03-06-2011, 06:49 PM
Suspension tuning is a way for many people to enjoy their car more. This is especially true with the 968. The car is designed with what Porsche thought was the best compromise between ride and performance for the majority of the market. As more people learn of the abilities of the car, more suspension tuning options become available, and new levels of handling can be achieved. This is not without compromise though, and determining what is the best package and setup for you can be a confusing and daunting task.
I will try to lay out some of the basics so that you can understand what things do, and how they can affect the handling of your car. I will also try to establish an understanding of the compromises you can expect, as well as the limitations of some setups. What you do for one application, is not necessarily what you do for another. How you set up a race car is not how you set up a street car. What you do for long tracks is not what you would do for short tracks, and what you do for autocross is entirely different than what you would do for either. What you would do for street is a whole different kind of setup. The most difficult choices are those that involve a dual purpose car. This is always a very real and limiting compromise.
I won't get into particular brands or models of components, as that is really too subjective, and constantly changing as new products emerge. Instead I will focus on the conditions and circumstances that would lead you to choices.
The things delineated here are for general reference only. Do not blame me if you go out and wrap your car around a phone pole because you read something here and made some change and then drove beyond the limits of you or your car.
Ride versus Cornering versus Handling
Ride
Ride is defined as the civility or lack thereof that a car exhibits as it runs over bumps and anomalies in the road. This is often described in terms like "smooth", "soft", "firm" or "harsh".
Cornering
This is the ability of a car to hold a speed in a corner. This is generally expressed in a measured factor in force of gravity.
Handling
This is the ability of a car to work through successive and changing corners and conditions. It is generally measured in terms of slalom times. There is something more esoteric about describing the handling of the 968, but we won't get into that here.
There are basically 3 factors that you try to control in any suspension setup. They are bounce, body roll, and weight transfer. These are conditions for acceleration, braking and cornering. Each of these needs something else to address a limit or condition. The trick is to find the balanced setup that adequately controls all 3 things in all 3 scenarios. For example, what you use for controlling bounce in acceleration may well work against you in cornering.
The components that address these conditions are springs, dampers (or shocks), and anti-roll bars (or sway bars). Each item serves a specific function, and has an affect on each other item. Determining what product or size of a given item takes a lot of math and a very clear understanding of what you are trying to achieve. Chassis setup and suspension geometry also play a role in how these things behave.
Chassis Setup
I won't get into the theory of Chassis Design, or how a car chassis works, other than to the point of how it relates to the suspension. That is an entirely different discussion.
The first order of business is to get the chassis set up so that there is controlled movement or flexing of the mounting points of the suspension. In a Unibody car like the 968, a certain amount of flex in the chassis is desirable. It allows stresses to be transferred across the entire chassis more evenly than in a ladder frame car. However, it becomes even more important to ensure that the suspension mounting points are fixed, so that limits can be placed on the geometrical changes that occur during weight transfer. This means that things like the castor blocks and strut tower points are key areas where improvement can be made, allowing the car's suspension to be tuned without making major changes in the geometry.
Suspension Geometry
The suspension Geometry is reflected in angular change in Camber, Castor, and Toe. Each has an effect on the way a car drives. There are of course limits to any angle applied, there are limits to what these settings can do.
This is at both ends of the car. As a car goes through corner, the angles of the geometry change. Controlling and limiting this change is how loads at each corner are managed. The goal is to maintain as even a distribution of weight at each corner as is possible, while maintaining the accelerative advantages of centrifugal force.
Camber
Camber is the lateral angle of the wheel. If the top of the tire is tipped in, it has Negative Camber. If it is tipped out, it has Positive Camber. Negative camber will help a car in a corner, as it allows the outside tire to gain contact area as the load increases. The down side of this is that the more load on the tire, the higher the heat on that tire. Also, too much Negative Camber can result in a loss of net contact patch area when factoring in the loss in contact of the inside tire.
It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.
