| THE BASICS - BALANCE |
| Tuning, if we were to sum it up in one word, is about balance. The first bit we are going to discuss about balance is the front to rear weight ratio or bias. |
| The vehicle centerline, for purposes of front to rear weight distribution, is the midpoint between the front and rear axles of the car. |
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| Every car has a different weight distribution. Ideally, a car with a 50/50 weight ratio is desired, but race engineers have been pretty creative along the way, in regard to the chosen path of vehicle design, to achieve it. |
| There are distinct reasons why we need balance in our tunes. In order to take this knowledge to the next level, we need to understand the forces of nature that are acting on our vehicles as soon as we take them from a static state and set them in motion. |
| As soon as you set your car into motion, there are physical forces acting upon it. The three basic types of motion are: |
| 1- Linear or straight line acceleration. 2- Linear deceleration or braking capacity. 3- Cornering force or cornering power. |
| Balance is most noteworthy when talking about cornering. A car in any state of motion is a complex subject. When it comes to road racing, fast laps are made (or not made) in the corners. In order to understand how balance while cornering is affected, let's analyze. |
| VEHICLE CENTER OF GRAVITY |
| "The center of gravity of any body is defined as that point about which, if the body were suspended from it, all parts of the body would be in equilibrium." |
| Carroll Smith - 'Tune To Win' - pg.29 |
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| Let's assume in the diagram above that the car has a 50/50 front to rear weight bias, as well as being evenly weighted from left to right a shown. |
| Why do we need to know this? Because the centrifugal force generated while turning comes from the car's center of gravity. And it is because of this, in part, that balance is affected. |
| CENTRIFUGAL FORCE |
| Centrifugal force is a product of a vehicle's lateral acceleration, generated as the car enters a turn, achieves (even if briefly) a steady state condition in mid-corner, then applies throttle on corner exit. |
| Centrifugal force, lateral acceleration, and cornering force of the car's tires are all measured in g-forces, and they're all closely connected. |
| In the accompanying diagram, the car is turning right. If we draw one line vertically from the contact patch of the outside rear tire toward the center of the turn, and another line perpendicular from the steering angle of the outside front tire's contact patch, they intersect at the turn's geometric center. |
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| The centrifugal force generated is the equivalent of attaching a rope from the geometric turn center to the car's center of gravity (as illustrated by the red arrow) and spinning it in a circle. |
| The force generated wants to throw the car outward away from the center of the turn, passing though the vehicle's center of gravity. |
| The only thing keeping this from happening is the opposing cornering force being generated by the tires. |
| CORNERING FORCE |
| Cornering force is all about the tractive capacity of the tires. We'll dedicate an entire section to tires, but for now, let's stay foused on how tire traction specifically affects vehicle balance. |
| In order to achieve a steady state, or state of equilibrium through a given corner, the tires must generate an opposing cornering force (measured in g's) equal to the centrifugal force being generated that wants to throw the car toward the outside of the turn (also measured in g's). Both forces push against each other in opposite directions. |
| In order to grasp the concept of cornering force it must be noted that even thought tires are inanimate objects, they DO generate force. If you recall anything from high school physics, if you put a 1 lb. book on a table, the table exerts 1 lb of force upward on the book to keep it stationary. Tires are no different. If the car applies 1g of centrifugal force on the tires, the tires must generate an equal and opposing force of 1g to keep the car in a state of equilibrium. |
| Just to add some simple numbers to the mix, let's say our car weighs 2,000 lbs, and the car is cornering at 1g. 1g equals the force of gravity, so the car is generating 2,000 lbs of centrifugal force. The tires, in turn, must apply 1 lateral g or 2,000 lbs of opposing force via the road surface to keep the car balanced. Sounds simple enough, but here's the kicker: |
| Centrifugal force is generated from one singular center of gravity, while the opposing cornering force of the tires comes from two separate pairs, (front and rear). It's here that the vehicle's balance gets thrown. |
| There are many, many factors that determine cornering capacity of the tires, namely gross vehicle weight, vehicle load transfer characteristics, and size and characteristics of the tires. |
| Without going off on a tangent about the variables of tire adhesion, what do you think will happen if the front tires are generating more cornering force than the rear tires? And what do you think will happen if the rear tires are generating more cornering force than the fronts? |
| OVERSTEER AND UNDERSTEER RESPECTIVELY |
| Look at the diagrams below and visualize a scale, with the CG (acronym) acting as the balance point. The scales simply tip in the direction of the most weight. Car balance is essentially the same. The only difference is that the forces acting on the car are lateral as opposed to a scale, where the loading is done vertically. |
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| If you can grasp the concept of balancing oversteer and understeer and apply it to tuning, you'll be well on your way to success. |