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Post by BurnOut on Aug 31, 2009 22:03:51 GMT 2
Ok saw this section (great idea btw!) and thought I'd open fire. I've been racing sims since 2000 and still havent fully understood how suspension works. I know what travel, bump and rebound are, but HOW do you set them up to work properly at a certain track? For example lets take the FXR at Westhill. Where do you start? Trial and error or 'scientific' method? Looking forward to reading everyone's input, but specially the setup gurus's
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Post by ksheppard on Sept 4, 2009 6:43:40 GMT 2
Expanded: ======================================
The springs are the foundation of your setup and can be simple to deal with in one sense and difficult in another. Springs are primarily used to dictate ride height and body roll, though they can also be used to transfer grip to an underused wheel in certain situations. Softer springs will extend the time that your car takes to respond to your inputs because your suspension is in "slow motion" of sorts. However, this does not necessarily mean it is a bad thing. A very experienced driver may want the faster response from a stiffer suspension, yet a less experienced driver may need the slower response so they have more time to catch and respond to the car. Neither is really going to be any faster or slower over the course of a lap in equal hands, they are just driven differently. For the sake of stability, softer is better. As your abilities advance, trying different spring packages may be worthwhile. When the mass of the car moves it creates energy. This energy is absorbed by the springs on the car. The energy created from the chassis goes through the springs before affecting the tires. This means that if your springs are stiff most of the energy from the chassis will be absorbed by the springs. This gives us faster chassis movements, but will reduce the amount of travel of the tire. So if you're running on a bumpy track, the tire will just skip over bumps instead of keeping contact with the ground.
As a rule of thumb, softer springs equal greater mechanical grip and a more forgiving chassis. You want your spring package as soft as possible without the car bottoming out from the downforce or the bumps in the track. We have all seen the images of a Formula 1 car shooting sparks out from the under tray at top speeds. This is the bottom of the car hitting the track surface and is the primary definition of 'bottoming'. Common sense tells us that when the chassis of the car is making contact with, or dragging on, the track surface you are not reaching your highest possible speeds. Even though your car is not dragging at slow speeds, your car has the added aerodynamic forces pressing it into the track at higher speeds. These aerodynamic forces can easily exceed the stationary weight of the car. The springs need to be stiff enough to prevent the bottoming, yet soft enough to maximize it's mechanical grip when the aerodynamic forces are not there to help you.
There is also another form of bottoming, known as 'bottoming out the suspension'. This happens when the chassis itself may not be making contact with the track surface (thanks to bump-stops or packers), but the suspension is compressed to maximum and the spring is prevented from compressing any further. If the chassis is riding on the solid bump-stops, there is no suspension movement and the car can not react to the track surface or weight shifts.
You want to prevent both versions of 'bottoming' in a stable and predictable setup. While this may take some tweaking to find the lowest ride height and softest springs, you can retain that information for the next track you visit. However, the differences in tracks themselves may force you to adapt, or evolve it into a track-specific setup. A high-speed track may require slightly stiffer springs due to the aerodynamic forces being generated at higher speeds. All-out speed may be more important then grip, so you may wish to sacrifice mechanical grip in exchange for less drag. A low-speed track will not generate the same aerodynamic forces and you rely even further on mechanical grip. This may cause you to run an even softer spring package than normal, but since the top speeds are lower, so are the aerodynamic forces that may cause the bottoming. Cars that rely heavily on aerodynamic grip may sacrifice mechanical grip by using stiffer springs and even lower ride heights. The underbody of some cars generate massive down force and the closer it is to the track surface, the greater downforce it is generating without the added drag that an increase in wing angle may bring.
Springs do also dictate the amount of grip allocated to each corner of the car, in a generic sense of the term. A stiffer spring is "pre-loading" the grip to that particular corner of the car by pushing the wheel into the surface of the track. The harder the spring pushes, the more grip that is "pre-loaded". However, while you may have more grip initially, you approach and exceed the point of overloading that wheel at a faster rate as well. Changing the springs at each corner changes your corner weight. Though the corner of the car may not literally weigh any more or less, springs can alter the weight at which the wheel is being pushed down into the track surface. As you are navigating a corner at full potential of the car and tires, the outside tires will be using their full amount of grip. Any additional weight on that tire will result in a loss of grip as the weight exceeds the capability of the tire.
