Bicycle Mechanics - Bottom Bracket problem?

Over the course of say a 10 minute period of riding, I will get this "clunk" noise from the bottom bracket one or two times. It feels and sounds like one of the crank arms is hitting a kickstand (I dont have one). Neither the pedals or the crank arms needed to be tightened. Also, there is no play in either.

The "clunk" doesn't happen at any specific crank position, or any specific amount of pressure. Sometimes it will be after a short coast, when I start pedalling again. Other times it will happen during normal pedaling (not mashing or sprinting or anything). I dont feel a skip in pedalling resistance, it just feels like one of the crank arms has hit something metal (but something like a kickstand that will move out of the way).

I really can't figure out what the crank arm may be hitting, and due to my fairly noobish understanding of the mechanics of bicycles and troubleshooting I can only guess that it may be a bottom bracket problem.

Thanks in advance.

Are you sure the "clunk" isn't coming from the chain skipping slightly on the cassette? A slight mis-adjustment of the rear derailleur or worn cogs and/or chain can let the chain semi-disengage from a cog, particularly while coasting or if you back-pedal slightly. It picks up with a jolt when you pedal again and that may be what you are feeling.

Worn chainrings could also cause something similar.

Possibly a freehub problem as well.

Are you sure the "clunk" isn't coming from the chain skipping slightly on the cassette? A slight mis-adjustment of the rear derailleur or worn cogs and/or chain can let the chain semi-disengage from a cog, particularly while coasting or if you back-pedal slightly. It picks up with a jolt when you pedal again and that may be what you are feeling.

Worn chainrings could also cause something similar.

The "clunk" sounds like its coming from direcly below me, although I am not 100% sure that the chain isn't skipping on the cassette. In the past I have ridden bikes with lose kickstands. That is, they may sag down a bit after a bump or something, and on the next revolution, the crank arm would hit the kickstand pushing it back into its normal resting place. The "clunk" I'm hearing/feeling, as far as I can tell, seems exactly the same as the sagging kickstand "clunk".

Would there be any way to narrow down the possiblities without having someone watch the cassette until it happens (sometimes can be a fairly long period of time).

While the chainrings/cassette dont seem to be worn enough for the chain to skip, I'm certainly no expert and unfortunately there aren't any bike shops around here. Closest area that has some is a hour drive away and I dont even think they deal with used bikes, they're just new dealers.

I can take some pictures if you guys think it would be worth while.

Thanks again.

I say pull the crank and tighten the fixed cup (drive side).

I say pull the crank and tighten the fixed cup (drive side).

will I need any bike/crank-specific tools for this?

yep, you'd need a crank puller and a bottom bracket tool. We might charge about $10 if you had it done at the shop. And the knowledge that the drive side is reverse threaded. (someone verify that, I ALWAYS forget.)

I plan on buying some bike-specific tools with christmas money. Assuming the problem will be solved by pulling the crank and tightening the fixed cup, will it be alright to ride it until I'm able to get the right tools (< 1 month)?

Concerning which side is reverse threaded, in another thread someone said:

On both sides, if you start with the wrench pointing up, to loosen, you turn towards the rear of the bike.

(which would make the non-drive side reverse threaded)

I'm going to weigh in with Hillrider and Pete Hammer (always good company to side with by the way). I doubt your crankset is the problem. If it were, it would likely happen at the same place of the pedal stroke. Bike noises are goofy. They all seem to sound like they originate at your crank.

Actually, my first bet would be your chain. Try turning the cranks backward with your hand and watching the chain when it comes out of the rear derailleur cage. My bet is that you'll find a place where the chain has a tight link.

I plan on buying some bike-specific tools with christmas money. Assuming the problem will be solved by pulling the crank and tightening the fixed cup, will it be alright to ride it until I'm able to get the right tools (< 1 month)?

Concerning which side is reverse threaded, in another thread someone said:

On both sides, if you start with the wrench pointing up, to loosen, you turn towards the rear of the bike.

(which would make the non-drive side reverse threaded)I wouldn't put it off because it may be something serious and you may end up having to spend some big bux to R&R. Simple enough to test and isolate whether it's a crank problem, just separate teh crank from the rest of the drivetrain. Do this by removing the chain from the crank. On the bottom side of the chain, push it inwards towards the bike, use screwdriver so you don't get your hands dirty. As you push it inwards, spin the crank backwards, the chain will slide off and rest on the bottom-bracket shell.

