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I got a road bike frame with vertical drops, so can't set the tension up. Tried to remove one link but then the chain becomes too short. My next idea was to change the front sprocket, currently have 46 tooth sprocket.

The question is though: how much will each next tooth affect the chain tension?

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    (It is pointed out by the answer below, but I really want to highlight it once more) Not only you need to set up the chain tension once, you'll have to maintain it throughout the life span of the chain. I found that after a very short while (maybe two weeks of riding), a freshly installed single-speed chain may require its first additional tensioning as it breaks in. Aug 2, 2020 at 20:29
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    You need to get a chain tensioner. Aug 2, 2020 at 20:32
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    @DanielRHicks: 1) please stop answering in comments, 2) chain tensioner is a no-go on a fixie, those are for single speeds only. Aug 2, 2020 at 20:40
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    @NazarPasternak "will it at some point stop stretching?" — No, it won't. The stretching is wearing, and it simply is losing the metal on chain rollers which reduces their diameter. There are no known preconditions to completely stop it nor to revert it, lest you replace the chain with a new one (which will require retensioning). Aug 2, 2020 at 22:51
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    @GrigoryRechistov: the reason a chain tensioner is a no-go is because a chain tensioner can't handle the force of backpedaling, or of pedaling in your hypothetical double-tensioner setup. Aug 3, 2020 at 1:01

3 Answers 3

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The short answer, is you generally cannot make a fixie out of a frame with vertical dropouts.

Not only do you need to tension the chain, but the spacing of the axle of the bottom bracket and rear wheel varies, depending on your selection of cog and chainring tooth counts.

Adjusting tooth counts will allow you to try to fine adjust the spacing, but it won't always get you there. The formula for how adding or removing a tooth will change your ideal chainstay length is not simple. You're removing a link from the chain, but you're removing it at an angle, so your reduction in chainstay length will not be the same as the amount of chain you remove.

Online, you can find many magic gear calculators [example] where you can try to pick a ratio that will exactly match your chainstay length.

You can also use a special half-link chain element, to adjust the chain by half as much as an ordinary link.

With all that said, the difference between "slack" and "tight" is a very small amount of chain stretch, and fixies in particular are pretty touchy for the tension. What I describe above I think works well for coaster-brakes and single speeds, but I would not recommend it for fixies.

If you are especially willing to throw caution to the wind, you could file the dropouts to be slightly wider, which might buy you just enough space to make one of the magic gear ratios work. The risks to safety should be obvious but that's up to you.

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    "With all that said, the difference between "slack" and "tight" is a very small amount of chain stretch" — to add some numbers: just merely 1% of relative chain stretch is considered terminal for a drivertrain. 0.1% of stretch is clearly visible on a single speed bicycle without a tensioner as a chain slack. Aug 2, 2020 at 20:35
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    Thank you for your answer, @whatsisname, helped me get more understanding of the whole process. Also, is it an option to redigest the dropouts to the horizontal ones with the aluminium frame? Aug 2, 2020 at 20:55
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    @NazarPasternak even if re-welding the dropouts would have not compromised their strength (which is very likely to happen given it is an aluminum alloy), the cost of such procedure at an experienced welder with Alu-specific skills and equipment will certainly be higher than what a couple of fresh new steel fixie frames would cost. Aug 2, 2020 at 21:13
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If you cannot control the rear cog's position, you can try moving the front chainring to tune the chain tension. That is, get an eccentric bottom bracket:

Eccentric BB in frame

By rotating it in the frame, the distance between rear and front cogs can be tuned.

The same idea is achieved by eccentric rear hubs, e.g. White Ind. Eno:

Eno

The hub choices below offer those of you looking to convert a frame with vertical dropouts, a choice of eccentric hubs.

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    It might be worth mentioning the BB standards where this is reasonably doable. An ordinary 1.27" English bb shell this isn't an option unless you shell out a lot of dough for some exotic parts Aug 2, 2020 at 20:44
  • Oh, there are also eccentric rear hubs that will work with vertical dropouts! whiteind.com/eno Yeah, they cost ~200 USD apiece Aug 2, 2020 at 21:18
  • The first option seems appealing, will do a research on that. Thanks! Aug 2, 2020 at 21:29
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    "An ordinary 1.27" English bb shell this isn't an option" — so true. The BB housing on the picture above is huge in diameter, likely to correspond to an Ashtabula-sized system. There is not much space to create eccentricity inside a BSA housing for a BSA spindle (maybe offset external cups? nah, that sounds crazy complicated to align). Aug 2, 2020 at 22:57
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    @GrigoryRechistov There may not be much space in a normal bb shell for eccentricity, but you don't need much eccentricity either. You need exactly one half link's length worth of eccentricity, a quarter link's eccentricity if you are prepared to use a half-link chain lock. I have no clue what eccentricities are available for the normal bb shell, though. With the screw-in system it would be hard to fix the position of the bb anyways. So, I guess, nice idea, but very likely not useful. Aug 3, 2020 at 9:43
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A chain tensioner will work. But you've got to set and adjust it to ONLY provide the needed tension to keep chain at 1/4 to 1/2 slack. In other words the pulley on the tensioner is not stressed to the point of damage or breakage but flexes and moves with the chain without excessive travel or tension. I have done this for years and I've never had a problem it takes care of chain slack as the fixie chain begins to loosen it's all in the adjustment making sure that the tension is just enough so that when you apply back pressure to the pedals the pulley is not stressed but the chain remains at the proper tension. This info is for all you vertical drop cyclists out there by the way. The tensioner should be at 3 o'clock position approximately.

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    How does the chain tensioner work when you brake?
    – ojs
    May 1 at 11:31

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