47 What are the pros and cons of gravel traps, tyre walls, armco & walls?
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This article is from the Motorsport FAQ, by A H Henry bspahh@midge.bath.ac.uk with numerous contributions by others.
47 What are the pros and cons of gravel traps, tyre walls, armco & walls?
When a car leaves the circuit, the aim is to slow it down as smoothly
as possible to reduce the peak deceleration on the driver. The car and
debris should also be kept out of the way from the oncoming traffic if
at all possible. The kinetic energy of a car is proportional to its
weight and to the square of its velocity.
The physics of the situation is comparatively simple. Generally a fit
driver can survive 50G impact if they are well restrained and so long
as it doesn't last for too long To slow a driver from 185MPH without
exceeding 50G, the shortest time in which you can stop the car is 1/6th
of a second which takes 22 feet at a steady 50g deceleration.
185 miles per hour = ((185*1609 m) / 3600 s) = 82.7 metres per second
decrease in velocity = deceleration * time
deceleration time = (82.7 m/s) / (50 * 9.8 m/s^2) = 0.169 seconds
deceleration distance = ((decrease in velocity * deceleration time) / 2)
deceleration distance = (82.7 m/s * 0.169 s)/2 = 6.98 m = 22.9 feet
It doesn't much matter what you crash into, as long as the driver is
protected from intrusions and that crucial 50G is not exceeded.
Realistically, whatever you hit is not going to slow you uniformly,
so you need to spread it out even more.
There are different ways to slow the cars, some of which are better
for some types of accident. The optimum solution for one type of
vehicle might be very bad for another.
Large grass run-off areas are cheap and easy to maintain and are
one of the best solutions for motorbike accidents. However, they mean
that the areas for spectators have to be set back a long way from the
track. They are obviously not an option at street circuits.
Gravel traps are commonly used on Formula One circuits. They work
very well for single seat racing cars and motorbikes. However, they
are not so effective for heavy saloon cars, as in NASCAR, where there
is a lot more energy to lose in an accident. Gravel traps are useless
when a car gets airborne. They also trap cars that stray into them so
that marshalls will not be able to push cars away from dangerous
positions. This means that tow trucks or cranes have to be available
for each and every race which makes them an expensive solution for a
long circuit - a problem if you want to run low budget racing series
at a profit. If a car manages to rejoin the track after visiting a
gravel trap, the circuit may get covered with gravel.
Tyre walls are a cheap way of cushioning walls. They work well for
low to medium speed crashes, but if a car hits one at high speed, it
can be bounced back onto the circuit, possibly into the following
cars. The car may also get snagged on the tyres and get thrown into
violent spins and rolls which may launch debris towards the
spectators. High speed crashes may also dislodge tyres from the wall
into the oncoming traffic. This problem can be solved by using
water-filled barriers instead of tyre walls like at Mickey Thompson
stadium off-road races. Another alternative is a large block of
expanded polystyrene. These are easy to move around for temporary
circuits.
Armco and concrete walls are good where cars hit them at a shallow
angle -- on a super-speedway for instance. The aim is not to absorb
the whole of the impact, but to deflect the car back along the track
to lose speed slowly. They are bad when a car hits them at a sharp
angle. The worst accidents on super-speedways occur when a driver
tries to correct oversteer by steering towards the wall. If he
regains grip, he gets sent into the wall at a sharp angle which then
causes a series of violent spins and rolls. Armco (crash barriers)
can absorb some impact. However, the barrier is still solid around
the supporting posts. Sometimes, single seat racing cars have been
able to `submarine' underneath the barrier which causes a very rapid
deceleration. Both types of barrier are bad for motorbike accidents.
Catch fencing was used for a while in the 1970s. Stretches of wire
netting was strung between lightly fixed posts. They worked well for
closed wheel cars, but if the nose of a single seater dipped
underneath the fencing, the drivers neck was left in a very exposed
position. They were particularly dangerous for motorbike accidents.
If adequate crash protection cannot be provided at a corner, the
track can be modified -- to allow extra run-off area or to slow the
cars with a chicane.
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