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Bikight tensioner

Is it perhaps installed next to the wheel, in the inner part of the frame?

Picture of chain tensioners.

  • Without attempting this to be an answer: Either with an appropriate bolt on the rear derailleur hanger or in the absence of that, on the rear axle. – Carel Sep 30 at 17:38
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That device looks like a chain tensioner that replaces the rear derailleur when converting a frame made for a derailleur drivetrain to a single speed setup. Hence, it would be mounted with a bolt in the derailleur mounting hole.

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In an image search on the web I found plenty of product pictures of these tensioners. Only one however where it is mounted to a bike: chain tensioner product image from aliexpress

The image is mirrored, the drive side is on the right. The bike appears to be a Dahon-like folder. The tensioner is mounted to the rear axle inside the dropouts. This also explains why the idler wheel is on such a long bolt. The chain has to pass to the left (rider's point of view) of the fixed member.

For mounting the idler on a derailleur hanger the chain needs to go between the fixed and the articulated member. Since there is plenty of space, and the idler is shown in some product pictures in that position, it might be built for that as well.

As a typical blingy Chinese CNC made device it is very unlikely that you find useful documentation on the net. A search for similar images didn't find anything this might be a copy of.

  • The part is rather cheap and you might simply give it a try and see if you can modify it for your purposes. 2 out of 3 times I got something usefull. Machining was always good. Strength of CNC milled parts doesn't compare to cold forged Shimano, Campa or even Tektro parts. – gschenk Sep 30 at 20:08

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