christopher heiser <christopher AT heiser DOT net>
Date Published: January 25th, 2007
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Finding the Center of Gravity

Over the years I've designed (and in one case helped build) aftermarket brake systems for cars. One of the more interesting aspects of this process is trying to determine the right size hydraulic and mechanical components to use to maintain an optimal braking bias between the front and rear axles. Over the past few years, automotive manufacturers have added computer-controlled systems that automatically adjust the bias dynamically--but these systems are usually tied directly into the main PCM and are nearly impossible to adapt to other cars.

So you're stuck with plain ol' brake systems. And one of the most important inputs into your overall design is height of the car's center of gravity (Cg). There's a nice webpage that explains the physics of braking in detail and shows how the Cg affects the optimal brake bias.

The problem is that car manufacturers don't publish this specifications for their cars, although they routinely claim that the Cg has been lowered by 1" compared to last year's model, and so forth. Very frustrating!

Well, enter the US government to make things right. Sort of.

Starting with the 'rollover' hysteria around the Suzuki Samurai in the late '80s, the National Highway Transportation Safety Administration (NHTSA) began testing cars to determine their propensity to tip over under emergency maneuvers. NHTSA developed two tests--a static test and a dynamic one. The static test was essentially a measurement of the height of the center of gravity compared to the width (track) of the car. By placing the car on an articulated platform, the Cg could be measured by noting the weight transfer as the car was rotated on the roll axis.

This is very cool for the play-at-home designers. Using the data on NHTSA website we can find the Static Stability Factor (SSF) for vehicles like the 2005 Miata. The SSF is a scalar value of the vehicle track divided by twice the height of the Cg. By simply looking up the track of the Miata on a random website we can do some simple math to determine that the Miata's Cg is 18.4" above the ground.

Which turns out to be pretty low! Here's a few more that I calculated, all 2005 models:

  • Ford Mustang: 20.4"
  • Mazda RX8: 18.5"
  • Nissan 350Z: 19.3"
  • Acura RSX: 21.0"
  • BMW Z4: 18.8"

The NHTSA site doesn't nearly cover all the cars on the road. They spend more time with high-volume sedans and SUVs than with sports cars. But it's nice to see some actual data--and it's not to hard to extrapolate from the available data to your own specific project.

Your tax dollars at work!

by Christopher Heiser on January 25 02:43
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