Caster
Caster is the longitudinal angle. If the wheel is tipped forward, it has Negative Caster. If it is tipped back, it has Positive Caster. Positive Caster helps return the steering wheel to center and maintain high speed stability. Larger Caster angles result in more steering effort required, but provide more straight line stability.
Toe
Toe is the relative angles fore and aft. If the fronts of the tires are closer to each other than the rears, they are "Toe-ed in". If they are farther apart than the rear, they are "Toe-ed out". Toe In helps maintain directional stability and reduce wander, while Toe Out promotes better turn in. Street cars generally have Toe In and race cars generally have Toe Out.
There is an entire thread about street Alignment settings for the 968 here:
http://www.968forums...s-for-your-car/
Understeer and Oversteer
Understeer is the condition where the front of the car wants to continue in a straight line in a corner. This is often as the result of too much weight in the front of the car relative to the rear, too stiff of a spring in the front, too stiff of an Anti-Roll bar, or incorrect tire pressure in front.
Oversteer is the condition that causes the rear wheels to swing out in a turn. This is often caused by too little weight in the rear of the car relative to the front, too stiff of a spring in the rear, too stiff of an Anti-Roll bar in the rear, or incorrect tire pressure in the rear.
Power Oversteer is the condition that causes the rear of the car to swing out under power.
Springs
Springs are there to control variations in the road and the loads applied to the corners of the car during cornering. Springs make arguably the biggest difference in improving a car's handling and cornering. Choosing the right ones can really make a car fly through the corners. Choosing the wrong ones can put you in a wall. There are basically 3 kinds of springs in a car suspension.
The first is a coil spring, which is basically a length of wire wound in the shape of a coil. This wire has a fixed and calculable resistance to compression that is generally expressed in pounds per inch of compression. That is to say that a 250lb spring will require 250 pounds of vertical force to compress the spring 1 inch. The spring rate is determined by the coil diameter, wire diameter, and free length. Typically coil springs are placed at the corners of the car, and generally fairly upright in orientation, making determining the wheel rate pretty straight forward or at least simple geometry to calculate.
The second type of spring commonly used is what is called a Torsion Bar. This has exactly the same effect as a coil spring. It differs from a coil spring in that it is a long straight bar, which is attached to the car at one end and resists twisting as the suspension moves up and down. Some cars have that bar running fore/aft, and some have it running left to right across the car, as is the case with the 968. The spring rate is determined by the bar diameter and the bar length. There is an added complication in determining the wheel rate, as you also must factor in the lever length (typically the control arm). Typically the installation is more difficult as well. This makes tuning with torsion bars much more complicated.
The third type of spring is called a "Leaf Spring" and we won't be discussing that here, as the 968 does not have them.
Springs have what is called a "Natural Frequency". This is the rate at which a car will bounce up and down, based primarily on the spring rate and the weight of the car. The goal is to have the Natural Frequency of all 4 corners be the same. This will allow the car to move evenly as it crosses over anomalies in the road, as well as transitions through corners. This requires some math, as well as other devices to help control them.
As a car goes through corner, it transfers weight to the outside of that corner. Springs resist that transfer, and attempt to maintain and even distribution of weight at all four corners. If there were no springs, centrifugal force would allow all of the force to transfer to the outside, and the car would abruptly slide when the limits of adhesion were reached, and would not accommodate additional changes. Springs allow weight to transfer more evenly.
Choosing springs is like choosing shoes. There is no single right answer. You have to decide how much load you are trying to control. This is determined by the kind of driving you will do, the weight at each corner of the car, and the ride you can live with. Choosing springs that are too stiff will cost you comfort, and can make the car less predictable. Choosing springs too soft will open the door for more body roll and leaning, as well as limit the speeds you can maintain through a corner, as it affects the weight transfer.
Dampers
Dampers are there for one reason and one reason only. They are there to control the oscillations of the springs, regardless of what type of spring. it is a common misconception that stiffening up dampers makes a suspension stiffer. In reality all it does is make the suspension less compliant. With the exception of the pressure of a gas charge, there is no spring rate attributable to a damper, so there is no stiffness added, regardless of setting. Setting them high may make the suspension "feel" stiffer, but all you have done is remove the damper's ability to control the spring. The result is poor handling and unpredictability.
