the crank spider to move chainline over a bit, although it will look ugly, so i'd rather not). The best part is that i now have even _better_ chain wrap than with a 'proper' SS setup.

The cheap tensioner folks should really have thought of this in the first place, though!
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Old 10-04-05, 07:06 AM   #15
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"The solution? Mount it backwards"....That's what you did the first time.
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Old 10-04-05, 10:05 AM   #16
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Quote:
Originally Posted by Don Cook
"The solution? Mount it backwards"....That's what you did the first time.

Umm.. no i didn't. Look more carefully. Not to mention that i had to take it apart and reverse the direction of the bolt to mount it backwards.

What's with the snarky one-line posts?

Last edited by robo; 10-04-05 at 10:31 AM.
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Old 10-04-05, 10:24 AM   #17
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Quote:
Originally Posted by robo
The cheap tensioner folks should really have thought of this in the first place, though!
Perhaps they should include supplemental instructions on this mounting alternative.
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Old 10-04-05, 10:28 AM   #18
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Quote:
Originally Posted by The Fixer
Perhaps they should include supplemental instructions on this mounting alternative.
Perhaps, but it's definitely a hack. The pin that holds the non-moving part of the spring housing in place is now sticking out into the wind (with just friction holding that housing in place), and because the arm is so close to the dropout, wheel removal will require a bit of wiggling (or just taking the nut off the QR before taking the wheel out.

What i really meant was that the company should have designed the spring to go backwards in the first place.

This tensioner design is pretty common. I've seen it sold in a bunch of places under a bunch of names, usually costing $20-30. Presumably there are a bunch of people riding around on converted singlespeeds with either massive chain tension and lots of friction, or a chain wrapped around 4 teeth on the rear sprocket..
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