Parallel Engineering

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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:
* Gearbox cradle rear isolastic tube is parallel to swinging-arm spindle tube
* Front and rear isolastic tubes checked for distortion from welding during manufacture
* With swinging-arm spindle and rear wheel correctly aligned and cradle assembled in frame the position of front isolastic bracket is checked        in relation to front frame mounting brackets.

The above image on the left 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.  The image on the right is 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 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 when the wheel has been deflected to the left or right by imperfections in the road surface. A  rolling chassis which is not in true alignment will create a left or right steering bias. This will cause the rider to steer the motorcycle to compensate for the bias which means the front wheel trail will be continually trying to bring the front wheel back to the centre position. This creates instability which increases as road speed and bump frequency and magnitude increase.

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 which is offset to the frame centre line due to inaccurate manufacture and or assembly, is angled within the swinging-arm then rear to front wheel alignment is achievable. However the steering bias caused by the offset rear wheel which is now in addition not parallel to the frame centre line will cause instability.  

 Above photos L to R: 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.


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