Dampers come in a variety of forms. They can be MacPherson Struts, or common shocks. The can be gas charged or oil filled. They may be fixed valve rate, or variable. They may or may not have adjustability. There are advantages and disadvantages to each type.
The least expensive is an oil filled non-adjustable shock or strut. This is what the 968 came with in standard form. These provide an excellent control of the springs in the car, and establish a very tolerable compromise between ride and handling. While these are excellent choices for a daily driver street car, they have very real limitations to what they can do in more severe applications.
Gas-filled dampers are most common in the performance arena. They resist movement by means of a gas charge passing through a series of valves. These tend to be quicker reacting, and immediately more resistant, making them desirable for performance applications. They are not without complication though, as the gas preload that is inherent with them can cause symptoms like a more jittery response during transitions, and a tendency for the car to bounce a bit when unsettled.
Rebound (or bump) and Compression are the conditions that dampers try to control. They can do this a number of ways. Less expensive units strike a compromise in how they control both. Some units have dual valving to try to handle each. Some have adjustability for one, the other, or both.
Rebound is the extension of the damper after compression. It is the rate at which it allows the spring to return to its static orientation. A firm rebound setting allows the spring to return quickly, but can result in the car being less controllable during transitions if set too high. This can result in understeer, oversteer, or a harsh ride.
Compression is the setting used to control the rate at which the spring compresses under load. Set too high, this results in the spring being unable to control the load applied to it, and can make the car slide or feel very harsh. Set too low, the car will tend to bounce like a pogo stick.
Finding the proper balance of Compression and Rebound settings can be a daunting task, requiring a lot of driving time. This is because many conditions need to be experienced, and they need to be repeated to establish the setting. They are also dependent on the spring rate. What you would set for a 200lb spring is not what you would set for a 400lb spring, even if everything else was equal. This often seems like more art than science.
Determining what damper is best for you depends on a lot of things. If you like to fiddle, and have the time and resource, and plan to drive the car competitively, then adjustable units may be right for you. If you just want to take the car to work, then you are probably best off with the oil filled fixed units. If you plan to do both, then perhaps a dual valve non-adjustable unit, or limited adjustment unit is right.
Anti-roll bars (sway bars)
These are essentially torsion bars that are attached to fixed points in the middle of the car, and have levers on each end that attach to moving suspension components like the control arms. They resist twisting during a corner as the car tends to lean. This helps maintain a flatter stance during the corner, and helps control weight transfer. The thicker the bar, the flatter the car will be. It is easy to go too far though, which can cause the car to lose adhesion at either end. Choosing the right bar depends on knowing the spring rates and the weight of the car at each corner. Stiffer springs result in higher speeds in the corners, and tend to require larger bars to control lean. Too stiff of a bar on too light of a spring, and the car will tend to be very unpredictable. Too light of a bar on too stiff of a spring, and the car will tend to break away abruptly. The key is to find a bar size that smoothly manages the weight transfer, and maintains the most even amount of load at each corner.
Tire Pressures
Tire pressures are likely the widest varying item on a car. The temperatures of the tires change dramatically which causes changes in pressures. Higher temperatures mean higher pressures. Maintaining as even a tire temperature as possible will help ensure the most consistent pressure possible, which will facilitate the best adhesion the tire can provide. What we try to achieve is the lowest differential between cold and hot temperatures. This will vary from day to day, dependent on weather, speed, and other factors. This is far less of a factor on a street car, but can mean everything on a track car.
Higher tire pressures on a street car can mean better fuel economy, but a rougher ride. Higher pressures also provide more load capacity.
Lower tire pressures can make for a smoother ride, but at the expense of tire life.
Tire pressures generally fluctuate about 1psi for every 10 degrees of change in temperature. This means that what you set your tires at cold will likely vary from season to season. It also means that you have to consider this when you set them cold for a track day, as they will go up dramatically from morning to afternoon.
