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.

The camber was initially set at 2 degrees negative (top of the tire toward center of the car). The caster was set at 3 degrees positive (top ball joint angled back from bottom ball joint). These settings were not mechanically changed on the chassis during our testing. Starting points always begin with the wheel on the floor. At each stage of testing, the ride height of the chassis was changed using wood blocks. All four corners were blocked level.

I started with an initial setup of 5 inches of ride height. This was measured at the ends of the side framerails. The static camber was set to 2 degrees negative while the caster was set to 3 degrees. With a jack under the lower A-frame (the spring and shock have been removed), the suspension on the right-front corner was raised until the lower A-frame contacted the chassis. At this ride height (5 inches) the suspension moved 1 5/8 inches, measured at the hub face. This we call bump travel. It is a small amount of travel. While it might be suitable for some smooth paved tracks, I doubt it works on a rough surface such as a dirt track.

Keep in mind that it's necessary to have enough bump travel so the suspension never goes solid, i.e., the A-frame doesn't contact the chassis. If any corner of the car goes solid, for whatever reason, then that corner will lose traction.

When I raised the suspension as described, the camber increased 1 degree and the caster also increased 1 degree. This is referred to as dynamic camber and caster, also camber/caster gain. This is a small change with a small amount of bump travel.

When the ride height of the chassis was raised to 6 1/2 inches, some other things happened. Our camber remained at 2 degrees. The caster, however, went to 6 degrees. For this project, I left the settings alone. If you were resetting the ride height of your car, then you would want to reset the caster.

At this new ride height, the suspension now has more room for movement. The bump travel measured at the hub face was now 4 1/4 inches. We found the dynamic camber had moved to 3 1/2 degrees while the dynamic caster increased from 6 to 11 degrees. There- fore, the camber gained 1 1/2 degrees while the caster gained 5 degrees.

So is this good, bad, or indifferent? It can be some of each. Camber gain is necessary so that the tire's vertical relationship to the track remains where you want it as the chassis rolls to one side in the corner. Caster gain, especially when more is used on the right than the left, can assist the car on corner entry. Excessive caster gain actually reduces crossweight, but don't spend much time worrying about it.

While moving the suspension about, I decided to check the bumpsteer. This is steering movement when the spindle is moved vertically but the steering shaft is held motionless. I used the Craftsman Laser Trac to throw a wide vertical beam alongside the chassis. I then clamped an aluminum angle to the brake rotor. Marks were made on the angle at 12 inches from the center of the spindle. As the suspension was moved up and down, I simply measured from these marks over the laser beam. I found this chassis to have more bumpsteer than I'm used to seeing on these metric frames. Usually, if all the stock suspension points are retained, the bumpsteer is close to accurate. Did my excessive bumpsteer come from the previously bent chassis? It might pay to check your chassis.

With bump travel of the suspension, there is camber and caster gain. There is more camber/caster gain as bump travel increases. There should always be enough chassis clearance so that the bump travel encountered on the track never allows the car to go solid on any corner. Springs and shocks will determine how much the car wants to move vertically. Chassis clearance will determine how much it can move.

Check camber at close-to-full bump travel to know where it is in relation to the track surface in a turn with the car's weight shifted. Also, check the bumpsteer and correct it as necessary. For paved tracks, it may need to be near zero. Dirt track racing is usually not as critical.

Remember your car might not show the same numbers or measurements. Just know that caster and camber change. It's most important that caster and camber are where you want them when the car is fully loaded into a turn.

SOURCE
Craftsman Tools
www.sears.com
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