Ok. So I have a model that I applied the Curve modifier to, and I set the object as my bezier curve. I am however having an issue… I’m wanting to make a loop, but when I try to make my track go upside-down, it sorta ‘flips’. Here’s an image:
Is that curve set to 3d or 2d? If it is set to 3d, you should see the arrows that indicate what direction it is going, as well as how much twisting is going on. You might try subdividing your curve and getting a gentle slope going, and really examine the twist in the curve.
Here are two versions, hope you figure it out. Open without the ui, I did my editing in 2.57b. One twists, the other does not. If you were wanting it to not twist, make sure you do your manipulations in front or side view, making sure to rotate the points naming the axis to constrain to. When they are 3d, they love to get wonky really quickly. I usually do my curve design in 2d, then rotate the whole thing at once after swapping to 3d.