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I would like to dicuss PROS and CONS of 2 method for chain tension for single speed.

  1. Chain tensioner with Bolt (left upper and left lower image).

  2. Chain tensioner with a kind of spring + arm + roller (right image).

What comes to mind, that 1 method is more minimalistic and more "esthetic". While its cons, that each time when chain stretches a bit, you need to adjust bolt and axle position and maybe also to adjust bit a V-Brakes.

What are CONS of chain tensioner with spring + roller ? Is there any other considerations ?

tension with bolt tension with spring + roller bicycle with bolt tensioner

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    One downside of sprung chain tensioner is it will not work with a fixed gear bike. The main upside of such a tensioner is it allows you to run a single gear on a bike not designed with that in mind (e.g. no track dropouts).
    – shox
    Apr 26, 2023 at 17:23
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    @shox, on the middle image spring tensioner is attached to track dropout.
    – Michael D
    Apr 26, 2023 at 17:26
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    yep, but it doesn't have to be on a track dropout. I thought we were talking in a general sense not just on bikes with track dropouts. To me it's weird to do that rather than tension with the track dropouts. On kids bikes and many bmx no tensioner is used, just pull the wheel back and tighten. Another possible benefit of the spring tensioner is you are able to run the shortest wheelbase possible and still have tension on your chain. Personally I would only use a spring tensioner if that's the only option. The tensioner can bounce around and gives a much larger chance of the chain coming off.
    – shox
    Apr 26, 2023 at 17:51
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    I don't think the tensioner is necessary in any of these photos. Back in the day when road bikes came with horizontal dropouts a QR was enough to hold the rear wheel in place for the strongest sprinters and a nut has much stronger grip.
    – ojs
    Apr 26, 2023 at 18:44
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    Terminology: Your "chain tensioner with bolt" is also called a tug nut. The purpose is to help with alignment and minimising the chance of slipping. The axle nuts do the main work of holding the chain via the wheel/sprocket.
    – Criggie
    Apr 26, 2023 at 19:46

2 Answers 2

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You should never use a sprung tensioner when you have a choice. They add fragility and complexity. They exist only to allow single cog drivetrains to work with dropouts that don't allow chain tension adjustment (vertical or thru axle dropouts).

It's true that sprung tensioners adjust dynamically as the chain wears. However, only neglect would make this a problem otherwise.

Sprung tensioners also make it a non-issue if the cog, cranks, ring, and/or hub selected have poor concentricity, which otherwise creates variance in chain tension. Components that have problems like this are poor quality and should be avoided in the first place.

Slotted chain tensioners don't do the same thing as sprung ones. There is not an equivalency there. They are for creating extra security against slipping and also for easing installation of the wheel.

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  • "against slipping" means slipping of wheel from dropouts or slipping of chain from chainring ? In may case with "tug nut" the chain has variance in chain tension. I don't know the reason. All components except the rear cog seem to be of decent quality....
    – Michael D
    Apr 27, 2023 at 8:04
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    I went a long way to make my single speed on a frame without horizontal drop outs work without a chain tensioner ('magic' gear ratio). If you can avoid having a sprung tensioner, life is easier without. Especially in cities where you can put your bike's rear wheel in a bike rack without worrying of the tensioner getting entangled, bent, or snapping off.
    – gschenk
    Apr 27, 2023 at 9:36
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    'slipping' when the nut is not tight enough and very strong force on the pedals pulls the wheel axle forward in its slot. That's especially annoying when the left side still holds the axle and the wheel misaligns and starts to steer your bike right all the time.
    – gschenk
    Apr 27, 2023 at 9:41
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    @MichaelD Identifying and fixing concentricity problems in single cog drivetrains is its own topic. In some cases it can be fixed by means such as squirming the ring into a slightly different position (mostly applies to one piece cranks). Most of the time, the problem is simply that one of the parts isn't up to the task. Note that some variance is normal. Apr 27, 2023 at 17:23
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Brakes

If you have a coaster brake, then a trackend is the only safe way to tension the chain. Otherwise your braking efforts are initially wasted in the tension spring, which can be damaged by the straight chain lifting it too high. You can also build up speed and shock the chain.

Minor consideration - Your purple frame looks like a very small wheel folding bike - there are some fold designs that need a bit of slack to operate the hinges.

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    My bike is red with v-brakes. The purple is taken from internet, just for illustration. Thanks!
    – Michael D
    Apr 27, 2023 at 8:06
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    Also not possible with a fixie (which is a very similar situation to a coaster brake). Essentially the same effect when back pedaling.
    – shox
    Apr 27, 2023 at 15:55

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