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.
In modern racing cars, there are two factors which contribute to this force, or adhesion: aerodynamic grip and mechanical grip. In 1967, however, Grand Prix cars had not yet begun to use aerodynamic downforce to augment the grip available from the tires.
In GPL, the focus is on the tires. We need to find ways to make the tires generate their optimum grip at all times. This means keeping the tire contact patch at its optimum shape, with the optimal vertical load distribution among the car's four tires.
A number of different adjustments in GPL's Car Setup menu impact the way the tire contacts the track surface at any given moment. We need to find the best compromise among these settings to give us the optimal levels of grip on each circuit.
We also want the car's handling to be balanced; that is, when it nears its limits of adhesion, we want both the front tires and at the rear tires to begin to slide at approximately 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.
In order to develop a good race car, we want to develop maximum grip in all directions. We also want it to be balanced and easy to drive. A number of factors enter into the equation.
The stiffness of the springs in the car's suspension perform a fundamental role. Together, the four springs support the weight of the car, and also resist additional vertical loads imposed by banked corners, dips, bumps, and so forth.
The ride height is also a crucial factor in the car's suspension behavior. The higher the ride height, the softer the springs can be. In general, and within reason, softer springs allow the tires to follow the bumps in the track surface better, so they give more grip.
However, higher ride height also means more weight transfer, which reduces the overall available grip. (More about this below under Weight Transfer and in The Tire.)
Therefore, we must find a compromise between high and soft, and low and stiff.
This topic is so important that I've devoted an entire section to it. If you hope to master the art of setup development in GPL, I strongly recommend you read Spring Rates and Ride Height.
Certain aspects of GPL's implementation make finding the optimum compromise a little more challenging than it might be in real life. In GPL, it's not easy to detect when we've overstepped the lower limits of the available combinations. If we do overstep these bounds, we permit the suspension to bottom at critical moments, destabilizing the car and drastically degrading its drivability and its cornering capability.
Fortunately, there are ways to cope with this; see How Do I Know when It's Bottoming? This is arguably the single most important page in this entire Help manual.
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 tires at that optimum slip angle 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 against 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.
In steeply banked corners, the additional vertical load will tend to distort the tire more, so we'll need to run higher pressures on steeply banked ovals than what we run on road courses and shallowly banked ovals.
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 typically we will run 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.
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 approximately 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, including camber, spring rates, anti-roll bar settings, damper settings, differential slip limiting settings, and, under braking, brake balance settings. 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 limited slip differential exerts a powerful influence over the behavior of the car in braking, in midcorner, and under acceleration. By limiting the amount of slippage between the rear wheels, we can dramatically alter the stability of the car in all three phases of the corner.
More locking effect makes the car more stable under both braking and acceleration. However, under acceleration at low speeds, this is true only up to a point; once the outside rear tire loses grip, the car becomes unstable no matter what the differential settings are. And more locking tends to make the outside tire lose its grip more suddenly.
On the other hand, more locking effect will increase the car's overall available traction, and therefore maximize acceleration out of slow corners.
The trick is to find a level of locking which delivers as much traction as possible without resulting in an unacceptably abrupt transition to power oversteer.
Again, the differential is a such critical factor that I've devoted an entire section to it, The Differential. I feel this section is also a must-read for anyone who does not already thoroughly understand the characteristics of the limited slip differential.
The amount of weight transferred onto each tire during cornering can 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.
This characteristic of tire behavior is a crucial element in adjusting the car's balance, as we'll see below.
In balancing the car, we have several factors at our disposal. As we've seen, we can adjust tire pressures, and camber to impact the grip at each corner of the car. 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 front and rear anti-roll bars are the primary adjustments used to affect the car's balance. These affect the roll stiffness; that is, how much weight is transferred to the outside wheel during cornering.
There is an anti-roll bar 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; soften the rear, and the car will tend towards understeer.
Changes that promote understeer will increase the car's traction under acceleration, while changes that promote oversteer will decrease traction.
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.
