All four-wheeled vehicles which steer from the front wheels have basically the same dynamics, whether it's an Indycar, a sports car, a family sedan, a dump truck, or a little red wagon. I'll focus here on the basic principles as applied to race cars.
In all vehicles, the force produced by the friction of the tires against the track or road surface is used to alter the car's velocity and the direction of its velocity. These alterations are called acceleration, cornering, and braking. With modern racing cars, there are two factors which contribute to this force, or adhesion: aerodynamic grip and mechanical grip.
In order to develop a good race car, we want to develop maximum grip in all directions, with minimum drag. This will give us the fastest speeds through the corners, coupled with the fastest speeds down the straightaways. Obviously we can't have both, so for each track we will need to find the optimum tradeoff between straightaway and cornering speeds; on short, twisty tracks, we'll be faster with a car that emphasizes good cornering, while on fast tracks with long straights we'll go quicker with a car that sacrifices some cornering for straightaway speed.
We also want the car's handling to be balanced; that is, when it reaches its limits of adhesion, we want both the front tires and at the rear tires to begin to slide at the same time, at the same rate. If the car is not balanced, either the front tires will begin to slide first, and we'll slide straight off the track, or the rear tires will begin to slide first, and the car will spin.
We also want the car to be forgiving. That means that its behavior is predictable and consistent, and when it starts to slide, we want the slide to be progressive rather than sudden. We'd like it to begin to let go gently, with some warning, and get fairly deep into a slide before it becomes uncontrollable.
All of these things require tradeoffs. Let's take a look at how we might break down the many factors affecting cornering, braking, and traction into manageable parts, so we can have a basis to work from when developing our race car's optimum setup for a given track.
Mechanical grip is the adhesion provided by the tires in contact with the track. This adhesion is influenced by such factors as tire pressure, tire compound, as well as the angle of the tire in relation to the track (known as camber). The percentage of the vehicle weight on a given tire is also a critical factor.
Aerodynamic grip is the extra adhesion provided by the additional downforce created by airflow over the wings and, in the case of Indycars, Formula One cars, and prototype sports cars, the body itself. Wings and underbodies undergo continuing development to find shapes that will create more downforce while producing less drag.
Traction refers to the grip available during acceleration.
Understeer is the condition in which the front tires are sliding more than the rear. The car is tending to go straight on, no matter how much we turn the wheel. It's also known as "tight" or "pushin'".
Oversteer is the condition in which the rear tires are sliding more than the front. The car's tail is coming out; if we don't correct with some opposite lock, the car will spin. This is also known as "loose".
Opposite lock means turning the wheel in the direction the car is sliding (you driving instructor may have called this "turning into the skid"). If you're in a right-hand turn, and the tail starts to come around - to your left - then you turn left to correct and, hopefully, prevent the car from spinning.
Slip angle refers to the angle of the tire's travel in relation to its centerline.
Camber refers to the angle of the wheel and tire in relation to the car's vertical centerline. Negative camber means that the top of the wheel leans inward in relation to the certerline of the car.
In order to develop a good race car, we want to develop maximum grip in all directions, with minimum drag. To do this, we must consider mechanical grip separately from aerodynamic grip. While aerodynamic factors affect cornering mostly at high speeds, mechanical factors affect cornering at all speeds.
Because their effects are masked at high speeds by aerodynamic downforce, we generally examine and develop the factors which affect mechanical grip at slow speeds, in hairpins or other slow corners. Race teams sometimes use skidpads for exploring their cars' mechanical grip, sometimes even running without wings so they can optimize mechanical grip without the influence of aerodynamic factors.
Once we've optimized mechanical grip, then we can tune the aerodynamics.
The angle of a tire's travel in relation to its centerline is known as its slip angle. Since a race car spends much of its cornering time near the limit of adhesion, understanding slip angle is crucial to understanding and optimizing the car's behavior. As the cornering forces on a tire increase, it begins to follow a track which diverges from its centerline. The difference between the tire's path and its centerline is known as its slip angle.
