You hear engineers and crew chiefs talking a lot about the racecar’s “center of gravity”. There’s a reason for all the talk. The center of gravity really is the point around which everything else on the car rotates.
Terminology: CG vs CM
You will hear people in racing use the terms “center of gravity” and “center of mass” interchangeably. It drives physics professors and people who work at NASA crazy. No one else really minds.
The reason for the two terms is that things weigh differently depending on the gravitational field they are subject to. If you are on the Moon, for example, your weight would be one sixth what it is on the Earth. Your mass – the amounts of ‘stuff’ that makes you you – doesn’t change, but the pull of the planet (or satellite) on which you’re standing makes you feel heavier or lighter.
The location of an object’s Center of Gravity is the same whether it’s sitting on Earth, the Moon, or Pluto. Each has a pretty constant gravitational field at their surface. If you work for NASA, you have to make calculations of things like galaxies, where the gravitational field changes throughout the object. That matters, and it means that the center of the mass and the center of gravity aren’t always the same thing.
However, if you’re driving a race car — unless something goes terribly, terribly, wrong — you will always be in a uniform gravitational field and the center of gravity will be the exact same thing as the center of mass. So we tend to use the two terms interchangeably.
We abbreviate them CM for Center of Mass and CG for Center of Gravity. The abbreviations are much short and it makes us look like we really know what we’re doing. Also, we use the cool icon at right to denote the CG. You may have seen this icon on crash test dummies (the real thing, not the band) before.
What is it?
The simplest explanation is that the CG/CM is the balance point of an object. Finding the CG/CM ob a uniform object is simple. Take a ruler, for example. If I asked you to balance the ruler on one finger, where would you put your finger?
Yep. Right in the exact middle. If I gave you a yardstick, the CG would be at the 1-inch mark. The CG of a meter stick is at the 0.50 meter mark. For anything uniform, it’s pretty easy – it’s at the geometrical center.
And you’ll notice from the donut in the lower right-hand corner that the CG/CM doesn’t have to be on the object. It could be out there in space. It’s an imaginary point.
Finding the CG for a racecar is a little more challenging because racecars are not uniform objects. There are heavy parts, light parts, and they’re all weirdly shaped. For most race cars that turn left, the CG/CM is located somwhere near the driver’s seat. (I mean literally the driver’s seat. His or her butt.) The diagram below is from an excellent book by Bob Emmons called the Racer’s Math Handbook.
You want the CG as low and as left as possible in the car (for oval tracks) because that’s the position that’s going to help you turn faster and circle track racing is won and lost in the turns. The location of the CG is often limited by your sanctioning body’s rules, either explicitly (specifying front/rear and left/right weight distributions) or implicitly (limiting how much ballast can go in the left frame rail).
Finding the CG height of a racecar is usually done experimentally by measuring how the weight is distributed between the front and the rear of the car, then raising the car and re-measuring. Longacre has a nice online calculator that not only tells you how to do the measurement, but explains why it works. Note that the CG of the car will be different with and without the driver in the car, especially if you have a heavier driver.
In Part II, I’m going to examine the relationship between CG and stability.