To determine how your tire pressure is working on a street tire, look at the wear pattern. If the center is wearing more than the outside edges, you have too much pressure. if the outside edges are both wearing more than the center of the tire, you have too little.
As a general rule on the track you lower the tire pressures to increase traction. There is a point at which this tips over to a loss of traction though. Believe it or not, often you have to RAISE the cold tire pressure to increase traction. If you start off too low, the pressure rise will be too high, and the tire will get "greasy" in the corners. However, if you start off too high, then it will take a while for the tire to come up to temperature, and you won't get the maximum traction.
To determine whether or not you are at the optimal pressure, experiment with pressures, adjusting a pound at a time, until you get the smallest differential cold to hot. Then, you can use a temperature gauge to see if you have the most even temperatures across the tire. Adjust accordingly until you do. As a general rule, if the inside corner of the tire is dramatically hotter than the outside, then you need to decrease the pressure a bit. If the outside corner is hotter, then you need to increase the pressure a bit. Do this until you can achieve the most even temperatures possible.
You can also draw a line of chalk or paint across the tread of a tire from one side to the other and watch the wear pattern. If you are wearing out the outside corner of a negative camber tire, then you need to increase the pressure, and visa versa, taking care that too much will make things worse. On a race car there can be a lot of other factors that cause front tire outside wear, and a lot more measuring of corner loads will be required to diagnose that.
What does this mean and how do I choose?
As much science as there is and math that can be applied, unfortunately often much of this is more like Black Arts. There is a fine balance to be achieved in setting up a car, and it can be the difference between a car that feels planted and predictable to a car that is nervous and feels like it's on marbles. How you drive is as much of a factor as what you put in the car. The only way to try to find out what is right for you and your car is to talk to people that drive where and how you do, see what they use, and drive their car. If you are lucky, you'll gain some insight into what different setups do, and be able to facilitate your decision. Budget plays a huge role for most people too, and you may find that you can strike a compromise between ultimate performance and what you can afford.]
My best advice is to set up the car for the 85th percentile. That means set it up for what you will do with the car 85% of the time. Leave the other 15% alone.
Hopefully this has helped you to understand the basics, and allow you to tune your car's suspension to help provide you with the best experience possible.
I will try to lay out some of the basics so that you can understand what things do, and how they can affect the handling of your car. I will also try to establish an understanding of the compromises you can expect, as well as the limitations of some setups. What you do for one application, is not necessarily what you do for another. How you set up a race car is not how you set up a street car. What you do for long tracks is not what you would do for short tracks, and what you do for autocross is entirely different than what you would do for either. What you would do for street is a whole different kind of setup. The most difficult choices are those that involve a dual purpose car. This is always a very real and limiting compromise.
I won't get into particular brands or models of components, as that is really too subjective, and constantly changing as new products emerge. Instead I will focus on the conditions and circumstances that would lead you to choices.
The things delineated here are for general reference only. Do not blame me if you go out and wrap your car around a phone pole because you read something here and made some change and then drove beyond the limits of you or your car.
Ride versus Cornering versus Handling
Ride
Ride is defined as the civility or lack thereof that a car exhibits as it runs over bumps and anomalies in the road. This is often described in terms like "smooth", "soft", "firm" or "harsh".
Cornering
This is the ability of a car to hold a speed in a corner. This is generally expressed in a measured factor in force of gravity.
Handling
This is the ability of a car to work through successive and changing corners and conditions. It is generally measured in terms of slalom times. There is something more esoteric about describing the handling of the 968, but we won't get into that here.
There are basically 3 factors that you try to control in any suspension setup. They are bounce, body roll, and weight transfer. These are conditions for acceleration, braking and cornering. Each of these needs something else to address a limit or condition. The trick is to find the balanced setup that adequately controls all 3 things in all 3 scenarios. For example, what you use for controlling bounce in acceleration may well work against you in cornering.
The components that address these conditions are springs, dampers (or shocks), and anti-roll bars (or sway bars). Each item serves a specific function, and has an affect on each other item. Determining what product or size of a given item takes a lot of math and a very clear understanding of what you are trying to achieve. Chassis setup and suspension geometry also play a role in how these things behave.
