What the flip happened here?

I reckon there’s a good chance you’ve seen this infamous photo at some point: Mark Webber’s Mercedes CLR flying into the air at a 1999 24 hours of LeMans practice session. And this was no isolated case. Incidents like these were occurring more frequently among the LeMans prototype series cars in the late 90’s – prompting an urgent revision of the regulations.

What caused the flips?

Pitch angle: A pitch angle is simply described as the angle between the underbody of the car and the road/track surface.
Typically, negative pitch angles are used on track cars for increased downforce on the front of the car for increased cornering speeds. Alternatively, if your aim is to reach higher top speeds, you would want a more neutral pitch angle.

 

Fig.1

However, pitch angles are a dynamic setup which means that the angle of pitch can change under different conditions such as acceleration, braking and (in this case) when driving over the brow of a hill. When these prototype cars would drive over the top of hills, it would cause the pitch angle to change from neutral to an excessive positive angle very quickly (Fig2.)

 

Fig.2

This causes a lift force on the front axle. You may experience something similar if you were to quickly slide a piece of paper off a flat table. You’ll notice that the paper will curve upward instead of falling straight to the ground.

When this lift force is coupled with the downforce on the rear axle, the car will pivot about the rear axle to further increase the pitch angle through several 360 degree turns AKA a flipped car.

Overhang: The length of the overhang (distance from the front of the car to the front axle) on Mark Webber’s Mercedes CLK GTR cars was the longest in the race. This essentially made the front of the car force multiplier for the lift force (AKA a lever).

How can the flips be prevented?

There are three ways flipping can be prevented:

1) Reduce the pitch angle

2) Reduce the front overhang length

3) Release air pressure at the front of the car

The FIA has since added a regulation that limits the legal front overhang length to between 750-1000mm and have told the teams to open up the front wheel arches to reduce air pressure in an effort to reduce the chances of cars flipping. Thankfully, these changes have been a success.

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