GT4 and Brakes

Discussion in 'GT4 Tuning' started by Scaff, Mar 18, 2005.

  1. Tornado

    Tornado

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    It was in an old Car and Driver I think. I'll look around for it.
    I know. I just didn't want you to think I was mocking you or anything like that, especially after the recent revelation.
     
  2. Scaff

    Scaff Staff Emeritus

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    I'd be interested to see it. :tup:


    No problem at all.

    Regards

    Scaff
     
  3. Mt. Lynx

    Mt. Lynx Premium

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    Wait a minute here... I must be misunderstanding something very basic here. Scaff, are you (and as it seems the motor industry) saying that it doesn´t actually matter how good your brakes are, it´s the tyre that matters???
    An example: Take two F1 cars (of the same make), set up in the exact same way, but provided with different brakesystems (let´s say one has state of the art F1 brakes, and the other has the brakes from a ´70 Dodge Charger.), these two cars will have the same stoppingdistance, if they have the same kind of tyres? I must say I have a very hard time beliveing that!
    Or are you (and the motor industry) saying that it´s pointless to have brakes that are better than the grip the tyre can provide (like having F1 brakes on a ´70 Charger with original tyres)? That is much more understandable in my ears!
     
  4. gregc

    gregc

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    That's exactly how I've always understood what Scaff says on brakes. The problem (or perhaps misunderstanding) when talking about brakes in GT4 is that on any given car, the stock brakes are almost always more than strong enough to overwhelm the grip of the tyres (after adjusting the balance controller if necessary) - so there's no point in upgrading to racing brakes.

    At least, that's my take on it...
     
  5. Scaff

    Scaff Staff Emeritus

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    The latter is the correct one (that I have highlighted in bold), as your stopping potential is always limited by the tyres.

    However I've never said it pointless to have better brakes, just that better brakes will not reduce stopping distances (unless the original brake system is very poor).

    The advantages of better brakes are many, they reduce brake fade, increase modulation and improve feel. However with this also comes problems, such as an increased need for cooling (and the drag caused by cooling ducts) and an increase in unsprung weight.

    The first example you used of the two F1 cars would cause confusion because the Charger brakes would most likely not be able to produce sufficient force to reach the tyres limits. However if you put say the brakes from a modern performance car (say a 911) on an F1 car they may be able to stop the car in the same distance as the F1 brakes if they could use all the tyres available grip. What they certainly would not be able to do is repeat the process more than once (brake fade would certainly set in here).

    Its this reason why the industry (both motor and motorsport) place such importance on the tyres, they are the ultimate limiting force for stopping distances. Once you reach a certain point larger, more powerful brakes will not stop you any quicker, but they will allow you to do it more often and with more control.

    Which is why Stop-Tech (who make motorsport brake systems) say

    Regards

    Scaff
     
  6. Mt. Lynx

    Mt. Lynx Premium

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    Thank you very much indeed! I´m now a wiser man going to bed!!
     
  7. FIDO69

    FIDO69

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    Hello All Brake-Nuts and Scaff!

    After a long hiatus, just wanted to drop by and post a Hello, and great job with the summary!

    I'm going to be linking all my new BMW Z4 forum friends to this discussion... I got in a bit of a discussion about improving brake performance on our cars...

    Keep up the good work, eh!
     
  8. Vince247

    Vince247

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    thanks Scaff for these facts!.:bowdown:
    You've learnt me a whole lot, and I haven't read the other guides yet!:scared:
    Sadly they don't have this stuff in physics class...(yet??)
     
  9. Scaff

    Scaff Staff Emeritus

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    Glad to hear this thread is still of interest and use :tup: and thanks for the feedback, I hope that you find the guides just as enjoyable/interesting.

    Thanks

    Scaff
     
  10. Exxon_Valdez

    Exxon_Valdez

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    I may be a bit of topic. And I'm sorry for my bad English but there is something that bothers me a lot about GT4- brakes setups and i dont know
    where else i can get this question answered. Why have even performance cars like the Toyota Minolta a braking force distribution of 3 to 3 thats somehow strange because this is the starting value when you built in the brake controler and its obviously not the setup is really used on cars like that. That makes adjusting the brakes difficult cause the braking performance on the minolta is quite good actually a lot better than it can be with this stupid default setup. Why had PD done such a thing its like with fully costum LSD it has always this standart setup of 10 40 20 and works even on the fastest cars. That is confusing to me. At least they called it the real driving simulator. In my opinion they missed something with brakes and default setups at least they should have been a bit more like the setups of the real cars. Thats pretty loosy when you think about the time and money wich was invested in the game.
     
