Shamelessly copied from the MGCars BBS news item:
http://www.mgcars.org.uk/mgcc/sf/000503.htm
having originally been published in the MG Car Cluib magazine.
Laws of physics when hitting a concrete wall.
Russ is an example of the effects of pure deceleration forces that were
marginally sublethal. The G forces (multiples of the force of gravity)
experienced during this type of event are calculated by G = 0.0039v2/d,
where v is the speed in km/h and d is the distance over which the
deceleration occurs, in metres. The MGB did not deform much during the
abrupt hC deceleration - this saved his legs from external crush, but the
lack of crumple increased the forces on Russ because the distance over which
deceleration occurred was shorter Let's say the speed was 120 km/h and the
M.G. had 0.75 metre of crush as it stopped, both I think reasonable
estimates. Therefore the deceleration was just under 75G. This represents a
potentially fatal injury - at much over 60G, either the heart ruptures or
the aorta falls off the heart. Russ's feet were up against the footwell and
decelerated at the same rate as the rest of the car, and indeed Russ has
fractures of bones in one foot. The footwell was not deformed and there was
no intrusion. It was purely the force of the weight of his foot against the
footwell that caused the fractures. If Russ's body had decelerated at this
rate, he would have died from multiple injuries. Why was he saved?
The harness stretched out forwards so that his torso moved over some
distance - perhaps a bit too far as the steering wheel was bent, but the
deceleration was spread over almost another metre. This gets the
deceleration down to below 40G, which is survivable for the human torso.
Then his head - this flew forwards another half a metre, so that the G was
force was down to under 20G. However, the stretch on the neck must have
transiently been much greater than this, as the neck would offer little
resistance to forward movement until it got right forward. If the terminal
deceleration on the neck was about 25G, then the force on the neck was
twenty five times the weight of the head plus the helmet. This injury, I
think, made Russ unconscious. The top of his brain was squashed against the
inside of his skull and his brain stem, lower down, was stretched. All this
occurred momentarily before the whiplash. You can see why the brain gets
scrambled, even if it swims around in a little pool of cerebrospinal fluid,
cushioning impacts. It is likely that some of the nerve-to-nerve contacts
(synapses) rupture or are disrupted during the impact, and the severity and
extent of this disruption determine the extent of brain injury. The brain
can look normal to the naked eye, but becomes unwired. Russ was unconscious
for about 10 minutes, not so bad as some other well known drivers - Mika
Hakkinen had a much worse brain injury in Australia in 1985, but, as with
Russ, there seems to be no lasting cerebral injury.
The safety message
Russ's survival in this impact was down to perhaps the most mundane article
of safety, which most of us take for granted. The safety harness is a
brilliantly engineered piece of kit that converts a lethal injury into
graded deceleration and survivable impact. So make sure your harness is not
old and has not been stretched in an accident before - they don't stretch
twice. Make sure your mounting points are secure - if Russ decelerated at
25G and it was probably greater, the shoulder mounts would have taken a
force of over half a ton each. Indeed, examination of Russ's car apparently
showed the metal around these mounts was deformed, but had not given way.
Russ commented to me that in his MGA, the mounting points would probably
have given way - so think about strengthening your harness mounting points,
and spreading the load area. When you buy a helmet, get one that is strong
and light - a heavy helmet can be bad news, so invest well. Make sure your
seat is firmly attached to the floor - you don't want that on your back.
Lastly, if you have a six point harness, do arrange the crotch straps
carefully when you strap yourself in. If your pelvis weighs 20kg and you
decelerate at 25G, you've dropped half a ton in the area of your testicles,
so your eyes might water.
Rob Higgins, Chief Medical Officer for the M.G. Car Club meeting at
Silverstone.
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