///M-Spec's Pocket Guide to GT3 Suspension Tuning

Discussion in 'GT3 Tuning' started by ///M-Spec, Aug 16, 2003.

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  1. ///M-Spec

    ///M-Spec Staff Emeritus

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    ///M-Spec’s Pocket Guide to GT3 Suspension Tuning

    This guide covers the fundamentals of suspension tuning for GT3. It gives you some important background information and hopefully will teach the beginning tuner how to make a car handle better. It presumes very little prior knowledge; so experienced car buffs may want to skip ahead to the Handling Problems and Solutions Guide.

    The tips in this guide are taken from real life tuning principles, but its important to remember that the GT3 suspension model is abstracted and grossly simplified compared to a real racecar. I also claim no special knowledge of the inner workings of GT3's physics modeling: I'm simply applying what I know about real cars to what you can do in GT3.

    In addition, I've also striven to ensure the accuracy of the information contained within, but obviously can make no guarantees as to the effectiveness of any proposed solution. In other words: I hope this information is useful, but Your Mileage (literally) May Vary.


    WHAT YOU NEED TO KNOW ABOUT TIRES
    Our discussion on tuning starts with the tire, because everything you will do to your suspension is for the sake of the four pieces of rubber attached at the corners of your car. Each tire can produce a tractive force (aka 'grip') that helps keep the car on the road. How quickly you can corner is dependant upon the distribution of work load to each of your tires. The goal is to make sure each of your tires is producing its fair share of grip to turn the car.

    When you corner, all the weight of the car will move to one side. Thus, the tires on the outside of your car will tend to take the brunt of the weight transfer. This is actually good to an extent, because a modern performance tire will produce more grip as you increase the amount of weight you put on it. The amount of downward force applied on a tire is commonly known as download.

    As download increases, a tire's tractive force will rise, allowing greater lateral acceleration (faster cornering). The ability to convert download into tractive force is known as a tires' coefficient of friction. In layman's terms, the better a tire can stick, the higher its coefficient of friction.

    The tire's ability to produce tractive force will peak at a certain point, however. After this point, it will drop off -- sometimes dramatically. When a tire gets to the edge of its ability to produce tractive force, it begins to slip. But a curious thing begins to happen at that moment: a modern performance tire grips best just as it begins to slip. Really. The amount of slip is commonly called the slip angle and refers to the angle between where the tire is pointing and the angle that the tire is actually moving in. The optimal slip angle for most performance tires are usually only a few (less than 10 but more than 1) degrees.

    There's a great deal of technical stuff available on why tires behave this way, but is outside the scope of this guide. Simply put, the harder you press down on a tire, the harder it grips, until it starts to give up and loses traction. Some tires grip harder in proportion to how hard you press on it. These tires are known as 'grippier' or 'stickier' tires. In GT3, you can help this simply by buying tires with a greater Traction number.

    Therefore, the goal of every race car driver and engineer is to put the stickiest set of tires on his car and get each of the tires on his race car to that magic optimum slip angle and stay there as long as the car is turning. Of course, this gets complicated in a hurry as we'll see soon in the next part of this series.
     
  2. ///M-Spec

    ///M-Spec Staff Emeritus

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    WEIGHT TRANSFER, CG, TRACK AND WHEELBASE
    We talked about weight transfer earlier, let's go back to it for a moment. As we discovered, the weight of the car pushing down on the tires will make them grip better. At one point, they have too much download on them so they 'let go' and begin to slip.

    Three factors determine the amount of download you put on tires: the car’s wheelbase, track and its center of gravity. Without going into the details of why, a car can attain higher levels of lateral acceleration when the CG is lower to the ground and the wheelbase and track is wider. All else being equal a tall, skinny car with a short wheelbase will overload its tires before a low, wide car with a long wheelbase. This is why F1 cars are so low and wide, and the tires are pushed out to the far corners.

    Obviously, at least two of these factors are out of your control in GT3, but we can talk about one that is: CG. You can change the ride height of any car with Full Race suspension. There's a simple principle here: go as low as you can, until the car begins to bottom out.


