Any ideas on why this happened, and so how I can avoid it. My friend has bought my old SR185. The other night, it threw the chain (just as she pulled up at the lights exiting the freeway!). I put it down to the fact that she's been pretty harsh on the gears while learning to ride a manual vehicle, combined with the fact that she hasn't once lubed the chain :? (or checked the (lack of) oil but that's another story). Thus the chain has stretched unnoticed, and fell off. Anyway, I tensioned and lubed the chain, presuming all was now right, except that it was thrown again the other night. Now the device for tightening the chain is a circular piece of metal, with notches in it. You pull this circular metal (on both sides) and the circle is shaped such that there is increasingly more distance between the centre and the outer side - ie it is not a true circle, but gradually expands in size as you turn it - and you tension it till the chain slack is good and a particular notch (numbered the same on both sides) sits/pushes tightly against a stub of metal. Anyway, the second time round, this tensioner came loose on the left side. I didn't do the axle bolt up too tightly, and I responded by making it quite tight (naturally cotter pinned both times) but I can't imagine that this would be a problem. While there was proper slack, at the same time the circular metal had sat quite firmly in place when I first tensioned it (it would not go any further without over-tightening the chain). Any ideas, as I can't make sense of what happened? Certainly it's not something we want to re-occur, esp at speed!, so I'd like to isolate the cause.