The front roll center location is extremely important to the performance of your race car chassis. It’s position will determine how your suspension will react to the forces and motions caused by negotiating a turn.

There are two reasons why we need to know the location of the roll center. One is to help determine the camber change characteristics and the other is to help determine how the dynamic forces will influence the handling of the race car. The height of the roll center (above or below the ground) affects the camber change characteristics. The position left or right of the centerline of the car will determine how the suspension will react to the dynamic forces which will cause the car to roll in the turns.

Lets measure a race car together so that we can analyze a typical double "A" arm front suspension system. We will use these measurements in a two dimensional (height and width) geometry software program. We want to determine were the roll center is located in order to determine how that location influences the geometric and dynamic characteristics of our race car.

It is important to accurately measurement these points if you expect to get accurate information back about the position of your roll center. Here are some helpful instructions on how to correctly measure the suspension points on your race car.

The first consideration is to find a level floor on which to measure. Professional teams use a surface plate made of thick, machined steel. Not all of us are so fortunate. So instead, lets find a portion of our garage floor that is flat and level. We only need and area that measures about 65 inches by 20 inches and that is level and flat to position the front suspension over.

The measurements will be much easier to take if the engine is out of the car. If you cannot get them while the engine is out of the car, try to "block" the car up, at the chassis corners, in order to provide easy access to the lower suspension mounting points. Use the same distance to block up at each corner. Remember to support the car safely and use jack stands as backups in case the cars shifts while someone is under it. You cannot use too much caution here. A good distance to block up is 10 inches. This is also an easy number to subtract from your initial height measurements in order to record the actual distance the points are from the floor if the car were at ride height.

We will need to establish a centerline of the chassis. To correctly analyze the roll center which will influence the camber change characteristics and dynamic properties of the chassis, we will want to use a centerline which is halfway between the tire contact patches of each tire. This is the centerline which the car will "feel" as the forces react on it in the turns. It will be important for us to know where our roll center is in relation to this centerline.

Ball joint measurements will be easier to take if the wheel is removed. Here is a method that will keep the spindle in the correct position after the wheel has been removed. Before you block the car up, measure and record the length of each front shock from bolt to bolt with the car at ride height with the race weight in it (with all fluids and the driver in it). Then block the car up as explained above and remove the shocks and springs (either the spring/shock combo in a coilover configuration or the shock and separate spring in a stock type big spring car). In place of the shocks, insert a piece of tubing or strap metal which has two holes drilled in the ends exactly the same diameter and distance apart as the shock lengths you measured.

Mark the center on the bottom of each tire and measure between them. Place a mark on the floor which is halfway between these two tire centers. Repeat this procedure for the rear tires. Then snap a chalk line over these two centerline points front to rear to produce the centerline on the floor that you will measure the width of each point to. Now you can remove the wheels and the spindle will be at the exact same vertical position as it was when the car was at ride height.

Now that the car is positioned so that we can get to the chassis points, we need to determine the center of rotation location for each point. This is easy for heim joints or mono ball joints, but many race cars use the OEM type ball joints. The center of rotation is not obvious for these type of ball joints and we will need a little help. Do Not "eye-ball" these points and guess at the location. Instead, use the information supplied by each manufacturer. Your supplier should be

able to get you this information. Be persistent and get this information before you start. Your roll center location will only be as accurate as your measurements.

Now we are ready to measure. First determine the height of each point. Measure from the floor to the center of rotation for each point. Remember to subtract the distance you blocked the chassis up from ride height from this measurement. We want a number which will represent the height of each point as if the car were at ride height with all of the race weight on it.

Getting the true height of the upper chassis mounts and ball joints is going to be difficult. You will not be able to measure at right angles to the floor directly to these points. There are suspension components in the way. So we need to be innovative. Again, Do Not "eye-ball" the measurements to these points. Use a small level to extend the height of the center of rotation out and beyond the suspension components so an accurate measurement can be taken. You will need a helper to get this done right.

To measure the lower control arm chassis points, use the front chassis mounting points on the chassis. Whether you have a strut type of system or an OEM type of lower control arm, you will need to use the front chassis mounting point for a two dimensional roll center analysis.

Now that we have recorded the heights of each point, lets measure the width each point lies from the centerline of the car. Use a plumb bob to project the lateral center of rotation down to the floor. Place a piece of paper tape on the floor to mark on. Put a crows foot mark on the tape and circle it so that it is easy to find. Measure from each point to the chalk line which marks the centerline of the race car. For this measurement, make sure the tape measure is at right angles to the chalk line. We want the shortest distance.

Once all of the points have been measured and recorded, we will enter this data in the geometry software program. For this example, we will be using the new double "A" arm geometry software produced by Chassis R&D. This WindowsÔ based software is made easy to use by showing us a picture of the cross section of the chassis. Each point height and width location is entered next to the picture point on the chassis diagram. The software will also allows us to quickly calculate the location of the roll centers and make changes to that location easily. The diaplay will show us the existing control arm angles, both upper and lower, as well as the lengths of the upper control arms.

To make the changes to the roll center locations, we simply type in the new control arm angles or upper control arm lengths and re-compute the roll center locations. The program automatically calculates the new chassis mounting point locations and it tells you how far each one has moved from the original location. You can then easily know where to move your chassis mounting points should you decide to change them.

It is important to look at the control arm angles which will determine the roll center location height and width, (both static and dynamic). To reposition the roll centers, we must change these control arm angles. For example, if we increase the right upper control arm angle and/or decrease the left upper control arm angle, we will move the roll center to the right from its original position. If we increase both the upper control arm angles, we increase the roll center height.