Chassis Setup
I won't get into the theory of Chassis Design, or how a car chassis works, other than to the point of how it relates to the suspension. That is an entirely different discussion.
The first order of business is to get the chassis set up so that there is controlled movement or flexing of the mounting points of the suspension. In a Unibody car like the 968, a certain amount of flex in the chassis is desirable. It allows stresses to be transferred across the entire chassis more evenly than in a ladder frame car. However, it becomes even more important to ensure that the suspension mounting points are fixed, so that limits can be placed on the geometrical changes that occur during weight transfer. This means that things like the castor blocks and strut tower points are key areas where improvement can be made, allowing the car's suspension to be tuned without making major changes in the geometry.
Suspension Geometry
The suspension Geometry is reflected in angular change in Camber, Castor, and Toe. Each has an effect on the way a car drives. There are of course limits to any angle applied, there are limits to what these settings can do.
This is at both ends of the car. As a car goes through corner, the angles of the geometry change. Controlling and limiting this change is how loads at each corner are managed. The goal is to maintain as even a distribution of weight at each corner as is possible, while maintaining the accelerative advantages of centrifugal force.
Camber
Camber is the lateral angle of the wheel. If the top of the tire is tipped in, it has Negative Camber. If it is tipped out, it has Positive Camber. Negative camber will help a car in a corner, as it allows the outside tire to gain contact area as the load increases. The down side of this is that the more load on the tire, the higher the heat on that tire. Also, too much Negative Camber can result in a loss of net contact patch area when factoring in the loss in contact of the inside tire.
It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.
Caster
Caster is the longitudinal angle. If the wheel is tipped forward, it has Negative Caster. If it is tipped back, it has Positive Caster. Positive Caster helps return the steering wheel to center and maintain high speed stability. Larger Caster angles result in more steering effort required, but provide more straight line stability.
Toe
Toe is the relative angles fore and aft. If the fronts of the tires are closer to each other than the rears, they are "Toe-ed in". If they are farther apart than the rear, they are "Toe-ed out". Toe In helps maintain directional stability and reduce wander, while Toe Out promotes better turn in. Street cars generally have Toe In and race cars generally have Toe Out.
There is an entire thread about street Alignment settings for the 968 here:
http://www.968forums...s-for-your-car/
Understeer and Oversteer
Understeer is the condition where the front of the car wants to continue in a straight line in a corner. This is often as the result of too much weight in the front of the car relative to the rear, too stiff of a spring in the front, too stiff of an Anti-Roll bar, or incorrect tire pressure in front.
Oversteer is the condition that causes the rear wheels to swing out in a turn. This is often caused by too little weight in the rear of the car relative to the front, too stiff of a spring in the rear, too stiff of an Anti-Roll bar in the rear, or incorrect tire pressure in the rear.
Power Oversteer is the condition that causes the rear of the car to swing out under power.
Springs
Springs are there to control variations in the road and the loads applied to the corners of the car during cornering. Springs make arguably the biggest difference in improving a car's handling and cornering. Choosing the right ones can really make a car fly through the corners. Choosing the wrong ones can put you in a wall. There are basically 3 kinds of springs in a car suspension.
The first is a coil spring, which is basically a length of wire wound in the shape of a coil. This wire has a fixed and calculable resistance to compression that is generally expressed in pounds per inch of compression. That is to say that a 250lb spring will require 250 pounds of vertical force to compress the spring 1 inch. The spring rate is determined by the coil diameter, wire diameter, and free length. Typically coil springs are placed at the corners of the car, and generally fairly upright in orientation, making determining the wheel rate pretty straight forward or at least simple geometry to calculate.
The second type of spring commonly used is what is called a Torsion Bar. This has exactly the same effect as a coil spring. It differs from a coil spring in that it is a long straight bar, which is attached to the car at one end and resists twisting as the suspension moves up and down. Some cars have that bar running fore/aft, and some have it running left to right across the car, as is the case with the 968. The spring rate is determined by the bar diameter and the bar length. There is an added complication in determining the wheel rate, as you also must factor in the lever length (typically the control arm). Typically the installation is more difficult as well. This makes tuning with torsion bars much more complicated.
