Bad History With Crank Arms

[Rossostopher]

Hi All,

Just looking for some advice and knowledge since I am relatively new to mountain biking and am having a few problems.

I have recently bought a Raleigh AT30 MTB to get a bit of practice on the trails. After riding if for about 4 weeks I found the left crank arm came loose. I had it tightened and decided to carry a spare allen key with me just in case it happened again. Low and behold it did, in fact it happens about 20 minutes after I do it up.

This isn't the first time this has happened to me. My last bike was a Trax Fearless (again a bike to try and get me into MTBing. I had the same problem with the left crank arm coming loose and falling off. I had this changed and had loctite used to keep it on. After about an hour of riding it came loose again.

My question is, as I have had terrible luck with this, could my problem be down to how I ride my bike or is it purely bad luck?

I tend to ride in low gears (granny gears as I call them), I hate the feeling of pedaling and going nowhere so I use higher gears where I can actually feel myself doing something and going somewhere. Not as problematic as I may think, but I also tend to point my toes outwards when riding, I doubt this actually affects anything but just a bit more info.

If anyone can give me advice or suggest a long lasting fix I would be grateful.

[trevgbb]

this was my reply to a previous question on a similar subject, most of it applies to you, especially getting the new one tight, very tight, forget torque settings, very tight...
This is a common problem and as others have said, you need a new / replacement left hand crank arm, not a new crankset, unless you have other problems.
The thread and the axle are fine, as probably is the nut.
Look at the shape of the square hole on inside of the arm, if this is at all bell mouthed, no amount of tightening will succeed.

A new arm should not cost much, here in the UK they start from about £8 for steel, and £10 for alloy, some LBS sell s/h ones for even less, but do check for wear on the taper and the thread for the pedal, and also make sure you get the same length as your ex one. Yours is an alloy Shimano, probably 170mm. long.
BUT, BUT it is very important to get the new fitting VERY tight from the start, or you will repeat the same exercise several times, if you use your LBS they will fit it for you for a modest fee.

[DaveM]

If you are generally strong/heavy/rough on the pedals, this is not an uncommon problem. But you should be able to avoid it. The key is never letting the crank arm get loose it the first place. If it starts to loosen and you ride on it, then the hole in the arm gets flared out of shape and you can never get it to seat properly again.

Get a new left crank arm. Very lightly grease the the BB spindle (axle) before installing. You can use locktite on the bolt threads. But the main thing is to get the crank installed to the right torque, not to prevent the bolt from unthreading.

If you can borrow/buy a torque wrench, they are helpful for getting the crank on correctly. If not, the trick isn't to crank the arm down as tight as humanly possible. More important is to get it tight and check it occasionally in case it has started to loosen up.

I'm not sure how much difference there is, but a better quality crank arm probably uses slightly better quality aluminum than a shop's $5.99 no name crank.

Lastly, watch how good riders clear obstacles. You may see when they spin the pedals in lower gears and when they crank on high ones. Spinning can make you a much better rider when used properly.