I need help too fix a problem. I try to make a simple rollercoaster but the cars always flip on the curve. I've got the same problem either when i use motion path or ik spline.
It has something to do with the tangent of the curve.
See my scene http://sauge1.free.fr/test.mb
Of course with an ik spline you can use the "roll" attribute to fix the problem or the "twist" with motion path but i what i really wonder is if there is is a clean way to do it instead. A way with no flip to deal with.
This problem always occurs when you want to make a motion path or ik spline with an "extreme" curve and i've never been able to fix it and nobody yet could tell me how.
I know that i could use the pointOnSurface technique but it's not very handy for a rollercoaster for example. Moreover i'm sure there is a easiest way.
Thanks a lot for your help.