A “Toolbox” for Forensic Engineers

70 Forensic Materials Engineering: Case Studies
Figure 3.7 Bent axle from the front wheel.
Figure 3.8 Brake disc, bent and bolts missing where trapped in the calliper.
the wheel to break up in this manner. Figure 3.7 shows the axle placed on 1mm
2 paper, clearly revealing that it is bent toward one end. A steel axle does
not deform in this way unless one side is restrained while the other is acted
upon by a force sufficient to deform it. This necessary displacement is
revealed by the way the fork stanchions have been twisted out of line, as
illustrated in Figure 3.8. The same force had twisted the brake disc, breaking
two bolts at one side, while it had been restrained by the calliper (Figure 3.9).
Thus the big force against the tire must have been transmitted through the
wheel into the axle and then to the forks, twisting them sideways, as illustrated
in Figure 3.10. This force must have then been reacted to by the head frame
tube of the chassis. All external forces on the forks are transmitted to the
headframe of the machine via the bearings at the top and bottom of the
steering tube.
The steering tube exhibited a ductile shear failure just below the top
bearing, as already shown in Figure 3.4, and had bent immediately above the
bottom bearing as a result of the bottom of the fork being pushed backward,
as illustrated in Figure 3.9. (Surprisingly, the first engineer had made no
mention of this in his report and had not taken any photographs of this
region. From the dimensions of the tube in the vicinity of the bend and