Inserts in the steering head or headstock of the frame can be used to adjust the rake and the weight distribution of the motorcycle.
MotoSPEC can specify these inserts in several ways, to accommodate all the different methods in use.
The 'Steering Head Adjustment' methods are found under CHASSIS>OPTIONS in the MotoSPEC ribbon.
There are several methods available to specify the adjustment of the steering axis in the frame:
'Steering Axis Offsets at Upper and Lower Bearings'
Steering Axis angle and offset at Upper Bearing / Mid-Headstock / Lower Bearing
Fork Angle and Steering Axis offset to Headstock Axis:
Example of 'Pivot about Mid-Headstock'
A common example of a model with head inserts is the current Kawasaki ZX10R.
It has inserts available to adjust the rake by +/- 0.5 degrees, and also +/- 1.0 degrees.
The pivot or rotation point of the angle is between the bearings, at the middle of the headstock.
The ZX10 also has inserts available to move the steering axis fore and aft by up to 4 mm from the standard position in the center of the headstock.
Below is an image showing -0.5 degree and +0.5 degree inserts applied.
You can see that even though the inserts adjust the steering axis by 0.5 degrees, the effect on the rake is only 0.4 degrees.
This is due to the rotation of the forks about the middle of the headstock. The front of the motorcycle is raised and lowered, and this reduces the amount of the rake adjustment.
By choosing 'CHASSIS' in one of the graph series and clicking on the 'TABLE' button, coordinates of a number of chassis points are displayed, as shown in the image below.
It can be seen that the vertical coordinate of the 'Frame lower headstock' point is raised and lowered 2 mm with the rake adjustment. This approximately equates to the loss of 0.1 degrees of rake, versus the intended 0.5 degree of the insert.
Other points are raised and lowered in proportion to their distance from the rear axle. The swingarm pivot is raised and lowered by approximately 1 mm, as this is approximately half way between front and rear axles.
It can also be seen that the distance from front axle to swingarm pivot (circled in purple) is affected by 4.2 mm in either direction with rake adjustment. This affects the amount of weight carried by the front tire, and therefore the weight distribution of the motorcycle.
In the example shown below, the second and third columns show the effect of a rake adjustment with an offset of the steering axis.
With -0.5 degrees and the steering axis offset by 5.0 mm forward, the wheelbase is the same as the first column, indicating that the position of the front tire and therefore the weight distribution is identical.
The same is achieved in the third column, with +0.5 degrees and -5.0 mm inserts.
Example of 'Upper and Lower Offsets'
With this method, the offset of the bearing cups are specified.
If spherical bearings are used, this allows for a range of rake and offset by using different combinations of insert offsets.
For example, the distance between the bearings for a ZX10 is approximately 180 mm.
Using offsets of +/- 0.8 mm at upper and lower bearings results in +/- 0.5 degrees of rake adjustment.
The example below shows how to specify a fore or aft offset of the steering axis, without a rake adjustment.
Even though there is no rake adjustment, a change in the rake can be seen because the offset happens in the plane of the steering head, and so raises and lowers the front of the bike slightly (approximately 0.07 degrees).
The example below shows how to specify a fore or aft offset, combined with a rake adjustment.
The second column moves the steering axis an average of 4.0 mm forward, with the +/- 0.8 mm difference in upper and lower bearings resulting in -0.5 degrees of rake adjustment.
The third column moved the steering head 4.0 mm rearward combined with effectively +0.5 degrees of rake adjustment.
With spherical bearings, inserts of different offsets can be used to produce a wide range of offsets and rake adjustment.