National Physical Laboratory

Measuring the Smart car

How many plastic balls can you fit in a car? Not a question you would normally have to answer, but the Materials team at NPL has done just that for Mercedes-Benz's Smart car division.

Smart car

Smart asked NPL to undertake measurements that would demonstrate the Smart's features, comparing them to everyday scenarios to which consumers could easily relate. There were several options available to the team. They could characterise the materials used to build the car, or measure the tolerances of parts of the engine with very high levels of accuracy. But whilst accurate, such precision measurements were unlikely to deliver meaningful results to consumers.

NPL's Materials team devised a series of tests that were indicative of the real life circumstances Smart owners could expect to experience and which would deliver results that could be easily compared to everyday situations. They tested the resilience of Smart's body panels by flexing them in normal environmental conditions and then after they had been super-cooled with liquid nitrogen. They dropped large ball bearings on the panels from great heights, and threw cricket balls at them to replicate the energies they would need to withstand during minor road collisions and prangs. They measured the vehicle's turning circle; they tested braking distances, and the temperature of the brake discs after heavy braking; they also assessed the amount of energy used by the car in its lifetime. One of the more unusual approaches came when the team needed to measure the internal volume of the car, as a way to assess the amount of room inside the car for passengers.

"This is a good example of where relevance became a priority over precision," explains Dr Nick McCormick, who led the project. "We looked at lots of ways to measure internal volume. We considered filling the car with tiny plastic particles or even water to ensure we reached into every nook and cranny. But it quickly became clear that we didn't need to use such excessive or potentially destructive approaches. Achieving meaningful measurements did not warrant total precision. We used plastic balls about the size of a human fist instead because they represent the realist sphere of movement of a person's hand inside the vehicle. In this application the degree of precision required means the balls are a more relevant representation of usable space for people inside a car, and this is what mattered in this case."

NPL's report on the Smart car delivered some interesting results - including a series of comparable facts about Smart cars that have since appeared in advertising campaigns throughout 2008 and have allowed the company to make some impressive claims. For example, that Smart car passengers benefit from the same legroom as a business class passenger on most commercial airlines; and that the car's internal safety cage can sustain a collision with a rhino at full charge.

So how many balls can you fit in a Smart car? 3,441 apparently...

For more information, please contact Nick McCormick

Read more about NPL's research in Materials

Last Updated: 12 Apr 2012
Created: 28 Jan 2009