As the slip angle increases, the grip available from that tire increases - to a point. After the optimum slip angle for a given tire is reached, the grip available begins to decrease as slip angle increases. Go very far past this optimum slip angle, and you've lost control.
The slip angle characteristics of each tire differ. Graphs of these characteristics are available for some tires. If we could choose an ideal tire, we'd like one whose slip angle graph showed a gentle increase to optimum, and, more importantly, a gentle decrease after optimum. The more gentle the slip angle curve, the more forgiving the tire will be to drive. A tire with rapid falloff of grip after optimum slip angle will be difficult to drive; it will seem to "let go" without warning.
For rear wheels, the tire's slip angle is roughly the same as the angle of the tire's travel in relation to the car's centerline. If the car is turning right, and enters an oversteer state, the rear tires' slip angle will increase as the car gets more and more sideways.
Note that once a wheel is locked under braking, and the tire is simply sliding, consideration of slip angle becomes rather irrelevant. The same is true once a car has entered a spin, or the driving wheels have been broken loose by engine power and are spinning under acceleration.
Our objective in developing a car setup is to find one which allows the driver to easily find the optimum slip angle for all tires, and keep the car there as much of the time as possible in corners.
For maximum grip, we need to optimize the tire's contact with the road. This means having as much of the tire in contact with the road as possible at all times.
If a tire's inflation pressure is too low, the middle of the tire will not press the road as hard as the outside edges. If it's overinflated, the outside edges may not even touch at all. Using tire temperatures, we can find the optimum pressure; the higher the pressure, the harder the middle of the tire will work, and the higher its temperature will be in relation to the edges of the tire.
Note that the important time for a tire is when it is on the outside, since this is when it gets the most vehicle weight on it, and therefore this is when it generates the most cornering force - and most of its "heat".
Once we've found a good tire pressure, we may find that the inner and outer edges of the tire are showing different temperatures. If the inner edge of the tire is higher than the outer, than the inside edge is doing too much work. We must lean the top of the tire out a little so the outer edge will be doing more work. We call this adding positive camber. If the outer edge is too hot, we'll change towards more negative camber.
Since tires always distort a little under cornering, and tend to lift their inside edges, we'll almost always have some negative camber. The only exception is on ovals, where sometimes we will run a little positive camber on the left wheels, since they are always turning left and therefore their outer edge is always to the inside of the corner.
Since softer compounds generate more cornering force, we always want to run the softest compound we can. However, since softer compounds heat up more, and wear out faster, we sometimes have to run harder compounds, especially in races. The only way to find this out is by trial and error, monitoring temperature and wear. If the temperature of a tire gets into the tire's critical zone (in ICR2 the tire temps turn red) then we have to use a harder compound on that corner.
Note that changing the tire compound can affect the balance of the car, particularly if we change compounds at only one end of the car. On ovals, it's not uncommon to run a harder compound on the right front, or even both right side tires, since these tires do so much more work. If we go from, say, a soft to a medium right front, we'll have less grip on that corner, and we'll have to find some way of increasing the grip from the front end, or else we'll wind up with understeer. The car will want to slide straight off the track instead of turning in.
That brings us to...
When a car is cornering, we'd like it to be balanced; that is, when it reaches its limits of adhesion, we want both the front tires and the rear tires to begin to slide at the same time, at the same rate. If the car is not balanced, either the front tires will begin to slide first, and we'll slide straight off the track, or the rear tires will begin to slide first, and the car will spin.
For the car to be balanced, the grip available at each end must be proportional to the percentage of weight at that end of the car. In other words, if the car has 60% of its weight on the rear wheels, it must generate 60% of its grip from the rear wheels.
Many factors affect balance. As we have seen, tire compound can impact the balance. A small change in tire pressure, by increasing or decreasing the grip available from that end of the car, can also change the balance.
