Sunday, August 2, 2009

Racing Helmets

The problem is not that of a standard inelastic collision, yet that is the essence of the problem: how to dissipate energy while conserving momentum, and how to reduce the acceleration of the head inside the helmet when the impulse being applied is not under your control. Complicating this is the need to keep the weight of the helmet down so that the helmet itself does not cause injury by increasing the forces on the neck in a crash (the problem that the HANS device helps solve as part of a coordinated systems approach to safety). More on the physics at the bottom of this article.

There is an excellent story on the Formula 1 website about the evolution of racing helmets, driven by the amazing survival of Felipe Massa after being hit in the helmet by a 1 kg spring that came off of Barrichello's car (at a closing speed of about 160 mph), although it doesn't give much credit where it is really due over the history of motorsport. The helmet they show Fangio wearing, which originated for use when playing polo, was similar to the one worn in a key death in the US that started the move toward today's safer helmets.

It was the Sports Car Club of America that was the first to require seat belts in automobile competition (1954), and it was an SCCA member who started the Snell Memorial Foundation in 1957 to provide testing for helmets used for automobile racing after the death of Pete Snell in a racing incident. Their page about the history of the organization and its current activities shows the crash that killed Pete Snell, discusses the physics of a crash, and shows the sort of testing that goes into certifying a helmet. The photo at the very bottom of this page shows a sample drop test of a helmet that tests for the sort of thing that happened to Massa.

The Massa incident was as close as it gets. Getting hit in the head by 1 kg spring at a relative velocity of about 160 mph would be fatal without a helmet even when the spring was deflected by the nose of the car and the bolster on the side of the cockpit. Even the helmet was put to the ultimate test, because the impact point was at the edge of the opening. You can see the effect in the AP photo that accompanies this news article. Higher resolution images of just his helmet and eye injury are available if you search "massa crash" on google images, but I don't recommend doing so.

The Physics

Some things about the collision of an object with a helmet are outside your control. The momentum of the incoming object is a given. The amount of momentum transferred to your head and helmet is somewhat under your control, but mostly depends on things like the angle of impact that you really can't do much about. Bouncing off (elastic collision) makes the momentum transfer worse for your head, so design can help a bit, but physics puts a lower limit on what engineering can do about this part of the problem.

The amount of momentum transferred to the helmet is what is called "impulse". You can reduce injury if the helmet or its lining is soft enough to increase the duration of the collision, thereby reducing the force applied to the head. This is also the job of seat belts and other safety systems, but only a helmet can protect you against the impact of an object or the road itself.

BTW, there isn't much that a helmet can do if something large (like a wheel) hits you at high speed. There are things that will kill you in motorsport. Based on one of Hemingway's rules, that is what makes car racing a sport. (If there is no chance that the animal you are hunting can kill you, he did not consider it a sport.)

The helmet has to provide an artificial skull, to protect your skull. (That means it has to be hard and strong, so it is the job of the lining to dissipate energy.) Even though the impact was right at the edge of the "eye socket", the helmet Massa was wearing did an amazing job. It appears that fragments from the helmet or visor injured his eye, although the damage could also result from a fracture as the helmet hit his head. That is the other thing the helmet has to do: absorb energy and redistribute the forces over the entire head. Massa's helmet just barely managed perform that task. He still had a fractured skull as well as a concussion from the forces that were applied to his head by the helmet.

Apparently he also had a fracture at the base of the skull (what killed Dale Earnhardt), which is supposed to be less likely with a HANS device. His roughly 120 mph impact with the tire barrier should not have produced this, as I understand the designs, so that might also have resulted from an unanticipated motion of the helmet from the spring impact. It also makes me worry about how the emergency people were moving him in the news photo I link to above!

And just to be clear:
Physics is not the entire story. Physics tells you the constraints of the problem. It tells you what physical principles apply and what forces MUST result from those principles under specified conditions. Engineering is the task of choosing materials that will handle those forces and dissipate energy without adding too much weight, so the forces that get to the head are within limits known from the analysis of deaths and injuries from past crashes. More will be learned from this one.

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