Were we simply talking drivetrains, I could write this post in two words because only the main culprit will make a “clunk”. “Ticks and clicks” take care of virtually everything else in the drivetrain. The two non-drivetrain related “clunks” are a quick release skewer being loose to a point of being dangerous and a loose headset. “Ticks and clicks” cover everything else there as well. The diagnosis of a loose headset is simple: grab some front brake and rock the bike back and forth. Place your right hand on the headset while you’re rocking the bike. If it’s loose, you’ll feel the slop in the system. Loosen the stem bolts that clamp to the fork post, tighten your stem cap bolt till the slop is taken up, tighten the stem bolts, and you’re done. Quick release is even easier. Give your wheel a side-to-side wiggle. If it’s loose, you’ll feel it. Tighten the quick release before you crash into something and don’t ever let your bike get like that again. You should be checking things like that routinely before you ride.
The main clunker is your crankset and/or bottom bracket bearings.
Road bike cranks tend to be relatively simple affairs nowadays. The pricier models, such as the S-Works crankset on my Venge, is ridiculously simple and impressively light. The cheaper the crank, the heavier and more complex they get, requiring special wavy washers and cap washers to keep dirt out. Cranks that require wavy washers are notorious for letting dirt into the system. They require regular cleaning to keep from clicking and ticking while the pedals go ’round. Your Dura-Ace, Ultegra, S-Works and SRAM Red and Force lines don’t require as much fuss.
We’re talking about clunking, though. Not clicks and ticks. Clunking is caused by a loose spindle or bearing. If the crank bolt isn’t tightened enough (and they require a hefty amount of torque, just look on the crankset and it’ll tell you how much), you’ll eventually get a little clunk at the top of the pedal stroke as power is transferred from one crank arm to the next – often in the left arm.
Now, on one hand there’s the simple fix; tighten the bolt. On the other, I like to go a little further and pull the crankset out, clean it, lube it and put it back together. If I’m going to be in there anyway, I may as well clean it out and do it right. I’m going to have to deal with it sooner or later anyway.