The main reason that roller coasters are able to stay on their tracks even when they're upside down is that as the cars in the coaster travel up into the loop, their inertia would keep them going up in a straight line. True, gravity by itself causes a downward acceleration, but the coaster is going so fast and the track curves so much that it is forced to accelerate down even faster than gravity would make it. The track provides the rest of the force to keep the coaster turning, so obviously the coaster stays in contact with the track. If for some reason a coaster went into the same turn much more slowly, it would indeed fall off.
However, as roller coasters become bigger and faster, desginers need to make them safer. And many modern coasters, and all the ones that go upside down, actually have three sets of wheels to keep the on the track at all times. Normal trains--or roller coasters that do not go upside down--just have one set of wheels that rides along on top of the track. But coasters that do go upside down have one set of wheels that rides on top of the track, a second set that rides under the track, and a third set that rides along the outer side of the track. The three sets of wheels fit very tightly to the track and hold the cars in place even while they’re moving very fast through turns and loops. That may not be a very clear description, but I think you’ll see what I mean in this picture:
Which I got from Cedar Point’s Webpage
, a popular amusment park in Ohio.
Hope this helps,
Tim (w Mike)
(republished on 07/12/06)