I would agree with Georges comments that although the hip joints may be aligned with the pivot transversely, they are not aligned longitudinally, ie they are displaced laterally and therefore will impart some lateral force on the BB. Also it seems that I am not the only one who has placed the pivot at the front edge of the seat. Looking at the gallery on the Python webpage it looks like the following bikes have done likewise....Torbens, Laurents, Rays, Marcels and also Jukka2, Jaculus, and BHP. So I wonder if any of them had PSI problems and how they solved them. And regarding the PX5, Yes Jurgen, I think it would be wheel flop which would be very uncomfortable as it would lead to rider flop! I played around with attaching the seat to the front section and it didn't feel good! Anyway I am now going ahead with my trike construction with a pivot angle of 53 degrees but I will of course raise the seat to its original position. This in turn will decrease the negative trail a bit, so I hope that wont affect the steering adversely.....it should improve it if anything, shouldn't it? I will write again to the Python mailing list after testing the new trike......hopefully with no further problems. Thankyou all for your assistance. Cheers Howard