Then hold the left crankarm (aimed towards the back) and push in and out towards the chainstay. Does it move? Repeat for right crankarm. Spin it by hand, how smooth are the bearings?

As for bottom-bracket cups, the drive-side fixed-cup is reverse threaded. To remove, spin it clockwise (looking from the right side). On the left adjustable-cup, spin it counter-clockwise to remove.

To put the chain back on, use a screwdriver and pull the bottom part of the chain down and out so it lines up with the small-ring on the bottom. Spin crank backwards to wrap chain onto small ring.

IConcerning which side is reverse threaded, in another thread someone said:

On both sides, if you start with the wrench pointing up, to loosen, you turn towards the rear of the bike.

(which would make the non-drive side reverse threaded)
No!!!! I was describing how to remove PEDALS, not bottom bracket cups. :eek:

Pedals have left-hand threads on the left side (non drive side)

Bottom brackets (English at least) have left-hand threads on the right side. (drive side)

Note that these components are exactly the opposite of each other. Be sure not to confuse the directions.

All of the previous suggestions are possibilities, but one that was not mentioned is the possibility of failing BB bearings, either from wear or from recent contamination. Have you done any riding in the rain recently? Sometimes the water gets past all defenses, including seals.

Does it move?

No. Both crankarms are solid as a rock.


how smooth are the bearings?

Smooth as silk. :)



No!!!! I was describing how to remove PEDALS, not bottom bracket cups.

Pedals have left-hand threads on the left side (non drive side)

Bottom brackets (English at least) have left-hand threads on the right side. (drive side)

Note that these components are exactly the opposite of each other. Be sure not to confuse the directions.

Oops. Sorry about that.


Have you done any riding in the rain recently?

No, but I Just got the bike (used) so the previous owner could have.

No. Both crankarms are solid as a rock.
Smooth as silk. :)Ok, I think you've ruled out the crank & BB as contributors to your clunking noise. Something that dramatic should've resulted in something you can feel by hand on your inspection. The irregularity of the noise also indicates it's somewhere else; BB problems tend to occur once per revolution.

Could be a kink in the chain so that it occurs only once per chain revolution, which doesn't coincide with crank or wheel revolution. But that would happen more often than once every 10-minutes...

Very odd, try to pay attention to all the factors when you experience this. Could be hitting a bump and getting a jerk on the chain. Or hitting a bump right when you're shifting and the chain could be flopping around and missing a cog. Could be debris being thrown up by the front wheel? Could be phase of the moon...

but one that was not mentioned is the possibility of failing BB bearings

I once took apart a bottom bracket that had an occasional clunk, and found one ball bearing that was sheared in half. It would turn smoothly as long as the two half bearings stayed out of each other's way, but when they got together wrong it went clunk.

Not that that's what's wrong with yours, 'cause I've only ever seen that once...just sayin'

Very odd, try to pay attention to all the factors when you experience this. Could be hitting a bump and getting a jerk on the chain. Or hitting a bump right when you're shifting and the chain could be flopping around and missing a cog. Could be debris being thrown up by the front wheel? Could be phase of the moon...

Definetly not because of a bump in the road. Also it hasn't happened during shifting either, but it has done it in at least 3 gears (i dont think i've ridden out of the middle chainring yet).

I suppose it could be debris, but I honestly dont think that's the case. The "clunk" is identical each time, I feel it in the pedals, and it makes a metal-on-metal noise.

I haven't been able to ride it much since I've gotten it. A few trips to the store and a couple of joy rides.

I may just order all the bike tools and such a bit early and use my old bike for now. I plan on taking everything apart (I need to know how to do this), cleaning, and regreasing. I'll probably order a new chain as well. Probably an SRAM.

I really appreciate everybody's help and will come back to this thread when I have a chance to go for a ride and can offer more information.

ctp. I'm imagining what a sheared bearing might feel/sound like when it goes "clunk" and that sounds like a reasonable explanation. If that's the case, I wont be able to find out until I get some tools anyway.

The smallest chainring is pretty shark-toothed so i'm guessing the previous owner did a lot of heavy loaded touring. The silky-smoothness in my mind may not be what you guys would consider silky smooth. I have been riding an old huffy from my childhood whose bottom bracket is not so smooth.

I took off the pedals cause the toe clips were hitting the ground and it seemed very smooth to me.

I have had a couple of bikes in that would do this if the drive side of the cartridge BB was just a little loose. You could not feel play with the rock test but if you pulled the cranks you could get maybe a quarter turn on the cup.