<img width="200" height="100" class="floatleft" src="www.motorcycle.in.th/images/articles/Understanding-the-Wear-of-Chain-sprocket_1.jpg" alt="" />Not so long ago, somebody asked me 'why do the drive chain and sprocket assembles on motorcycles wear in a manner that produces tight spots and loose spots in the chain as it rolls?' You would expect that both the chain and sprockets wear out uniformly. Another frequent question is 'how much of a difference between the tight and loose spots means that the chain is worn out?'
There are several reasons that chains and sprockets can wear unevenly. First of all, you might think that the delivery of thrust from the engine to rear wheel is a smooth flow of power, but your driveline would disagree. The engine speeds up very slightly with each combustion stroke, then slows down until the next firing; and while those changes in speed usually are impossible for the rider to perceive, they impose ever-varying taut-slack-tut loads on the chain.
To a lesser degree, the same thing happens in reverse on trailing throttle as the rear wheel pushes the pistons through their compression cycles and makes the cams overcome valve-spring pressure.
<img width="230" height="154" class="floatright" src="www.motorcycle.in.th/images/articles/Understanding-the-Wear-of-Chain-sprocket_2.gif" alt="" />Next time you ride alongside a chain-driven motorcycle, watch the chain and you'll see that even when the motorcycle is moving at a steady speed and constant throttle setting, at least some part of the chain is usually fluttering up-and-down. With single-cylinder engine motorcycles that have almost two crank revolutions between combustion events, or V-twins with irregular firing intervals, the fluctuations are even more pronounced.
These fluctuations change according to engine speed, road speed, gear selection, throttle opening and riding style, and they are not synchronized to the ratio of the final-drive gearing. Consequently, the cumulative wear they impose on the chain and sprocket almost can't help being uneven.
Other factors compound this basic wear pattern. Water and dirt usually do not accumulate in equal amounts all around the chain and sprockets; a little more corrosive material in one place or another will cause uneven wear. Plus, after a ride when the chain is warm, some part of it cools on the straight runs between sprockets while the rest of it does so while wrapped around the sprockets. Depending upon the nature of any contamination that has accumulated on the chain, as well as the lubrication on or in it, that difference can easily cause a slight kink that usually goes away during the next ride but, in the process, still causes a little extra wear in those kinked areas. And once a wear pattern starts in one place on the chain or sprockets, the rate of wear tends to accelerate more there than on the rest of the chain.