Corner-weight techniques can help spread the workload across two tires, offering more overall grip in corners of that same direction. An example of a track that you might have different springs on the left and right sides of your car is Lime Rock Park. Since Lime Rock has all but one right-hand corner, you can focus solely on the right-hand turns. Having stiffer springs on the right-side of the car will allot more grip to the inside tires and in turn will allow the outside tires a larger grip "budget" because the inside tires are sharing more of the workload. You will be sacrificing grip in that single left-hand corner of course. For the most part, you do not have to be overly concerned with corner-weighting your setup. Focus on a symmetrical and balanced spring package and come back to corner-weight techniques later down the road.
When you are first developing your baseline setup, use the softest springs available on the rear of your car for maximum grip under acceleration, then balance the front springs to match grip between front and rear for a neutral setup. Typically, your rear springs will be softer than your front springs for rear-wheel drive cars.
The ride height setting adjusts the measurement between the ground and your car. A lower setting will lower the centre of gravity of your car, which will help overall handling. However, running a car too low will make the car touch the ground or bottom out the suspension, especially over kerbs and bumps. This will result in loss of speed and control.
An antiroll bar is used to resist body roll while cornering. Body roll will exaggerate the camber changes from weight shift and wreak havoc on the aerodynamic forces that the underbody of the car may generate. The larger the ARB, the less body roll. However, this does not necessarily mean that less body roll equates to more grip.
The antiroll bar attempts to compensate for body roll by pulling the inside wheel up into the wheel well. By pulling that wheel up, the chassis of the car can stay more level with the track surface. However, that inside wheel being pulled up and off the track greatly reduces the grip that wheel is generating. As a general rule, a softer ARB will equal more grip to the front or rear of the car depending on which ARB you are adjusting. This is probably the easiest setup adjustment to make to regain the neutral car feel. While it may be very normal for me to run very soft ARBs both front and rear, it is always wise to test the full spectrum. I have found that some vehicles will respond better to the less body roll of very stiff ARBs.
After changing your ARB settings, recheck each of your tire temperatures and make pressure changes as needed. You may need to adjust camber and caster settings as well.
So, when you drive through a corner, the mass of the car moves to the side of the car. The anti-roll bar works so that it connects to left and right suspension to keep the suspension geometry aligned with the amount of mass moved so that the tires have maximum contact even when the car is tilted to the side. The anti-roll bar adds to the roll resistance without resorting to an overly stiff spring. A properly selected anti-roll bar will reduce body roll in corners for improved cornering traction, but will not increase the stiffness of the ride, or reduce the effectiveness of the tire to maintain good road surface contact.
How we use this tool. In simple terms this controls the amount of mass moved from side to side. If you have a stiff anti-roll bar setting it will increase stability but you will instead lose some of the traction available. The anti-roll bars are very good tools to adjust the overall balance of the car. If you want to change a specific part of the corner balance you use damper settings but if you need an overall balance change you adjust with the anti-roll bar.
TIP: Softer ARBs will help equalize left-to-right side tire temperatures.
TIP: Stiffer springs will increase tire temperatures and even tire wear. Dampers counteract the springs' natural compressing and decompressing by resisting those forces. Dampers do this via hydraulic fluids being pushed through small valves inside the damper while the damper is in motion. A higher damper value equates to greater resistance.
A wheel's maximum grip level will be achieved when the spring is fully compressed. This is when that wheel has maximum weight being transferred through the suspension, tire, and to the track. In a generic sense that weight equals grip. How quickly or slowly that spring is allowed to fully compress is the job of the damper. Springs may dictate HOW MUCH weight is transferred, dampers dictate HOW and WHEN that weight is transferred.
The easiest way to begin to understand dampers is in a straight line, under braking or acceleration:
Under braking, much of the car's weight will shift from the rear of the car to the front. The front springs will compress while the rear springs will decompress (or rebound). The dampers do the same and will compress (front) and decompress (rear). The faster the front springs are allowed to achieve their most-compressed state, the faster the front tires will have maximum grip for that all important braking. A softer compression setting will give the least amount of resistance to the spring compressing, allowing weight to transfer very quickly once the brakes are applied. The rear damper compression setting will have no effect on what happens here, but the rebound will. A greater rebound setting will resist against the rear springs decompressing. If the spring is not allowed to rebound quickly, the rear tires will be somewhat lifted off the ground (exaggerated of course). Softer rebound settings in the rear will allow the rear tires to stay connected with the road and offer more rear-grip during that weight transfer to the front.
