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Dynamics Acting on the Car

Let’s face it…regardless of the power plant that you have in your car or truck, if you can’t get that power applied effectively to the track and get around corners, you could easily come in behind an underpowered car that has their suspension tuned to the track. You also need to think about tuning your suspension so that it is putting you in the proper position to have the quickest exit out of the corner to take advantage of whatever your power plant has to offer you to get you back up to speed faster on the straights.

In this article we will discuss the forces acting on the car's suspension and chassis, and how they change under different circumstances.

Physics (Don’t worry…No math)

It’s no hidden secret that the tires of the car are the contact point between the car's suspension and the track surface, and between all four tires, the entire weight, or mass, of the car's suspension and chassis is supported. With the car sitting still (no motion), all of the force acting (gravity pushing the weight of the car straight down) on the four tires is a constant. The actual weight on each wheel may be different; however it is not changing when there is no motion because the acting force (gravity) is constant.

Now, once the accelerator is pushed, it accelerates, or forces, the car forward. I’m sure your saying “no kidding, that pretty stupid, I already knew that before I learned to drive!.” I know it’s basic, but fundamentally important. The big thing to take away from this is what happens to the weight/mass of the car when this happens.  When the car accelerates forward, some of the weight (or mass) of the car is transferred to the rear tires, and decreased on the front. And this is regardless of a Front Wheel Drive (FWD) car or Rear Wheel Drive (RWD). The total weight/mass of the vehicle has NOT changed, just the location of it over the four tires. The faster the acceleration which forces the car forward, the more weight/mass will shift to the rear tires, and at a quicker rate.

This explanation of how the force of the acceleration affects the weight/mass of a car is a perfect example of Newton’s Second Law of Motion. This law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass/weight. So in our example, the acceleration of the car (our object) is directly proportional to the force applied by the drive train (net force), and inversely proportional, or opposite, to the weight/mass of our car, causing the weight to be transferred to the rear tires as we accelerate forward. This is why the front of the car tends to rise up under acceleration.

The same holds true under braking. The weight/mass of the vehicle is already in motion, and when the braking force is applied, the deceleration of the car causes the weight to transfer over the front tires and decrease over the rear tires. This is also seen while braking. The front of the car will dive as the braking force is applied. Again, the total weight of the car doesn’t change, just the proportional amount of the total weight on each of the tires.

So far we have covered the weight transfer under acceleration and braking. What about turning, or cornering?  This is a touch more involved as far as how the forces work, and I will cover that a bit later in the series, but for now let’s go over the basics. The basic concept is relatively the same as under acceleration and braking, except that during cornering at a constant speed, the weight will be transferred either to the left side tires or the right side tires.

Remember our discussion of Newton’s Second Law above? The mass reacts opposite to the force of acceleration or deceleration that is applied. Let’s make a left turn. We gradually change the direction (basically lateral acceleration in theory) of our car by turning the tires to the left. This now begins to move the direction, or lateral acceleration, of our car to the left, which begins to transfer the total weight proportionally to the tires on the right side, and proportionally decrease the weight on the left side tires. Again, we haven’t changed the total weight of the car, just the amount of the total weight on each of the four tires. If we make a right turn, we would change the direction of the car to the right, and the transfer would increase the weight on the left side tires, and decrease on the right side.

One of the best ways to observe these forces in action is to take a piece of string or fishing line, about 12 inches or so in length and put either a fishing bobber, a tiny fishing line weight, or something similar (you don’t need anything very heavy or large. Just something to add a bit of mass to the end of the string) at the end of it and hang it from the rear view mirror mounted to the inside of the windshield of your street car/truck. With SOMEBODY ELSE driving, observe how the weight on the end of the string reacts in relation to accelerating, braking, and turning. You will notice that the weight will always want to go the opposite direction that the force is being applied. Again, make sure that someone else is driving so you can observe!

The important thing to remember is that these forces act equally on ANY type of vehicle, whether it’s a Formula 1 car or a dump truck. What makes the difference is how the suspension is designed and built to deal with these forces and to make the best use of the forces to make you faster. All of these forces act around a ROLL AXIS or ROLL CENTER, and that is determined by how your suspension is designed and mounted on your chassis.

In our ‘Control the Roll’ series will continue to discuss and explain how these forces affect your suspension and handling and how to get your car or truck to stick to the track better.

What you will learn in the ‘Control the Roll’ series that we are going to be making available very soon:

• How to ‘Balance’ your suspension set-up
  • For either Street Performance or Racing
• How to Find your Roll Center and Moment Center
• Recommended ‘Base’ set-up before hitting the track
• Learn about Scrub Radius, Sprung Weight and Unsprung Weight, Ackerman, Ball Joints, Springs, Shocks, Spindles, Sway Bars, Proper Weight Distribution
• How to see what your tires are telling you about your suspension set-up.
• How to tune your chassis to the track, or road
• Camber, Caster, Bump Steer, Toe Settings and how these change with suspension travel and what it does to your speed
• What to do if you are limited by your rules on the suspension “pick-up” points and how to do the best with what you have.

If you want to be put on the email list to be notified as we complete sections of this series please email us at info@dj2motorsports.com and put Email Notify in the Subject line.

© 2012 - DJ2 Motorsports LLC

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