Solutions for Motorcycle Frames
The following checks are required to establish complete alignment:
-
Engine mounting bolts and brackets are perpendicular to the steering head axis
-
Swinging-arm axis is perpendicular to the steering head axis
-
Swinging-arm rear wheel axle slots are parallel to the swinging-arm spindle axis
-
Position of rear wheel rim centre line is parallel to and in line with the frame centre line
-
Fork yokes are parallel
-
Front wheel rim centre line is centred to the steering head axis.
In addition to the above checks when aligning Norton Commandos:
-
Swing-arm bushes shimmed to eliminate side movement between cradle and swing-arm.
-
Front and rear isolastic tubes machined to remove distortion from welding during manufacture
-
Front isolastic assembly position checked in relation to front frame mounting brackets.
Above image is of a 1973 Norton Commando showing the rear wheel severely offset to the left of the frame centre line. This fault is a result of inaccurate factory manufacture and assembly. It is not limited to a particular year or batch but is typical of many classic bikes. For accurate steering the rear and front wheels need to be in-line with the frame centre-line.
Above image is on the same bike after having the frame corrrectly aligned at Parallel Engineering utilising an engineering grade granite surface table.
Why use a Surface Table?
Because in conjunction with parallel engineering blocks they provide fixed horizontal and vertical surfaces to use as datums. From these datums I can establish to a high degree of accuracy the relative position of for example, the rear wheel centre line to the frame centre line and the swinging arm spindle axis to the steering head axis.
Front Wheel Trail
This is defined as the distance from the tyre contact patch centre to an imaginary line from the steering head axis extended to a point on the ground. When the motorcycle is in motion the trail brings the front wheel back into the centre position as the wheel is deflected to the left or right by stones and bumps in the road surface. A rolling chassis which is not in alignment will create a steering bias which means the rider has to hold the handlebars in a position to counter the steering bias. The riders grip on the handlebars reduces the effectiveness of the front wheel trail and leads to instability. The situation worsens as road speed and bump frequency increases and in extreme circumstances leads to total loss of control (tank slappers).
Swinging-arm Spindle Axis
When this is not at 90 degrees (perpendicular) to the steering head axis as the swinging arm moves up and down the rear wheel will move side to side across, instead of moving in line with and parallel to, the frame centre line. This produces the same symptoms as worn swinging-arm pivot bearings and creates variable front to rear wheel misalignment and therefore instability as the motorcycle is ridden.
Wheel Alignment
Aligning the front and rear wheels to each other is no guarantee that either wheel is aligned to the frame centre line. If a rear wheel is offset to the frame centre line due to inaccurate manufacture and or assembly and is angled within the swinging-arm then rear to front wheel alignment is achievable. However the offset rear wheel which is now in addition not parallel to the frame centre line will cause a steering bias which leads to instability.
Checking swinging-arm spindle is perpendicular to steering head axis using set square.
Centering front wheel rim to steering stem and checking for run out;
Checking position of rear wheel in relation to frame centre line using a vernier height gauge.