If you've been reading my little cycling blog here for a while, then you may remember my skipping chain issues last spring, including an epic near miss and a really nasty bike crash. Tachikoma, the bike which was giving me a problem, has been sitting idly in the stable for some time now, but I recently fully disassembled it. It during this disassembly process that I realized what root of the chain skip problem was. It was chain stretch.
Chain StretchWhen you hear of 'chain stretch' you might picture the side plates of a chain being stretched lengthwise. Of course this is not the case. Chain stretch is caused by the internal rollers of the chain rubbing up against whatever-it-is they rub up against and the chain internally milling itself, causing slack inside of each link. The primary causes of chain stretch in bicycles are a lack of lubrication and grit, sand, dirt and such inside the chain. This is why it is so important to keep your chain lubricated properly with a chain-specific oil. WD-40 only strips existing lubricants off of chains, due to the bicycle chain's internal movements, pressures, and workload. This exacerbates the problem considerably.
Anyway, check this out:
You can click the image for a higher resolution picture. What we're looking at here are two bicycle chains. The one on the top is a brand-new chain, the one below is Tachikoma's old chain. On the top, at the 13-inch mark, is the 26th link of the top chain. On the bottom, you can count 27 links. That's a nearly 4% difference--chains should be replaced when it is a .5% difference. With there being an extra link every 26 links on the old, worn, stretched bike chain, and with a large chain ring usually having between 52 and 54 teeth, it's not surprising at all that the chain was skipping! All road chains are exactly ½ inch between each link axis, so it's easy to tell if you chain is significantly worn or stretched without taking it off the bike--just hold a tape measure up to it.
Chain maintenance, especially regular lubrication, is essential to keeping your bicycle running smoothly and safely. Take it from me, I have the scars to prove it. If you want to know more about bicycle chain maintenance, check out Sheldon Brown's Chain Maintenance page.