The third type of spring is called a "Leaf Spring" and we won't be discussing that here, as the 968 does not have them.
Springs have what is called a "Natural Frequency". This is the rate at which a car will bounce up and down, based primarily on the spring rate and the weight of the car. The goal is to have the Natural Frequency of all 4 corners be the same. This will allow the car to move evenly as it crosses over anomalies in the road, as well as transitions through corners. This requires some math, as well as other devices to help control them.
As a car goes through corner, it transfers weight to the outside of that corner. Springs resist that transfer, and attempt to maintain and even distribution of weight at all four corners. If there were no springs, centrifugal force would allow all of the force to transfer to the outside, and the car would abruptly slide when the limits of adhesion were reached, and would not accommodate additional changes. Springs allow weight to transfer more evenly.
Choosing springs is like choosing shoes. There is no single right answer. You have to decide how much load you are trying to control. This is determined by the kind of driving you will do, the weight at each corner of the car, and the ride you can live with. Choosing springs that are too stiff will cost you comfort, and can make the car less predictable. Choosing springs too soft will open the door for more body roll and leaning, as well as limit the speeds you can maintain through a corner, as it affects the weight transfer.
Dampers
Dampers are there for one reason and one reason only. They are there to control the oscillations of the springs, regardless of what type of spring. it is a common misconception that stiffening up dampers makes a suspension stiffer. In reality all it does is make the suspension less compliant. With the exception of the pressure of a gas charge, there is no spring rate attributable to a damper, so there is no stiffness added, regardless of setting. Setting them high may make the suspension "feel" stiffer, but all you have done is remove the damper's ability to control the spring. The result is poor handling and unpredictability.
Dampers come in a variety of forms. They can be MacPherson Struts, or common shocks. The can be gas charged or oil filled. They may be fixed valve rate, or variable. They may or may not have adjustability. There are advantages and disadvantages to each type.
The least expensive is an oil filled non-adjustable shock or strut. This is what the 968 came with in standard form. These provide an excellent control of the springs in the car, and establish a very tolerable compromise between ride and handling. While these are excellent choices for a daily driver street car, they have very real limitations to what they can do in more severe applications.
Gas-filled dampers are most common in the performance arena. They resist movement by means of a gas charge passing through a series of valves. These tend to be quicker reacting, and immediately more resistant, making them desirable for performance applications. They are not without complication though, as the gas preload that is inherent with them can cause symptoms like a more jittery response during transitions, and a tendency for the car to bounce a bit when unsettled.
Rebound (or bump) and Compression are the conditions that dampers try to control. They can do this a number of ways. Less expensive units strike a compromise in how they control both. Some units have dual valving to try to handle each. Some have adjustability for one, the other, or both.
Rebound is the extension of the damper after compression. It is the rate at which it allows the spring to return to its static orientation. A firm rebound setting allows the spring to return quickly, but can result in the car being less controllable during transitions if set too high. This can result in understeer, oversteer, or a harsh ride.
Compression is the setting used to control the rate at which the spring compresses under load. Set too high, this results in the spring being unable to control the load applied to it, and can make the car slide or feel very harsh. Set too low, the car will tend to bounce like a pogo stick.
Finding the proper balance of Compression and Rebound settings can be a daunting task, requiring a lot of driving time. This is because many conditions need to be experienced, and they need to be repeated to establish the setting. They are also dependent on the spring rate. What you would set for a 200lb spring is not what you would set for a 400lb spring, even if everything else was equal. This often seems like more art than science.
Determining what damper is best for you depends on a lot of things. If you like to fiddle, and have the time and resource, and plan to drive the car competitively, then adjustable units may be right for you. If you just want to take the car to work, then you are probably best off with the oil filled fixed units. If you plan to do both, then perhaps a dual valve non-adjustable unit, or limited adjustment unit is right.
