Upside-down object placement using cross product?

This article helps greatly: http://wiki.roblox.com/index.php?title=Cross_product

But what if you want to place it on the bottom of a surface? This is an example of what I want to be able to do with cross product and this type of math. I was never that great at applied math, so even figuring out the basics of this and how to manipulate it took a bit of work. Even though I did end up figuring out how to do it above a surface, raycasting upwards to place an object on an upside-down surface still doesn't help. If you're curious, I'm trying to use this as a way of skateboarding around a loop.

What happens when I try it with this approach: https://gyazo.com/7c58729302851596b2ce100ae6f8ddc6 https://gyazo.com/7ee23ce7fa291f9e3e857f79624ce389

In the skateboard example, I never reach the angle of the next segment of the loop. How do I do this?