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1/8" chain and Sturmey Archer chain tensioner

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1/8" chain and Sturmey Archer chain tensioner

Old 05-10-21, 09:25 AM
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1/8" chain and Sturmey Archer chain tensioner

Replacing a worn chain on my electric cargo bike, and I went with a 1/8" KMC e101 (https://www.kmcchain.com/en/product/...1-single-speed) since it seemed like the best chain for the job and it's a tough job being an electric cargo bike that often is nearly 300lbs fully loaded.
The problem is, the bike uses a Sturmey Archer chain tensioner (https://www.sturmey-archer.com/en/products/detail/cts86), and turns out this chain tensioner is only designed for 3/32" chains. The 1/8" chain can move through it, but makes a clacking noise as the links hit the sides.

Is there a way to increase the spacing of this tensioner, or should I just replace it or run a narrower chain?

Last edited by robo; 05-10-21 at 09:59 AM.
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Old 05-11-21, 07:20 AM
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Presumably, the original 3/32" wide chain worked well enough for you to wear it out. You might be able to cobble something up to make the SA tensioner wider (spacers between the pulleys and the cage, perhaps?), but I suspect that a decent 5-6-7 speed 3/32" chain would work fine without any modifications.
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Old 05-12-21, 09:22 PM
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just go with a magic gear setup IF you don't have a mid-drive motor setup (i suspect you have a singlespeed setup and therefore a motor hub).
in order to use this you need to have the chainstay length in mm. very rarely is it a non integer value for a vertical dropouts frame so it's not very hard to get it in case you don't find the geometry numbers on the webs.

it's hard to explain how you can accurately measure the chainstay length using either a new chain or even a used chain (supposing you have a caliper at hand). it involves measuring a low enough chain slack - so you don't have too much error. then apply the a^2=b^2+c^2 thing. and if you have a chain (or different chains) that is not new then you can use a caliper. mount the chain so that you have a bit of tension in it, just hang some light load on one pedal. get the smallest possible distance measured and then the largest one (meaning some pairs of links more). then you get the actual length of the chain, after you do all this, having counted the pairs of links of that chain.

i have less than 0.1% chain wear after 5000 miles of pedaling with a singlespeed bike because i use a proper lube.
hebie chain glider

one more thing... if the electric motor is in the hub then you don't really need a 1/8" chain. chainline alignment is almost always impossible to get accurate enough for a 1/8" chain. the only advantage 1/8" chain has is being able to work with higher loads (useful IF having mid-drive motor with a geared hub) but then again... the frame should be rigid enough and the chainline should not be off by more than 0.1mm, otherwise you would have a discomforting noise and vibration. 1/8" chain is harder to clean/lube too.
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