SoSmellyAir

On my 5 year old Trek FX 2 hybrid bike with fairly low miles, when I am pedaling, I can hear (very softly) and feel (very slightly) a clunk in the bottom bracket once every crank revolution. No clunk when freewheeling. The crank spins freely in both directions. For example, if I flick the pedal in the backpedaling direction with my foot, it spins multiple revolutions with no clunk. Also, no play in the bottom bracket. I read that this bike has a English threaded bottom bracket. Any idea what (if anything) is causing the clunk? Thank you in advance.

rossiny

The pedal? Are you sure its in the crank or bottom bracket?

Gresp15C

I fixed a bottom bracket noise recently on my daughter's bike, though it's an ancient thrown together single speed. First thing I did was to isolate the noise by flipping the bike upside down (don't worry, flat bar bike on grass), and put my weight on the pedals. Then turned the crank 1/2 turn, and put my weight on the pedals again. Long story short the BB was simply a bit loose. The amount of play was so miniscule that the only way to really notice it was for one of us to feel it move with a finger while the other one pushed on the pedals.

Another possible cause is a loose pedal. Clunks have a habit of seeming like they come from one place when they really come from another, so a general overall tightness check on the whole bike is probably justified before doing major surgery.

The downside is that you need two special tools to get at it, so: 1) It would help to be really sure that you've isolated the problem. 2) Depending on how handy you are, well, a bike shop will have those tools and can make swift work of it, assuming I'm correct about the cause, and if I'm incorrect, they can probably set it right anyway.

SoSmellyAir

Yes, I think so. This clunking has persisted over multiple pairs of Race Face pedals; I recently swapped from Chester to Ride pedals so I can focus on riding with my son without gashing my shins.

SoSmellyAir

Thanks. l really don't think it is the pedals. (See my prior post.) I guess I will bring it to my local Trek shop where I had bought the bike.

Bill Kapaun

With that group, you'll have a lower level BB such as a UN-26.
I don't mash, but my UN-26 in my hybrid is starting to feel "grainy" w/ about 1800-2k miles.
Your may be simply reaching the end of its useful life.
Ride it until it gets worse & change.

SoSmellyAir

That is what I have read somewhere else too.

What do you mean by grainy? Like there is fine grit in your bottom bracket that is causing friction? I am not experiencing any friction.

The hybrid has really low mileage (probably < 1,000 miles), so hopefully not.

Yup. I have neither the skill nor tools to be messing with bottom brackets.

Bill Kapaun

By "grainy" I mean it's just "less smooth", but not "chunky" which is really bad.
It's still got a "few" hundred miles left in it. maybe a lot more.
I'll just ride it until it gives actual problems. Not mental ones.

SoSmellyAir

The more I learn about bikes, the more of the latter (i.e., mental problems) I get.

pdlamb

Assuming the bike hasn't been laid down on its side in the sand (you don't do that, do you?), my experience would be that you've got many more miles in the BB. I'd go over everything that could loosen: pedals, crank fixing bolts, and then check the BB tightness itself. Of course if you get to the bottom bracket, you'll then have to re-re-tighten the crank bolts.

Litespud

check that your drive-side crank arm isn’t catching your front derailleur cage. Look for a mark on the inside face of the crank arm

curbtender

The first thing I check is if the cable end got mashed down and is touching the crank. It happens.

Mr. 66

Chain ring bolt maybe

SoSmellyAir

All good suggestions, and I have checked those points this morning to no avail. It is a low pitch clunk which I could feel.

Sonofamechanic 

Ok.....I bet one of your crank bolts is loose— a “thunk” is exactly what you feel. Grab one crank in each hand and pull/push them in opposite directions from each other—if there is even a slight slip that’s your heavy clunk.

SoSmellyAir

Maybe? I tried tightening as much as possible using a 8 mm metric hex key. It is part of a set that is built into a single plastic handle, from which you can flip them out (usually one at a time). I kept the 8 mm key at a 90 degree angle to the handle to improve the leverage, and I was able to tighten each crank bolt by about 1/16 of a turn. The "clunk" feels like it got slighter, but that could be my imagination (because I just came home from a ride on my road bike). I do not have a 8 mm hex bit so this is about as much as I can tighten these bolts.

I tried this both before and after doing the above but did not detect any slip.

Sonofamechanic 

What if you go the opposite way...loosening them up a bit and see if that same clunk becomes more pronounced. Then just remove the cranks and grease the connections—re assemble and tighten.

SoSmellyAir

That sounds like a good suggestion. After watching a YouTube video on how to remove a square tapered crank, I think this has to wait until the weekend. Thank you.

SoSmellyAir

Bill was right! It was the cheap stock VP bottom bracket. I replaced it with a Tange Seiki LN-3922 (made in Japan!) bottom bracket as part of a 1X conversion (Converting Trek FX2 to 1x - Bike Forums) and the problem went away.

Sonofamechanic 

brilliant! Thanks for sharing the final conclusion. Good to know for next time.

SoSmellyAir

Thanks. Hopefully no next time for me.