I recently purchased a Trek FX 7.2 and about 2 weeks after purchase I was riding and I noticed a creaking sound and shortly after the crank fell off. I brought bike back to where I purchased it and they put the crank back on and put the chain derailleur back on as it fell off. Great, no problems with the bike until about 1-2 weeks after where I was out riding and I noticed a creaking noise on the same spot, the left crank. As soon as this started I went right home and tightened the crank the creaking went away. I guess what I am asking is what can I do to the bike so that it does not creak and fall off every 2 weeks. I am a large person standing 6 foot 1 and about 270 pounds. Is the problem I have just a heavy man problem where I need to bring the allen key with me always and tighten it every 2 weeks or can something be done to avoid this entirely?
Square taper cranks are easily damaged if they a ridden loose. You may find that the only fix is to replace the crank. If after tightening to the correct torque they continue to loosen, they must be replaced. Over tightening, while tempting, is not the correct solution and will lead to maintenace problems down the track. (Essentially someone will have to remove the crank for some reason, it will keep coming loose after that and eventually be replaced. The process of fixing will take several attempts as the presumption will be the crank is in good condition when it is not)
Take the bike back to the the shop. As the crank has come loose again, ask them to replace it. If they refuse, be prepared to take your bike back every two weeks until they install new ones. Do not "fix" the bike yourself. The LBS might claim it was something you did that caused the problem.
The Trek FX series is their fitness line, and these have a max rider capacity of 300 lbs. Don't worry, it isn't your weight. And I applaud you buying a bike to get in shape. It's why I bought one too.
The crank arm bolts must tightened properly using a torque wrench. Too loose and they can work themselves free and too tight and you can strip threads.
I would have that bike shop take the crank apart, make sure that the threads are all still intact and then to tighten it to the proper tension and perhaps using the appropriate threadlock if applicable.
Note that it's very hard to get the requisite torque with an Allen key. I have to use a hex bit in my torque wrench and use some force to get to the right torque.
If the crank bolt coming loose is the only issue, perhaps try some locktite on the bolt...go medium strength so you can remove it in the future if needed.... i had problems with my 2009 Trek 4500 with the crank creaking. Ended up using a plain old washer on the bolt as a spacer...worked a charm and no issues in the last 18mths. Currently have a trek stache with the creak issue. Bolt is thicker so haven't found a washer that will fit on it to see if that will fix the issue as well...