The Dimension Basis user preference was added in HVE version
8.10 (Dec 2010). Its purpose is to allow the user to assign certain
vehicle-fixed dimensions (CG to Front Axle, etc.) according to either the sprung
mass CG or the total mass CG. There is a difference in sprung and total mass CG
location, and while the difference is rather small for most passenger vehicles,
it can be significant for on-highway trucks wherein the unsprung masses (wheels
and tires) represent a larger fraction of the total vehicle
weight.
When executing a 3-D simulation, the equations of motion
require all vehicle-fixed coordinates (e.g., wheel locations, connection
locations) to be relative to the sprung mass CG. Therefore, HVE has always
displayed and stored these coordinate data relative to the sprung mass CG. When
HVE is executing a 2-D simulation, the equations of motion require all
vehicle-fixed coordinates to be relative to the total mass CG (technically, the
reason is that the unsprung masses are included in the total vehicle mass for a
2-D simulation). Prior to Version 8.10, it was assumed that the difference was
small and could be ignored. However, as mentioned above, for heavy trucks the
difference could be significant. So we introduced the ability to switch between
total mass and sprung mass dimensions. Each dialog in HVE’s Vehicle Editor that
displays coordinate data automatically switches from sprung mass basis to total
mass basis when that preference is selected. (In Version 9.00, the current
setting is displayed in HVE’s status bar at the bottom of the
screen.)
Here is the takeaway: You can use either preference in the
Vehicle Editor. While executing an event, a 3-D simulation (e.g., SIMON) will
always use the sprung mass dimensions (as it should). However, 2-dimensional
HVE-compatible programs (e.g., EDSMAC4) are designed to use either the sprung
mass or total mass CG location (as determined by the current user preference) in
order to maintain compatiblity with earlier versions of HVE. So while executing
a 2-D event, you should use the Total Mass preference. The Dimension Basis that
was used while executing a 2-D event is displayed in the Program Data output
report.