The amount of weight transferred onto each tire during cornering can also change the balance. This turns out to be a useful fact. A good rule of thumb is, the more weight that is transferred away from a tire, the worse that tire will grip. This rule does not apply in direct proportion to weight transferred to a tire; a tire having more weight transferred to it will not gain as much grip as a tire having the same weight transferred away from it.
In other words, if we are going through a certain corner with 250 pounds on the right front, and 200 pounds on the left front, and we make a setup change that puts 270 pounds on the right and 180 pounds on the left, we will lose some from the total grip being produced by the front end of the car. The increase in grip on from the right front will be more than offset by the decrease in grip from the left front.
In balancing the car, we have several factors at our disposal. As we've seen, we can adjust tire pressures, compounds, and camber. But we want to find the optimum for each of these, and not degrade a tire's performance just to reach balance.
Fortunately, we have several chassis adjustments that will allow us to tune the balance. These adjustments impact the weight transferred to a tire during cornering.
The first is the spring stiffness. ICR2 and N2 lump the spring stiffness together with shock stiffness into one factor. This isn't quite realistic, but fortunately in Papyrus' driving model it works well. It also simplifies our task.
Basically, if we stiffen the springs at one end of the car, that end will give up grip in relation to the other end. So if we make the rear shocks stiffer, the car will tend to oversteer more. If we soften that end, the opposite will happen.
If we stiffen the spring on only one corner, we will get a different effect when we are turning one way as opposed to the other. For example, if we have two slow hairpins on a given track, and they both go to the right, we might want to stiffen the right rear so the car will turn right better. This will also make it tend to understeer in medium-speed turns, so if we have some left-hand sweepers, the car will be more stable.
Be warned, however, that it can be very easy to get lost in the effects from asymmetrical changes to the chassis. Make sure you make only once change at a time, and keep copious notes!
In the cockpit, we have two more adjustments which affect the car's balance: the front and rear anti-roll bars. These affect the roll stiffness - i.e., how much weight is transferred to the outside wheel during cornering - at each end of the car. If we stiffen the front bar, the the car will tend to understeer more; if we soften it, the car will tend more towards oversteer. Stiffen the rear, and the car will tend towards oversteer.
We want to arrive at a basic setup that gives us some adjustment range in these bars, because as we race and burn off fuel load, the car will tend to go from understeering to oversteering. We want to be able to set the rear a little stiffer on full tanks, and soften the rear or stiffen the front to maintain the balance as we burn off fuel.
In the cockpit, we can also adjust brake balance. This will adjust the amount of work each end of the car does under braking. Too much braking by the rear wheels, and the car will have a tendency to spin under braking, because if the rear wheels lock they no longer have any directional stability. Too much the front, and the car will tend to go straight under braking, and will be less efficient as well.
Transient states refers to the moments when the car is changing from one condition to another - acceleration to braking, cornering to straight, etc. The most important of these is the transition from straightline to cornering. At this time, the car transfers weight from all four wheels to the outside wheel. How it does this - how quickly, and the impact of this on the car's responsiveness and feel, as well as stability, is critical to the drivability of the car.
Transient behavior is mostly impacted by the shock absorbers, or dampers. Since ICR2 and N2 both combine the damping adjustment with spring adjustment, we can't do any independent tweaking of the shocks. Since I don't understand this area really well anyway, I'll move on to the much sexier topic, aerodynamics.
At high speeds, aerodynamics has an enormous impact on the car's cornering and braking potential. Yet, from the standpoint of a race engineer, it is one of the simplest factors to deal with. Given a set of wings, we must simply find the optimum angle for each wing. As a basic principle, we want to run the lowest wing angle possible while still getting good cornering. Less wing means less drag, which translates into faster straightaway speeds.
However, less wing also means less grip. If we crank both wings all the way back to minimum angle, we'll be really fast at the end of the straight, but we'll have to corner more slowly, so our entry onto the straight will be slower. We need to find the optimum angle, so we have a high cornering speed without too much drag. The stopwatch will tell the story: the best setup is the one that produces the fastest lap time.
