As a reminder to all stock car teams, be they dirt or asphalt racers, we present a list of the top 10 most important setup parameters each year. Each new season brings changes in the goals related to setup and to how the racer will go about meeting those goals. And so we tune each preseason list with thoughts on the current trends and how the steps might be looked at differently based on those trends.
Each trend must prove itself in order to stand the test of time. It must show a consistent gain in performance and a logical ideology to the bulk of race teams. And, it must be economically applied.
I have always believed in priorities because you can make gains faster when you solve the high priorities problems first. And, some aspects of chassis setup build on other aspects. So, here are, in order of logic and importance, a list of setup parameters we need to address to make our cars fast and consistent.
We always start with the front end geometry on any race car, be it dirt or asphalt. The settings, including the moment center location, really do dictate how all of the other parts and pieces of setup will work. If this component on your car isn't right, then the whole car will suffer, no matter what else you do.
Front end geometry involves...
Front end geometry involves not only toe setting, camber and caster, but also the moment center design. Many teams are hearing about a new approach to race car dynamics involving jacking forces acting on the instant centers of the control arm design. We will address this trend with a round of real world testing in the near future.
A newer terminology for understanding the dynamics of the front end involves jacking force related to the location of the instant centers in the geometric layout. We'll be going into much more detail about how the moment center technology relates to jacking force in future issues.
The dynamics of the moment center and the effects of camber change have been explained in past articles. We have continually pressed these issues because of the extreme importance they have. Long gone are the days of saying that the MC is not important.
The dirt car moment center design is different than that of an asphalt car. On dirt, the average g-force is much less than on asphalt because the track just doesn't provide as much grip. So, the MC needs to be located farther to the left in order for the car to work well.
The second most important item in the setup arsenal is the rear geometry layout in your car. The components that locate the rearend must be evaluated and set correctly. The control arm angles affect the rear steer and the third link, lift arm, and so on and can redistribute load upon acceleration. On a metric four-link car, the four control arms determine the rear moment center height too.
Rear alignment and rear steer...
Rear alignment and rear steer are important issues for both dirt and asphalt racers. The angles of the trailing arms must be thought out carefully and in-shop testing and measuring can ensure you will have the rear geometry you need and what the car needs.
It's not advantageous to have the rearend steer to the right at any time on asphalt. A slight amount of rear steer to the left has been shown to help provide more traction at the rear and bite off the corners where it's needed. But the most useful rear steer will only occur on acceleration and not at mid-turn.
The dirt car rear geometry layouts are varied and usually highly adjustable. Each car needs to be evaluated for where it's to be raced and then set correctly. The trailing arm angles affect the rear steer and bite and the pull bar or lift arm can redistribute load upon acceleration and deceleration.
The steering system in your car must be evaluated and any negative characteristics must be eliminated. Negative aspects might include excessive bumpsteer, excessive Ackermann, and incorrect steering quickness.
Front steering geometry involves...
Front steering geometry involves toe change as the steering wheel is turned. It needs to be evaluated in the preseason and remember that for dirt, the Ackermann must be tested for both directions of steering. We have explained how to do this in past articles.
Eliminate most of your bumpsteer and Ackermann and install the correct steering ratio for your track that would suit the driver. Ackermann is easily checked by using a laser system or strings. If all of these issues are evaluated and corrected, then you can move on.
Dirt car steering systems must be designed to work the same in both left and right turn attitudes. Mechanical affects such as Ackermann could be more beneficial on dirt than on asphalt, but again only to a small degree.
The Ackermann must be developed in the design of the tie rods angles from a top view and not differences in steering arm length. That way the wheels always keep the same toe or toe gain in equal amounts while turning right or left.
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.
Shock technology has evolved...
Shock technology has evolved and now includes designing for control of bump rubber spring rates. There is an art and science to developing the setups we now see where on asphalt the cars run on bump rubbers on both sides and on dirt on the right front, and possibly right rear.
Once you have evaluated all of the above and feel fairly confident that the car is set up correctly, you should then work to tune the transitions into and off of the corners with the shocks. The overall work that a shock does is to resist the rebounding of the springs and control the speed of compression. Since the spring promotes rebound and resists compression as inherent properties, then the shock rate of compression control must be less than the rate of rebound control.
The amount of difference you need is directly influenced by the installed motion ratio of the spring and the spring rate. A very soft spring would need more compression rate and less rebound rate, whereas a stiff spring would need a lot of rebound rate and much less compression rate.
These are the general rules and the exception would be when running on bumpstops or bump rubbers. The spring rate of these devices is quite high, and so the shock must be designed to control that high rate with equally high rebound resistance.
Shocks affect the speed of the motion of the corners of the car and therefore the placement of loads during transitional periods. If one corner of the car is shocked stiff, then as that corner desires to move in compression, more load will be retained by that corner as well as the opposite diagonal corner of the car during the compression cycle only.
If the same stiffly shocked corner is in rebound, less of the overall load will be retained by that corner, and its diagonal corner as well, during the rebound cycle only. That is the essence of shock technology related to handling influences. Plan your shock layout by comparing the stiffness of one to the other corners and to the spring stiffness at the corner you're trying to control.
Dirt cars show a lot of travel as they negotiate the four turns. This extreme amount of wheel travel means that shocks are able to do more work than with other types of race cars.
Each corner of the car might need a different shock characteristic. The amount of difference is directly related to the installed motion ratio of the spring and the spring's rate and amount of motion.
