Sturmey-Archer CT-S80 chain tensioner - quick review
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Sturmey-Archer CT-S80 chain tensioner - quick review
This is one of Sturmey's more modern bits, but seeing as not everything C&V involves horizontal dropouts, I figured I'd post this short review - or analysis, rather. Also, I already took pictures of the thing and I couldn't post the pictures to the Amazon review...so here goes.
This tensioner happens to share the same limitations that have given its Shimano Alfine cousin a negative rap, but I have a feeling most people considering buying one of these (yours truly included) are not and were not aware that it (apparently) was designed for a fairly specific application.
Functionally, this thing is an identical copy - by dimensions, not appearance - as Shimano's Alfine tensioner. Thus, it shares the same limiting factor in its design: Despite the extensive upper adjuster threads for left-to-right adjustment, the pulley cage sits so far inboard that it is nearly impossible to space the tensioner outboard enough if you're working with a bike with a singlespeed or single-cog IGH hub. The B-pivot stop is also shorter than the threads, making about 4mm of what appears to be adjustment room unusable.
Keep in mind they don't come with spacers - the one you see installed is a modern Sturmey axle spacer, repurposed.
I tried to wrap my head around why both Shimano and Sturmey made tensioners like this, then it hit me: The CT-S80 and Shimano Alfine tensioners are designed exclusively for singlespeed conversions where an offset cassette hub is retained on a fairly wide rear triangle, e.g., 135mm. Problem is, both Sturmey and Shimano forgot to market them as such (my guess is Shimano's legal team has no intentions of letting the DIY crowd know what they can use it for, and Sun-Race just copied the thing without thinking).
At any rate, the rear cog is likely to be spaced inboard of the dropout a lot more on a cassette conversion with a bazillion spacers, while a bike with a purpose-built singlespeed or IGH hub will have a rear cog sitting fairly close to the dropout.
I might add that the upper body of the tensioner is a bit roughly finished under the already-rough (but acceptable) flat black finish. It's underwhelming, and given that this one is usually more expensive than its Shimano counterpart, not worth the extra cost.
-Kurt
This tensioner happens to share the same limitations that have given its Shimano Alfine cousin a negative rap, but I have a feeling most people considering buying one of these (yours truly included) are not and were not aware that it (apparently) was designed for a fairly specific application.
Functionally, this thing is an identical copy - by dimensions, not appearance - as Shimano's Alfine tensioner. Thus, it shares the same limiting factor in its design: Despite the extensive upper adjuster threads for left-to-right adjustment, the pulley cage sits so far inboard that it is nearly impossible to space the tensioner outboard enough if you're working with a bike with a singlespeed or single-cog IGH hub. The B-pivot stop is also shorter than the threads, making about 4mm of what appears to be adjustment room unusable.
Keep in mind they don't come with spacers - the one you see installed is a modern Sturmey axle spacer, repurposed.
I tried to wrap my head around why both Shimano and Sturmey made tensioners like this, then it hit me: The CT-S80 and Shimano Alfine tensioners are designed exclusively for singlespeed conversions where an offset cassette hub is retained on a fairly wide rear triangle, e.g., 135mm. Problem is, both Sturmey and Shimano forgot to market them as such (my guess is Shimano's legal team has no intentions of letting the DIY crowd know what they can use it for, and Sun-Race just copied the thing without thinking).
At any rate, the rear cog is likely to be spaced inboard of the dropout a lot more on a cassette conversion with a bazillion spacers, while a bike with a purpose-built singlespeed or IGH hub will have a rear cog sitting fairly close to the dropout.
I might add that the upper body of the tensioner is a bit roughly finished under the already-rough (but acceptable) flat black finish. It's underwhelming, and given that this one is usually more expensive than its Shimano counterpart, not worth the extra cost.
-Kurt
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Thanks, this makes me feel better about not buying one a little while ago.
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ZERO chance I would use it, the way it droops like on defailleurs.
Got to be something better with one pulley.
Got to be something better with one pulley.
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I ordered one of these; should have the necessary adjustment, though I'm not sold on durability.
-Kurt
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Why not use a rear derailleur?
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I used a RD on my IGH bike with a Shipmano Alfine and a CampI triple front.
