Chain question...

[Max-M]

I run a #41 chain on my bike (to match the sprocket on its EZM Q-Matic drive). As seen in the attached photo, I also use a spring-loaded chain tensioner, with a 17-tooth #41 idler sprocket which smoothly runs on ball bearings.

My 56-tooth drive sprocket is mounted very concentrically to a Manic Mechanic hub adapter. And my chain is quite new.

But the difference in chain tension as the rear wheel rotates (measured on its upper run) is pretty dramatic; probably 1/4 inch of slack or less when tight, and 3/4" of slack when loose. When the bike is on its center stand with the engine running and revved up a bit, the chain tensioner bobs up and down pretty rhythmically as the rear wheel spins! When I'm slowing down from a full-throttle run, there's a definite pulsating sound from this variation in chain tension.

And as I mentioned, this happens with a very new chain. (And, I've experienced this with several new chains.)

My question is, what could be causing this variation in tension? The sprocket seems very concentric. And I know that chains "elongate" from wear of their pins and plates (often erroneously referred to as "stretch"), but this is happening with very new chains.

Does anybody have any thoughts on what might be happening? And/or am I getting over-concerned about the variation in chain tension that I'm experiencing?!

 

[Fabian]

The 10 tooth sprocket might seem concentric but it most certainly doesn't work out that way in practice.

This video gives an idea of how badly machined the 10 tooth sprocket is:

 

[Fabian]

With a brand new chain, the tension will go from, tight as a drum to having half an inch of slack, through each revolution of the 10 tooth drive sprocket.

It seems impossible because the sprocket doesn't 'visibly' seem to run out of true.

 

[Max-M]

Thanks for your input, Fabian.

I've got a Huasheng 4-stroke, with an EZM Q-Matic drive. So in my case, there's a pulley attached to the engine's driveshaft (which attaches to a pulley on the drive's centrifugal clutch, with an idler pulley in between). The clutch used by the Q-Matic is a heavy-duty, American-made Max Torque model: http://www.maxtorque.com/

I'd like to think that the 10-tooth sprocket on this clutch is properly machined. So I imagine that the 56-tooth drive sprocket may be the Chinese-made culprit. Possibly due to less-than-perfect drilling of its three mounting holes. I've never had another rear sprocket on this bike, and when I find one sized for #41 chain and install it, it'll be interesting to see if my varying chain tension issue persists.

 

[Fabian]

I was talking about the 10 tooth sprocket that is attached to the standard Chinese 2-stroke bicycle engine.

I have no idea about your situation with the 4-stroke engine etc. Forgive me for making the assumption that you were using a standard Chinese 2-stroke bicycle engine.

 

[Ludwig II]

Something else which bothers me about these small engines is the use of sprockets with less than 12 teeth, which I've always regarded as a minimum number for smooth operation and minimising wear.

How long, eith in miles covered, or operating hours, do the 10s, 11s etc last, and how does the chain hold up?

 

[Fabian]

the 10 tooth sprocket on the Chinese 2-stroke bicycle engine lasts forever - i got 6,000 kilometers out of a sprocket and it wasn't badly worn, so it could potentially have a life span of 10,000 kilometers, powering a SickBikeParts shift kit.

What is surprising is the the eccentricity or maybe acentricity is not reduced with wear. I would have thought that over 6,000 kilometers of use, the gear would have worn itself to some level of concentricity but this was not the case.
With a new 10 tooth sprocket or a worn 10 tooth sprocket the chain tensioner wildly bounces around with equal enthusiasm.

 

[Max-M]

No problem, Fabian. Just this morning, I believe I've found the thick #41 56-tooth sprocket that I'm going to buy as a replacement for my current thin one. The new sprocket will require the drilling of three hub adapter holes, but I'm glad of that because I'm going to have a good local machine shop drill them, assuring accuracy of at least that spec.