Center of Gravity, Part II

Last time, I explained what the center of gravity (CG) is. This time, let’s look at why we care.

A fast reminder – the grip you have on each tire depends on the force pressing down on that tire. The force pressing down depends on the weight on that wheel, plus the aerodynamic downforce. Today, I’m ignoring aerodynamic downforce for the sake of argument.

Let’s start by trying to get the car so that the weight is distributed equally on all four tires.  (Yes, I know that’s actually not what you want for ovals, but I’m trying to make things a little simpler here, okay?)  Let’s assume a 3600 lb car+driver, so that would be 900 lbs on each wheel.

Here’s the problem. That weight changes when you brake, accelerate or turn.  You can divide a car into two pieces:  the sprung mass is that part of the car that is supported above the suspension.  (You often include part of the suspension mass in there as well). The unsprung mass is the wheels, tires, and lower half of the suspension.

The unsprung mass is more or less tied to the ground. It responds directly to any bumps or wiggles. The sprung mass, however, is attached via mushy, springy things like springs and shocks, which means that it doesn’t respond directly to changes at the wheels. This is good in the sense that a suspension isolates the passengers from a rough road; however, we know that in racing, the goal is speed, not comfort. The moving around of the sprung mass complicates things because it changes how much weight pushes down on each tire. In other words, the sprung mass changes how much grip you have.

The tippy truck sign (at left) is a good general breakdown of sprung and unsprung. The top half of the truck (the rectangle) is the sprung weight and the unsprung is the axle and wheels.

It’s not a perfect analogy, though. I hate to break it to you, but the sign is wrong. The truck doesn’t just rigidly rotate. The wheels stay on the ground (unless you’re taking that curve really fast), but the sprung mass does shift.  When you break, weight shifts from the rear wheels to the front. When you accelerate, it’s the reverse. Weight shifts from the front to the rear. When you turn left (as the truck in the picture is doing), the sprung weight shifts to the right.

That means that the grip on each tire also changes every time you speed up, slow down or turn. That’s why a car can be loose coming out of a corner and tight going in. The set up is the same – but the top part of the car is shifting constantly.

And here’s why the center of gravity is important. The amount of the shift – is proportional to how far off the ground the CG is .

The car in the left picture has a lower CG than the car in the right. That means that when the right car goes around a corner, it will experience a bigger shift in grip from the inner wheels to the outer compared to the car with the lower CG.

This is why SUVs and semis are much more likely to tip over than sports cars. When the CG shifts outside the box formed by the wheels, the vehicle will tip over. Stability means that your CG is firmly over your base.  Football players use the three-point stance (crouched low) because it does two things: it forms a wide base (the triangle formed by the two feet and the one hand) and it lower your CG. This is one you can try at home. Especially if you have little brothers because, face it, that’s what they’re there for, right?  Try to knock one over when they’re standing up with their feet together. Then have them move their feet apart. Finally, let them crouch down like a football player on the line and try to knock them over now.

In addition to tuckering yourself out, you’ve managed to show that a lower CG is more stable. For a race car, that translates into less weight shifting on acceleration, braking or cornering. This was a big issue with the COT (Gen-5) car because the CG was much higher (a few inches if I remember right) than in the previous versions of the car. Just raising the CG messed up everything the engineers had figured out in the old car, never mind all the other changes.

This is why the ballast for the race car is placed in the (left) frame rails. You don’t want to raise the CG. More weight lower in the car brings the CG closer to the ground and helps improve the grip on all four wheels around the corner.  Teams are even using carbon fiber composite (a very expensive material) in dashboards and seats to try to save weight and keep that CG as low as possible.

So that’s why keeping the CG low in the car is so important. The weight of the car doesn’t change – but the distribution over each wheel does and weight translates directly to grip. You can only go as fast as your least grippy tire, so the less change in weight transfer as you turn, the faster you’re going to be able to go.