Cracked frame?

Question

I've got an old aluminum MTB (Gary Fisher Tassajara) that makes a metallic popping sound when I bounce on the seat. The sound is coming from the frame at the top of the seat tube. It still does this with a different seat/post. There are no visible cracks anywhere. Everything is greased and torqued properly. Can this be something other than a cracked frame? It has an over sized seat tube that tapers down at the top. Can something be cracked internally?

Answer 1

Like the others have said, weird sounds can come from virtually anywhere and be transmitted by the frame, so it can be very hard to isolate the source. If it is a crack, it will show. The highest-stress area on the bike will be around the bottom bracket, especially the bottom of the seat post and the chain stays near the bottom bracket.

Regarding the warranty, Gary Fisher warrants the frame for the lifetime of the original owner. Cracks in the welds are a defect, same as a crack in a tube. Aluminum has a limited fatigue life; when it bends it work-hardens until it becomes so brittle that the least stress can crack it. That's why aluminum frames are engineered to be stiff - so they don't do that. A properly-designed aluminum frame will not break.

If you are not the original owner, many manufacturers will sell you a reduced-price replacement frame. You might contact them and ask, it can't hurt.

Answer 2

Aluminum frames often make creaking noises but it is better to be safe than sorry.

Did you lubricate the seat post before installing it? Before going crazy, take a fingerful of lithium grease and wipe it into the seat tube then install the post. Remove the stem from the steerer and do the same thing. Check to make sure the bottom bracket is properly seated and the crank arms are installed correctly. Take off the pedals and lube the threads.

Look very closely at the following areas:

- all along the seat tube

- where the chainstays meet the bottom bracket

- the chain stay reinforcement

-the rear dropouts

A crack may appear as a very thin long scratch so it is probably a good idea to use a magnifying glass for inspection.

If the frame is cracked it likely will NOT be covered under warranty. Most cracks that are warranted are tubing cracks, not cracks at welds.

Aluminum has a limited life and as such will eventually fail no matter how you ride.

Answer 3

That's a common area to crack, but it's likely something else if you can't see anything.   You might try doing the seat-bounce thing with a helper so that one of you can carefully examine the welds and bottom of the clamp-slot on the tube (if it has one)....under pressure you may spot a crack opening up.  

It could just be a creaky seatpost....very common.  Grease it up with some thick grease and see if the problem is still there.  If so, then grease the saddle rails and seatclamp bolts, too.  Failing there, do the bounce thing again and have your helper look at the welds at the bottom bracket and chainstays, maybe even feeling underneath the tubing with a finger.  

It's possible to have an internal crack, but usually if aluminum is going to crack it'll show through the surface right away.  I'm guessing it's just metal-to-metal contact with a component.  Work through each part systematically (not forgetting the bottom bracket cups....tight-n-lubed) and you'll find it.  Don't overlook silly things like cable housing ends creaking in their holders.  It's amazing how sounds can travel and magnify through an aluminum frame to where they sound like something entirely different.

If it does turn out to be a cracked frame, Trek/GF should be good about either relacing it or getting you a discounted crash-replacement frame.   Hope this helps...good luck!   :o)

Answer 4

Unfortunately with alum. the noise could come from unsuspecting places as the alum. tubing transmits noise rather well. If you can not find any cracks or alum. dust check your BB and your head set. I have a Tassajara and the last time I had a strange noise it sounded like came from the BB but was actually a worn out head set,