Calculating the internal structure of a FrankenMech was already complicated - it had to be done on a by-location basis involving the donor unit's mass, the donor's internal structure type percentage (endo steel = 05%, reinforced = 20%, etc) and the location's percentage of the body.
For bipeds and quads, the body location percentages as given (from Strategic Operations, p.189) are:
05% Head
25% Center Torso
15% Left and Right Torso
10% Each Arm, Each Leg
However, Tripod mechs have a ninth body location, the "Center Leg" so we can't use the same percentages for these units. While it's not overly difficult to calculate these figures on my own (reducing each location by 9.1%), I cannot provide official data. Is there any word from The Powers That Be on what body location percentages should be used for Tripod FrankenMechs?
