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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 12-27-06, 08:06 PM   #1
Krink
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Tensioner rant

I've read at least some of the helpful threads here about tensioners/idlers/singulators, thanks.

But, I still don't understand why tensioners are modeled after rear derailleurs--the two pulley type tensioners especially. Don't ders compromise efficient tensioning in favor of efficient shifting? Ders are hung right next to the cogs and force the chain to travel around relatively sharp angles to line it up with the cogs.

All the tensioner has to do is tension the chain, and usually only tense it a wee bit. Shouldn't the tensioner meet the chain at the midpoint between where the chain leaves the crank and the point where the chain hits the cog? It seems to me that the tensioner should be hung on the chainstay, so it can meet the chain in the middle of its run, creating very shallow turns and a line as close to the ideal as possible. On many bikes the chain crosses directly beneath the chainstay at a reasonably middling point, which would make hanging a tensioner even easier.

Also, I think the tensioner should pull the chain upward, so the contact point is above the idealized line of the chain, not below it. I could explain why, but this note is long enough.

I can't see that there is much downward pressure to deal with, other than the weight of the chain. And why do tensioners have teeth on the pulley, since there aren't any timing issues related to shifting...perhaps there needn't be a pulley at all...? It seems to me something simpler and lighter would perform just as well. Current tensioners point directly into the driving force of the chain. Hanging them perpindicular to the chain would seem a better alternative. Let the chain flow.

Any way, thought I'd throw out some raw meat to you mechanical experts. I know the brains at Surly, Melvin etc must be right, but I don't see why I'm wrong.
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Old 12-27-06, 08:59 PM   #2
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get a bike with track end or horiz drop outs and quit worrying your pretty little head
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Old 12-27-06, 09:30 PM   #3
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Have you seen the Rollenlager? It seems to agree with you on at least some points.
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Old 12-27-06, 09:45 PM   #4
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Just one quick thing about why tensioners aren't mounted in the middle of the chainstay...

You ever tried mounting ANYTHING to the middle of a chainstay?

Why invent a new system that's not guaranteed to work when you've got handy little holes already on the dropout?
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Old 12-27-06, 09:57 PM   #5
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i think you're right. make one and take pictures.
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Old 12-27-06, 10:00 PM   #6
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There was a tektro tensioner which mounted on the chainstays. It's somewhere on the Sheldon Brown website. There was also something about the reasons they were discontinued. It was apparently very hard to keep them there and keep them working.
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Old 12-28-06, 09:39 AM   #7
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I've seen at least one tensioner that mounted on the chainstay, it was basically a band of metal in a loop, a roller, and a screw to hold it in place. I forgot what company made it, tho.
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Old 12-28-06, 10:02 AM   #8
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That Rennen thing looks pretty cool except for the pulley.
But bolting something halfway down the chainstay is a recipe for a crushed chainstay and a slipping tensioner.
As for the pulley, if you ever heard the racket that a DH bike with those kind of pulleys makes...you'll know right away
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Old 12-28-06, 11:50 AM   #9
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Most tensioners mount to the derailleur tab because it's there, and it will keep the tensioner from trying to rotate. If you mount it in the middle of the chainstay and it begins to rotate around the chainstay, it will derail the chain. It will also scratch the finish on the chainstay.

Many tensioners use a smooth wheel instead of a gear. A gear will probably wear less quickly, and if you get a flat spot on a smooth wheel, it could cause some problems.

The simple matter is that, if the chain is the right length, it won't be making very sharp angles for the chain, so it's not really an issue.
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Old 12-28-06, 12:04 PM   #10
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Before the Surly tensioner came out I tried using a Kore downhill chain tensioner that mounted on the chain stay. It was designed to stop the chain from whacking the chainstay and keeping the chain tight for shifting. Bottom line, it sucked and didn't work.

I've had very good luck with the surly in the push up mode on a road bike.

