Weight Distribution

Weight distribution is one of the important aspects of suspension tuning as the car's weight distribution affects car handling, acceleration and traction. If a car's weight is not evenly distributed on the four tires, the traction on the tires will not be equal, and the handling characteristics will not be balanced, from left to right, making the car unpredictable when negotiating left and right hand corners. For good car handling it is important that the suspension is tuned so that the car's weight is distributed in the same proportion front-to-rear on the left and the right sides of the car.

Checking the weight distribution on a Corvett

You need to measure the weight on each wheel to determine the weight distribution but this must be done with the normal driving condition of the car, that is, with the driver in place and an average amount of fuel in the tank. You can measure the weight on each wheel by placing platform scales under the four tires. The scales must be of the same height to ensure that the car is level. If the car is not level then the readings from the scales will be inaccurate. Once you have the weight on the four tires, you can add them to find the total weight of the car.

Now you can determine the front-to-rear distribution of the car by adding the weight on the front tires and dividing that by the total weight of the car. This can be converted to a percentage by multiplying by 100 as follows:

percentage on front tires = weight on left-front tire + weight on right-front tire
total weight of the car x 100

We can calculate the percentage of weight on the rear wheels in a similar manner:

percentage on rear tires = weight on left-rear tire + weight on right-rear tire
total weight of the car x 100

The percentages on the front tires and the percentage on the rear tires should add up to 100% and will give you the front to rear weight distribution. Ideally, the front-to-rear weight distribution on the left side of the car should be the same as the front-to-rear weight distribution on the right side. In other words, if the front tires carry 60% of the total weight of the car, then the left-front tire should also carry 60% of the weight on the left side of the car, and the right-front tire should also carry 60% of the weight on the right side of the car.

ADJSTING THE WEIGHT DISTIBUTION

You can adjust the weight on any one wheel by adjusting the ride height at that end of the car. For example, if you want to increase the weight at one wheel, you have to raise the ride height at that end of the car. However, when you adjust the weight at one wheel, the weight at the other three wheels is also affected and the weight needs to be redistributed. Therefore, if you want to adjust the weight at one wheel, you need to make a quarter of the adjustment at each wheel. Say, for example the front-left is 40 lb too heavy because of the driver, what you actually want to do is redistribute the weight equally by making the left-front 10 lb lighter and each of the other three corners 10 lb heavier. This means lowering the ride height on the left-front wheel until a decrease of 10 lb is reached, and raising the ride height on the right-front, left-rear, and right-rear wheels until the weight on each of those tires has increased by 10 lb.

With the weight distributed more equally, the car handling will be more predictable and the car will be easier to drive. However, changing the weight distribution also affects the car's center of gravity. In our next section we look at moving the car's center of gravity and the affect it has on the car's handling

Car Handling Basics

Body roll on a Lotus Exige

There are a number of aspects to your car's handling and cornering ability that we need to understand in order to fully appreciate our discussion on suspension tuning. We'll begin by looking at the various conditions and properties that affect your car's handling, and the different characteristics of road holding.

UNDERSTEER, OVERSTEER, BUMP STEER AND ROLL STEER

The terms understeer, oversteer and bumpsteer refers to the car's steering characteristics that are affected by the car's suspension settings. Understeer and oversteer occurs under cornering conditions, while bumpsteer (and roll steer) occurs when negotiating rough road conditions.

• Understeer occurs when the front wheels of the car tends to lose grip and drifts towards the outside of the turn, giving the impression that the car is straightening, or not turning in enough. The suspension setting on most stock cars tend to favor understeer. This is because understeer is seen as a safer option than oversteer for the average driver as the correction is instinctively accomplished by reducing speed or turning more into the corner.
• Oversteer occurs when the rear wheels the car tends to lose grip and drifts towards the outside of the turn, giving the impression that the car is turning in too much. The driver must counter oversteer by straightening the car a little otherwise the car will spin if it is pushed to its limit. Oversteer on rear wheel drive and all wheel drive (AWD) cars is sometimes useful for the experienced driver who can use the power to the rear wheels to balance the steering through the corner, allowing the driver to come out of the corners at higher speeds.
• Bump steer results from the suspension geometry occurs when the toe angle of the front wheels change as the suspension moves in an upward (bump) or downward (drop) motion as it negotiates the bumps over a rough road surface.
• Roll steer is similar to bump steer but occurs when the toe angle of the front wheels change as the suspension moves in an upward due to body roll.

For good handling and road holding, the car should not have a strong tendency for either understeer or oversteer, but should rather be at a point between the two, a point often referred to as neutral steer. Achieving neutral steer is one of the aims of suspension tuning, and is usually accomplished by increasing the traction on the wheels that lose grip.

