Tips Archive:

Hot Tip of the Month- February 2004: "Why do my Crank arms keep coming loose?"
The crank arms on your bikes endure a lot of force. On the upstroke, on the down stroke, depending on the weight of the rider, hundreds of foot-pounds of pressure are applied, every day from every rider. To that end the cranks should be checked and retorqed every month. Typically, the torque foot pound (TFP) level is 35-40 pounds although different manufacturers have different TFP levels. Check with the manufacturer to see what they recommend. If a crank arm gets loose and the bike is still being used, it will compromise the "taper" in the crank arm. The taper is the inside of the crank arm where it fits onto the bearing assembly inside the frame. Generally speaking, the material of the bearing assembly or "bottom bracket" will be harder than that of the crank arm itself. So to that end, when the two components aren't tight, it's like sandpaper on wood, albeit, this may take some time. So when you re-tighten the crank arm and it comes loose over a short period of time, it's most likely because the taper has been compromised and the crank doesn't "press onto the spindle of the bottom bracket spindle the way it's supposed to. The solution? Replace the crank arm.