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Sprocket Dramas 250 Ninja

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by gee-aye-dee, Nov 13, 2013.

  1. Having a bit of a mechanical drama atm though with the front sprocket i bought. Some of the teeth are 3.5mm wide at the tip and some are just over 1mm at the tip. O.D is the same but it seems this difference in tooth width is causing the chain to go tight in some spots as the rear wheel is turned. Similar to a stretched chain (chain is new). Am i right i thinking the sprocket is the cause? or could i have a bent output shaft or something?


     
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  2. Is this a new sprocket? Take it back, or throw it away. It's crap.

    Badly formed teeth are not so uncommon in really shit sprockets, but of greater concern is eccentricity (where the hole is not exactly in the centre)

    If the ever so thrifty owner fits one of these, and is not careful to adjust the chain slack at the tightest place, it is a recipe for a bent output shaft. At best, you can get away with it, but expect the sprocket to not last for nuts and cause increased wear in the chain and rear. It will be rough and noisy in use and puts additional stress on your output shaft, the seal and bearing behind.

    Always buy really good quality sprockets and chains and replace them in sets.

    To check whether the output shaft is bent, take the sprocket off and rotate. It should not wobble as it rotates.
     
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  3. Bought a new sprocket from kawa dealer today. Adjusted it up and it is still a little tighter in one spot than the other side of the chain but not nearly as bad. before it would go from normal to almost as tight as a bmx chain. Now it probably gets a few mm tighter at most. From what ive read, i take this as somewhat normal?
     
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  4. #4 jstava, Nov 13, 2013
    Last edited: Nov 13, 2013
    Is it the sprockets or is it the chain? Chains wear, but don't actually stretch. They can wear unevenly if the lubrication is lost and replaced by grit in a section of chain. Mark the chain rotate, watch. If its always tight/loose when the chain is in the exact same position, it is the chain. If the tight spot moves, it is one or both of the sprockets. (and possibly the chain) If only one sprocket is eccentric then a front sprocket eccentricity alone will show up as several tights corresponding to the ratio of the number of teeth on it to the number of links in the chain in one revolution of the chain. If its only the front sprocket then this will be a number like 6 or 7, if it is only the rear, it will be a number like 3 or 4. If it is both, it will be inconsistent over several revolution of the chain.

    The danger to the output shaft occurs when the sprockets line up with their eccentricities in opposition, both pulling the chain tighter at the same time. You will be lucky to find this by turning the chain through only one or two revolutions. Most people, in adjusting their chains, don't explore this much, turn it around once, find the tight spot as they see it, and don't ever see the tension when it is at the maximum it can be at when the sprockets happen to line up in a particular way. This could take many chain revolutions, as the sprockets turn at different rates. They find the tightest spot (faith in the roundness of the sprockets, thinking they are always spot on, and its always a stretched or worn chain) and the output shaft finds the "tight spot" for them after they ride off.

    Its a bit of a can of worms. Also it is the best rationale going for replacing chain and sprockets as sets, with quality stuff, never pressure washing the chain, exposing the o-rings to strong solvents during cleaning and being positively anal about chain lubrication. Oh and if you need to get the tension a little wrong, a little loose is way better than a little tight.

    Another thing. Do not necessarily assume that the OEM product is the best there is. After market products exist (at least the better ones do) because it is possible to improve on the brand named product. The cheap aftermarket ones exist because it is possible to manufacture to a price, rather than a standard. You usually get what you pay for.
     
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  5. Yeah, i think i got what i payed for with the first lot. I had all 3 as a set and the front sprocket had major quality issues. The rear was actually a good item but now i am second guessing the chain also just because "if the other parts were that crappy can i trust the chain" i guess we live and learn and a few dollars wasted is better than a lacerated leg from a broken chain.

    Also the dealer sold me a JT sprocket i assume for the simple fact they are probably cheaper than oem as a single piece unit i.e w/o the fancy rubber dampener.

    I did take the chain through a few revolutions this afternoon with the new front and it was good. But ill make sure to mark it and check for consistency.
     
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