Momentum overload

These days, if a manufacturer launches a vehicle that doesn’t attain five stars in the tough 64 km/h EuroNCAP crash test, eyebrows tend to be raised. This is because research has proved that the more stars a vehicle is awarded in the laboratory, the better protection it offers in real-world crashes.

But I have a question… If you read EuroNCAP’s crash test protocol, you’ll discover that it simulates the effect of hitting an oncoming vehicle by means of a carefully-constructed deformable matrix attached to a solid (and stationary) laboratory obstacle.

Although this method does allow for representative and comparative testing, it has one drawback: in real life, it assumes that the vehicle concerned will be hitting another vehicle of identical mass. If you happen to drive something like a Mercedes-Benz E-Class, you’re in a pretty heavy car. But since heavy cars are in the minority on the road, the chances are that whatever you hit will be lighter than you. Which may be good luck for you because momentum is working in your favour; but it’s usually extremely bad luck for them…

In recent years there has been a vast swing towards smaller cars – meaning that if you drive one, whatever you hit is likely to be larger and heavier than you. So your Yaris’ five-star rating doesn’t mean much when you get swatted by a BMW 760Li coming from another direction. U Tube is full of novelty crash tests between small and large vehicles which demonstrate the effect of this “momentum overload” on small vehicles; even those that perform well in the solo crash test.

I believe the Euro NCAP test needs to be adjusted to correct the problem so that star ratings between vehicles of different masses are more comparable within reasonable limits; in other words, that the total energy expended in the crash is similar for all vehicles. The way to do this would be to have the test vehicle collide with a mobile solid object which travels (on rails perhaps?) at a speed adjusted to simulate the average weight of vehicles out there when compared to the mass of the test vehicle. I reckon 1.4 tons is a good average. This would factor extra energy – representing extra mass – into the crash without having to change the standard 64km/h impact speed of the test vehicle.

For instance, a heavy car might actually require the obstacle to be moving away from it slightly to create a fair comparison, while a lighter vehicle would require an obstacle approach speed of perhaps as much as 20km/h. It sounds like a strange proposal at first glance – but if we did it that way, five stars in an E-Class would mean the same as five stars in a Mini and we would be able to start coming even closer to real-world crash performance without leaving the laboratory.

No doubt vehicle manufacturers – especially the manufacturers of small cars – would cry and moan like they did when the original EuroNCAP star ratings were introduced; but it’s the next logical step in the improvement of vehicle crash-worthiness.


Rob Handfield-Jones has spent 20 years indulging his three passions: vehicles, road safety and writing. He heads up, a company which offers training in economical and safe driving.

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