If the top bars on a four-bar...
If the top bars on a four-bar car are set all the way to the top of the mounts, then as the car rolls up on the left side and down on the right side, the rear end will steer to the right quite a bit.
There are four types of rear suspensions used in most dirt cars that are significant to study regarding rear steer. Characteristics of the metric four-link and the leaf-spring systems are the same as discussed under the headings related to asphalt cars, so let's expand on the other two systems
Dirt Late Model cars can be designed with a considerable degree of adjustment for rear steer. Many teams use varying amounts of rear steer to adjust to constantly changing track conditions, a product of variations in moisture content that is so common in dirt racing. Other teams may just stick with a fixed location for the mounts in the rear end and adjust handling by other means.
A car may test fast in practice, qualifying, and maybe the heat races, but when the track changes it's out to lunch come feature time because of improper rear steer. Here's how each system functions and how they can be adjusted for the degree of rear steer.
The Standard Four-Bar System. The four-bar suspension is highly adjustable and can be made to steer wheels in both directions. The rule about never steering the rear end to the right on an asphalt car does not apply on a dirt car. There are times when we definitely want the rear to steer to the right.
This helps the driver get...
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.
Depending on the angles of the trailing arms or bars, each rear wheel can be made to move to the front or rear. The roll angles and vertical movement on a dirt car can be very pronounced. With so much movement, we can plan our rear steer just about any way we need it
The bars can be mounted on one side of the car so only the wheel on that side moves to create the rear steer. If both sides are configured to move in opposite directions, then rear steer can be extreme in magnitude. The trend in the past few years has been to limit the degree of rear steer, even on dry-slick tracks.
On a tacky track, the team would do well to eliminate rear steer on both sides of the car. These conditions call for a driving line that is more straight ahead. When the track goes slick, especially dry slick, some rear steer may be needed. In the past, drivers would set the car up for exit off the corners by throwing the car sideways and breaking the rear tires loose.
In more recent years, teams have been setting the car up so the left side rises quite a bit. When limiting LR wheel movement, the LR tire does not tuck under the left front of the wheelwell as much as in the past.
If the bars on a four-bar...
If the bars on a four-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 on which rear steer cannot be used and the thrust needs to be pointed straight ahead.
There are limits to how far we go in steering the car this way. One disadvantage was pointed out by Keith Masters. He said, "High left-rear loading does not increase traction." As the left side of the car travels up, the front of both of the trailing arms are angled upward so the LR tire tries to drive up under the chassis, loading the LR tire considerably.
We can have too much weight on the LR tire and lose traction and/or cause the car to push off the corners because of all the forward thrust concentrated in the LR tire. In racing, we have the maximum amount of traction from a pair of tires on the same axle if they are equally loaded. Excess loading of either the LR or RR tires decreases traction in most cases.
The Z-link rear suspension, or swing arm as it is sometimes called, is another system used on dirt cars. Compared to the four-bar system, it has more limited adjustment for rear steer and has worked well on the tighter and more highly banked racetracks 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.
According to Richards, the Z-link or swing arm suspension can have nearly as much rear steer as the four-link without the excessive loading of the LR tire. This comes with adding more adjustment holes and is beneficial if rear steer is needed.
A Z-link suspension system...
A Z-link suspension system uses a link extending from under the rear end forward to the chassis and one from the top of the rear end rearward to a mount on the chassis. The axle mounting points are on a birdcage that rotates on the axle housing. Most designs use mounting holes that would enable the team to make adjustments for the amount of rear steer in those cars. Note that the spring is mounted directly on the front link.
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 rear end in relation to the chassis. Therefore, the rate that 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 100-pound spring in a four-bar car.
The chassis travel in a Z-link is enhanced compared to the four-link suspension when using the same installed spring rate, and this causes quite a bit of chassis travel and related rear steer. Teams need to take this rate difference into account.
We should learn to read the conditions of the track and tune the amount of rear steer. We need less for tacky and wet conditions and can add more rear steer as the track becomes more slick. In extreme dry-slick conditions, use some rear steer to the right to help the car rotate. This is accomplished by causing the RR wheel to move back and the LR wheel to move forward as the car rolls. Soft springs, a left-side 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 you need to, and 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 the changing conditions.