jeffpoulin

I have a 2008 Trek 7.7FX which I use for commuting and day trips. In 8 years, I've put just over 40,000km (25,000 miles) on it. For the past few months, I've noticed a cracking (or crackling) sound coming from what I think is the rear of the bike. It's intermittent. The bike will make this sound for a second or two, then run quietly for a few minutes, then make the sound again.

I can't figure out what's causing this. I get the sound whether I'm standing up or sitting, pedaling or coasting. The sound gets louder and more frequent when I go fast (for example, riding downhill) which makes me nervous. Hitting a bump will sometimes make the sound too.

At first, I thought my rear hub was shot (it's an aftermarket wheelset with an XT hub). I tried a different rear wheel, but the bike still made the same noises. I checked all the bolts to make sure nothing was loose, but all the bolts were tight.

The bike has never been in an accident, but I've ridden it over plenty of bumpy roads and in all kinds of weather. The frame is mostly aluminum with carbon forks and seat stays. I suspect there may be a crack in the frame somewhere, but I can't find one.

My questions are: does this seem like a crack in the frame to you? If so, how would you try to find it?

CliffordK

I'd look at bearings and spokes, but it sounds like you've been evaluating that a bit.

So, the 2008 Trek 7.7FX has carbon seat stays + monostay. I'm not sure how you would determine if that is flexing a bit at the joints. Certainly look for scratching around the joints. Maybe putting some masking tape over the joints, you'll either see the tape moving, tearing, or bunching up. Or try painting something like a clear fingernail polish around the joints. If they are flexing, you'll see cracking in your enamel.

Rollfast

Pictures would probably help, or at least you'd find a crack first.

jeffpoulin

Clifford, thanks very much for the ideas!

Here are some photos of the rear triangle, where I suspect the noise is coming from (right and left sides). Apparently, I can't inline my photos from google drive. :-(

https://drive.google.com/open?id=0B6...WtGb1JKc2lTZWs
https://drive.google.com/open?id=0B6...3ozc2l3cGw5RzQ
https://drive.google.com/open?id=0B6...mZTMDZZT1dVakU
https://drive.google.com/open?id=0B6...TJPUWVFbkdrNDA
https://drive.google.com/open?id=0B6...UNXUTJ0LUZSeGM
https://drive.google.com/open?id=0B6...TV3VFZZUnM5OUE
https://drive.google.com/open?id=0B6...lhkc0dLcFVTZGM
https://drive.google.com/open?id=0B6...WRTQ0hZRWI5M0E
https://drive.google.com/open?id=0B6...mV6eF9qeS1lMGc

pinsonp2 

This may be a bit off the wall, but I once went through that with a carbon bike. Turned out to be the seat clamp where it attached to the rail. When I moved, pedaled or hit a bump I heard a noise that made me think I had a crack. I read a tip on this forum and put a touch of grease on the clamp. No more sound.

P2

CliffordK

I've got a squeaky seat. But the squeaks go away when standing.

Looking at the photos... do you have a rack on the bike? Have you checked that?

I'd carefully see if the top of the CF B-Stay is moving in the tube where it attaches to the frame.

jeffpoulin

Yes, I have a Tubus Cargo rack on it. I checked the bolts and it's very solid when I wiggle it by hand. I do ride with about 5kg (11lbs) of weight on the rack while commuting, so that may place extra force where the rack is attached. I'll try removing it and the fenders too and give it a test ride. It may not make a difference, but in that case, at least I'll know what it's not.

jeffpoulin

UPDATE

I removed the rack and fenders and went on a test ride. The sound was still there.

Thinking back to what Clifford said in his first post (about bearings and spokes), I checked the front wheel. To my surprise, the hub felt a little rough. Spinning the wheel fast, I could hear some grinding noises. I have a spare front wheel, so I put it on and went for another test ride. The bike was blissfully silent. I was so convinced that the sound was coming from the back of the bike that I didn't even bother to look in the front.

So, it's not the frame after all. I'm glad since I love this bike and didn't want to have to retire it. It also gives me peace of mind to not hear metallic cracking noises when riding fast. Many thanks to CliffordK for nailing it in his first response. Now I've got some hub repacking to do....

sch

Referred sound is a favorite bike trick. Singletons are bad enough, strange creaks on tandems are even harder to track down.

CliffordK

Glad you found it...
I was going to also suggest the fork.
But, I couldn't figure out how it would make noise at the back of the bike.

It is a good point, however, that it can be tough to localize odd sounds.