    SO WHY DO WE HAVE SUSPENSIONS?
    Reason one: bumps. If the world was as smooth as a billiard ball table, we could probably mount the wheel to a solid beam that bolts right to the car's frame. But a car with such a stiff suspension would knock its driver and itself senseless in the real world. The real problem is that if the car encounters a bump while cornering, it would bounce and go airborne -- this sudden reduction in download would do truly terrible things to a car's handling and probably send it right off the road.

    The second reason why suspensions exist is to help distribute download across all the tires. We can't change the amount of weight transfer when a car corners, but we can change where it goes and how quickly. To a degree.

    So the suspension in a race car has two goals: 1) to ensure the amount of tire download does not change dramatically when the car encounters a bump, and 2) to ensure the best possible distribution of that download across the available tires.

    There are 3 basic factors in your suspension that help you achieve these goals. The Spring Rate, Damper Stiffness and Stabilizer Stiffness.


    SPRING RATE
    We all know what springs are and what they do. Let’s change our thinking about them slightly. Each spring holds a certain portion of the car’s total weight. Thus, if we change the stiffness of the springs, we can change (to a degree) how much weight is distributed to each tire when the car corners.

    As a general rule, you want to set your car to run with the highest spring rates that the course will permit. The reason is that high spring rates will allow you to set the ride height low while reducing the possibility of bottoming out. As we discovered earlier, the lower, the better. As you encounter bumps or the rumble strips (sometimes called aprons), too firm a spring rate may cause the car to lose download.

    You can also use spring rates to trim car balance, which we'll talk about in the next section.

    DAMPER (Shocks)
    A shock absorber or damper resists motion. If you've ever held one, you may notice it’s not too hard to compress , but it is very hard to compress it quickly. The same is true when a shock absorber extends.

    When a car hits a bump, the spring absorbs the shock of the bump instead of transmitting it to the rest of the car. After the initial impact, the spring will extend to its original height with a similar amount of force. If this force is not contained and controlled, a car that hits a bump will continue to bounce several times, losing a little energy each time. Clearly this will do bad things to the amount of download on our tires, so we have a shock absorber that is designed to let the car absorb the initial impact of a bump, then re-extend to its original height just once. The shock absorbs the excess energy contained within the spring. Thus, the shocks have a supporting role with the springs and should be thought of as a supplement to good spring rates.

    As a general rule, the stiffer your spring rates, the higher your shock stiffness should be. It is very important to match the amount of shock stiffness with your spring rate. Too stiff and your car will be jittery over bumps. Too soft and your car will oscillate after bumps.

    You can also use shocks to augment spring rates in trimming car balance, which we'll talk about in the next section.

    STABILIZER (Sway-bar, or anti-roll bar)
    A sway bar is a simple torsion bar that transfers download laterally. During cornering, a certain portion of the download on the outside of the car is transferred to the inside of the car. The stiffer the bar, a greater portion of download is transferred. This ensures that the inside tires are retaining some amount of download so as not to overload the outside ones.

    As a general rule, you should use a sway-bar to eliminate excess body roll that the springs do not eliminate, or to trim car balance, which we'll talk about in the next section.

    Avoid over-reliance on sway-bars. Too stiff a bar will hurt you in the long run because you are in effect linking two sides of the car together, thus, making your suspension LESS independent.

    In the next post, we'll talk about triming car balance with these suspension parts.
     
  3. ///M-Spec

    ///M-Spec Staff Emeritus

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    CAR BALANCE
    Now that we have an overview on the major suspension components and what they do, we can talk about car balance. Most of us are already familiar with the terms understeer and oversteer, but let's start thinking about them in a different way now. With an understanding of how tires produce tractive force and why, let's define understeer as when we are putting too much download over the front tires and oversteer as too much load over the rear tires.

    When a tire gives up, the car will begin sliding with the end that loses traction first leading the way. If the front end of the car loses traction first, your ability to steer the car may decrease --you turn the wheel, but the car won't turn. This is called understeer. If the one of the rear tires give up first, your car may begin to yaw into the direction of the turn. The tail end of the car sticks out and you may find yourself spinning. This exciting looking situation is called oversteer.

