Hi everybody, over the last 10 Years I had again and again Honda XR200 and some other Dirt Bikes, which are mainly driven on the Roads, in my Shop.
I observed the following Mystery: this Bikes have either O-Ring Chains or standard Chains Type 520, the Front + Rear Sprockets are not the same configuration (like:F=13R=42/F=14R=42/F=13R=46)
what I noticed is that the Chains lenghten during their lifespan but they seem to do this uneven, like when i turn the Rear Wheel the Chain will be very tight in one position and loose on another making it impossible to make an even Chain Tension adjustment.
What’s the reason for this, is there some mathematical explanation or what?
Thanks
Best guess: uneven wear of the chain.
Suppose you have a 30-segment subsection of the chain in which the pins/bushing are worn slightly more than the rest of the chain. When you position the chain so that this worn portion is hung between front/rear sprockets, you can move the rear wheel further back to take up slack.
Now rotate the rear wheel until this loose section of chain is wrapped around the rear sprocket. The looseness of that section is now entirely contained on the rear sprocket, and a less-worn subsection (also of 30 segments) now hangs between the front/rear sprockets. This 30-segment subsection is slightly shorter than the worn subsection mentioned in the previous paragraph. With the worn section wrapped around the rear sprocket, all of the tension from that free (unworn) section is taken up by the last bushing that’s in contact with the rear sprocket (and the last bushing in contact with the front sprocket). Now the chain will be really tight.
Second-best guess: uneven sprocket wear. If rear sprocket is less worn over one portion (say from zero to 180 degrees) than another portion (say from 180-359 degrees), then chain will be less slack when it’s being carried by the less-worn portion of the sprocket.
I don’t think there is a math reason for this because the relative position of the chain changes over time as you shift gears. My right leg causes more problems for me and both were made at the same time too. Maybe some extra dirt got on sections of the chain causing greater wear there. Maybe the bushings of the chain came from different lots. There certainly is a specific reason somewhere, but we’ll never know. And I doubt it relates to the bike mechanism. I’ve never noticed this phenomenon and I wear chains until they cause problems.
The torque you apply to the chain when riding stretches it. Sometimes more drastically in certain spots than others. Think about when you really hookup and let out the clutch with the throttle revved. This will stretch the chain. Then you have about 1/3 of the chain that is loose. If you happen to apply hard torque again in this area, the slop will have an effect on even more stretching.
Just my opinion having been riding for over 40 years… YMMV
later, Tom.
I don’t believe the chain segments undergo any significant tensile strain. I’ve always been under the impression that chain lengthens over time because wear increases the amount of slop in each pin/bushing fit-up (the holes become elongated over time into quasi-elliptical bores); more slop in those joints means that when all that slack is taken up, the distance between the distal two pins of a pair of adjoining links is increased, even though the absolute overall length of any given link doesn’t increase.
Bicycle chains develop the same slop over time (crappy fit on sprockets), and I’m quite certain that I never actually stretched them.
If I’m not mistraken, once uneven wear begins (and it probably began with the hungover worker on the assembly line), it will tend to worsen over time, unless you correct it,which you can’t. Rotating you tires helps to even their wear, but there’s no practical way to even out the wear on the chain and sprockets.
Given that you are on a trail bike, there is an issue over chain tension and swingarm movement which can result in a part of the chain being severely stretched, you do tend to need more slack on trail bikes than road ones, unless its one of the BMW trail bikes which has the piot poivnts and the drive points in the same axis exactly.
Thanks for all the suggestions, I am still not happy with all of them, I mean the Bike is moving all the time at different speeds, a lot of Pull-Pull action by stop and go but the Chain is always in a different position on the Sprockets, is it not? I have to admit that the Chains are probably of poor quality as we buy them here localy in the Philippines, most likely made in China.
Is the rear hub or crank bent? Are they out of alignment with each other?
IME, its usually the sprocket’s fault when you are not able to have chain-tension remain relatively uniform as wheel rotates. At least that is what I found as I replaced sprockets on a Husqvarna dirtbike years ago. Of course, the chain does wear out, but usually not as quickly as the sprocket does (ime). Next time you take off a sprocket that has been used heavily, roll it on a flat floor real fast - bet it bounces from being out of round 
I had same thing happening on bicycles (single-gear), but not to the extent that torquey engines produce.
And like outlierrn says, once the wear begins, it worsens almost exponentially, kinda focusing the wear on the already worn area(s).