Why do the crank set screws keep coming loose?

Question

I bought a Trek 4500 about a month ago

1. On the first evening I owned the bike I got about three miles into my seven mile journey home when the left side pedal came loose and I couldn't ride the bike.

2. Went in to bike shop the next day and they called Trek and got the bottom bracket upgraded to a power spline and a new crank under warranty.

3. Okay so this part is my fault I rode over some rocks and annhilated the crank so I opted to just replace the whole crank.

4. The bike shop owner replaces the crank after getting the wrong one in twice and charging me twenty dollars more than the crank is worth but oh well I am a fan of local business and i planned on doing more business with them so I paid it.

5. day after crank is replaced the left side pedal falls completely off...set s***w was gone within 8 miles of riding. They put it back on with loctite

Today it came lose agian, what gives

crank was upgraded to a Bontrager Select

why is this a continuing problem?

Answer 1

I wonder about this. Your problem has been on the left (non-drive side) each time. I would hope that the shop has been actually replacing the whole crankset and not just the left crank arm. If they were doing just the left, then it is quite possible that the threads have been crossed, making any attempt to replace the bolt or arm a waste of time and money.

If they are replacing the whole crankset, then maybe the mechanic isn't fitting it correctly. Maybe it's time to get a second opinion from a different shop.

Answer 2

try Red loctite

Answer 3

Are they using a torque wrench to tighten the bolt or just guessing? Have they checked to make sure the bottom bracket shell of the frame is square?

It's fallen off on you three times. I'd be screaming bloody murder to have the entire bike replaced under warranty. Call Trek yourself and complain.

Answer 4

stop upgrading the crank, take it to the shop and ask what you can do to stop it. if they tell you you have upgrade say no cigar. try www.sheldonbrowncycling.com and search in the C section for crankset. that guy is pretty much the jesus of cycling. he might be able to help u