    Neither of these situations is the fastest way around a corner. Big tail out oversteer may look exciting and on the edge, but you have lost a significant portion of your grip when this happens, and you will always end up cornering slower than if you were just at the edge, rather than over it. Remember from our tire discussion that a tire produces maximum grip when it is just barely slipping.

    So how do we fine tune car balance? The first important concept before you tune is to understand the goal: to get all four tires working to turn the car. This probably sounds like a simple concept with a simple solution, but it is complicated by the second important concept: everything you do to a suspension is a compromise between two conflicting requirements.

    As a general rule, making one end of the car softer will decrease roll resistance on that end, promoting better grip on that end.

    Conversely, making one end of the car stiffer will increase roll resistance on that end, decreasing grip on that end.

    Thus, on a car that is understeering a lot, reduce the spring rate and shock rate in the front or increase same in the rear. If you want to use your sway bars to help trim, stiffen the rear bar while softening the front bar.

    The important thing to remember is there are always trade offs when using spring rates to trim car balance. Going to a lower spring rate (softer) on one end of the car to promote better grip may work to a point. But after that point, you will need to deal with increased body roll possibly changing suspension geometry or the possibility of bottoming out on bumps. These effects can hurt you in the long run.

    Finally, understand that just because the car feels good doesn’t necessarily mean you are going any faster. Sometimes you will simply have to accept certain handling traits of a car and change your driving style.

    For example, say you have a car that understeers heavily. You have already made the front softer by reducing spring/shock rates. You decide the car is still not turning in quick enough, so you make the rear much stiffer by maxing out spring/shocks. You also go full soft on the front sway-bar and full stiff at the rear. Now the car is very loose and you find you can easily rotate it simply by adding throttle and powersliding all the way around the turn.

    In this case the car may be more fun to drive, but it is unlikely you’ll gain any speed through the corners. Powerslides feel good because you feel more in control of the car, but in this case, all you’ve done was compromise grip on the rear in order to deal with the front end washing out. You have, in effect, lost total grip. It is important to know when you can trim the car balance to gain time, and when the car simply has no more to give. Sometimes, you will have to come to terms with the limits of tuning and simply adjust your driving.

    ADDITIONAL NOTES RELATING TO CAR BALANCE

    First, keep in mind that a tire has a maximum amount of grip available, regardless of how much download is on it. Think of it as a "grip bank". As you increase download (trying to corner faster), you make deposits into the bank. The more download you deposit, the more grip is generated. (think of it as a conversion rate)

    However, this conversion rate is a downward sliding scale. The more you put in, the less gets converted. Your balance is still rising, but not in proportion to how much download you are depositing.

    This conversion rate hits critical mass at some point and suddenly drops to below negative. You are trying to deposit too much download into the bank at once. If you continue to add download, the grip bank will turn them into withdrawls. Suddenly, the more you put in, the less your balance is. It will do this until you reduce your deposits.

    Hope that makes sense...

    As for the softer end of the car producing more grip: sounds strange, but its true! As a general rule, more roll resistance = great amount of weight transfer. Here's how you can think of it...

    Say we have a hypothetical race car. This car weighs exactly 2000 lbs. It has a weight distribution of 40% F, 60% R (it is MR). Here's a top view of our car. At rest, each front tire carries 400 lbs of load while each rear tire carries 600 lbs.

    /-400lbs-----600lbs]
    <-front-------rear--+
    \-400lbs-----600lbs]

    Let's say that in this car, if we approach a long, sweeping left hand turn at 50 mph, exactly 400 lbs is transfered to the right side of the car.

    /-600lbs-----800lbs]
    <-front-------rear--+
    \-200lbs-----400lbs]

    Now, even though the tires on the left side of the car are perfectly capable of handling 600 and 800 lbs of download, they are not being asked to do much.

    Now, let's pretend we stiffen the front springs. What does this do to our car when we take that turn?

    /*700lbs*----800lbs]
    <-front-------rear--+
    \-100lbs-----400lbs]

    The stiffer front springs have caused MORE weight to move to the outside front tire. Suddendly, we have a problem: the front tires can only handle about 600 lbs of download before they start to slip. 700 lbs. is far too much. RESULT: our race car understeers.

