The same principle that allows an airplane to rise off the ground by creating lift from its wings is used in reverse to apply force that presses the race car against the surface of the track. This effect is referred to as "aerodynamic grip" and is distinguished from "mechanical grip", which is a function of the car's mass, tires, and suspension. The creation of downforce by passive devices can be achieved only at the cost of increased aerodynamic drag (or friction), and the optimum setup is almost always a compromise between the two. The aerodynamic setup for a car can vary considerably between race tracks, depending on the length of the straights and the types of corners. Because it is a function of the flow of air over and under the car, downforce increases with the square of the car's speed and requires a certain minimum speed in order to produce a significant effect. Some cars have had rather unstable aerodynamics, such that a minor change in angle of attack or height of the vehicle can cause large changes in downforce. In the very worst cases this can cause the car to experience lift, not downforce; for example, by passing over a bump on a track or slipstreaming over a crest: this could have some disastrous consequences, such as Mark Webber's and Peter Dumbreck's Mercedes-Benz CLR in the 1999 24 Hours of Le Mans, which flipped spectacularly after closely following a competitor car over a hump.