To answer your questions about the 2 pulley type. I believe you are talking about Paul. Paul's tensioner is made to handle enough chain slack so you can use 2 chain rings in the front with one cog in the rear.
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Old 12-28-06, 12:04 PM   #11
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but op is correct since ive seen many tandems which have a tensioner along the chainstay. the difference is that the chainstay has a tab brazed on so the little tensioner will stay put and not spin around the stay.
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Old 12-28-06, 02:32 PM   #12
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I agree that the thingy would need a solid connection to the chainstay. Not sure how the chain would crush the chain stay. Luckily, my muscles are too weak to cause much damage to my bicycle. It must suck to a superhero, Fixxie.

Thanks for pointing to the picture on Sheldon Brown's site, that one is definitely mounted on the chain stay. However, it's not in the middle of the stay and it pushes the chain down. That's kind of like the boy who bragged about kissing a girl, but the girl turned out to be his sister and his mom made him do it.

I hadn't seen the toothless one, it probably would make more noise. Still noodling on the efficiency aspect. Back on the farm we would have just welded a pipe at the right spot for the combine chain to flow over and say "this may not surpass OSHA standards for sound dampening."

I agree the chain should be short as possible, but I don't see how adding a pulley or two in the der style can be done without adding more length than pulling up (or pushing down) at the midpoint of the chain.

Another advantage of mounting on the chainstay would be that you'd be able to peddle backward in fixed mode, eh?

I keep seeing the problem differntly despite these good explanations. Rather than fight the force of the chain head-on, and then dramatically divert the chain, I would avoid fighting the chain as much as possible and then nudge the chain just the little bit necessary. I mean, we're talking less than a link here, right?

As far as a mental model...tack a little thread spool onto a popsickle stick and play with it. I kind of think the chain would flow best if the stick could rock within a limit and the spool could swivel as moved by the chain travel. If the popsickle stick could also adjust up and down, you could use it for different combinations (though that would be more than one link...)

By the way, I like the way the Melvin one sounds, I'm just saying it's a mini-derailleur. I found a picture on MTBR of someone who used it with a triple crank. I'm actually trying to think up a low-maintenance light-weight 3-4 speed that doesn't use a geared hub.

Not that I'm prejudiced against hubs either. Some of my best friends are hubs.
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Old 12-28-06, 02:44 PM   #13
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Quote:
Originally Posted by Krink
I agree that the thingy would need a solid connection to the chainstay. Not sure how the chain would crush the chain stay. Luckily, my muscles are too weak to cause much damage to my bicycle. It must suck to a superhero, Fixxie.
I think he meant that the act of tightening a clamp onto the chainstay would crush it. Much like the clamp on kickstands tend to do.

Quote:
Originally Posted by Krink
I agree the chain should be short as possible, but I don't see how adding a pulley or two in the der style can be done without adding more length than pulling up (or pushing down) at the midpoint of the chain.
I just meant that, because of the short amount of excess chain, the angle of the chain going over, or under the pulley, is not sharp enough to cause a noticeable loss of efficiency. I don't think your arguments are wrong, i just think you are looking for a solution without a problem.
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Old 12-28-06, 02:57 PM   #14
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Quote:
Originally Posted by Retem
get a bike with track end or horiz drop outs and quit worrying your pretty little head
++

I once bought an apple and painted it orange. I couldn't peel it like an orange. It didn't taste or smell like an orange.

Should I:

A) Blame the Apple
B) Blame the paint
C) Get an orange to start with
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Old 12-28-06, 04:25 PM   #15
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So the Surly Tuggnut tensioner is just so you have a bottle opener attached to your bike? That's not a bad thing.
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Old 12-28-06, 04:39 PM   #16
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Quote:
Originally Posted by Retem
get a bike with track end or horiz drop outs and quit worrying your pretty little head
Unless you already have a frame with vertical dropouts

If you just want a single speed (not fixed gear), then the tensioners work fine.
I love my Rollenlager. Now I actually want to ride my old mountain bike
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Old 12-28-06, 06:29 PM   #17
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Quote:
Originally Posted by carleton
++

I once bought an apple and painted it orange. I couldn't peel it like an orange. It didn't taste or smell like an orange.

