Driver and team owner Robbie...
Driver and team owner Robbie Jordan says one of the keys to getting tons of forward bite while your car still turns is properly adjusting the length and location of the rear suspension bars
The Dirt Late Model is arguably the most interesting race car. With all pretense of "stock" removed from this class of stock car racing, the Dirt Late Model is stripped of absolutely everything that doesn't convey speed. There's no "stock-style" front clip based on old Chevy Nova dimensions like in the NASCAR Late Model Stock class. There are no requirements to run a stamped metal body that resembles that of an over-the-road car. There are even transmissions with no more than two forward gears and an insanely small clutch that wouldn't last a day in a passenger car.
But that doesn't mean the state-of-the-art Dirt Late Model (DLM) isn't a complex piece of racing equipment. A well-designed DLM chassis offers more adjustment options than any other chassis. One adjustment that simply isn't available on most other styles of stock cars (using truck arm, or three-link suspensions) is the arm length and position of a four-bar rear suspension setup.
Because gaining forward bite in DLM racing is so critical, the rear suspension setup has become highly evolved. There are many other factors involved in the rear end, including shock and spring position, weight placement, the Panhard bar, and even pinion angle, but here we will only concern ourselves with the suspension arms in the four-bar setup.
The baseline setup at ride...
The baseline setup at ride height has both lower bars approximately level, and the attachment point for the right-side upper bar is higher on the chassis than the left-side bar. There is a difference in location for the upper bars to prevent pushing.
For more information, we visited the shop of Robbie Jordan Racing, in Waxhaw, North Carolina. Jordan and crew chief Neil Wilson race mostly MastersBilt chassis and were more than willing to share what they have learned. It is important to note that they typically run a left-side shock-behind setup (the shock on the left side of the car is located behind the rear axle while the shock on the right side of the car is in front of the axle). But these tips will work for most four-bar setups.
Lower Bar Location
Most four-bar systems have level lower bars as a baseline. Normally, there are three attachment locations on the chassis for the lower bars. Ride height should be set so that the bar is level when it's in the center hole. Reattaching the bar in the lower hole drops the angle 5 degrees, and raising the bar to the upper hole raises the angle 5 degrees. Wilson says the team rarely bothers with moving the bar location where it attaches to the birdcage on the rear end.
On the right side of the car, raising the bar location will free the car up (loosen it) getting into the corner. Likewise, dropping the bar to the bottom hole will tighten up the car on turn entry. "This is a pretty dependable change with how it affects the car," Wilson says, "so it is one we use a lot. Usually, we'll move the bottom bar on the right side between the heat races and the main, depending on how the track is changing as it dries out."
Most four-bar Dirt Late Model...
Most four-bar Dirt Late Model cars have multiple options for attaching the suspension bars to the chassis. This changes the amount of leverage the suspension places on the rear end as the rear end tries to twist up under power.
On the left side of the car, making changes to the lower bar has much the same effect, but it's on turn exit instead of entry. Raising the mounting point on the chassis provides more rear steer. "It doesn't really provide more rear traction, but it feels like it," Jordan says. "Don't let it fool you." Lowering the inside mounting location on the left-side bar actually provides more traction and tightens up the car coming off the corner.
Upper Bar Location
The upper bars on a DLM play two distinctly different roles. Still, both are manipulated by changing the location where they connect to the chassis, not the birdcage. Raising the right-side top bar will help keep a push out. As a result, Wilson says he and Jordan almost always run the right-side top bar at least one hole higher than the top bar on the left side of the car.
On the left side, the top bar can be used to control traction on turn exit. Dropping the bar increases traction and forward bite, but it can also create a push when the driver picks up the throttle. Because of this, you should generally try to run the upper left-side bar as low as possible until driving the car out of the turn becomes a problem.