However, as soon as any crease or chipping of the chrome is evident on the fork tubes they are history. A frame's function is basically to connect the steering head and swing arm pivot points in as rigid and economical a manner as possible and also to provide mounting points for various ancillaries. For our purposes, the question of whether the chassis is aligned or not condenses now to whether the frame's steering head is perpendicular to the swing arm axle. We will return to the question of measuring the steering head angle in part 3 of this series of articles. We therefore represent the frame as in figure 3.
Some machines have asymmetrical frames with respect to the center of the swing arm axle.
However, in all cases, the steering head MUST lie in a plane normal to the swing arm axle. In addition, for a perfectly aligned chassis, the steering head must lie directly vertically above a line connecting the center of the tire contact patches when the wheels are aligned and vertical.