Anti-roll bars (sway bars)
These are essentially torsion bars that are attached to fixed points in the middle of the car, and have levers on each end that attach to moving suspension components like the control arms. They resist twisting during a corner as the car tends to lean. This helps maintain a flatter stance during the corner, and helps control weight transfer. The thicker the bar, the flatter the car will be. It is easy to go too far though, which can cause the car to lose adhesion at either end. Choosing the right bar depends on knowing the spring rates and the weight of the car at each corner. Stiffer springs result in higher speeds in the corners, and tend to require larger bars to control lean. Too stiff of a bar on too light of a spring, and the car will tend to be very unpredictable. Too light of a bar on too stiff of a spring, and the car will tend to break away abruptly. The key is to find a bar size that smoothly manages the weight transfer, and maintains the most even amount of load at each corner.
Tire Pressures
Tire pressures are likely the widest varying item on a car. The temperatures of the tires change dramatically which causes changes in pressures. Higher temperatures mean higher pressures. Maintaining as even a tire temperature as possible will help ensure the most consistent pressure possible, which will facilitate the best adhesion the tire can provide. What we try to achieve is the lowest differential between cold and hot temperatures. This will vary from day to day, dependent on weather, speed, and other factors. This is far less of a factor on a street car, but can mean everything on a track car.
Higher tire pressures on a street car can mean better fuel economy, but a rougher ride. Higher pressures also provide more load capacity.
Lower tire pressures can make for a smoother ride, but at the expense of tire life.
Tire pressures generally fluctuate about 1psi for every 10 degrees of change in temperature. This means that what you set your tires at cold will likely vary from season to season. It also means that you have to consider this when you set them cold for a track day, as they will go up dramatically from morning to afternoon.
To determine how your tire pressure is working on a street tire, look at the wear pattern. If the center is wearing more than the outside edges, you have too much pressure. if the outside edges are both wearing more than the center of the tire, you have too little.
As a general rule on the track you lower the tire pressures to increase traction. There is a point at which this tips over to a loss of traction though. Believe it or not, often you have to RAISE the cold tire pressure to increase traction. If you start off too low, the pressure rise will be too high, and the tire will get "greasy" in the corners. However, if you start off too high, then it will take a while for the tire to come up to temperature, and you won't get the maximum traction.
To determine whether or not you are at the optimal pressure, experiment with pressures, adjusting a pound at a time, until you get the smallest differential cold to hot. Then, you can use a temperature gauge to see if you have the most even temperatures across the tire. Adjust accordingly until you do. As a general rule, if the inside corner of the tire is dramatically hotter than the outside, then you need to decrease the pressure a bit. If the outside corner is hotter, then you need to increase the pressure a bit. Do this until you can achieve the most even temperatures possible.
You can also draw a line of chalk or paint across the tread of a tire from one side to the other and watch the wear pattern. If you are wearing out the outside corner of a negative camber tire, then you need to increase the pressure, and visa versa, taking care that too much will make things worse. On a race car there can be a lot of other factors that cause front tire outside wear, and a lot more measuring of corner loads will be required to diagnose that.
What does this mean and how do I choose?
As much science as there is and math that can be applied, unfortunately often much of this is more like Black Arts. There is a fine balance to be achieved in setting up a car, and it can be the difference between a car that feels planted and predictable to a car that is nervous and feels like it's on marbles. How you drive is as much of a factor as what you put in the car. The only way to try to find out what is right for you and your car is to talk to people that drive where and how you do, see what they use, and drive their car. If you are lucky, you'll gain some insight into what different setups do, and be able to facilitate your decision. Budget plays a huge role for most people too, and you may find that you can strike a compromise between ultimate performance and what you can afford.]
My best advice is to set up the car for the 85th percentile. That means set it up for what you will do with the car 85% of the time. Leave the other 15% alone.
Hopefully this has helped you to understand the basics, and allow you to tune your car's suspension to help provide you with the best experience possible.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
(This post was last modified: 03-07-2011, 07:51 AM by flash.)
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