  11. Greycap

    Greycap Premium

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    The default values are only meant as a starting point for you to begin your own work from. Setups of real race cars are impossible to replicate as every driver uses different setups on different tracks, even in different weather conditions. In short races the setup can be very different to that of long races, let alone very different course profiles. On a slow twisty course a rear-biased setup can be worth gold whereas on a fast course with hard brakings it could be fatal.

    To be honest, very few default settings work perfectly. Pretty much everything needs tweaking if one wants the car to drive really well, the stock LSD is good for FR cars of around 500bhp but drive anything else and you will find that fine tuning is needed.

    They didn't save any money or time when doing the setups, the fact is just that no car will fit everybody. There would be many people complaining about bad default values even if the values were those of the real cars. One of the main points of this game is making the car drive like you want it to, and you have the chance of doing it.

    - R -
     
  12. Scaff

    Scaff Staff Emeritus

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    Its simply because no such thing as the 'right' set-up exists, take a look at every default value for any upgrade in GT4 (and the entire series for that matter) and everything has a standard set of starting values (if I recall correctly only spring rates vary from car to car, but even these increase as the suspension gets more race orientated).

    What they do is put in place a set of default values simply because something has to be put in place, however you will need to work with these values, you driving style, any modifications you make to the car and the track you are racing at to get the 'right' value for you. Any 'default' value will be wrong from this point of view.

    I suggest that you don't worry about the default values and simply get the car set-up for the track you need.

    Regards

    Scaff
     
  13. Orion

    Orion (Banned)

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    The Stabilizers vary from car to car also, as well as ride height. :tup:
     
  14. shangoob

    shangoob

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    yes it is same thing happening on aids setting , it's have two defaults setting ones 10/10/7 and the other 10/10/5

    how can I fix aids setting (oversteer and understeer)?
    where is the best track to try my setting ?

    I really need to learn that ,who knows maybe it will make your race easier.
     
  15. Greycap

    Greycap Premium

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    By turning them off. Seriously, all aids you will ever need is the occasional "1" of TCS and even that isn't needed in cars with less than 800 bhp, not even in them if you have good throttle control.

    The aids use brakes and throttle to control the car. Even my logic says that if the brakes are engaged, the speed will be going down and that's not what we're trying to do. They will make the driving easier to a point, but also greatly reduce from the experience and in many cases actually make the car difficult to drive. We've heard of cars accelerating by themselves thanks to the ASM, that doesn't sound normal to me anymore. The biggest problem of the aids is that they allow a certain amount of slip and then cut in just as the driver is beginning to correct the line. As a result you get double correction and an about 30% power loss as the system begins to brake the slipping wheels. By driving yourself you would only have made that one correction and continued at full throttle.

    Got the idea? ;)

    - R -
     
  16. happyhondaman

    happyhondaman

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    Greycap is right just the other day I was driving the 1989 Toyota 88C-V in the Sears point (Infineon) Raceway 2h:45min endurance race and started off with
    965 HP on Hard racing slicks wihtout any driver aids and that was okay I could handle it but it wore the tires out too fast. I liked the controllable oversteer and steering the car with the throttle but there are so many slower corners on that circuit that I had to constantly feather the throttle out of the slower 2nd and 3rd gear corners to avoid lighting up the rear tires every time. So instead I did the race with TCS on the 1 setting adn my tires lasted 4 laps longer and my lap times went down by 2 to 3 seconds per lap! So when you have a car with that much power you may need a little TCS on the slower circuits where you need a very fast race car, or you may not, just experiment hey if you get frustrated just hit the reset button on the PS!
     
  17. Alleycat

    Alleycat

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    I am not sure if anyone reads these threads anymore, but I would like to add my 2 cents...

    Since I am a new poster to this forum, you might not see me as an "expert", especially since you work in the industry (so do I btw).

    I think you missed the point of MR-GT.

    The "friction" a tire creates is based upon a coefficient and the vertical "load" on that tire. Take a 2000 lb car. at 50/50 distribution, it puts 500 lbs on each tire. Now if each tire is a "low-grade" tire only capable of 1.0 G of force at this weight, this means each tire produces 500 lbs of stopping force, which summed up, produces a total of 2000 lbs, to slow the car at -1.0 G. This is the "coefficient" mentioned above.

