MotoSPEC v5.6.1 added a setting that will indicate the forward reference for the swingarm length.
Swingarm length is typically measured from the swingarm pivot axis to the rear axle. On most motorcycles, the swingarm pivot is visible and accessible.
For Ducati Panigale V2 and V4 models with adjustable swingarm pivots, the eccentric adjustment occurs inboard of the swingarm bracket - so the swingarm pivot axis is not directly accessible for measurement.
The calculations for swingarm and chassis geometry now consider the offset between the frame center and the actual swingarm pivot.
In the case of the Panigale V2 and V4, the 'swingarm length' - the value entered in each column of MotoSPEC - is measured to the center of the adjuster located in the frame.
Starting with MotoSPEC v5.6.1, the 'Swingarm Length Reference' settings is found in CHASSIS>OPTIONS.
'Swingarm Pivot' is the typical setting, and the default that is applied to all earlier MotoSPEC files.
'Frame Center' is the setting relevant for the Panigale V2 and V4 models with adjustable pivots.
The setting can be applied or changed by the user as necessary.
The difference between 'Frame Center' and 'Swingarm Pivot' is minimal when the pivot adjustment is strictly vertical. However, the effect is greater when there is a horizontal pivot move, as caused by the eccentric swingarm pivot adjusters.
If you are measuring the swingarm length to a feature or tool located on the frame (rather than the actual swingarm pivot), you can apply the 'Frame Center' setting.
Further explanation and drawings of the setting are below.
MotoSPEC will refer to the 'Swingarm Length Reference' setting when calculating the swingarm and chassis geometry, and also the gearing options presented in the Gearing Table.
CALCULATION OF SWINGARM GEOMETRY
The geometry of motorcycles with eccentric rear axles is more affected by swingarm length than motorcycles which have the usual sliding axle blocks.
MotoSPEC considers the swingarm length and dimensions, and also the chassis options settings for eccentric position and swingarm length reference, when calculating the swingarm and chassis geometry, and the rear suspension characteristics.
MotoSPEC uses the measured swingarm length to determine the position of the eccentric. The distance between the swingarm pivot and the rear axle must be within the range of the 'center-to-center' distance (596.9 in the image below) +/- the eccentric radius (14.0 below).
The rear axle of the eccentric adjuster may be positioned above or below the line between the swingarm's pivot and eccentric center (blue line in image above).
'Eccentric Lower Half' = rear axle is below this line
'Eccentric Upper Half' = rear axle is above this line
The position of the eccentric is normally set by the manufacturer of the motorcycle or swingarm.
For each swingarm length, there is are two possible eccentric positions - one with the eccentric above the swingarm center-center line, and another below.
Knowing the swingarm dimensions and the eccentric setting, MotoSPEC calculates the swingarm geometry.
SWINGARM LENGTH REFERENCE: 'SWINGARM PIVOT' vs 'FRAME CENTER'
The Swingarm Length Reference is necessary so that MotoSPEC can correctly calculate the swingarm geometry as specified by the user.
The image below shows the typical arrangement, with a 'Swingarm Pivot' length reference, where the dashed line indicates the measured swingarm length between the rear axle and swingarm pivot.
The next image below shows the 'Frame Center' length reference, where the measured 'swingarm length' is between the frame center and the rear axle.