How much is too much? There are limits to how far we go in steering the car this way. One disadvantage is pointed out by Keith Masters. He says, "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 upwards so that the LR tire tries to drive up under the chassis loading it considerably. This is a trend that now sees much less use.
We can have too much weight end up on the LR tire and lose traction and/or cause the car to push off the corners because all of the forward thrust is 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 System
The Z-link rear suspension, or swing arm as it's sometimes called, is another system used on dirt cars. Compared to the four-bar cars, it has more limited adjustment for rear steer and historically has worked well on the tighter and more highly banked racetracks because the direction the rearend is pointed is 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. Mark Richards told us that today, the Z-link suspension can have nearly as much RS as the four-link, without the excessive loading of the LR tire.
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 that the car feels is much less, usually around 50-55 percent, than the actual installed spring rate.
A Z-link suspension system...
A Z-link suspension system uses a link extending from under the rearend forward to the chassis and one from above the rearend rearward to a mount on the chassis there. Most designs use very few mounting holes that would enable the team to adjust for high amounts of rear steer in those cars. Note that the spring is mounted directly onto the front link.
A 200-pound spring in a Z-link car feels more like a four-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 four-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 wheel 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 car. The front of the car felt the actual 400 pound rate while the rear "felt" like 200-pound springs. The track in this case was banked and had a lot of grip. So the car was set up and driven more like an asphalt car and it was fast. Those conditions rarely exist on dirt.
Along with the chassis adjusting...
Along with the chassis adjusting holes, the birdcages have numerous holes so you can make changes to the link angles to regulate the rear steer. We used this combination of positions to design our 2008 Dirt Late Model for minimal rear steer. The car was very easy to drive and had plenty of forward bite.
The Metric four-link System
The metric four-link is a widely used system that comes with some models of stock automobiles. It has become the standard for Street Stock-type dirt cars. It uses four links, as the name implies, that aren't parallel to the centerline of the car. The top links are angled from a top view with the front pivots wider than the rear pivots. The lower links are angled from a top view with the front pivots narrower than the rear pivots.
Leaf-Spring Systems
The leaf-spring rear suspension system locates the rearend fore and aft as well as laterally using the leaves. There can be a small amount of RS as the chassis rolls and squats, but it's both minimal and mostly fixed as far as adjustability. The advantage of this system is that it keeps the rearend squared up and the thrust under acceleration straight ahead, if that's what is needed.
The height of the front eye of the leaf spring in relation to the axle centerline determines the amount of rear steer. Spacing the leaf different heights from the axle tube can be a way to change the height relationship of the front eye, which will change the amount of rear steer for this system.