    Now in your world, quoting newton, this coefficient is static. The problem, as Mr-GT tried to explain, is that this coefficient is NOT static, and actually varies upon load.

    Now let's say this -1.0g decel transfers 100 lbs onto each front tire. One now has the rears only loaded for 400 lbs, while the fronts each have 600 lbs. This still totals to 2000 lbs of the car, but the traction coefficient for the rears has (probably) improved slightly, while that for the front has decreased. Let's say the rears went to 1.1G and the fronts dropped to 0.9g. This puts a total of (400*1.1*2 = 880 rear + 600*0.9*2 = 1080) 1960 lbs total for a REDUCTION to -0.98G of stopping power.

    Now with a heavier car, lets say a 2400 lb car, the tires START at 600 lbs each, so the BEST it could do is -0.9G as each tire becomes "overloaded".

    These numbers are not very accurate and are just used to illustrate the fact that as tires deform under weight, their friction reduces, so one does not get a 1:1 ratio between added force and added friction. Physically, as weight is added, the tires generate a LESS than proportional amount of added grip.

    So as in pure turning grip, braking power reduces as the total weight of the car increases. The reasons for the Land rover are prolly as the other guy stated.. better/bigger tires, maybe better ABS (for more consistent stops if the magazine took samples).. or some other factor.

    I will agree that Tires/weight transfer play a big role, but TOTAL weight does too.. just not as much as one would think.
     
  18. Scaff

    Scaff Staff Emeritus

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    Thanks for the reply, it is more than welcome. It would however appear that you have read part of the thread or has scan read it (both of which given its size are understandable), which has lead you to a few in-accuracies regarding my views on this topic.


    I'm fully aware of this and its discussed here at length and while I understand that you use the number for simplicity a tyre capable of 1g of grip would most certainly not be a low grade tyre at all. On road tarmac most standard mass market tyres would struggle to generate much above 0.8g.


    If you take the time to read this thread fully you will see that point is not dismissed at all, however you yourself have hit upon the small but rather fundamental point in that it is not curb weight that is important here but load.


    It all depends how you use the figures, now assume all the above is true, but the static weight distribution of the car is 40% front and 60% rear (classic mid-engined), and 100 lbs is moved forward under deceleration again (on each tyre), we now have our full 2000 lbs of braking force back.

    Static = Front 40% (400lbs each tyre) and Rear 60% (600lbs each tyre)
    Under Braking (100lbs transfer per tyre) = Front 500lbs * 1 * 2 = 1000lbs and Rear 500lbs * 1 * 2 = 1000lbs. Total = 2000lbs


    Switch that around to a FWD layout and we get 60% front and 40% rear, resulting in 700lbs on each front tyre and 300 lbs on each rear tyre under braking and a reduction in braking force avaliable to use (and the numbers here would be less than the initial 50/50 static distribution).

    Static = Front 60% (600lbs each tyre) and Rear 40% (400lbs each tyre)
    Under Braking (100lbs transfer per tyre) = Front 700lbs * 0.8 * 2 = 1120lbs and Rear 300lbs * 1.2 * 2 = 720lbs. Total = 1840lbs



    Now without a single change in the curb weight we have three very different sets of figures.


    However we can also look at differing curb weights and get some interesting results.

    Car A
    Curb weight - 2400lbs
    Static dist. - Front 40% (480lbs per wheel) Rear 60% (720lbs per wheel)

    100lbs per tyre forward under braking gives

    Under braking - Front 580lbs per wheel and Rear 620 lbs per wheel

    Using 580lbs = 0.87g and 620lbs = 0.93g (consistent with the scale you are using)

    Front = 580 * 0.87 * 2 = 1009lbs
    Rear = 620 * 0.93 * 2 = 1153lbs

    Total = 2162lbs



    Car B
    Curb Weight - 2000 lbs

    Static dist. - Front 60% (600lbs per wheel) Rear 40% (400lbs per wheel)

    100lbs per tyre forward under braking gives

    Under braking - Front 700lbs per wheel and Rear 300lbs per wheel

    Using roughly 700lbs = 0.8g and 300lbs = 1.2g (consistent with the scale you are using)

    Front = 700 * 0.8 * 2 = 1120lbs
    Rear = 300 * 1.2 * 2 = 720lbs

    Total = 1840lbs


    So here we have a lighter car exerting less braking force (considerably less) than the heavier one, because load distribution allows the heavier car to make better use of it.

