I was at Volusia Speedway Park recently for the DIRTcar Nationals and observed the top cars in the Dirt Late Model divisions. All of the top cars kept the left front tire on the track. Most of the top cars exhibited much less body roll and hike and some looked more like asphalt cars, in that they drove more straight ahead and flat.
Rob, our editor, observed at East Bay Raceway Park the week before that a fast line around that track was up high and the cars drove through there with a very straight ahead attitude and were very fast. I spoke with Billy Moyer, often referred to as Mr. Smooth, and he has quite a bit of disdain for the trend of hiking the left front off the track. He told me that he would never do that again.
Types Of Rear Suspension On Dirt
There are four types of rear suspensions used in most dirt cars that are significant to study regarding rear steer. Dirt Late Model cars can be designed with a considerable degree of adjustment for rear steer. Many teams use varying amounts of RS 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 rearend and adjust handling with other means.
One of the reasons why a car will test fast in practice, qualifying, and maybe the heat races, but when the track changes be out to lunch come feature time, is a result of improper rear steer. Here's how each system functions and how they may or may not 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 both directions. The rule about never steering the rearend to the right on an asphalt car does not apply on a dirt car. There are times when we may want the rear to steer to the right.
That being said, the trend over the past 5 years has been to set up the dirt cars with much less rear steer. With the introduction of meaningful technology on the front moment center and its overall role in front-end dynamics, many of the Dirt Late Model and Modified builders are designing better front ends that help the car to turn. This reduces the need to introduce rear steer to get the car to turn.
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 out our rear steer just about any way we need it.
The bars can be mounted on one side of the car so that only that wheel moves to create the rear steer. If both sides are configured to move in opposite directions, then RS can be extreme in magnitude.
On a tacky track, the team would do well to limit 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, rear steer is needed. In the past, drivers would set up the car for exit off the corners, throwing the car sideways by breaking the rear tires loose. Now, in more recent years, teams have been setting up the car so that the left side rises up quite a bit.
The left rear suspension is designed so that when that corner rises up, the arms are angled so that the LR wheel is pulled forward, toward the driver. This produces quite a bit of rear steer to the right and the rear of the car moves to the right, just like when we used to throw the car sideways. The difference is that now we can maintain rear traction having never broken loose and the car is angled somewhat sideways and pointed in the right direction to get off the corner.
The high angle of the bars...
The high angle of the bars on the left side cause the LR wheel to move forward as the left side of the chassis rolls us through the corners. This can be extreme, whereas the LR tire moves well forward in the wheelwell. We are seeing less and less of this attitude in the top divisions of dirt racing.
If the bars on a four-bar...
If the bars on a four-bar car are set all the way to the top of the mounts on the right side, then as the car rolls down on the right side, the right rear wheel will move back. This, in conjunction with the left rear wheel moving forward will cause the rearend to steer to the right quite a bit. This can help the driver get the car turned to prepare to exit the corner and keep the rear tires in contact with the track surface under extremely dry slick conditions.
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 rearend will compensate for fore and aft movement resulting in zero rear steer. This is best for tight, wet tracks where we don't need rear steer and where we need the thrust to be pointed straight ahead.