Ballasting is one of the oldest techniques in racing; while largely supplanted by modern suspension and tire engineering, it still has a place in certain applications and in racing classes that require a minimum race weight. Where you put the ballast is just as important as how much you use -- more so, in most cases. Chassis engineering isn't a simple science, but bearing these ballast tips in mind will help you to decide where and how heavy your ballast should be, and whether you should have any at all.
A tires' traction effectively increases with the mass placed on it. Ideally, all cars on earth would use all-wheel drive, four-wheel steering and maintain exactly 25 percent of the car's mass on each tire. This arrangement would hypothetically distribute traction perfectly to all four tires, ensuring that no one tire is ever over- or under-loaded under static conditions. However, automobiles don't work on a static basis; movement in any direction changes how everything works.
Acceleration causes a car's weight to transfer backward, braking shifts weight forward and turning shifts weight to the tires on the outside of the turn. Rear-drive cars do their best work under acceleration, when weight shifts backward and shoves the rear tires into the ground. When deciding whether or not to add ballast to the rear of your car, you need to determine what your primary goals are. Adding weight to the rear will aid in acceleration, but may not benefit the car under other circumstances.
Where you place your ballast has a major effect on how it will affect your chassis dynamics during load transfer. Drag racers will typically relocate the car's battery to the trunk, placing it right over one of the rear tires. Because of the direction of axle rotation, a chassis will always twist to one direction or the other. A clockwise-rotating driveshaft (when viewed from the front) will tilt the chassis toward the driver side and try to lift the passenger-side rear tire; a counterclockwise-rotating driveshaft will lift the driver-side tire. Placing weight over the lifting tire will give it a bit of traction bias during launch, helping to keep it planted.
Front-to-rear ballast position is very important. Weight placed behind the rear axle will have a far greater effect on rear traction than weight placed directly over it. It will also, however, cause the car's body to act like a see-saw and try to lift the front tires. Think: wheelie. Weight directly over the axle won't increase rear traction as much, but it will reduce front-end lift. The closer the ballast gets to the midpoint between the car's axles, the less it will contribute to rear traction and the less impact it'll have on front traction.
Ballast placed closer to the axle's center-line will increase rear traction by weight alone, while a higher ballast position will act as a lever to shift more weight from the front. Think of the ballast as a hand on the end of a lever connected to the rear axle shaft. The longer that lever, the easier it'll be to rotate the front of the car body upward and shift weight to the rear. To put it simply, placing weight higher will allow weight to shift more quickly backward during acceleration, and more quickly forward under braking. It will also cause the rear of the vehicle to roll more while turning, and will increase the amount of time it takes for the rear to get level again.
Polar moment of inertia is a very important factor when it comes to placing ballast. Polar moment is the distance of a car's mass from its center of rotation (or center of gravity). Imagine holding a 10-pound bowling ball close to your chest, then spin quickly clockwise; stop suddenly and spin counter-clockwise. Because the ball is very close to your center of rotation, you can change direction quickly. Now, imagine holding the ball out at arm's length and doing it again. Odds are that you'll twist your spine out of alignment if you try to move too quickly. The same logic applies to a car -- the closer you place a weight to the car's center of gravity, the quicker the car will be able to change direction and the more smoothly it will transition from acceleration to deceleration and back.
All else being equal, weight is almost always the arch-nemesis of performance. Increasing a car's weight forces the chassis and engine to work harder to maintain control, and requires larger tires to more evenly distribute that weight on the ground. Moving-required weight from one end of the car to the other is fine if that weight has to be there anyway (as in the battery example above), but adding weight is generally bad for all-around performance. That being said, a bit of ballast can be helpful in traction-limited applications where increased acceleration and handling and braking balance are more important than outright braking or handling performance. Examples include off-roading and drifting (where the tires will regularly lose traction during handling and braking no matter what) and top-speed racing where vehicle weight, handling and braking are far less important than horsepower, accelerative grip and aerodynamic profile.