    Now while the initial coefficient of the heavier car is not 1.0g, it must be kept in mind that this figure only plays a part for a very sort amount of time and that full braking is not achieved instantly (you don't go from 0g to 1g instantly) but as soon as you start braking the load shifts. As a result the 'under braking' values are far more relivent than the initial ones (and if you go from accelerating to braking then the initial values are completely irrelivant).


    The only time you can say that a lighter car will always make better use of its tyres is if every single other value is identical, and lets be quite honest that is never going to happen. In an earlier post I did mention and example of this, take a car and prep it for racing. So strip out the seats, interior and anything not needed and it will be lighter, add in a roll-cage (which will almost certainly be needed) and while this does push the weight up a bit the car is now much lighter overall. However far more than the weight has cahnged, we have totally changed the cog of this car and almost certainly changed its static weight distribution, all factors that will effect how much load is transfered under braking, cornering and acceleration.


    Again I don't disagree, but its the load that causes this to occur not the weight itself (as the example of three differeing static distributions for a single curb weight illustrate).

    Now while I have not taken this into account in the figures I have used above (that is a simple linear scale), if I had it would actually be more of a problem for the lighter FWD car than it would be for the heavier Mid-engined car, once again illustrating that how load is distributed is far more important than the weight itself.

    Simply saying lighter = more braking force is overly simplistic and assumes that all other factors are exactly the same, as soon as any other variable changes the load transfer and distribution become far more important than the weight.


    Again I have said exactly this repeatedly throughout this entire thread, you have simple picked on one example in which I was 'trying' to get a single point across, If you have the time to read the whole thread I think you will find that our opinions differ very little.


    Regards


    Scaff
     
    Last edited: Nov 12, 2008
  19. Alleycat

    Alleycat

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    I am glad to see that this is still alive and well :)

    You are right in that I only read about 1/3 of the post 30 pages!! :) ) I will make an effort to try to read the entire post as it seems the concept I am talking about is rather well known around here.

    I still think a piece here is missing.

    While the 2400 lb car can exert a higher force, it takes more to stop it. Using your figures we see:
    Car A
    2162 lbs braking force/ 2400 lbs static weight = -0.9008333 G Decel

    Car B
    1840 lbs braking force/ 2000 lbs static weight = -0.92 G Decel

    So even in your example.. the lighter car stops quicker..

    I am sure there are cases (like one where the CG is high enough to cause significant transfer and the "heavy" read-biased car balances out to 50/50 where the "light" front bias car puts about 80-90% of its weight on 2 tires) where a heavier car will outbrake a lighter car given the same tires and braking systems (one that is "good enough" to lockup all tires), but I would say that this is not "normal".

    the heavier car starts out at a disadvantage, and it takes a good deal of weight distribution "sloshing" to overcome this disadvantage. Given the "perfect" circumstances where both cars are 50/50 under braking, the lighter car generates -1.0g of braking force while the heavier car generates -0.9g. This *can* be overcome by favorable/unfavorable weight distribution, but it is still a hill to overcome.

    -----------

    I will come back to this again when I finish reading the entire thread. I am sure you understand these concepts, but others were not quite grasping them. In particular, the case where one thought weight matter because it was easier to push his remote with a heavy object instead of a lighter. The same is true for cars in that it takes more force to stop a heavier car than a lighter one. The point you are making is that most brake systems are strong enough to stop all but the heaviest of cars, thus it was the tires that is the "weak link". I mostly agree with this (though I dunno if modern braking systems can generate the 3.5-4.0 G of force an F1 car can utilize braking at 200 mph), but I was trying to add the subtleties of tire deformation under load starts to question some of the assumptions that were being made.

    I am sure that after reading all of the thread this point will probably already been made and taken into account :)
     
  20. Scaff

    Scaff Staff Emeritus

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    Not quite.

    The above would be true is weight were the final part of braking, if F = M * A was the final factor in braking. However its not the case, as the following two links cover....

    http://www.stoptech.com/tech_info/The Physics of Braking Systems.pdf

    http://www.teamscr.com/grmbrakes.html

    http://www.stoptech.com/tech_info/wp_brakebiasandperformance.shtml


    .....the figure you use here are vital in specing the braking system but don't ultimately determine stopping distances.


    Scratch that - going mad and posting while busy is not a good idea.

