Mystery Bike Thunk Debunked

Yesterday in the midst of my recollections of right-hooking drivers, I made mention of a couple instances where I was pedaling and from out of nowhere a pretty violent slippage and thunk of the bike’s drivetrain would occur.

What I failed to mention was that the drivetrain was in its second day of use. Over the weekend I’d replaced the maddeningly creaky 108mm TruVativ bottom bracket with a far quieter 107-mm one by Shimano; installed new 170mm cranks to replace the 165mm TruVativs; and also opted to drop in a new 48-tooth chainring, down from the 52-toother I’d been using.

So I knew the thunks were being generated from that part of the bike. Trouble was examination was somewhat exasperating because I found nothing remiss. Everything was tight. Nothing was warped or bent or kinked.

Then on the way home last night as I was cranking hard up an incline along the Ballona Creek Bikeway, the chain jumped off the rear sprocket, and the cause began to manifest itself.

See on bikes an important aspect of the drivetrain is something called “chain line,” which is almost self explanatory: it’s the line the chain makes between the gear on the rear wheel and the chainring. On road bikes with two or three chainrings and eight or nine gears in the back there’s always some derivation from a straight chainline as you gear up or down, but ideally on a single-speed (either freewheel like mine, or fixed gear) you want the chain line to be as straight as possible because an angle in or out will create stress on the chain and that in turn affects how it travels around the cog.

Knowing that, I considered whether the 1mm difference between my previous 108mm-wide bottom bracket and the new 107mm one would be enough to cause a problem, and I decided it wasn’t. Five, six or seven millimeters, yeah but one lousy thousandth of a meter? Not bloody likely.

So all I could do last night after loosening the rear wheel and reinstalling the chain on the freewheel was to take up as much chain slack as possible and hope that would help keep things in place the rest of the way home. It did.

Getting home I had planned on devoting some time to the solution, but instead I devoted time to the television. So it wasn’t until this morning that I took a good look and found that I was entirely at fault: I’d bolted the 48-tooth chainring on the inside of the cranks, instead in its proper seat — in effect providing that five or six millimeters of shift to the inside, which would explain why the chain dropped off on the wheel-side of the freewheel (as opposed to the frame side). It also explains the slip/thunks as being cause by near derailments wherein the chain managed to regain its grip on the freewheel.

No time to fix it, I’ll be riding it as-is this morning and at the end of the work day I’ll turn 8-Ball tires-up in my office, move the chainring to its proper place on the crank, and hope to enjoy a thunk- and/or derailment-free ride home.

UPDATE (10.2): As hoped my ride home was entirely uneventful.