Now, on to matters at hand:

Newton discovered something most people knew but didn’t bother to talk about much: To make something move, you’ve got to push it! And, the bigger it is, the harder you have to push it! And, once it’s moving, it also takes a push to stop it - or to change its direction. No big revelation here, boy! But these basic laws of nature answer the most important question in a slot racers life: How can I get my car moving very fast, very quickly, and get it to go around corners instead of choosing to ignore them (as in crash and burn)? Of course, the laws of nature are a bit more complex than this, but there are many aspects of Newton’s laws that we can understand and use to our advantage. The first is the concept of center of gravity.

Everything has weight. Because the term "weight" involves gravity, the scientist refers to it as mass instead (things have mass even if there is no gravity). A slot car is no exception. The amount of energy that it takes to change the speed or direction of something in motion is proportional to its mass. From this idea, lets pretend to be scientists and lay down simple rule number one:

But it is a mistake to think of the slot car simply as a mass. For one thing, the mass is not evenly distributed throughout the car. The motor is certainly heavier than many other parts of the car, and each part adds mass where ever you put it on the car. Does this uneven distribution of the masses in the car make a difference? Standing still, probably not. But start moving and it becomes very important indeed!

Try this experiment: Remove the body and tie a piece of string to your car somewhere around the middle of the chassis. Hold the car in the air by the string. Does the car balance, or does the end with the motor in it hang lower? If you tie the string at various places until you find the perfect spot where the car hangs level, you have found the center of gravity. Well, sort of. What you have found is two of them - but there are three! You have moved the string to various places on the chassis until the car hangs level both nose-to-tail (the longitudinal center of gravity or "CG"), and side-to-side (the lateral CG). That’s two dimensions; but this is a real slot car in a 3D world! The third dimension is less obvious, but is the most important of them all! If you try to hang your car with the string tied along, say, the left side of the chassis so it hangs with the axles vertical and the right wheels toward the ground, you will find that it takes adjustment of the string to make the car hang exactly vertical. Moving the string forward on the car makes the back end hang lower, and moving the string rearward makes the front and hang lower. That’s the same longitudinal CG adjustment we made before. But what if the car hangs so that the plane of the chassis is not vertical, but tilted? This means that the string must be tied above the chassis or perhaps below the chassis to find the vertical center of gravity. This may not seem too important, but I’ll explain why it is.

When a car goes around a corner, what changes it’s direction? Well, the guide flag applies a force which re-directs the front end, but the mass of the car pushes against the tires, and traction prevents the heaviest part of the car, the rear end, from sliding right off the track or making the car go into a skid. That means that the tires can be thought of as if they push the car around the corner. But where is this push applied? If the tires bite, then the force of the tires is applied to the chassis where the connection is made - at the axle. This is called the center of thrust.

If you, in the heat of race-battle anxiety, were to give your arch rival (who nerfed you really badly in the last race and is trying to do it again now) a good hard shove at the knees, his body would remain about where it is and his legs would go out from under him. That’s because your rival’s center of gravity is most likely somewhere around his midsection, and the thrust (or push) that you have so rudely provided is being applied well below this - at the knees. If you wanted to move him, you’d have to push him at or near his CG to do the job. Otherwise, you’ll just spin him around. (As I get older my own CG tends to move lower, I’ve noticed.) When the tires on your car apply their much-needed force to get the car around the corner, they should to do the same - they should apply their force near the vertical CG of the car.

Let’s say that the car has a high mounted motor and so the vertical CG is about 3/4" above the track surface, and the tires are applying their push to the chassis at the axle height, about 1/2". Going around a corner, the tires bite, the axle holds, and the tires push the bottom out from under the car - just like you pushing your rivals legs out from under him! The car rolls right out of the slot because the center of gravity is higher than the center of thrust. The tires just shoved the car into a roll, and off the track!

Now, let’s say we do a little soldering-iron-surgery. We lower the motor down to the legal limit, we take some weight that we had high above the chassis and lower it, we take a chassis plate off the top of the piano wire and set it instead down between the piano wire, and we even move the lead wires down hugging the chassis - everything as low as it can be. We even take a few decals off the high parts of the body - it couldn’t hurt! The more it weighs, the lower it must go! Let’s say that all this work gets the vertical CG down to 1/2". Is it any wonder that the car suddenly corners much better? Now, when the tires push the car, they push at the CG. The car no longer has weight above the axles trying to roll it out of the slot. Instead of cornering like a Samurai four-by-four (ie: rolling like a barrel), it corners like a fine, sleek, low-profile sports car. (Now you know why they are low-profile!)

So, rule number two for our scientific blackboard:

Okay, one last item. What about that guide flag? It pushes, too, right? Of course it does! And, what’s worse, it pushes even lower than the tires and axles do! That means that the weight (mass) in the nose of the car must be even lower than the weight (mass) in the back! Or, in other words, it means that we should keep the nose as light as we can and move the weight rearward whenever possible. So, rule number 3:

By the way: Many racers feel that the front tires should stay up and out of the way so that they don’t steal any downward force from the guide flag and braids. But remember that once the car begins to tip the CG rises up higher and makes the situation worse! Using front tires to control this tipping action can help keep the CG low and get you through the corners.

That does it for center of gravity. Newton would be proud, I think!

Next time we’ll talk about diaplanes and airflow, and I’ll show you a little invention of mine that you air-control folks, and perhaps even non-air control folks might get a kick out of.