Research on shock influences in dirt racing have shown that there are a lot of gains to be had by concentrating on your shocks. This is evidenced by the influx of new designs of shocks into the dirt car market. The age old truth is that we need to perfect our setup first before working with the shocks.
Once the setup has been balanced and the shocks are decided on, we need to evaluate the turn entry characteristics and brake bias, a very important influence in this segment of the track.
Brake bias influence can be easily determined by entering the corner with medium to heavy braking first and then entering with light braking to see if there is a difference. If there is, try to adjust the brake bias to eliminate the adverse condition.
Once you have made the entry to the corner balanced, check to see if the adjuster is centered. If it's too far to one side, then changes to the brake master cylinder sizes and/or the pad compounds might need to be made in order to maintain a centered bias adjuster. Off-centered adjusters can be very inconsistent.
Turn entry on dirt is important and dictates how well we are able to negotiate the middle of the turn. So, we need to evaluate our turn entry characteristics related to brake bias. We may want to try to solve turn entry problems with the brake bias on dirt.
More and more drivers are using braking to slow the cars on entry on dirt. We have observed top Late Model, Sprint Car and Modified drivers who win a high percentage of the races they enter, entering the turns straight ahead and braking to slow the car.
Bite off the corners can be...
Bite off the corners can be accomplished in several ways. Pull bars, lift arms, or spring third links can help reduce the amount of shock the tires experience when the driver picks up the throttle.
In situations where the exit portion of the track provides less traction and/or the corner is more flat, we might need to develop more rear traction on acceleration. Just giving the car more rear traction, period, does not help us if the car becomes too tight in the middle of the turns. We could end up with the reverse of what we need by going to a tight/loose condition.
Another bite enhancing technique...
Another bite enhancing technique that is not new at all is the placement of the left rear spring ahead of the axle tube and the right rear spring behind the tube. In a system where the rearend is allowed to rotate, the left spring gains load (as does the right front tire) and the left rear spring loses load. This increases the crossweight, or left rear weight to tighten the car on exit off the corners.
We must develop more rear traction on acceleration only. There are ways to do that without changing the handling at other points around the racetrack. Rear steering the rearend is one way. This must be accomplished only while the car is accelerating and not in the middle portion of the track.
We could always use more bite off the corners on dirt. To give the car more rear traction, we need to understand a little about the dynamics at work on the car when we are accelerating. One way to reduce wheelspin is to reduce the "shock" of sudden application of throttle and torque to the rear wheels.
We can use lift arms and pull bars with various stiffness of shocks and springs to soften the application of power. More and slower movement is needed for slick conditions while less movement is correct for the tackier conditions. Another way to gain bite involves the use of a spring-loaded push-rod that allows a certain amount of forward right rear wheel movement to steer the rearend more to the left.
Antidive and antisquat are mechanical influences that can help our transitional phases of entry and exit. Antidive helps prevent sudden nose dive on entry by mechanically resisting the downward motion of the suspension using the rotational forces created through braking.
Antidive geometry is useful...
Antidive geometry is useful when you want to slow up or eliminate front end dive on entry under heavy braking. Newer setups encourage front end dive as a means to getting on the bump rubbers quicker.
Antidive is useful for both dirt and asphalt cars, but the trend for both types of racing involve promoting front end dive on entry to get the nose down for improved aero efficiency and downforce. So, we see teams actually using pro-dive on the left front of the cars to quicken the movement down.
Antisquat results from the third link trying to straighten out or become more horizontal as the car accelerates and as the rearend desires to rotate. The more third link angle you have, the more anti-squat there is. The lateral location of the third link will affect the distribution of load among the two rear tires that results from acceleration and antisquat.
Excess antisquat can be detrimental to corner entry. So, there is a limit to how much you can get away with and still have a decent corner entry. Roughly 8 to 10 degrees of third link angle is sufficient to promote antisquat and not hurt your corner entry.
The very last thing you need to worry about is your aero package. I'm not saying this is not important to some degree, but on many short tracks gains in turn performance outweigh gains in aero.
Aero designs of modern bodies...
Aero designs of modern bodies take advantage of the newer low profiles generated by the setups. Testing in wind tunnels has proven that a low nose produces more downforce. Now if we can make sure we have a proper footprint and load distribution at that low attitude, we can truly go fast.
Still, the goals of a lower front end resulting in tighter spacing from the front valance to the track surface will enable a lower CG, resulting in less load transfer and a higher downforce loading. Both of these are positives and proper goals as long as the overall grip in the four tires is not compromised.
Try to understand how aero downforce is created and then configure your car so that you take advantage of every area where you could produce more downforce. Remember that drag is an important aspect of aero design. Do not seek aero downforce at the expense of aero drag increase.
Dirt car racers discovered the need for better aero designs some years ago. Just look at the Dirt Late Model cars and how they have evolved. The front ends are wedges that scoop the oncoming air up and over the car. The wheelwells are shaped to route air out and away from the front wheel wells creating a lower pressure under the hood and therefore added downforce.
The degree that you need to get involved with aero for your car depends a lot on what you run and where. Aero influence varies with the speed of the vehicle. There is an algebraic increase in both drag and downforce associated with increases in speed through air.
The ideas we have presented and the methods we preach have a basis. I would never promote technology that I haven't seen work. I would rather not say anything rather than mislead. No one has to buy into any of what is printed here, but you do need to think out your particular setup and how you attack your racetrack.
To be more successful, it helps when you get your car setup correctly for the basics of: front and rear geometry, alignment, and balance. Develop a proper approach to the setups for the tracks that you intend to race at and be prepared and willing to make changes to your setup when the track conditions change. That ideology will never change.