I'm pretty sure the backup envelope of the semi-trailer tracks differently than the forward one. There was a University study done, I think out of Canada, in the mid-90s that developed templates that the commercial program AutoTurn adopted in version 3.0. I could review a few enevelopes and email them to you Lars if you'd like? Hopefully I can find a copy of the study posted online, nchrp_rpt_505.pdf is a good reference but looks to be forward movements only.