Should I:

A) Blame the Apple
B) Blame the paint
C) Get an orange to start with
Blame the paint, it wasn't orange flavoured. Also, if you did the paint job badly enough it'd look like orange skin too.

I'm getting a banana.
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Old 12-28-06, 09:26 PM   #18
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Quote:
Originally Posted by Krink
I still don't understand why tensioners are modeled after rear derailleurs--the two pulley type tensioners especially.
A two-pulley system gives a greater range of adjustment while still providing good chain wrap on the rear sprocket.

Quote:
Originally Posted by Krink
Also, I think the tensioner should pull the chain upward, so the contact point is above the idealized line of the chain, not below it.
That's true as far as chain wrap is concerned, and this is often an issue with singlespeed conversions. However that does make wheel removal/installation more difficult.

Quote:
Originally Posted by Krink
I can't see that there is much downward pressure to deal with, other than the weight of the chain. And why do tensioners have teeth on the pulley, since there aren't any timing issues related to shifting.
Because they use readily available derailer pulleys, rather than go to the expense of toolint up to manufacture a specific pulley. Derailer pulleys have teeth so that they can better apply lateral force to the chain to make it shift. The teeth do no harm.

Quote:
Originally Posted by Krink
..perhaps there needn't be a pulley at all...?
Any pulley-type chain tensioner on a non-derailer bike is an inherently mono-buttocked kludge. Straight chains are best and simplest. This technology was all figured out well over a hundred years ago.

See also: http://sheldonbrown.com/vertical

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Old 12-28-06, 10:52 PM   #19
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Did I say anything about hanging this tensioner from a vertical dropout bike?

That's the thing about apples and oranges: I like mangoes.
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Old 12-28-06, 10:55 PM   #20
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they used to have tensioners for a chainstay that you could use but theyve mostly stopped making htem.
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Old 12-28-06, 11:31 PM   #21
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so then why are you concerned with pulley type chain tensioner design if you don't have verticle drop outs
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Old 12-29-06, 02:54 PM   #22
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Quote:
Originally Posted by Retem
so then why are you concerned with pulley type chain tensioner design if you don't have verticle drop outs
First, because I like all kludges. Kludges put the humanity in the machine, and usually provide a story. When I saw pictures of the three speed Melvin (I'd call that a tringle) I felt like I knew the rider.

Second, because I live in a dream world with only a vague appreciation for mechanical reality. In other words, I'm like most people. I 'solved' my tensioner problem abstractly, then couldn't understand why the real world didn't solve it in the same way, and instead was only selling a product that wouldn't work for me. It's not easy to live with my brain. But I am glad to learn that my imaginary tensioner did once exist and may still exist in tandems. I'll look for that.

My other disappointment with pulley tensioners is that, like ders, they don't appear to disengage easily and completely. I'm guessing they are just in the way when not engaged. Is that true? My proposed tensioner should be easy to disengage--just move it down until the tension is lost. Fold it up against the chainstay.

I think your larger question is probably--why think about tensioners on horizontal dropouts at all? Aren't the dropouts enough to tension the chain?

As I see it, the recent history of bike design has Shimano evolving toward a 150 cog cassette and a chain the width of dental floss. In reaction, there are single-speed pietists with diminishing knee cartilage. In between are geared hubs, the Melvin, flip-flop wheels, cog/ring alignments. All seem like good ideas to me (Shimano and ss too), depending on what you want to do.

I'm just trying to dream up a kludge that retains one pure ss drive but compromises on purity to get wider gearing choices for other speeds than the options I've seen would allow. Not saying it will work. Then again, maybe such a beast already exists.

Even dumber--I'm dreaming a kludge for interchangable front rings...does that exist?
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Old 12-30-06, 04:50 PM   #23
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have you seen these?

http://cgi.ebay.com/NEW-A-MAZING-DRI...QQcmdZViewItem
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