During the sorting-out process, we can use the tachometer or, if we've got one, the speedometer, to check our exit speeds from the corners, and the speeds at the end of the straights. We'll try to find a wing angle that gives good corner exit speeds without sacrificing too much top speed. The longest straightaway is probably the place to concentrate on here.
Just as roll stiffness, tire pressures, and other factors can affect the car's balance by increasing grip available at one end of the car or the other, the wings can affect the car's balance. Mechanical factors tend to impact the balance more in slow corners, while aerodynamics tend to impact the balance more at high speeds. Get the mechanical balance right first - go for a fairly neutral mechanical balance, so you can get around slow corners without too much pushin'.
Once the mechanical balance is right, tweak the wing angles so the car is stable in faster corners. If it's loose or unstable in fast corners - oversteering - then first try taking out some front wing. Add rear wing only if you really must, since the rear wing tends to generate more drag. If it's pushing - understeering - too much, take out a bit of rear wing, again to try to minimize drag.
Generally a setup that is neutral in slow corners and a bit tight - understeering - in faster corners is best, since the car will be nimble in the hairpins but stable and predictable in the sweepers.
The most elusive quality in race car setup is drivability. The grippiest car in the world will not be a good race car if it is unpredictable, or if its limits are razor-sharp. As we discussed earlier, its behavior must be predictable and consistent, and when it starts to slide, we want it to begin to let go gently, with some warning, and get fairly deep into a slide before it becomes uncontrollable.
A car like this will be easier to drive, and it therefore will be faster over the course of many laps, since the driver will be able to drive it near its maximum a greater percentage of the time. If the driver is not occupied with reacting to nasty moves from the car, and using a lot of mental effort just to keep it on the track, she/he will also have more attention to devote to other things, like dealing with traffic, monitoring fuel load and tire wear, and so forth.
A critical factor in drivability is the tire's slip angle curve, which we discussed above. If we have a choice, we want to choose a tire with the gentlest slip angle curve, particularly on the downside. Once we've chosen a tire, we want to maximize the chassis to also have these characteristics.
Unfortunately, this is the area which is most difficult to pin down. It becomes almost a black art. For example, Shane Pitkin feels that his use of stiff rear shocks/springs lends his setups their distinctive "tossability" - a combination of mild oversteer, and stability in a slide.
I've found that sometimes it's better to run a bit more wing, particularly rear wing, than will produce the fastest lap times. Fast and loose works - for qualifying. But for a race setup, where you're dealing with all kinds of distractions and trying to stay on the track for lap after lap, a little more downforce, and a little more understeer, can have you looking good.
Obviously it's not as simple as I've made it sound. All of these factors interact with one another. For example, if you increase the downforce at the front, the increased downforce will squash the tires down more firmly. This can affect the temperatures, or require more negative camber. Changing almost any of the factors we've reviewed can impact on other factors. It's very easy to get into a situation where you are chasing your tail. Professional racing teams refer to this as "getting lost".
Change one thing at a time, and keep copious notes!
I hope you find this overview of race car vehicle dynamics helpful. Race car setup is a very complex subject, and, though the fundamentals haven't changed, new knowledge is being added all the time. Good race engineers and crew chiefs are among the most critical - and well-paid - personnel in top racing series.
There are many books available on the subject of race car vehicle dynamics. I suggest Race Car Engineering & Mechanics, by Paul Van Valkenburg, and the massive 992-page Race Car Vehicle Dynamics, by William F. Milliken and Douglas L. Milliken. These books are available at the online bookstores Amazon Books and Barnes & Noble, as well as Classic Motorbooks at 800/826-6600.
Note: Doug Milliken tells me that Amazon and Barnes and Noble don't stock his book, so it can take a long time to get it if you order from them. He recommends ordering directly from the Milliken Research Associates site or from the SAE. He also notes that many other good racing books are available from the SAE site.
Also, check out Shane Pitkin's setup advice and setup advice from other sim racers. And have fun!