I know this is a bit of an old thread, but I am fairly new to MN and this discussion is something I feel strongly about. Basically, rear facing is much much safer, and we all need to stop making excuses and start thinking of our children's safety.
The argument about rear shunts being the most common form of crash is not a valid one! Surely for every rear shunt, there is another car which is hitting you from behind (and that car is driving forwards). The mechanics which explain why rear facing is much safer is described below and can all be very easily explained using Newton?s three laws of motion (GCSE level Maths / Physics, so don?t be scared).
The three laws state that:
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Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
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The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma.
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For every action there is an equal and opposite reaction.
So what do they mean?
Well, let?s look at the first law. All it is saying is that in the event of a crash (let?s say a car doing 60 mph hitting an immovable wall or tree ? a pretty horrendous scenario), the child?s head will try to continue to move in the same direction and at the same speed (60 mph travelling forwards), unless a force is exerted on it from elsewhere. The only place this force can come from is the only thing it is attached to, the neck.
Moving onto the second law: In a crash, the car decelerates from 60 mph to 0 mph (a deceleration is just a negative acceleration). It is the rate at which this speed changes that defines the acceleration. Since the force on the child?s neck is equal to the mass of the child?s head times by the acceleration F = m x a, this means that as the acceleration term increases (i.e. the car slows at a quicker rate), the amount of force on the child?s neck increases proportionally.
All the third law does is says that when all these forces occur, there is always an equal force on another object. So the force of the car hitting the wall, in effect has an identical but negative force in the form of ?the wall hitting the car?.
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Taking the case above of the 60 mph car hitting an immovable wall or tree, and adding some simple maths for a forward facing car seat case, we need to calculate the rate at which the child?s head slows down (the acceleration of the child?s head). If the car has a crumple zone of 1m (i.e. the car will be 1 m shorter after the crash than before the crash due to all the bumper and metal compressing) and the child?s head continues to travel 0.5 m forward during the crash (due to the stretching of the seatbelts, the child seat harness and of the neck itself), then that gives a total distance of travel (for the child?s head) of 1.5 m from the start of the crash to the end (when speed = 0 mph). Some simple maths state that
v2 = u2 + 2 aS,
where:
a = acceleration
S = distance travelled during crash
u = Speed prior to the start of the crash.
v = final speed
Since v = 0 (i.e. the car and child come to a complete stop at the end of the crash), this can be re-arranged to:
a = -u2 / 2S
using our value of:
S = 1.5 m
u = 60 mph (converted to m per second gives 27 ms^-1)
When worked out, this means that during the crash the child?s head decelerates at roughly 243 ms-2, given that gravity (also an acceleration) is equal to 9.81 ms-2, this means that the child?s head will decelerate at roughly 24 g (24 times that of gravity), which in-turn means the child?s head will in effect ?weigh? 24 times it?s normal weight during the crash (with ?weight? being a force and in the direction of the car?s travel rather than towards the ground).
If a child?s head normally weighs 2.5 kg, then in the crash it will momentarily weigh 60 kg which is about the same weight as a medium build woman, hanging from your child?s neck!
Looking at the rear facing case, the total distance travelled by the child?s head is reduced, which actually makes for a higher deceleration and higher forces, but what actually happens is rather than this force all being exerted upon the child?s neck in the form of stretching, it is spread out along the back of the seat, which acts like a re-enforced spine for the child. This means that the actual load on any individual part of bone or body structure is relatively small as the load is shared amongst far more body structure.
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Looking at a 60 mph crash into a stationary vehicle. The calculation as above is identical, but all that happens is that the distance ?S? is increased. This is for two reasons, firstly because the second car also has a crumple zone and secondly because the car, when hit will be moved and as such, the energy of the crash will be absorbed over longer distances and time which means the acceleration to the child?s head will be smaller and hence forces reduced. This distance will be dependent upon many influences, such as the weight of the other vehicle the road surface and weather the vehicles brakes are on or off.
Using some example numbers:
Crumple zone of your car = 1 m (same as before)
Movement of child?s head = 0.5 m (same as before)
Crumple zone of static car = 1 m
Distance moved by static car during the crash = 10 m
All of this gives a total distance of S = 12.5 m
Put into the previous equation this gives an acceleration of 29 ms^-2 which is about 9 times lower than for the crash into the immovable wall or tree. This means the child?s head would ?weigh? 3 times more than normal, not insignificant, but in no-way as large a force than with the previous example. However, the reality is that the deceleration calculated here is an average over the whole crash, and it is likely that the vast majority of the deceleration would be achieved in the early parts of the crash, so this simple calculation will give artificially low values. It is probably reasonable to say that the acceleration would be about 10 g, giving 10 times the child?s head weight (using engineering judgement).
