Metric Chassis Dynamics - Motion Equals Change

For every action on a race car there seems to be an odd and a seemingly unequal reaction. The actions/reactions of the front suspension of a stock car fall into this category. With a double A-arm stock car suspension, the wheels do not move absolutely straight up and down. This may show up as caster or camber gain when the wheel moves up in relation to the chassis. Wheels can also twist on their steering axis during vertical travel. This is called bumpsteer, or as one fellow put it, "bum steer." The point of all this is that when a wheel moves up or down in relation to the chassis, it also moves other ways.

To see what happens with all this movement, I went to the local salvage yard and procured a GM metric chassis sans engine, transmission, and body. This chassis is the basis of many Modified, Street Stock, and Hobby Stock class cars. The GM Metric refers to the '79-'86 Monte Carlo, Buick Regal, Olds Cutlass, and the Pontiac Grand Prix. These cars all had the same frame/suspension underneath; only engine/body combinations seem to differ.

The first step after cleaning the chassis was unbending it. These frames are very weak in the side rails and at the front frame turnouts when not supported by a rollcage. At the salvage yard, a large forklift picked up the chassis, now bare except for the engine and transmission on one end. Know when to hold 'em and know when (not) to fold 'em applies here.

The frame was straightened by using my Craftsman Twin Cutter to saw into the fold. This unique saw has contrarotating carbide-tipped blades. This allows the frame to be pulled back into line. Some steel straps were welded in place to keep the alignment. Alas, this chassis will not be suitable for race car use.

I'm not sure where others measure ride height, but I use points just off the ends of the side rails. These points seem to be consistent-as consistent as any points on these frames. Note that these frames are made of many stamped-out steel parts. They are jig-welded in what must be rather loose jigs. I suppose assembly line speed is important here. I've found from measuring a number of them that linear dimensions can have a tolerance of 3/8 inch in some cases. Diagonal dimensions, measured like an X, can have a tolerance of as much as 3/4 inch on some frames. This means you could also have variations in the location of suspension mounting points. Considering the chassis weakness, some of this disparity might be the result of freeway adjustments.

Once the chassis was leveled and blocked up on the shop floor, I made a faux wheel. This bolted to a wheel lug. It extends to the floor to simulate an 83-inch-circumference tire, something similar to an IMCA tire. This made it possible to always support the spindle at the same height during these procedures.

What we're trying to show is the relationship of vertical wheel movement to changes in caster and camber. A simpler way of explaining it might be: What happens to caster and camber when the chassis goes up and down?

On an A-frame car such as the GM Metric chassis, when the wheel goes up (bump) the caster and camber change. This is referred to more correctly as camber/caster gain. To achieve these ends, the bare chassis was leveled and blocked to a beginning ride height of 5 inches. Our faux wheel (steel plate) was attached to the right-front hub.

The net result of this is equal to the car sitting at a ride height of 5 inches with an 83-inch tire in place. This might not be a practical situation for a dirt track car, but it might work on paved tracks. In this case only 1 5/8 inches of bump travel is available. Remember that tire size will affect chassis height but not bump travel. When the wheel rises 1 5/8 inches (measured at the hub face), the lower A-frame will contact the chassis. Should track rules allow more bump travel, the chassis contact point can be modified for additional clearance.