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Automobilista - How to Set Your Car Up!

Discussion in 'Automobilista Setups' started by Chris, Mar 10, 2017.

  1. belevezero

    belevezero

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    This is fantastic! Thank you.
    Have to say however that I am now more confused about diff preload than I was before. iRacing describes this setting much differently in their tool-tip, and I'm having trouble squaring this description with iRacing.
     
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  2. Chris

    Chris
    Administrator Staff Premium

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    TBH I'd quite like some clarification on the Diff Power and Preload settings as they apply to AMS, because I've read conflicting things on it, so I am also somewhat confused.
     
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  3. SimJim70

    SimJim70

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    Beautiful work. Any chance of a PDF? Please, please and pretty please??? Looking forward to the info updates too. :)
     
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  4. Leynad777

    Leynad777

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    The Preload is the minimum torque difference after which the differential get's unlocked. It's just a controlled friction for the intertia of the differential. You explanation is okay.
     
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  5. mancSLO

    mancSLO
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    Is there eny list of optional tyre temperature for Automobilista cars?
     
  6. -Alf-

    -Alf-

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    Pin this guide, Chris. It's very helpful, and not only for AMS.
    Amazing job, well done!! :thumbsup:
     
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  7. Ross Garland

    Ross Garland
    R3E & AMS Club Manager Staff Premium

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    A brilliant setup guide in plain English, great work Chris! It's already given me a few things I want to test out on-track. :thumbsup:
     
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  8. m_mirk

    m_mirk
    Premium

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    This is perfect Chris. Thank you so much!!
     
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  9. johnnymat

    johnnymat

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    This is the best guide I've seen yet. Thanks for all of your hard work.:)
     
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  10. Heitor Facuri Cicoti

    Heitor Facuri Cicoti

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    Back to business:

    A common misconception about suspension (which took me some years to realise was wrong) is that harder springs prevent weight transfer to occur. They don't (at least we should think they don't). Weight transfer depends only on CG height, lateral (or longitudinal) acceleration, and how wide (or long) the car is. The CG position in a race car during maneuvers does not move significantly for roll (or pitch) to be considered important to weight transfer. They start to matter on off road stuff or Super Trucks with lots of suspension movement, but not on low rigid race cars.

    Harder springs are actually a result of the primary intention to lower the car. With short travels you need them to keep the car where you want for aero purposes and not touching the ground. Their most noticeable impact is on how fast you will have to react to keep up with the car. Harder springs means higher frequencies, which means faster reactions. Sometimes it would indeed be better to have softer springs, so our (lack of) abilities are on par with the car's difficulty, but it's pretty much never possible.

    Another very important contribution of springs is to the roll stiffness. And that's where I wanted to get since the start. I think the following is the MOST IMPORTANT MINDSET you can have in order to improve your setups, or even to improve your driving. To me everything in racing comes down to: Which will be the first to give up, the front or the rear? That's always the battle I try to fight while driving. Will I suffer from under or oversteer? In the end one will show up. Will I lose time with the car not turning, or will I lose time with the car spinning? And there are two things we need to know if we want to solve this mystery: (Oh boy, this is going far)

    1 - A more loaded (with normal force) tyre won't gain as much grip as the less loaded one lost. (Video from Niels explaining this)

    That's also know as "the coefficient of friction decreases when normal force increases". All that means is that a pair of tyres will have their highest combined grip when loaded evenly. Whenever there is load transfer from one to another (weight transfer) their overall grip will be lower, because one lost more than the other gained. So weight transfer = always bad. (And remember harder springs don't prevent it!)

    2 - A car is a hyper-static structure. :D

    Do you know that chair with one leg slightly shorter than the others, that keeps rocking back and forth, side to side? That's a hyper-static structure, because it has more legs than what's needed for it to stay up - which would be 3 legs. The car is the same thing. Tricycles stay happily upright. Bicycles don't. Cars didn't need the fourth wheel, yet it's there and we have to deal with it. All of this to say that, during a maneuver, one wheel will tend to be forsaken by the physics Gods, just like that chair leg in mid air. That doesn't mean one wheel will always be in the air. Chair legs are rigid, car wheels are softer, so they settle better to the ground. But one wheel will have its load more transferred than the others (remember load transfer = bad). The thing is: Will that wheel be in the front or rear axle?

    This brings us back to roll stiffness, finally. Think about it as the difficulty to roll the body of a car by pushing it from the side (which is basically what lateral acceleration does to the car). The overall roll stiffness doesn't matter much. What REALLY MATTERS is how the front roll stiffness is compared to the rear roll stiffness. This will help us see where that forsaken wheel is.

    An imagination exercise in order to visualize it: Try to picture a car with the front shockers welded - no movement for the wheels at all - and regular soft rear suspension. Now start to push the side of the body with increasing force. As soon as the car starts to roll, one front wheel will lift off from the ground, because it's welded to the body. And what about the rear? It will still have both wheels to the ground, because the soft suspension let them sag and follow the ground shape. This means that we now have 100% load transfer at the front (hugely bad!), so the front axle overall grip is dramatically reduced, while at the rear things almost haven't changed regarding overall grip. This car is probably going to suffer from terrible understeer. The front may give up before the rear. I said "probably" and "may" because some other things (worn tyres, suspension geometry, driving style, etc) still can make the rear be as (or more) shitty as the front.

