Safety: It’s not just how fast you go – it’s how fast you stop.

One of the biggest concerns in motorsports wrecks is peak force – that’s the maximum force experienced by a driver in a collision.  As the diagram to the right shows, the force you experience in a crash varies with time.  When you hear someone talking about how ‘big’ a crash was,  the number they’re relaying is almost always the peak force. The goal of most safety devices used in crashes is to ensure that the driver experiences the minimum peak force possible.  It’s better to experience a smaller peak force for a longer time than a higher peak force for a shorter time.

Not surprisingly, the force experienced in a crash is determined by how fast you go — but it’s also determined by how fast you stop.

This equation tells you two things about a collision where a car hits something else and comes to a stop:

  1. The faster you’re going, the more force you’re going to feel when you come to a stop*
  2. The faster you stop, the shorter the time and the more force you’re going to feel.

* The caveat is because the change is speed is the critical parameter.  I stipulated that the car comes to a stop because then the faster you’re going, the greater the change in speed.

This is why dashboards are padded and gymnasts work on mats instead of bare floor.  The padding elongates the time during which a person is slowing down.  If you take twice as long to stop, you experience half the force.

Surprisingly, those wild crashes at Talladega where the car rolls a couple times are actually the better kind of crash to have (if you must have a crash):  every time the car hits, there’s a small change in speed and a small force.  While the driver might want the crash to end as quickly as possible, the fact that the car comes to a stop over a longer time is actually responsible for his not experiencing a larger force.

The SAFER (Steel and Foam Energy Reducing) Barriers have a lot of different functions, but the one I’m going to focus on here is how they elongate the time of collision.  SAFER barriers are made (surprisingly) of steel and foam.

The outer wall of the SAFER barrier is made of stacked 8″ square hollow steel tubing with 3/16″ thick walls.  This tubing is strong enough that it is hard to bend, but not so strong that it won’t flex.  That motion is the key to safety.  The time during which the barriers move is that much more time that the collision is being extended.

Because the wall is made of long continuous sections, the wall flexes instead of moving.  The same idea is in effect with tire barriers – the elasticity of the tire stacks allows them to move and that makes the time of contact longer.

One of the problems with tire barriers is that a crash at virtually any speed requires re-assembly of the tire barrier. What if a second car is involved in the crash and it hits an area where the tires were already destroyed by the first car?  One of the challenges with the SAFER barrier was designing something that could withstand all but the most brutal of hits and not require repair.

When they show crashes in slow motion, watch to see how far the barrier flexes.  You obviously can’t just stack up the steel tubing against a concrete wall – there’s no room to flex and it defeats the purpose.  The foam triangles between the outer steel wall and the concrete barrier give the tubing room to flex.

They also absorb energy.  Foam is made of a lot air pockets.  The air pockets are separated by walls.  It takes energy and time to crush those walls.  As the foam is compressing, it also contributes to elongating the time of collision.

So why don’t they just put SAFER barriers all around every track?

Yes, it’s expensive.  I think the last quote I heard was something line $1100/linear foot installed.  That’s not the primary reason, though.

At some courses — notably road courses — speeds can get down to 40 mph in some of the tight turns.  Even a 3600-lb stock car (driver included) doesn’t have the momentum to displace a steel wall if it’s not going fast enough.  Jeff Gordon once noted that collisions with the SAFER barriers actually hurt more in lower-speed collisions because it is just like running into an immovable wall. The steel walls don’t flex, the foam doesn’t compact and you don’t increase the time of collision relative to a fixed wall.  Despite the fact that SAFER barriers save lives in high-speed crashes, putting them indiscriminately around a track could actually cause more problems that it solves.

The standard ARMCO barrier – the type you see on the highway – is more low-tech, but still very efficient barrier technology.  ARMCO barriers were designed to work at lower speeds – it takes less momentum to move them.  On the downside, they can take a long time to fix when destroyed in a crash.  Most importantly, ARMCO barriers are made of fairly thin steel.  If a barrier is broken (or a car hits the edge of a barrier), you have a very thin piece of metal hitting the car at high speed.  Take a look at the Robert Kubica’s rally car after it hit an ARMCO barrier – the barrier embedded itself in the cockpit of the car.  Kubica escaped with very serious injuries, including a partially severed hand where the barrier entered the car.  Twenty-four-year-old Intercontinental Rally Car co-driver Gareth Roberts died very recently in an accident in which he was impaled by a guardrail that entered the car.  Given the huge number of sportscar and rally car races that take place at road courses, perhaps its time to re-examine this technology, too.

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