Bicycle Mechanics - bottom bracket hum?

When I pedal, especially when going w/ a lot of torque on the chain ring, I feel a slight hum or buzz coming through the mechanism. I say feel, b/c I really can't hear it and it seems to be coming from the bottom bracket rather than from the chain or pedals, etc.

[I've noticed it on several bikes I've ridden in the past, but now that I notice it on my brand new Trek 2100, I'd really like to figure out what it is so I can talk to my lbs about the problem.]

Also, how can you tell when the BB should be adjusted or overhauled (perhaps I have just discovered the answer??)

some bottom brackets can't be adjusted, they simply just get replaced. you find these on low end bikes.

as for you strange sound, I have no idea without riding the bike myself. it's possible that the front derailer needs to be adjusted. if the chain is off just enough, when your torquing down on the pedals (while riding) it could emphasize that fact that the chain is slightly off on the front sprockets.

Thanks. I figured on my older bikes that poor basic quality was unable to withstand the force of my thunderous muscles. :D I don't reallly think it is the front derailleur as that is there is no external rubbing and the front derailleur doesn't seem to be involved.

Hmmmm I've noticed a similar type hum while riding my bike. I even took it back to the LBS and had them check it out. They put it on the stand, ran it through all the gears and even adjusted the the bottom bracket. He even took it for a ride and said he could not hear//feal anything. But <sigh> the noise is still there. It is speed sensitive on my bike. By that I mean the noise starts when i get going 14-15mph, below that no noise. And it will happen in either the middle or big chain ring as long as the speed is about 14-15mph. I thought at first maybe the knobby tires were causing the vibration//hum. (I ride my mtb alot on the pavement).

it only makes this hum when pedaling, correct?

Yep on my bike it only makes the noise when i'm peddling, and starting at around 14mph. I can hear it more when i'm peddling next to a wall or fence, somthing that bounces the sound back to me. My bike is still under warranty so I'de really like to find out what the noise is before it becomes a problem. My luck somthing will break the day the warranty runs out :P I'm not complaining about my LBS NOPE. I took one of the peddles back in (after taking it apart) and showed them the cracked housing where the bearings ride. They took another set off of one of the bikes on display put em on my bike no questions asked. :) Hoping for a peddle and tire upgrade from santa :) LOL Already puttin the hints in.

Sorry bout buttin in on somebody elses thread for information :)

It is most likely caused by the chain running over the sprockets - transfer of power is not perfectly smooth, especially since the chain is essentially a series of straight lines that approximates the curves of the gear wheels. I have occasionally noticed a hum similar to that described and this was certainly not caused by the chain rubbing anywhere through mis-alignment. The hum is probably more noticable at some speeds where there happens to be a resonance in the bike frame or by the chain behaving like the stretched string in a musical instrument.

I really don't think this is a mechanical problem at all.

Cheers,

Ed

I will have to check that out. A sympathetic resonance certainly does make some sense. It only seems to happen for me on the two smallest gears, and then when I am really cranking hard, not just leisurely pedalling.

Ed Holland I think that is what is happing you hit it right on the head when you said "resonance" in the frame. I could not for the life of me think what was happing until you described it that way, now its obvious..... THANKS :)