![]() ![]() However, the common cause for threaded fasteners loosening is simply lack of tension during initial assembly. It is commonly believed that bolts and nuts often come loose “on their own”, for no apparent reason. For example, a fully tightened crank bolt can provide over 14,000 Newton force (3,000 pounds force) as it holds the arm in place. The amount of pressure applied by a thread can be substantial in order to hold the joint secure. The crank-to-spindle interface receives quite a lot of stress, making larger threads (M8, M12, M14) a better choice. Even if the bolt were as tight as possible, it would not provide enough force to hold the arm secure to the spindle. For example, the M5 bolt of a water bottle cage bolt would not be a good choice for holding a crank. Generally, engineers will specify a thread size large enough to handle the anticipated stresses. Tension in the fastener depends largely upon the amount of torque, the tightening, and the size of the thread. Loose bolts and nuts are also generally the source of various creaking on the bike. This can damage components, such as a crank ridden with a loose mounting bolt. Too little preload will mean the fastener will loosen with use. Too much tightening will deform the threads or the parts. Each fastener is designed for a certain range of tension. This stretching is not permanent, but it gives the joint force to hold together, called “preload,” or tension. As a fastener is tightened, the fastener actually flexes and stretches, much like a rubber band. Threaded fasteners, such as nuts and bolts, are used to hold many components to the bike. ![]()
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