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Now is probably a good time to think about a rear shunt impact, since we have just considered a 60 mph car hitting a static vehicle, let?s look at things from the point of view of that static vehicle.
In the crash, the car is accelerated from 0 mph to a certain speed (depending upon the car weight, road surface and if the brakes are on or off). It is reasonable to assume that the effects on the child?s head would be the same as for that of a child in the car which is hitting the static car, but in an opposite direction (Newton?s 3rd law).
Thus, a rear shunt in a car with a forward facing seat would mean that the child?s head would ?weigh? 25 kg (based upon 10 g) but the force would be opposed by the back of the child?s seat, distributing this load more evenly with the seat back acting like a re-enforced spine.
Therefore, a rear shunt in a rear-facing seat would be the equivalent force on the child?s neck for the passenger of the static car, as a child in a forward facing seat in the 60 mph car.
So what can we learn from this? Well, if you are in a traffic jam and are rear shunted, then you can reduce the forces by keeping your brakes ON (including your foot brake since the handbrake only holds the rear wheels), and by keeping a small distance from you and the car in-front. This will decrease the acceleration of your vehicle (and more importantly your child?s head) since your car will effectively have more mass, especially when your car hits the car in-front the mass of the two vehicles combine (since Force = mass x acceleration, increase mass for the same force, you reduce the acceleration ? Newton?s second law). However, this action will without question make the acceleration of the car which is was doing 60 mph prior to the crash higher, thus increasing the forces on their body parts, but hey, that is their problem. It may also mean that you damage the front of the car if someone hits the back of your car - but who cares about the car.
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Now, let?s look at the case of 2 cars both doing 30 mph and having a head on collision. To analyse this, you basically have to think of Newton?s 3rd law. So for every action there is an equal and opposite reaction. If we consider both cars are the same mass and design (so have the same size crumple zone). Then we can consider the events for both cars to be like crashing into a ?crash mirror?, both experience the same forces. This means that the event is essentially identical to a crash into an immovable wall. Therefore, using 30 mph instead of 60 mph, we can calculate the acceleration and forces using the equation and data from the very first example:
a = -u2 / 2S
With this in mind, and using our value of:
S = 1.5 m
u = 30 mph (converted to m per second gives 13.5 ms^-1)
When worked out, this means that during the crash the child?s head decelerates at roughly 61 ms^-2. This means that the child?s head will decelerate at roughly 6 g, which in-turn means the child?s head will in effect ?weigh? 6 times its normal weight during the crash.
As you can see, this is very much lower (4 times lower) than a crash of 60 mph into a tree! It is also NOT the same as crashing at 60 mph into a stationary car.
However, a head on crash with both cars doing 60 mph would give the exact same results as a car doing 60 mph hitting an immovable wall.
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So what does all of this tell us? Well basically, the highest energy accident you could ever have would be a head on crash into a static and very large object (eg a tree, wall, crash barrier, slow moving lorry), or into an object of similar size travelling in the opposite direction. In these events, the forces on a child?s neck due to the neck decelerating the child?s head are unacceptable high! Having your child in a rearwards facing seat would vastly reduce the force on individual body parts, distributing the loads and by providing a ?virtual spine? for the child to do all of the work.
Rear impacts are definitely lower energy and you can reduce the force on the child?s neck, for both forward and rear facing seats, by keeping your brakes on when stationary, having a heavy car and staying close to the car in-front in a queue, however this makes things worse for the occupants of the car which is moving when it hits you.
Without question, a forward facing seat provides better protection in a rear shunt than a rear-facing seat, but the forces are much lower for this event than for the head-on crash events discussed.
As to the likelihood of a frontal, side or rear impact, you will have to ask a statistician. All of the information above is heavily simplified and energy transfers to heat and sound energy are assumed to be negligible and ignored. It also does not include the event of items from the boot continuing forward and impacting the car occupants, for which a rear facing seat provides no protection (but you could just buy a dog divider)
I hope this helps and I look forward to other comments.
The poster accepts no liability for the accuracy of this post or for how the information within the post is used for decision making purposes.