Toe angle showing slight toe-in

WEIGHT DISTRIBUTION AND LOAD TRANSFER

Weight distribution and load transfer also affects the car's road holding. Load transfer is often, but incorrectly referred to as weight transfer due to their close relationship, however, load transfer is the effect of inertia or centrifugal force on the car's center of gravity rather than the actual transfer of weight. When a car accelerates, the traction force between the tire and the road surface moves the car forward. This force is acting at the road surface. But the car is also subject to an inertia force that acts against the traction force. However, the inertia force acts against the car's center of gravity, which is located above the road surface. The effect of these two forces, one pushing the car forward at the road surface, and the other pushing the car back at the higher center of gravity, causes load transfer to the rear of the car and results in the tendency for the front of the car to lift. This is called fore-and-aft load transfer or longitudinal load transfer and occurs during acceleration and braking. There is also lateral load transfer that occurs during cornering when centrifugal force acts sideways at the car's center of gravity, in a direction away from the center of the corner. Again, the effect of traction force at the road surface, and the centrifugal force at the higher center of gravity causes the load to be transferred from the inside tires to the outside tires.

So why is load transfer a problem? It's got to do with traction and tire grip. While the load transfer is always proportional so that the amount of load transferred from the front of the car is the same amount of load that is transferred to the back of the car under acceleration, the amount of traction or grip that is lost due to reduced load is not proportional to the amount of traction or grip gained by increased load. The amount of grip lost is always more than the amount of grip grained.

Load transfer is amplified by the height of a car's center of gravity, and by the amount of body roll of the car. It can thus be countered by lowering the car's center of gravity, which can be accomplished by lowering the car, and by reducing the car's weight. Load transfer can also be minimized by increasing the stiffness of the car's suspension, which will reduce body roll, and by increasing the car's track width and its wheelbase, in other words, increasing the distance between the wheels.

BODY ROLL

Body roll is another important consideration in suspension tuning. A car with excessive body roll affects the amount of power a driver can use during cornering as the angle of roll affects the camber of the wheels, which in turn affects the grip of the tires. As mentioned earlier, a large amount of body roll will also amplify lateral load transfer, particularly on a car with a high center of gravity, and will adversely affect tire grip. For these reasons, a race car will have very little body roll, but this translates to a much harsher and less comfortable ride. On production cars, ride comfort is usually more important, which is why most production cars have too much body roll.

Suspension Tuning

Front Suspension Coilover Kit

As you probably know already, the car's suspension system is responsible for driving comfort and safety as the suspension caries the car's body and transmits all forces, including power and torque, to the road in what is usually called traction. The suspension is a part of the chassis located between the body and the road, and includes the shock absorbers, or dampeners, the leaf or coil springs, they control arms the wheel and tires, and the steering system. Because the suspension transmits power and torque to the road, modifying and tuning the suspension has a profound effect on car handling and will allow you to get the most out of the performance modifications you've made as increased engine performance will change the physical dynamics involved when cornering and accelerating.

However, many people just lower the car and fit stiffer springs, believing that the lower ride height will improve handling; unfortunately the suspension and its modification and tuning is no simple matter. As with your engine, your suspension is a complex combination of parts that are designed to perform in a specific manner under different conditions, and the setting for each part can be modified to change the handling characteristics of your car.

If car modification is a compromise between performance and drivability, and it is none more so than when it comes to modifying the car's suspension. For example, the simple fitting of a lower profile tire improves steering response, and thus improves handling and road holding, by reducing the flexing of the tire walls. This allows for more precise cornering but it also transmits more of the road noise and vibrations as there is less rubber to absorb noise and vibration. Furthermore, a suspension modification aimed at improving one aspect of road holding can and a negative impact on another aspect of road holding. For example, fitting wider rear tires to improve grip at the rear end can also lead to oversteer. So suspension tuning is a bit of an art, especially when you need to set up your car for your style of driving, and given the different types of suspension systems in use today! Indeed, a car often has a different type of suspension at the front of the car and a different type at the back of the car!

In this section we discuss the full suspension, suspension tuning and suspension modifications, as well as their impact on drivability, in great detail. We'll also look at the different types of suspensions, and the different types of suspension springs. Well being by discussing car handling basics, before moving on to the unsprung mass, in other words the wheels and tires, before moving on to discus suspension lift, suspension lift kits and adjusting the ride height, modifying and adjusting the shock absorbers, and tuning the front suspension and rear suspension for improved road holding and handling.