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I ride an SE Lager fixed.

How tight should the chain be and what is the best way of measure it? I have a chain tug so I can be pretty accurate in dialling in the right amount of tension.

I don't like to have much slop. Part of the joy of riding fixed is having a direct connection to the road. I like to have it adjusted so that there's no significant movement of the pedals when the bike is stationary. This often equates to about a centimeter of movement in the chain if I squeeze the top and bottom of the chain mid-way between the cog and chainring but it's hard to judge how hard I should be pushing to get that movement.

As always, Sheldon Brown has a lot of information, mostly about centering the chainring(!!) and recommends having the chain as tight as possible without binding.

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7 Answers

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The simple answer is, as Sheldon brown says, as tight as possible without binding. But define binding. The noises you described on your chain in your previous question were symptomatic of binding or of a worn chain. The description you've given there of how you run your chain tension is tighter than I would recommend.

To clarify the tensioning - When I replaced the chain I adjusted the tension. There was a very noticeable adjustment required. But then when I subsequently adjusted the rear cog I didn't adjust the tug from it's setting with the new chain. I agree that I might have changed the tension marginally just be removing and refitting the wheel.

1 millimeter of movement is too tight, in every case I've seen. The manufacturing tolerances on the components (cog, chain, and chain ring, ans well as the hub, and BB axle), would need absolute perfection to allow that tight of tension to function without damage or noise. I've never seen good enough all at once, and you would need to be amazingly lucky or diligent to find that mix of perfection.

Typical tension on a fixie or single speed is roughly 1/4" to 1/2", or 5-10 millimeters, when pressing on the chain from the top, and from one side. A single speed requires slightly less tension, because the bearings in the freewheel will bind, and it won't roll at all, where a fixie will just be noisy and prone to wear. Make sure you check for the spot where the chain is tightest, and do the adjustment there, as well.

You don't need to be that tight, and your drive train will still maintain good contact between pedal and wheel.

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I set it to 1 centimeter of movement, not 1 millimeter. So it's about right based on what you're suggesting. –  Mac Aug 2 '11 at 23:49
    
I agree, a centimeter is much more appropriate. Did I misread or did you mistype? I'm not finding the measurement you commented with before. –  zenbike Aug 3 '11 at 2:57
    
I don't think I ever mentioned 1 millimeter. I promise I didn't edit that out just to mess with you :) –  Mac Aug 3 '11 at 3:51
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Keep in mind that the tightness of the chain varies through the rotation of the pedals. However loose you decide is enough, the measurement is at the tightest point in the chain's rotation. Simply checking that it's within 1cm at some point can still easily leave you with a chain that's far too tight. –  Stephen Touset Aug 3 '11 at 3:59
    
Of course, it should be the same all the way around, but no cog is perfectly round, and, more to the point, the crank may be a hair off-center, or the chainring may not be bolted on perfectly centered. –  Daniel R Hicks Aug 3 '11 at 5:05
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I think you've answered your question - as tight as you can (I'd throw in by 'hand' because that would seem to feel right, once you use vices and so on, it's going above and beyond reasonable tightness).

Bear in mind that you need a little slack because the teeth on the two cogs won't match exactly so you need to allow for some give to line them up independently, but enough to pinch a small amount seems fine.

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Better a hair too loose than a hair too tight..

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That's a very useful generic answer; sometimes one way, sometimes the other ... ! –  Unsliced Aug 2 '11 at 13:50
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A loose chain is a fast chain.

The proper technique for getting proper chain tension is to pull the wheel back in the dropouts and tighten the nuts a little past finger-tight. Don't worry about alignment just yet. Rotate the wheel and "feel" the slack in the top half of the chain with a screwdriver until it's at its tightest point. Here, it should still have some slack in it. How much? Perhaps an inch of total vertical movement is a good ballpark. Essentially, anywhere between "falls off the chainring" and "binds even slightly" are all equivalent, but this is "enough" slack while still being tight enough to minimize latency when reversing pedaling direction.

Back to tightening. If the chain is not ideally tightened, loosen the drive side nut. If the chain is too tight at this point, push the front of the wheel to the left, so the drive side of the axle slides forwards in the dropouts. If the chain is too loose, pull the front of the wheel to the right, so the axle slides backward in the dropouts. Retighten the nut a little past finger-tight. Now your wheel will likely be out of alignment. To remedy this, loosen the opposite nut and push or pull the front of the wheel until it's aligned. Retighten and recheck the chain tension. Repeat this process until the chain is to the desired tightness and the wheel is perfectly straight in the dropouts. Crank the nuts down tight and you're done.

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I have a tugnut so for me it's more "tighten the tugnut, and adjust the non drive side position". Thanks for the comment that anything looser than binding is equivalent. So I just need to make sure it's not binding and then find the tightness where I'm happy with the latency. –  Mac Aug 3 '11 at 3:53
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Just keep Daniel Hicks' advice in mind and err on the side of looseness. You'll get used to the latency. –  Stephen Touset Aug 3 '11 at 4:03
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As Daniel hicks said and also mentioned at the top, you would need 100% perfection in all your drive components to run a chain tight tight. Its just not possible to have all your machined parts to match for an over tight chain, something will be off somewhere! As mentioned if its not rolling off, its tight enough!

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I probably run my chain tighter than I should but a measurable flex of 1 to 2 cm should be okay.

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I have recently adjusted my new bikes chain with 3 speed internal hub gears and slightly oval chain ring. The bike had the chain cranked up extremely tight and it didn't feel or sound right. I'm sure it would have damaged the bearings or damaged something in the rear wheel internal hub over time. I kept slackening the chain until I found no noticeable binding at the point where the chain is tightest due to oval chain ring. The chain will have some some movement about 10-20mm halfway between the chain ring and back wheel, with this movement I could feel the chain had no slack. To dial out the drive train binding I lifted the back wheel of the ground and turned the pedals by hand, I felt the resistance and could here it too. I dialed this binding out bit by bit. I believe this binding equates to increased mechanical loss and more wear and tear on moving components. I think there should be no tension in the chain and as little as possible slack.

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