What I find from talking to hundreds of teams throughout the season is that many racers know more about his/her chassis and how it's built than ever before. That is because the competition has moved ahead in its knowledge. Just to keep up, let alone pass the winners, in any class be it Dirt or Asphalt, we need to have knowledge.
The beginning of your success starts with knowing all of the technical details about your chassis. This is true for all stock cars no matter what class or racing surface you run on.
Let's compare your chassis to your motor. If you have the luxury of having a good engine man tuning your motors, you will know that he/she is aware of every detail of how that motor is built, the components installed, and what is important to have for maximum power output.
In stark contrast to engine knowledge, most teams know very little about their chassis construction. That situation is rapidly changing whereby many top- and midlevel teams and more and more car builders are aware of the critical components and design issues that must be incorporated into the chassis.
Just a few years ago, if you visited most chassis shops and asked where their moment center (roll center) was located, they would either tell you it wasn't important or that they didn't know and didn't care. There was a very good reason for that. At that time, no one, including myself, knew what the MC did, much less where it should be located. Things have changed dramatically.
The MC syndrome is just one example of how technology has rapidly advanced over the past few years to where we can now define many important design issues related to the race car chassis. If we can define those areas, then we can make changes to our chassis to make them better. The chassis builders too can incorporate the proper design into the new chassis they build. And believe me, they are doing just that across the board, for Dirt and Asphalt chassis.
It's a wonderful time to go racing when, given the right knowledge, we can properly set up our cars to be competitive right from the start. In the past, it could take several months or years to find the sweet spot for our setup and some teams have never found it. Let's take a look at the most important aspects of chassis design and why they affect our handling and performance so much.
Moment Center Design
I don't care if you race on dirt or asphalt or ice for that matter, the front moment center design is the most important aspect of performance in your chassis. Sure, it's not the only aspect, just the most important. Up front is where it all begins.
Do you want to know how I know that? I know that because I continually hear from racers from all walks of racing say the same thing over and over again. When they took charge of their frontend design and corrected any deficiencies in the design, the car literally came alive. Once the car turns well, all of the other important aspects of setup can be properly dealt with.
The MC must be in a certain location for each type of racing and racetrack. The two most important considerations are camber change for both front wheels and MC static and dynamic location. We must develop a frontend design that will offer the least camber change while also positioning the MC where it will do the most good to promote the proper dynamics for the frontend.
With the softer spring setups, we will have a greater degree of camber change, but we can still minimize it to a certain extent. Frontend design is much different for those setups than with more conventional setups. That is why you can't successfully change from one to the other without a redesign of your front suspension.
The most basic design of your chassis will largely determine how much success you will hav
The position of the moment center is critical to how the front suspension will perform. A
A MC located more to the right of centerline is proper for higher-banked tracks with more
We have offered articles on proper location for the MC in the past, but suffice it to say that the lateral location of the MC belongs somewhere near the centerline. We see good results on flatter and slicker tracks with it in a more left position from centerline. For higher-banked tracks and ones with more traction, we need for it to be somewhat right of the centerline, but not too far.
The height of the MC is a product of upper control arm angles mostly and the best height is directly related to the correct camber change design. Like we said, different setups require different control arm angles to minimize camber change. When we achieve a good camber change layout of the arm angles, the MC height will usually fall in somewhere above ground and below 4 inches. We never shoot for a particular height number as opposed to the defined numbers for lateral location for a particular application.
The point is, we now have the tools to know where the MC is located and to redesign our cars to improve on the MC design. There are several fine geometry software programs that will tell you where you are. If it takes a cutting torch and welder to get what you need, then so be it. I have taken brand-new cars, plenty of times, and just cut them up to relocate the pickup points on the frontend. Remember that once you buy your car, it is yours, not the car builder's. Do what is necessary to provide your team with what it takes to win.
The car builders have taken note of the above. Many cars today are designed so you can adjust the heights of the pickup points to fine-tune the MC. If the car builders have done their jobs, the MC should be close and will need minor tweaking to compensate for different track configurations as well as different setups.
Proper Steering System
A short-track steering system must have certain "qualities" in order for the car to track well and the driver to be able to comfortably drive the car the entire race. The way we look at frontend design has changed somewhat with our new view of chassis setup and design.
In the past, when teams ran un-balanced setups, the left front tire did very little work as evidenced by the cooler tire temperatures. The rear suspension "out rolled" the front suspension and excess load was then transferred to the RF tire in the turns. If we introduced Ackermann Effect (added toe when we turn the steering wheel left) to the front steering design, it did help make the LF tire work harder to help turn the car and that was fine for those times in racing history. The overall setup picture is much different now.
In both dirt and asphalt racing circles, we see more and more teams working to balance their setups. The dirt cars now keep the LF tire on the track through the turns. The asphalt teams are seeking a balanced conventional setup and many are opting for the big bar and softer springs setups. The large sway bars used in the BBSS setups force the LF tire to carry more load than ever before.
Since the LF tire is working harder, we must eliminate the Ackermann Effect. If we do not, the front tires will fight each other and both will lose traction. So, Ackermann is not needed when we start using the LF tire. Make sure when you improve your frontend geometry and balance your setup that you check for the presence of Ackermann and get rid of it if you have it.
Caster and Camber
Caster and camber split is not designed the same as in years past either. With the advent of power steering on a large scale, we need less overall caster and less split than was used in the past to help us turn the steering wheel. We now see caster in the 1-3 degree range, and with asphalt cars and the split is 2-3 degrees at most. On longer, banked tracks, the split is reduced even more.
Steering geometry is an area where we can give up a lot of performance if not designed cor
Our overall objective in designing and setting up our race cars is to end up with a dynami
Caster and camber settings in the front suspension affect the feel of the car as well as t
The use of anti- and pro-dive is also being experimented with. Antidive works great to reduce frontend dive on entry when applying the brakes. It works using the force of the brake pads gripping the rotors. The beauty of antidive is that the more you need it, the more there is.