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I wish I had read this before I bought it, lol. I’m having that exact same issue. At least I know not to spend any time trying to resolve it. It was more of an experiment than a need anyway. I have an 8 speed hub, so I can get by with 1 chainring on this bike.
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I've received the alternate one since. It has the necessary left-right adjustment, but I'm not entirely convinced. First off, it required some pretty liberal greasing to work properly - nothing from factory.
Secondly, the upper/B-pivot spring keeps it very far rearwards, as if they expected it to be used with a large cog. Results in very little chainwrap on the Twenty build I'm working with (and no, it isn't the brazed-on derailer stop that's the problem - it's at the correct angle). I might just re-engineer the upper pivot before I'm happy with it.
Come to think of it, why TF does a chain tensioner need a B-tension spring in the first place? That's what the pulley cage is there for. This thing doesn't need to ride over cogs. Have to figure out another solution.
-Kurt
Secondly, the upper/B-pivot spring keeps it very far rearwards, as if they expected it to be used with a large cog. Results in very little chainwrap on the Twenty build I'm working with (and no, it isn't the brazed-on derailer stop that's the problem - it's at the correct angle). I might just re-engineer the upper pivot before I'm happy with it.
Come to think of it, why TF does a chain tensioner need a B-tension spring in the first place? That's what the pulley cage is there for. This thing doesn't need to ride over cogs. Have to figure out another solution.
-Kurt
Last edited by cudak888; 05-12-20 at 07:15 PM.
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That does look like it would work a lot better with bigger cog. I have a 25T, but that’s probably still too small. I’m assume they don’t sell enough of these to make more than 1 size? I’m guessing you are going to need to shorten it?
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I could spend $50 and 3D print a new, fixed steel upper pivot, but at that price, I might as well get a Paul tensioner - which is exactly what I plan on doing!
-Kurt
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I spent a lot of time last night trying to modify it so it would hang further forward - and fixed, no moving upper pivot. Didn't work.
I could spend $50 and 3D print a new, fixed steel upper pivot, but at that price, I might as well get a Paul tensioner - which is exactly what I plan on doing!
-Kurt
I could spend $50 and 3D print a new, fixed steel upper pivot, but at that price, I might as well get a Paul tensioner - which is exactly what I plan on doing!
-Kurt
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This is a good application for a used Campagnolo Valentino derailleur.
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Please let us know how that works. Looks like a nice piece. When I held my older model SA tensioner in place, I really wasn't very happy with how smoothly the chain went through anyway. This looks like a much better alternative to just putting a derailleur on, which I really don't want to do either.
Gotta put it in the ISO thread and hope something pops up.
-Kurt
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I just think this is too much expense and thought and trouble for something so simple. Or is it?
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Also, the concern over "expense, thought, and trouble" long flew out the window for this build
Yes.
-Kurt
Last edited by cudak888; 02-09-22 at 10:10 PM.
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Aha.
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Sorry to disappoint everyone but @noglider, but...
The limit screws on the Dura-Ace 7900 have enough travel to lock the RD off in both directions while set to perfect chainline.
-Kurt
The limit screws on the Dura-Ace 7900 have enough travel to lock the RD off in both directions while set to perfect chainline.
-Kurt
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Sorry, but
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It actually looks badass.
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And some 2 years later, I decided to put a double chainring upfront on a bike with 126 OLD with an an 8 speed IGH. I remembered this thread, as I too had previously failed to use the Sturmey Archer CT-S80 because it wasn't even close to lining up, and was just going to put a RD on it. Oddly when I held a RD up to the hanger, it looked like I was going to have to put an awful lot of turns on the limit screw to make it line up, and probably need a longer screw. I dug the CT-S80 out, and to my surprise it lined up perfectly. My best guess is that chain tensioner is made for 126 OLD? I think the original bike I tried it on 2 years ago was 120 OLD, which might explain it not lining up. Seems like it was off by more than 6mm, or would it be 3mm (not going to think that hard, as you have to factor in dish, etc.). Someone previously mentioned they thought it might be for a single speed 130-135OLD, so I may just have gotten lucky this one time. Just thought I'd update in case anyone else reads this with that same issue. BTW, I've put about 100 miles on this setup, and so far the CT-S80 seems fine.