    The main point I'm simply trying to make is that talking about weight alone is and can be very misleading, which is why every technical paper on vehicle dynamics, every braking company white paper or tech paper pretty much ignores it and focuses on the transfer of load.

    Weight distribution and load transfer are far more important that the actual weight and by focusing on that area (weight distribution) we get both a more accurate picture and a better understanding of what is occurring.



    I hope you enjoy it....:tup:

    Please do keep in mind that in a lot of cases simplified examples are used to try and get across what is a very complex subject in the most straightforward manner.


    Regards


    Scaff
     
    Last edited: Nov 12, 2008
  21. Alleycat

    Alleycat

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    yup.. was just about to go into how those articles actually show that F=MA IS what determines how fast one stops... Furthermore, the StopTech PDF not only gives the formulas to walk through what we have done (summed up the corner forces and divided them by static weight) it even hints at what I am talking about:

    "Note that in this analysis it is assumed that Upeakf and Upeakr are independent of deceleration, when in practice they are sensitive to the loaded changes brought about by the weight transfer phenomenon. Consequently, as weight is transferred the total vehicle deceleration capability is diminished by a small amount."

    I have also found the part where this was brought up before by Scant on page 13, and he DID do a rather good job explaining things :) (especially the part where a loaded truck takes longer to stop than an unladen one)

    He also tried to clear up the bogus response to "wide tires" that a High-school physics teacher (B.S.) "answered". While theoretically the teacher is correct in that the coefficient of friction shouldn't depend on area (pressure), tires, being deformable and all, DO change characteristics based upon pressure.

    The elastic phenomenon that causes tires to lose grip when they are deformed under high load, is the same as what happens when a smaller tire has a larger average psi than a "large" tire. Pressure = load.. decreasing the pressure is the same as lightening the load, and is the better reason that larger tires give more grip over the "higher chance of making contact with the road" excuse (though there IS merit that a wide tire maintains it's contact patch better under camber change than a smaller one, so in part, it is true).
     
  22. Scaff

    Scaff Staff Emeritus

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    That's what happens when I try and post while getting my youngest daughter ready for Cub Scouts. Family and things about physics rarely go hand in hand.




    If I recall correctly I believe I put a link to that post of Skants in the first post, it is a great summary.

    Having had a chance to read more of the whole thread I hope you can see that views are not that different. Its not that weight doesn't play a role, more that a lot of other factors are just as, if not more significant in the braking process.

    :tup:

    Scaff
     
  23. winoscar

    winoscar

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    I couldn't agree more, The first things I get for my car if any is racing brakes and soft tires for decent traction. Winston
     
  24. TOURNIQUETFAN

    TOURNIQUETFAN

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    HEY SCAFF I HAVE A SMALL LIST OF CARS I'D LIKE TO HAVE TUNED.NOT SO MUCH FOR DRIFTING BUT BETTER PERORMANCE.:confused::boggled::odd::dunce:
     
  25. Alltidxx

    Alltidxx

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    The first four links under "Links" in the threadmaking post are broken.
     
  26. Scaff

    Scaff Staff Emeritus

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  27. Alltidxx

    Alltidxx

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    Ok cool. My main idea though was that you might wanna update the first post since the thread is a sticky.
     
  28. kweiss

    kweiss

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    At 30 pages, I'm sure this has been discussed.

    Can we talk about when to brake and full throttle at the same time? I noticed some people setting incredible GT4 lap times on youtube doing this.

    Wikipedia has this to say:
    Surely it's not wise to do this for every sweeping turn in every car. So what decides when this is the proper technique?
     
  29. nomis3613

    nomis3613 Premium

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    I'm still kinda new here, but anyway here's my 2 cents.

    We shouldn't get too hung up on the maths and theory of braking (especially since we don't know for sure that the BBC is raw braking power to each axle- maybe Polyphony have already factored in static weight distribution for example).

    I used to tune brakes by taking them to the test track and finding the setting that gave minimum stopping distance (250-100km/h). Most of the time this resulted in 24 rear brake strength. I have since found that this approach causes strange corner entry behavior, even though the rears are below the ABS threshold.

    So these days I tune the BBC mainly on how it feels during braking and turn-in. Stopping distances into tight hairpins are longer, but it is well worth it for the improved balance in every other corner. I reckon this is proof that there are more important things that raw stopping power. Let's say you work out that a front heavy car will stop best with 80% front brake bias. I don't think this is much use if it sets the car up for plough understeer throughout the corner.
    </rant>