We all know a little bit about how cars handle, and the various components that comprise a suspension system, but how do you calculate the ability of your car to hold the road? First, we have to understand what g-force is. We all know the Earth's gravity is what keeps things on the ground. The weight of an object being pushed to the ground is one g. For example, if you weigh 180 lbs, then every 180 lbs you are pushing against the ground equals 1.0 g.
Another factor to keep in mind is the physics definitions of velocity and acceleration. If you are traveling 20mph in a straight line, and maintain that speed around a turn, your velocity has increased, meaning you accelerated. The force acting on your car is known as centripetal, or lateral, acceleration. All of this gives us the formulas we need to compute our lateral holding capacity.
 The first step is to find some remote area where you can mark off a relatively large circle. If you have a friendly auto-x nearby, you can probably talk everyone into helping you set it up. Just have someone stand in the center of the area with a large tape measure, and take one end of the tape and walk out at least 50 feet (FYI: most skidpads have a 100 ft radius). Place a cone or marker and continue in a circle, leaving cones until you feel confidant the boundary is shown. Mark a starting line and you are ready to go.
Once you have your skidpad marked off, it is time to start the driving. Have a friend with a stopwatch keeping an eye on the starting line. Drive around the circle until you feel comfortable making your run. Signal your friend to start timing the next time you pass the starting line. You can either do one timed run at a time, or compute an average over several laps. Once you have that average, or single time, whip out your calculator and plug in the numbers for the following formula:
Lateral g-force = 1.22 * radius of track / (lap time * lap time)
Make sure radius is in feet, and time is in seconds. The number you are left with is your maximum lateral g-force.
While driving, your front tires should be on the verge of breaking loose, unless you have done enough tweaking to your setup to cause oversteer. Gentle inputs on the gas and steering wheel should help you get better times. The last thing to know is that your holding capabilities may change when you go in different directions. Compute your times going both directions, then average those for your real results. These numbers will also change drastically with different pavements and tire compounds. If you hit anything above .95 g, you have a serious handling machine.
- Greg
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Posted 4/11/2003 1:30:55 AM
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