text: Type in "roller coaster loop" into Google or Yahoo and you will get about 5-10 really good sites.  See these sites for free-body diagrams that will explain the forces acting on the coaster train while in the loop.  These sites will also have more details on the stuff I am about to explain.\n\nG-forces are just a force that you feel when you are being accelerated (in the case of the roller coaster or in a car changing direction).  Usually this is expressed as multiples of gravitational acceleration on Earth (eg: pulling 3 G's means you would feel like you weigh 3 times as much as normal).\n\nThe main feature that many people don't know about roller coaster loops is that they are in the shape of a "clothoid" (not a circle).  Basically, the coaster train enters the loop at a fairly large radius of curvature (gentle upward slope).  Near the top, the radius of curvature tightens up.  Then, as it exits the loop, the curvature becomes larger again.  You will certainly notice this the next time you look at a coaster loop from the side.\n\nThe reasoning for creating a loop like this is because it subjects the passengers to more uniform forces than with a circular loop.  At the bottom of the loop, the train is moving faster than at the top, so the radius of curvature changes accordingly (smoother at the bottom, tighter at the top).  This design subjects the train to more or less constant "G-Forces" and creates a more optimal loop size.\n\nAs a comparison, the first looping roller coasters (way back in the mid 1800's to early 1900's) used circular loops with the same curvature all the way throughout the loop.  With these, you had to either make the loop smaller or send the train through the loop faster in order to make it around the top of the loop (due to slowing down near the top).  Many people complained about the rather large forces they would feel at the entrance and exit of the loop because of having to send them through so fast.\n\nSo that's a bit of info I know about the standard loop.  See the web as I mentioned before to get more detailed info/math if you want.  \n\nBut that is only on the "standard" vertical type loop.  In today's coasters (such as in one of my favorites "The Raptor" at Cedar Point) there are many upside down variations such as a corkscrew, barrel roll, cobra roll, half-corkscrew and more.  Also, there are "overbanked" turns such as on the Millenium Force where you are pretty much tilted upside down, good stuff.  Check out those kinds of roller coaster tricks as well.