Harder application of the brakes might produce more chassis dive. It also produces more antidive effect and therefore, more control of chassis dive. But what about when you need chassis dive?
Teams have been experimenting with using pro-dive on the left front suspension and antidive on the RF suspension. This promotes dive on the LF, thereby lowering the LF corner of the car for a more efficient aero-downforce effect. This is very tricky and other components, including shocks, must be adjusted to compensate for this effect in order to keep the car stable on entry. This is definitely not for the novice racer to try.
Changes to the camber must be made when balancing the setup. The LF tire will carry more load and do more work requiring more positive static camber. The RF tire will do less work and require less negative camber than when running an unbalanced setup. Be sure to stay on top of your tire temperatures and/or tire wear for dirt racers, so that you can adjust the front cambers to match the new dynamics of the balanced setup.
Rear Moment Center Design
The rear moment center location is important to the dynamics of the rear suspension. There seems to be a misunderstanding of the effect that the RMC has on the dynamics of the rear suspension. Here is how it's defined in automotive dynamics books written 60 years ago. The "theory" presented here has been proven many times over.
The angle of the Panhard bar can have a profound effect on the redistribution of loads on the four tires at mid turn and this affects the handling of the car in a separate way from MC dynamics.
The race car "feels" the rear MC midway between the top of the two springs (top mounts of the shocks on coilover systems) and the height as an average of the height of the ends of the locating device. A Panhard bar or J-bar has two ends and the average height of those ends is the rear MC's height.
Experiments have shown that lateral placement of the rear locating device does not change the dynamics of the rear suspension. That is not to say that how the device is mounted and where doesn't change the handling of the race car. There are several mounting trends that produce marked changes to the handling of the car due to artificial load redistribution.
Mounting the J-bar or Panhard bar to the left side of the chassis has a loosening effect when the car corners. This helps a tight car, but is one of those crutches we sometimes talk about. On dirt cars, angling the bar with a left-side chassis mount, and the left end higher than the right end, does have a profound effect on handling.
If the bar is angled inline with the right rear tire-contract patch, a lot of force is directed onto that tire. This helps cut through the dry powder on extremely dry slick tracks. If the track were more moist and tacky, it would be better to flatten out the bar and maybe move it over to a righthand chassis mount. This has been done with good success on touring Dirt Late Models under those conditions.
Rear Steer and Rear Alignment
How we design and place our rear trailing links has a lot to do with how our car will handle. As the car moves vertically, the rearend will most likely steer to a certain degree depending on the design. Rear steer design goals are very different between dirt cars and asphalt cars. We also need to make sure the static alignment of our rearend is correct. No matter how we end up at mid turn, the rearend should be squared to the centerline of the car when the chassis is at ride height.
Many car builders are incorporating adjustable upper control arm plates and slotted lower
The rear moment center function and dynamic properties were defined 60 years ago. That par
When we mount the Panhard bar to the left side of the chassis, we introduce a jacking effe
On asphalt, we can only tolerate a very small amount of rear steer and most of the time we are better off with close to zero rear steer. On dirt, some teams incorporate lots of rear steer to the right into the suspension. The degree of steer is directly proportional to the amount and direction of vertical movement associated with the right and left rear suspension systems.
We can simulate the degree and direction of rear steer in our race cars by duplicating the movement of the rear suspension. If we take visual reference to the position of the wheels in the wheel wells during cornering, we can get sufficiently close to replicating the suspension attitude with the car in the shop. We could then support the chassis at levels similar to the way the car looks on the racetrack and then measure how far each rear wheel moved and in what direction. It may surprise many how far the rearend steers under some conditions.
If more or less steer is desired, changes can be made to trailing arm angles, and so on as you are simulating the rear steer and the results can be measured. This is an excellent way to learn how arm-angle changes relate to rear steer magnitude.
Spring Location and Angles
It is an engineering fact that the less angle from vertical that we have in our shock/spring design, the more efficient and consistent our setups will be. Many cars are designed with excess coilover shock angle. This results in a low net wheel rate and also a variable rate spring with diminishing rate over compression travel.
On many four-bar dirt cars, the rear coilovers are mounted at a high degree of angle. This does two things: 1) High angles with the tops in closer to the centerline than the bottoms reduce the spring base that the chassis sits on and increases the roll angle of the rear suspension. 2) It reduces the wheel rate causing increased roll angle due to the soft spring rate. That is exactly why some cars will lift the LF wheel off the track more easily than other cars. Moving the top of the springs out toward the wheels will offer more stability in the car.
With high spring angles in the rear, as the car rolls, the angle increases and the rate decreases. With high spring angles at the front or rear, with every inch of chassis movement vertically, the spring will move less providing a lower rate of resistance to chassis roll. This is a variable that we just don't need in our race cars. Variables mean that handling will change based on different entry lines and the different banking of the higher and lower grooves.
If we can get to know our cars before the season starts, we can then make critical decisions on the front and rear design that can produce more consistent handling and better performance. All it takes is a little time and some effort. This winter, think about your overall chassis design. Work on the areas we have discussed here.
Remember, it's your car to do what you want with. We are all scientists and inventors at heart, so live up to your natural tendencies and get to work on that car. Cut and weld if necessary. A can of paint will hide the work done and keep curious eyes from discovering why your car works better than others of the same make. That's the fun part of doing it yourself and experiencing the positive results.
We should always align our car so that the rearend is square to the centerline of the chas
Rear steer can be adjusted on an asphalt car by changing the angles of the trailing links.
Spring angles have an effect on the roll stiffness of each suspension and therefore the ro