Materials and Structure The front fork is another component that experiences an off-axis force and hence a bending moment. The diagram shows the force at the wheel axle for a statically loaded bike and this makes an angle of approximately 19.50 with the centerline of the front fork (blue line). the force can be resolved into a component along the tube, FP = 196 cos(19.5) = 185 N, and a force normal to the tube, FN = 196 sin(19.5) = 65.4 N. The parallel force puts the tube in compression, the normal force gives rise to a bending moment that has its maximum value at the bottom head-set bearing. If the fork is approximated by a straight beam of uniform cross-section that has a length of 480 mm, the bending moment increases linearly from zero at the axle to a value of:                    M = 0.480 FN N.m = 31.4 N.m at the bearing. The maximum stress in the tubes of the fork is given by smax = 0.5(MD/2I), where the factor of 0.5 is due to there being two tubes and D/2 is the maximum distance of tube material from the neutral axis (blue line). Using an elliptical tube of 1.34 mm wall thickness and diameters of 31.8 and 19 mm gives a moment of inertia of 1.53 x 10-8 m4, and a maximum stress value of 16.3 MPa, i.e. (smax/sy) = (16.3/180) = 9 x 10-2. From: Apps, "The Bicycle Book," Smithmark (1996)