The reason why the shock travel method might not match the predicted loading is because of the compliance of the components during high loading due to stress. Many of the components including control arms, bushings, spindles, crossmembers, axle tubes, brackets, and more, bend under high load. This affects the shock travel and the way we load the chassis affects those changes.

Real World To Test Rig In the real world, the loads affecting the chassis are spread out throughout the entire chassis and include the effects of parts bolted or welded onto that chassis such as the engine. It's virtually impossible to replicate that on a machine, but we can come close enough to gather some important data.

On a test rig, we have to attach to the chassis at specific points that are easily accessable. So, we introduce loads to those points in excess of what they would see as the car negotiates a turn. Nonetheless, we accept this deficiency and work with the simulation to help us understand what goes on with the chassis. The same is true of any current model of dynamic analysis apparatus.

Both the simulation of loads and the measurement of compo-nent movement are valid tools for replication of the event of cornering. I might be inclined to use a combination of both, utilizing the exact science of dynamics along with the somewhat imprecise science of translating a measurement although the measurement method helps to eliminate errors in the application of the loading from the track to the rig.

Uses For The Pull Down Rig Once we understand the way the rig works, we can think of many uses for it. Here are just some of the ways a team can utilize the rig to better understand how their chassis is working and to allow better design of components and setups for specific tracks. From the Mittler information sheet, "Typical studies, actions, or items of interest include:

• Stroking the chassis through complete travel to check for binding or tight clearances
• Study toe changes over the full suspension motion range
• Study camber changes over the full suspension motion range
• Evaluate corner weight effects of sway bar changes
•Study clearances to shock bumpstops
• Evaluate load distribution across tire
• Prepare for rapid testing/practice spring changes
• Accurately align chassis spring ends in mid corner positions
• Evaluate feasibility of unusual bumpstop setups
• Review mid-corner dynamic wedge percentage
• Evaluate accurate front splitter height and/or valance setting
• Study rear steer and camber changes
• Good substitute where sanctioning body testing bans are imposed"

Here are my thoughts on some of these evaluations and how they apply.

Load Distribution One of the most critical uses for the PDR is to measure and evaluate load distribution. As the chassis is compressed and rolled, the sway bar influences the load distribution just as the camber changes and coil binding and/or compression of the shock bump rubbers do. It is of great importance that we know what these changes are.

There is a complicated rearrangement of the loads that takes place as the attitude of the chassis changes. We could never predict or calculate the result of all of the influences. But the PDR shows us exactly where the loads go and that is translated into actual wheel loadings and crossweight percents.

If our crossweight does change through various increments of dive and roll, then the handling balance will necessarily change. It is possible with the PDR to make component changes and reach a point where the load distribution change is minimal and the balance stays consistent. Handling is all about how the loads are distributed and re-distributed and this use of the PDR seems to me to be of utmost importance.