It has been found that the misalignment of your tires/wheels presents serious drawbacks to a finely tuned chassis and setup. Alignment issues are defined as: A. rearend alignment, B. contact patch alignment, C. driveshaft-to-pinion/transmission alignment, and D. engine alignment.
We are still fighting the...
We are still fighting the geometry battle it seems. Many teams still don't realize the importance of control arm angles in relation to camber change in dive and roll of the chassis. The ones that do definitely outperform these misguided teams.
The rearend needs to be aligned at 90 degrees to the centerline of the chassis and/or to a line through the center of the right side tire contact patches. The right-side tire contact patches will also need to be inline with the right front tire pointed straight ahead.
The driveshaft alignment is critical from the standpoint of mechanical efficiency. Loss of efficiency can rob power from the drivetrain due to the generation of vibrations and harmonics that are also damaging to the bearings.
The overall general rule is that the angles between the driveshaft and both the pinion shaft and the transmission output shaft need to be equal and in opposite directions. The less angle the better.
Alignment issues present just as serious a drawback for a dirt car as with an asphalt car. There really is no reason to misalign the rearend. In tests we have participated in, we have run the same lap times with the car "sideways," due to excess rear steer, as when running it straight ahead with no rear steer.
I believe that the rearend doesn't need to be any different in alignment than at 90 degrees to the centerline of the chassis and/or to the right side tire contact patches, and those patches need to be inline, even on dirt.
The setup we choose needs to be arranged so that the dynamics are balanced between the front and the rear suspensions. Each suspension system desires to do its own thing when lateral forces are introduced from the car going through the turns. These desires are directly influenced by the spring stiffness, location and spring split, the sprung weight the system has to support, along with the moment center locations and other settings.
The most successful teams...
The most successful teams now set up their cars to run on all four tires and the drivers drive mostly straight ahead. We have been saying this for some eight years now and the validation is in the winner's circle.
The bottom line is that at mid-turn, each end will want to roll to its own degree of angle. That is the best description of the result of the dynamic force that influences each system. If those desired angles are different, then we term the setup "unbalanced." On asphalt, we need for those two angles to be identical in order for the tires to carry the optimum loads.
Does a dirt car really need to be balanced? Of course it does. The balance, though, will need to be adjusted for the track conditions. On dirt we can manipulate that balance relationship to adjust the car to different conditions. If the track is tacky, then the balance needs to be more like what we would do for asphalt and that is to more closely match the desires of the front and rear suspensions.
The setup for slick tracks is with a controlled difference in balance in the front to rear relationship with the rear desiring to roll more so than the front. This provides more rear traction to give us more bite off the corners. If our MC design is correct, the car should still turn through the middle, but have better traction off the slick corners.