Under straight-line acceleration the complete opposite is happening, with the rear dampers compressing and the front dampers decompressing. Surely you will want maximum grip on the rear tires under acceleration, but the front tires may need grip adjustments to prevent understeer oversteer. You can adjust this condition by adjusting how the rear suspension compresses or how the front suspension rebounds.
The same philosophy can be applied laterally (side to side) as well. Long sweeping corners that do not involve large braking or accelerating will shift weight to the left and right of the car. How fast you allow that weight to transfer is up to you and can be adjusted via the left and right dampers, but keep in mind how that will also effect your front to rear damping.
Corner Entry: Front=compression damping, Rear=rebound damping
Corner Exit: Front=rebound damping, Rear=compresion damping
The softer you make a shock absorber the faster the weight is transferred. The harder you make it the slower the weight is transferred. So if I have a car that is understeering going into the turn I need more weight on the front end. You can change this by either making the front end softer so it squats more or loosen the rear rebound damping, so the rear end lifts up more and thereby transferring more weight to the front end. If I have an oversteering car out of the corner I can make the rear compression damp harder so that less weight is transferred to the rear, and I can change the Rebound dampening in the front so the front end doesn’t come up so much and there by less weight is transferred.
If your car is equipped with fast-damping adjustments, everything above still applies but only when the suspension is in "fast motion". An example of your suspension moving in fast-motion is when you are hopping over curbing, something I like to call "curb smacking". This is when you are shocking the suspension into movement in a very short time frame. Hitting a curb at speed (like you might at Monza, or the final chicane at Magny-Cours) is forcing your suspension to compress or rebound in a much shorter time frame than normal weight transitions. This is where fast-damping comes into the mix.
'Diagonal' damping is where things start getting a bit more complicated, and fun. Imagine what is happening to the right-front suspension of your simulated racecar as we go barreling into a left turn. As you lift off the throttle and apply brakes the weight of the car is being shifted from the rear of the car to the front. Added to this is a left-to-right shift as you begin to turn in to the corner. The right-front spring will compress under the weight transfer, and the shock will do the same. A low compression setting will allow that spring to compress quickly, giving that tire its full available share of grip very quickly. A higher compression setting will force the spring to compress more slowly, meaning the weight of the car will not transfer as quickly and maximum grip will still be available, just slightly later in the phase of the turn. The spring will fully compress no matter what the shock’s compression setting is, but you can dictate WHEN and dictate the handling of the car during this phase of the corner.
The opposite is happening to the left-rear of the car as it is losing weight both from braking and turning as that spring and shock is in a rebounding phase. How quickly that suspension is allowed to rebound dictates the grip for that wheel during this transition. If the shock is set to a low rebound setting then the spring will be allowed to easily decompress and maintain maximum grip. If set to a higher rebound, the spring will not be allowed to rebound as quickly and the grip will be reduced as the wheel is being pulled off the racing surface. You can imagine how this might affect the car during this corner-entry phase and how you can adjust it to suit your needs.
As the car approaches the middle of the corner, the springs are already at full compression and/or rebound, and the dampers would be as well. Dampers do not play a roll in the handling of the car once this has occurred, as dampers need to be in-travel (either compressing or rebounding). If the dampers are not in motion, they can not offer resistance.
As we pass the point in which the car is at a settled state in the middle of the turn, you begin to apply throttle for maximum exit speed. Applying the throttle will shift weight to the rear of the car, and as we ease out of the steering the weight will begin shifting back towards the left. As you apply throttle the right-rear shock begins to go into more compression, though would already be near its maximum compression. Since we can’t make this spring and shock compress any further, actions can be taken from the diagonal corner (left-front). Altering how that spring deals with the weight shifting will increase or decrease overall front grip.
There are hundreds of variables in dampers, even though there are only four dampers on the car. Changing one will alter the other three as well. One adjustment to the right-front rebound will alter how the weight is shifted to the left-front wheel (Lateral weight transfer in quick succession turns like chicanes or long sweepers), the left-rear (diagonal weight transfer in corners while braking or accelerating), and the right-rear (Longitude weight transfer under braking and acceleration). You have to be aware how one change will affect the other aspects. As usual, any setup change is a compromise between a gain in one area and a loss in another.
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