    Now, let's pretend we return the front springs to normal, but stiffen the rear springs.

    /-600lbs---*1000lbs*]
    <-front-------rear--+
    \-200lbs-----200lbs]

    The stiffer rear springs have now overloaded the rear outside tire. With 1000 lbs of weight over it, the RR lets go in mid-turn and the race car spins. RESULT: our race car oversteers.

    Now, I know exactly what you're thinking: Why don't we just soften all the springs so that only a minimal amount of weight is transfered when we turn?

    Ah, grasshopper, because springs that soft would cause our race car to have too much body roll. Too much body roll has the very unfortunate effects of 1) bottoming out our race car 2) not allowing us to set the ride height low 3) slowing down weight transfer too much --the car is floaty and hard to drive (like your grandmother's Lincoln Town Car) and 4) impacting suspension geometry in a very negative way.


    BRAKE BIAS
    This is an often overlooked but critical part of the tuning process. Maximizing your braking performance is just as critical as getting every last horsepower out of the powertrain. Unfortunately, properly adjusting brake bias is a daunting task in GT3 because there is virtually no way to tell which tire is locking up first under braking.

    The goal is simple: each tire must do its fair share of slowing the car down. Ask the front tires to do too much and they will lose grip and you will understeer at corner entry. Ask the rear to do too much and they will lock up, causing the rear to get very light. Do this in a corner, and you suddenly have a trail braking oversteer situation perhaps even leading into a spin.

    All else being equal, you will want to set your front brakes slightly higher compared to your rear brakes. This is because under braking, much of the weight transfers to the front tires, which carry the most burden of slowing the car down.

    When braking and turning in at the same time (aka trail braking), you will want the car to be stable, with perhaps a slight amount of oversteer to help rotate the car into the turn.

    With this in mind, begin tuning by setting both front and rear to maximum and testing the car’s behavior. I’ve found the easiest way to do this is brake into a high-speed turn like Turn One at Apricot Hill or Midfield. If the rear end starts to slide out under braking, try lowering brake bias on the rear until the behavior gradually lessens to an acceptable level.

    On certain tight courses, such as Complex String or Deep Forest, you may want the car to rotate under braking to help set up for the next tight turn. Raise and lower the rear bias to suit each course.

    In our next installment, we’ll look at the remaining suspension settings and talk about suspension geometry.
     
  4. ///M-Spec

    ///M-Spec Staff Emeritus

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    TOE, CAMBER AND TIGT3WDKOCC
    Camber and toe are wheel/tire geometry settings. That is, they affect the relationship between the wheel/tire combo and the rest of the car. These are tough settings to get right in real life because they are dynamic by nature. That is, they change naturally as the suspension moves up and down through its range of motion. So when you adjust a car's camber or toe, you do so in its static state, i.e., Sitting in your garage with your alignment gear around it. Out on the road (or on a track), it changes as soon as you take a corner or hit a bump.

    In real life, different cars' suspensions have very different characteristics. Old British sports cars like with swing axles are well known for making positive camber on the inside rear wheels under cornering. BMWs and Porsches, with their front MacPherson struts are known for their limited ability to make negative camber under load. Live axle cars like Mustangs and Camaros are often ridiculed for virtually uncontrollable rear geometry because a single bump upsets both rear tires instead of just one.

    In GT3, these suspension characteristics are abstracted and we have no control over them. I call this TIGT3WDKOCC. That is..

    Things In GT3 We Don't Know Or Can't Control. The developers have taken suspension geometry completely out of our hands in GT3. This means we really have very little information to go on when we try to decide what the best camber and toe settings are. We can only experiment.

    As a general rule, increase the amount of negative camber to increase grip in cornering. Adding too much can reduce grip. Too much negative camber can also increase braking distances (very bad) and make the car twitchy or jittery. In addition, you should avoid positive camber settings, which have almost no redeeming values to a race car.

    As a general rule, FRONT toe IN increases stability at speed. FRONT toe OUT can help the car turn-in quicker. I find most cars work well with just a tick of front toe-out.

    As a general rule, REAR toe IN can reduce oversteer. REAR toe OUT can increase oversteer. I find most cars work well with neutral rear toe(that is ZERO rear toe) or with a slight toe out.

