Monday, December 20, 2004

You're A Good Man Charlie Brown

Remember how Lucy would always snatch the football away just before Charlie Brown tired to kick it? Well the Insurance Institue for Highway Safety does the same thing to automakers. Last night Dateline NBC broadcast a segment on their latest test report. So today the Kia Spectrum is getting pounded in the mainstream press. When you read the report, you’ll note that the government safety standards require the ability to protect the occupants in a 35 mph crash. The IIHS runs their test at 40 mph. At one time, the respective speeds were 30 mph for the NHTSA and 35 mph for IIHS. Are you starting to suspect a bait-and-switch?

The speed is important because it determines the amount of kinetic energy that must be dissipated in the crash. The higher the speed the more energy. In fact, the kinetic energy is proportional to the square of the velocity. So given a fixed vehicle mass, we can scale the energy dissipated versus speed. Squaring the respective speeds we get:

30 x 30 = 900
35 x 35 = 1225
40 x 40 = 1600

Examining the data, we observe that the amount of increased energy absorbed going from 30 to 35 mph is 325 units (1225 – 900). The ratio is 1.36 (1225/900) meaning the percentage increase is 36.1%. Similarly, going from 35 to 40mph is 375 units (1600 – 1225). This ratio is 1.31 (1600/1225). So the actual energy dissipated in the current test is greater in absolute terms than before (375 vs. 325) and starts from a higher initial point (1225 vs. 900). The engineering naif is prone to assume a 5 mph increase is the same whether you are going from 30 to 35 or from 35 to 40. He would be wrong! Note that the increased energy going from 35 to 40 mph is equivalent (375 units) to a 19.4 mph crash!

So Kia meets the government standards as written, but cannot absorb the equivalent of an additional 19.4 mph crash. Are you really surprised? Do you think that the weight saved might have been used to increase fuel economy? Which is more important to you, energy independence or the ability to drive into a wall safely at 40 mph rather than 35 mph? We report, you decide!

P. S. So my advice to Charlie Brown is this, if Lucy keeps pulling the football away, kick that which remains stationary, LUCY. She’ll learn really fast not to mess with a Good Man.


Post a Comment

<< Home