Clearances Once we have determined the correct loading and/or suspension travel, we can measure the crossmember clearance, ride heights, front valance clearance, spring coil spacing and degree of binding, spring angle to top and bottom supports, sway bar angle and alignment, shock piston position, spring-to-shock clearance (for a coilover car), tire-to-fender clearance, component deflection, and more.

Now, those are a lot of areas to look at and you might be able to think of more. Most team goals these days involve attaining an attitude of the chassis and body that will be low and level to the racing surface to achieve maximum aero efficiency. With the correct input and pull-down travel, you can know exactly what the clearances are for many areas of the chassis.

When we calculate ride height changes due to the dynamic and aero forces, we often forget to include the tire deflection. The tire has a spring rate and thus a compression amount when a load is applied to it. So, we have a ride height change associated with tire compression that must be added to the ride height change due to spring compression. On the Pull Down Rig, all of that is taken into account and visualized.

Alignment One of the functions of the PDR is a system for measuring the wheels, front and rear, for camber change and steering angle changes. As the chassis is loaded, we need to take precise measurements at the wheels and the PDR does that, and the information is transferred to the computer for evaluation.

As the chassis compresses under loading, the suspension parts move and changes occur to the geometry. Cambers and casters change, bumpsteer-associated angles change with the control arms as well as the tie rods and even the drag link in that system. We can do a bumpsteer evaluation any time and at any chassis attitude or steering angle in our shop, but that doesn't take into account the compliance of the parts and pieces due to loading.

On the rig, we can do dynamic measuring of front end toe changes, Ackermann, camber and caster change, rear steer, and rear toe all under real track loading conditions. One of the least understood aspects of alignment involves the rear solid axle system. Under high loading, we often see with high lateral and vertical loading, there is compliance of the axle tubes and control links.

The Mittler PDR has the capability to measure cambers and toe changes as the chassis goes through its loading phase. With the late-model cars, the rearend will rotate as the chassis is compressed and that changes pinion angle, rear-wheel toe, and cambers. Imagine being able to physically measure all of that with the chassis loaded as it is on the racetrack. That is exactly what you can do with this rig.

Measuring The Effect Of Changes Each change we make to spring rates, sway bars, and so on comes with peripheral affects. One change can affect other settings and load distribution. Making changes and then evaluating those with the PDR can alert us to the consequences of our actions so that we are not surprised when we go to the racetrack.

Let's say we want to make a sway bar change to a larger bar. We can see the physical effect that change has on ride heights with the same tire loading as before the change. We can tune certain component selections to affect the chassis's attitude on the track. Our overall loading may be the same, but the attitude of the chassis and body to the racing surface can be fine-tuned with different combinations of springs and sways.

We may find that we need stronger components in certain areas of our suspension to reduce compliance and we may be able to reduce the thickness or strength of certain components that show to be less stressed to save weight. The beauty of the PDR is that you basically take a picture of the components and the overall chassis while it is fully loaded similar to the way it is on the track, but not going 165mph. There may be some things that will surprise you when you make your observations.