    Remember that each car will react to static camber and toe settings differently. For example, a Ruf 3400S (Boxster) or BMW 328 can benefit a great deal from 3.5, maybe even 5 degrees of negative camber. This setting may be overkill for a Corvette C5R or Toyota GT-One, which should have plenty of capability to make negative camber under load. Tune carefully.


    A NOTE ON TIGT3WDKOCC
    Ever wonder why GT doesn't let you change simple and rudimentary things like .. oh... Tire Pressure?? Or tire width and height? GT3 models tire wear and heat to a basic level, but doesn't give you access to important information like pyrometer readings --something that would make suspension tuning significantly easier. Obviously, the developers decided to leave them out for game play reasons... which leaves car buffs like us wonder just how the physics modeling works and what it takes into account.


    In our next and final installment, we'll post a Handling Problems and Solutions Guide.
     
  5. Duke

    Duke Staff Emeritus

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    Here are some additional words on TOE as portrayed in the game. These are reposted from this thread, which has some more in-depth discussion of toe.
    Toe OUT is when the wheels of the car are pointing away from each other as they roll forward. This is NEGATIVE toe. The degrees referenced in GT3 indicate the total angle by which the wheels are not parallel. In other words, at zero toe, the centerlines of the wheels are exactly parallel with each other and the centerline of the car. At -0.5 degrees of toe, each wheel is pointed -0.25" (away) from the centerline of the car, for a total of 0.5 degrees.

    Toe IN is, therefore, positive. The wheels are pointed slightly toward each other as the car rolls forward.

    Now, to get a handle on what toe does, consider what happens as the car turns. As the car settles into a turn, obviously weight transfers to the outside wheels, as we all know. That means, since those tires are handling the brunt of the cornering duties, we want their alignment to best fit the desired path of the car. If the front wheels are toed OUT and the rear wheels are toed IN, the car looks like this:

    Code:
      ^
    \ - /
    |   |
    |   |
    / - \
      =
    
    As you can see, either way the car turns, left or right, the outside wheels are going to be biased slightly against that turn. Now if you reverse that, and put positive toe IN at the front and negative toe OUT at the back, you get the configuration you're looking for:

    Code:
      ^
    / - \
    |   |
    |   |
    \ - /
      =
    
    Too much toe, however, adds friction on the straights, because the tires are continually crabbing slightly. So you don't want to run more toe than necessary to get good handling in the turns. As I said, rear toe OUT is more effective than front toe IN due to locations of the forces involved. Consider this example: Push a grocery cart forward, with the steering casters at the front, and make a turn. Feels like a normal car, with the back end following the front around. Then, push the cart backwards and make a turn. The end you're pushing will oversteer like crazy, because the rear wheels are steering to the outside. It's not precisely the same effect as a car, but it illustrates the point. The effect of the cornering force acts through the center of the front wheels, so adjusting toe there has a smaller effect than it does at the rear, where the effect is amplified by the distance from the rear wheels to the front.

    The Ackerman toe that I mentioned above applies only to the front wheels. Suspension geometry is designed so that toe out increases as steering input increases. In other words, the farther you turn the steering wheel, the more the front tires toe out. This designed in to accomodate the fact that the inside wheel must take a tighter-radius curve than the outer wheel. Without it, the inside tire would have to crab in order to accommodate the outside wheel following its true path. So if you set too much toe OUT statically (meaning when the car is at rest), then you get too much dynamic toe out when the car corners (due to motion of the suspension).

    That being said, there can be a reason to run toe out in front. A small amount of toe IN at the front improves steady-state cornering because of the bias described above - the outside tires are already steering around a circle. However, some toe OUT in the front can improve turn-in response at the cost of some steady-state speed. If the front wheels are toed out when the car is at rest (static), this amplifies Ackerman toe at very small steering inputs. So in other words, as soon as you move the steering wheel a little bit off center, the front wheels are already adopting a more-aggressive turning geometry. This speeds up the response time of the front end of the car. However, once the transition state at turn-in ends and the car enters a more steady-state cornering (as in a sweeper) the front toe out is reducing the amount of front end grip available.

    Duke
     
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