We can also adjust spring stiffness to tune the balance of the car. 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 springs 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. This is known as an asymmetrical setup.
For example, if we have two slow hairpins on a given track, and they both go to the right, we might want to soften the right front 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 of asymmetrical changes to the chassis. Make sure you make only once change at a time, and keep copious notes!
Also, changing the spring rates significantly from those which match the car's weight distribution can make the car more prone to bottom its suspension, especially if we go softer at the rear and/or stiffer at the front. There's more on this - and why it's a bad thing - in Spring Rates and Ride Height.
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 dampers (inappropriately referred to as shock absorbers in the US). Stiffening the dampers at one end of a car is roughly equivalent to stiffening the springs or anti-roll bars, but the effect is transient; a stiffer damper has most of its effect on handling during the moment when the driver applies a control input, such as applying the brakes, turning the wheel, or applying throttle, or when the car encounters an irregularity in the track surface, such as a bump or a dip.
Because damper stiffness impacts the way the car behaves over bumps, damper settings that work great on a relatively smooth track like Monza may make the car far too nervous over the bumps at a bumpy track like Zandvoort.
Dampers in GPL can be adjusted over a range of 1 to 5 clicks. This range is from very soft - almost no damping - to very stiff. Further, damping in bump can be adjusted independently from rebound.
In transient states while cornering, generally only two of the four dampers have a significant effect, and only one direction (bump or rebound) on each of those two dampers have much effect.
For example, under acceleration, making the outside rear bump setting stiffer will tend to promote power oversteer, as will a softer inside front droop (rebound) setting. This is because as the car squats under acceleration, it will tend to drop down at the outside rear, and rise up at the inside front. Making that outside rear stiffer is like putting in a stiffer spring or anti-roll bar - but only for a moment.
To help visualize this, envision a chair with a matchbook under one leg. If we're turning right, put a matchbook under the left rear leg. Now the chair can rock on the right front and left rear legs.
When we accelerate, the chair will rock toward the right rear and touch down on its right rear leg. The dampers that would resist that motion are the left front in droop, or rebound, and the right rear in bump. When we brake, the chair will rock toward the left front. The dampers that would resist that motion are the left front in bump, and the right rear in droop, or rebound. Resisting the motion gives additional stiffness at that end of the car for a moment.
See the Damper Tables for a rule-of-thumb guide to damper adjustments for tuning transient behavior.
Perhaps 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 condition, and so forth.
Obviously driver skill level enters into the equation. A very skilled driver at the Michael Schumacher/Juan Montoya level will be able to handle a car that requires more work to keep it under control, while a novice driver will benefit from a car that is more stable and docile.
I've found that I prefer to run a setup that understeers a bit. Fast and loose works ok for qualifying. But for a race setup, where I'm dealing with all kinds of distractions and trying to stay on the track for lap after lap, a little more understeer makes me much more comfortable and permits me to deliver more consistent performances with much less chance of a big mistake. The payoff for sacrificing a small amount of absolute speed potential is that I get much better race results.
Obviously it's not as simple as I've made it sound. All of these factors interact with one another. Changing almost any of the factors we've reviewed can impact on other factors.
Be aware of this as you make changes. Be sure to go back and recheck tire temperatures, for example, after you change roll stiffness, spring rates, differential slip settings, ride height, or anything else that might impact the shape and loading of the tire contact patch.
It's very easy to get into a situation where you are chasing your tail. Professional racing teams refer to this as "getting lost".
Until you're very, very comfortable with the impact of changes to the various parameters, change only one thing at a time, and keep copious notes!
I hope you find this introduction to 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.
This introduction only skims the surface of this large and complex body of knowledge. There are many excellent books available which discuss race car vehicle dynamics in much more depth.
If you're serious about understanding this vital aspect of racing, you owe it to yourself to acquire some of these books. See the References page for a list.
Since GPL's physics model is so detailed and so true to life, almost everything you read in these books will apply to developing setups for GPL. And almost everything you learn about developing setups for GPL will have relevance to real-world setups, if you're lucky enough to get your hands on a real-world race car!