Wednesday, April 21, 2010

I felt like a real mechanic

Today I had to call a guy and tell him, "you need pads and rotors."

"There's a grinding noise in the rear brakes" the customer told me when he dropped the bike off. I guess there is. Somehow, the retraction spring that holds the pads apart had gotten sucked in between the pads and rotor. It had been ground into the mechanism, scoring the disc. Then the rotor had rusted.

Disc brakes bring yet another jigsaw puzzle to the world of bikes. Rim brake pads are all made on a few basic formats. Not so, the proprietary disc pads. We had several types of pad in stock. We just didn't have the right pad. Only replaceable derailleur hangers have as much pointless variety.


Ham said...

The question is, did you find a real mechanic, and if you did, what did you do with them?

Probably not the best preparation to asking if you have any idea what is making a clicking noise on my road bike. It comes from around the bottom bearing, and is associated with crank speed. Appears to come from the non-cog side, and can be heard once a revolution when the pedal is around 11 o'clock. doesn't happen without weight on the pedals and can't be felt. The sort of noise a piece of metal might make falling a short way, as if the cranks are hollow and there was a piece of swarf inside. But then, it doesn't happen at rest. Seen anything like that before?

cafiend said...

Yes, way to butter me up.

In answer to your final question, "Seen anything like that before?" the answer is "all too often."

Sound travels in carbon and aluminum frames. The rider can't always tell exactly where it's coming from. It's even hard for an outside observer to pintpoint.

Common causes of BB noise include cups or cartridge working in the BB shell (re-torque with grease or anti-seize) or worn bearings in the BB assembly (repack or replace depending on design of unit). The noise can also be in pedal bearings or indicate a pedal not fully tightened into the crank arm.

Oh wait, it could also come from one or more of the chainwheel bolts, the crank arm bolts, or a crack in crank or frame.

It could even be something silly like the seatpost collar on the frame or the bolt(s) holding the saddle to the post. Or a slightly loose seat rail.

Seatpost collars can make noise even when you're off the saddle, as the frame flexes. The front derailleur clamp can also make noise.

Replaceable derailleur hangers can also work against the frame.

All these are documented from repairs we've done. I've probably forgotten one or two.


Ham said...

Well thanks for that. The frequency suggests that it must be crank related. Shouldn't be wear, as it is only about 1500-ish miles on the bike. Time for a poke about.

Steve A said...

I had (actually HAVE) a similar sound on my commute bike. I consulted with the bike shop and they basically told me they could fix it if I wanted to spend a lot of money, or I could ignore it forever with no risk. I find that if I turn up the earphone volume a bit, it is much less irritating and I still can't actually FEEL anything nearly 4000 miles later.

Anything that is a SERIOUS noise will eventually become obvious as to cause and cure. Just make sure it isn't a carbon seat post which can spear you in the, well, we all have imaginations.

OTOH, cafiend is an EXPERT in mysterious noises, and he's mentioned most of the usual suspects in his comment, though I have never seen a loose front derailleur clamp myself.

cafiend said...

It's true. I AM an expert on mysterious noises and equally expert at denying I produced them.

The front derailleur clamp wasn't loose exactly. It was just the last thing we hadn't tried yet. After cleaning and re-tightening the front derailleur, the noise finally went away.

This fits with Steve A's contention that the noise is innocuous. It depends on how neurotic you are, and how neurotic your riding buddies (if any) may be. An annoying noise can be used as a psychological weapon.