If the bars on a 4-bar car...
If the bars on a 4-bar car are set all the way to the top of the mounts, the rear end will steer to the right quite a bit as the car rolls up on the left side...
As the left side of the car travels up, the front of both of the trailing arms are angled upwards so that the left-rear tire tries to drive up under the chassis, loading the left-rear tire considerably. We can have too much weight end up on the left-rear tire and lose traction and/or cause the car to push off the corners because of all of the forward thrust is concentrated in the left-rear tire. In racing, we have the maximum amount of traction from a pair of equally loaded tires on the same axle. Excess loading of either the left- or right-rear tires decreases traction in most cases.
Dirt Track Aero
One theory related to having the attitude of the car sideways involves the use of the aerodynamic aspects of the car. If you look at a modern dirt Late Model, the sides are made up of big, flat panels similar to the sides of a Sprint Car wing. We can see the effect of the Sprint Car wing in the turns-as the cars actually roll left due to the pressure differential developed on the flat wing sides-as the cars go sideways at a high speed. On a Late Model, this air pressure difference may help keep the car on the track by virtue of having the car go through the air at an angle, causing both a high pressure on the leading (right) side and a lower pressure on the trailing (left) side. Masters added, "Any time we can press air against the sides of the car, we can help the car to turn." His company has experimented with dirt track aero in the past.
...and down on the right side....
...and down on the right side. This helps the driver get the car turned to prepare to exit the corner, and keeps the rear tires in contact with the track surface under extremely dry, slick conditions.
The reason rear steer aero might be important comes on dry, slick surfaces, where the tires do not grip well. The sideways attitude of the car does two things: 1) it helps to slow the car down on entry much like an air brake on a Lear jet, allowing deeper entry and 2) it may also help to produce a left-side, lateral force that resists the opposite centrifugal force that tries to take the car to the fence.
The Z-link System
The Z-link rear suspension, or swing arm as it is also known, is another system used on dirt cars. Compared to the 4-bar cars, it has more limited adjustment for rear steer and historically has worked well on the tighter and more highly banked race tracks because the rear end is pointed more straight ahead. Some manufacturers have added multiple mounting points on the front and rear chassis mounts. This helps make the rear steer characteristics more adjustable for the changing conditions. Richards added that today, the Z-link or swing-arm suspension can have nearly as much rear steer as the 4-link, without the excessive loading of the left-rear tire.
Spring Motion Ratio
If the bars on a 4-bar car...
If the bars on a 4-bar car are set in the correct holes, the movement of the top and bottom mounts at the rear end will compensate fore and aft, resulting in zero rear steer. This is best for tight, wet tracks where we cannot use any rear steer and we need for the thrust to be pointed straight ahead.
Most of the Z-link systems utilize a spring mounting system that attaches the coilover spring to the front link. This produces a motion ratio that causes the spring to move less than the chassis per degree of roll and/or inch of squat. Therefore, the rate the car feels is much less, usually around 50 percent, than the actual installed spring rate. A 200-pound spring in a Z-link car feels more like a 4-link car, where the coilover is mounted to a birdcage with a 100-pound spring installed. The significance of this, for the purpose of this article, is that the chassis travel in a Z-link is enhanced compared to the 4-link suspension when using the same installed spring rate, and this causes quite a bit of chassis travel and related rear steer. So, teams need to take this rate difference into account.
There are reports from the past of a team winning a race using four 400-pound springs on a Z-link type of car. The front of the car felt the actual 400-pound rate while the rears "felt" like 200 pound springs. The track in this case was banked and had a lot of grip. The car was set up and driven more like an asphalt car and it was fast. Those conditions rarely exist on dirt.
Tuning With Rear Steer
A Z-link suspension system...
A Z-link suspension system uses a link extending from the rear end forward to the chassis, and one from the rear end rearward to a mount on the chassis. Most designs use very few mounting holes that would enable the team to adjust for the amount of rear steer in those cars. Note the spring is mounted directly on the front link.
We should learn to read the conditions of the dirt tracks and tune the amount of rear steer-less for tacky and wet conditions, with more rear steer as the track gets slicker. On extremely dry, slick conditions, use lots of rear steer to the right. This is accomplished by causing the right-rear wheel to move back and the left-rear wheel to move forward as the car rolls. Soft springs, a left chassis-mounted track bar, and easy-up shocks on the left side all promote the body roll that produces rear steer to the right.
On asphalt, do not make large changes to components that influence rear steer. Make small adjustments if you feel a need. When you find the correct amount of rear steer, stay there and tune the handling with the other components. When racing on dirt, watch the conditions and be prepared to make changes accordingly, not just to the setup, but also related to rear steer. That way, the car will stay as fast and balanced as it can be throughout all of the changing conditions.