My crank is always coming loose ?

Question

i have truvativ isoflow chainset but i dont know what bottom bracket i have, Anyway my left crank is always coming loose, is this to do with the bottom bracket or the crank itself, and if it is to do with the crank will this one fit on to my bike, my bottom bracket is a standard sqaure size http://www.sjscycles.co.uk/src/froogle/product-Shimano-Left-Shimano-Alivio-FC-M410-Left-hand-Crank-Arm--Black-14545.htm

thanks a lot

Answer 1

This is a common problem with bikes, the left crank taper takes more strain than the right, because it has to transmit all its drive through the axle, while the right transmits most of its force directly through the chain wheel.

Once your left crank has loosened and been ridden hard while loose, the softer metal of the crank will be chewed up by the harder metal of the BB axle, this will permanently ruin the crank. The only thing you can do at this point is to get a new left crank.

When you put the new crank on, clean the BB axle, apply grease to the surfaces, and semi-hardening thread-locking compound to the crank bolt. Tighten to the torque recommended by the manufacturer, or check the following table:

http://www.mechanical-fitter.co.uk/torqu...

Lastly, get in the habit of pedalling with a proper cadence, you should be doing at least 80 RPM, and using the gears, rather than gritting your teeth and heaving the pedals round slowly, this puts too much strain on the whole drive train, including your knees.

Note: some Truvativ cranksets use a shallower taper angle than the standard 2 degree taper, if you have one of these pushed onto a standard tapered axle it will loosen much more readily. Look up the model specs, or whip out your vernier gauge and compare the taper measurements.

Answer 2

Link still doesn't work.  The bottom bracket could be worn also.

If the bike is new take it back.

Answer 3

If your crank has the Powerspline interface, then you have that type of bottom bracket and you're stuck with it.  Check out the sale items at http://www.chainreactioncycles and you can pick up a whole crank for cheap. They had some Truvativ Hussefelts for a great price in several options.  

As for the one you have now, be sure that you're torquing the crank bolt to spec (about 35 foot pounds).  If it's still coming loose with proper torque, then you could put a drop of red loctite compound on the threads and that should hold it in place (clean and degrease the threads first).  It may be that the crankarm splines have been worn or damaged and they won't hold with normal installation anymore.  Truvative seems to have had a lot of left-arm loosening problems in various cranks.  I have three sets of them (Stylo, and two Hussefelts) and they've all been good for me, but I hear about this alot.  

If you buy a new crank or bottom bracket, just remember that they have to be the same type of interface (square, Shimano Octalink, ISIS, Powerspline, etc) and you should be good.  

(and yes.....if the bike is new, I'd first take it back to the shop to see if they can help with proper installation or a warranty replacement or something.  They should be willing to take care of you.  Good luck!)