    So that's the mindset I use. The axle with higher roll stiffness will be injured in this battle of axles for grip. Always one relative to the other. Both high or both low can have the same effect. Remembering that during a corner is very helpful in order to understand what's going on with the car. I have to say that roll stiffness is influenced by springs, anti roll bars, and suspension roll center (the latter can't be changed by setup parameters, so don't worry). But try to focus springs for bumps and maintaining ride height, and leave the anti roll bars to do the roll stiffness job.

    The last thing is about caster and kerbs. Another caster function always overlooked is that it lowers the inside and raises the outside wheel when you steer. That is like growing one leg of a chair, forcing the chair to tilt. In a car (specially karts), that results in the rear inside losing load, because the car rolls, the front is now levelled with the road and the rear has to comply with that distortion. The exact same thing happens when you hit kerbs with the front wheel. It is like an instantaneous load transfer from the rear inside to the rear outside, decreasing rear axle overall grip, which induces oversteer and makes the car unstable.

    I think this is all I wanted to say. Sorry for the long post. Now I'm in peace with myself.
     
    Last edited: Mar 19, 2017
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  11. Heitor Facuri Cicoti

    Heitor Facuri Cicoti

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    Highlighted text should be the opposite. From my comment above: more load transfer = bad! (read with Trump voice). And the Higher values would actually limit the suspension ability to comply with the road, increasing load transfer and the chance to lock the wheel up.

    Edit: "Greater mechanical grip" on lower values is OK, but not because of increased load from body roll.
     
  12. belevezero

    belevezero

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    This is what I can't wrap my head around in regards to certain RWD cars. For example, the 911 Cup car in iRacing seems to handle better with stiff rear and soft front ARB, but I can't make logical sense of that. If the power is coming from the rear axle, wouldn't the handling benefit from that axle having the priority of the softer ARB? My only guess is that it's because the engine is at the back, so the weight is keeping the rear wheels planted regardless, but that's an uneducated guess.
     
  13. Heitor Facuri Cicoti

    Heitor Facuri Cicoti

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    That's exactly what you said. The roll stiffness does not tell the whole story about the battle. There are other weapons each axle uses to be the grippier - CG position being a big one. Also, absolute numbers for ARB doesn't say which end is stiffer. You can have 100 at the front and 40 at the rear, and the rear still being stiffer. That's because suspension geometry, front and rear track, and spring rates play a significant part on roll stiffness as well.
     
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  14. Nismogtr

    Nismogtr

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    Very nice work ! Thanks.
     
  15. arniesan

    arniesan
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    Anybody still believe in trial and error. :)
     
  16. Chris

    Chris
    Administrator Staff Premium

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    Are you sure about that mate?
    From what I can tell, if the ARB is keeping the car more level, then the suspension on both sides will have more room for compression to absorb the bumps, and the loading of the two fronts is more even.

    I don't understand how it can be reversed as with a softer ARB, the inside front wheel will surely be unloaded when turning as there is greater lateral weight shift to the outside tyre, thus the inside tyre is far more likely to lock up. Is it perhaps the way I've worded it that's confusing?
     
    Last edited: Mar 18, 2017
  17. Heitor Facuri Cicoti

    Heitor Facuri Cicoti

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    The total car weight transfer isn't changed by the ARB. It would change a bit if the CG moved significantly, like in a Super Truck, but on regular race cars, it is negligible. The big thing of ARBs is how they relate to the other end of the car. If one end is stiffer, it will have more load transfer, and the other will have less, but the car's total transfer won't change. Swap both stiffnesses, and the total will be the same.

    Setups with similar roll stiffness balance between front and rear (both stiff or both soft) will have the same tendencies regarding to under/oversteer but, in an overall stiffer car, bumps and changes of direction will translate more quickly to the body, meaning faster load transfers (but still, the total is the same). This, coupled with the last part of my comment about caster, shows that a bump on the front axle will probably induce a lock up at the rear, and vice versa.
     
    Last edited: Mar 18, 2017
  18. schlitty

    schlitty

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    Thanks to everybody contributing to this thread. Great stuff. Always appreciate people who share knowledge.

    Just about everybody. Having an idea of what a setting does doesn't remove the trail and error part at all!
     
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  19. TylerDurden4321

    TylerDurden4321

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    Great article and thread.

    Now I what I always wanted is some science about tire pressures, better cooling of a tire with higher pressure but also lowered contact patch. Peak temperatures will be similar?!?? This topic is really hard to understand for me.
    I wouldn't mind some kinetic gas theory and thermodynamics on the topic.

    About the ARB thing from Heitor and Chris:
    When you say "softer ARB will lead to more mechanical grip", aren't you forgetting the load sensitivity of the outside tire? That tire will have more grip, yes, but because of the load sensitivity a 100% higher load will only result in 90% more grip or so, meaning that the overall grip of both wheels will be reduced if you get most of the load on the outside tire.
    What also hasn't been mentioned is how a softer rear ARB under acceleration out of the corner will transfer more weight off the front inside wheel to the rear outside wheel, which will result in some amazing on-power oversteer(or better: amazing "yaw rate") at the prize of overall grip.(edit: also this doesn't work in cars with very low CoG or where the roll center is very modern-close to the CoG)
     
    Last edited: Mar 18, 2017
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  20. David Wright

    David Wright

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    A softer ARB will reduce the load on the outer tyre during cornering. Maybe you are confusing roll with load transfer. It is counter-intuitive. With a softer ARB you get more roll but less load transfer from inner to outer wheel. With a stiffer ARB you get less roll but more load transfer from inner to outer wheel.
     
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