Brake performance wrong in GT4 prologue!?

Discussion in 'Gran Turismo 4: Prologue' started by Buggy Boy, Jun 5, 2004.

  1. Rapidone

    Rapidone

    Messages:
    75
    No, but I studied the first two years of a mechanical engineering degree, before comming to my senses and quitting for somthing eaiser and more interesting.



    You may find the following table interesting, it gives stopping distances for around 250 different cars.

    http://www.movit.de/htm/tabstop.htm

    The lotus elise is shown as taking longer to stop than most cars in the list, and longer than every Mercedes in the list.



    However, after reading the following, I doubt testing was done properly.
    http://www.elises.co.uk/diary/2001/august.html

    'They do mention the chassis protection but only manage to stop the elise in 27.18m (in the dry) from 40mph, making it the worst car tested. In the text however they say they managed to get it down to 18.67m after practice, making it the 3rd best car! Surely a pre-requisite for this test is that the driver knows how to drive the car? Stamping on the pedal to lock the wheels is not going to provide very good stopping distances. '


    So who do we believe? Didn't the GT4p game creators test the cars to get their performance figures for the game? I hope they didn't use press released figures.
     
  2. Buggy Boy

    Buggy Boy

    Messages:
    660
    Well, you might have made a good choice :)

    Interesting what you found there. And who knows what PD used? I saw pics of sound recordings (and wonder what they did with it as I hear the sound of the BMW 120d), of photographing cars and tracks, of drifting, of track driving, but never of a brake test.

    It is worth noting that when Auto Express test drivers repeated the test without locking the wheels the car recorded the third best stopping distance of the day, confirming that the brakes are indeed more than adequate.
     
  3. Buggy Boy

    Buggy Boy

    Messages:
    660
    I've been thinking about that article.

    The ultra low mass of the car means that only relatively modest brake dimensions are necessary to deliver truly stunning stopping power.
    At least this seems to contradict the low performance in GT4P.

    The Elise was one of two cars without ABS tested but it looks like future European legislation is coming in to make ABS a compulsory feature.
    This scares me :yuck: Why do politicians always have to choose for the easiest solution "make the law by considering the most stupid people and then just apply it to everybody". :rolleyes:
     
  4. Sambert

    Sambert Premium

    Messages:
    1,304
    ---
     
    Last edited: Jul 14, 2018
  5. Rapidone

    Rapidone

    Messages:
    75
    Hi, again. I've been thinking about this problem a little more, infact I couldn't sleep last night until I understood what is going on with tyre grip, weight and breaking performance.

    I noticed that some of the cars in the table of stopping distances seemed to have very low 60-0mph distances. So I began wondering how long a car would take to stop if the tyres had perfect grip, a coeficient of friction of 1.0. That would give 1g of decelaration, so I'm lying in bed and do the caculation in my head, since I can't be bothered getting up. I calcutated that the stopping distance at 1g decelaration is about 45 metres.

    Now many of the cars on the list stop faster than that, which would mean a coefficient of friction greater than 1.0, which is not possible. It's imposible to explain with the theory of static friction on a smooth surface. So what is going wrong?

    The static friction and coefficient of static friction theory is based on the asssumption of a smooth surface (the road). But roads arn't smooth. Roads are rough and the soft car tyre moulds itself into the surface of the road due to the weight of the car on the tyre. This gives added grip above the coefficent of friction between road and tyre. Imagine two gears running toggether with their teeth meshed together, one gear is the road and one is the tyre.

    This extra component of grip is still not independant of the weight of the car, since a car without weight won't push the tyres into the road at all and won't develop any grip. However there is a point beyond which adding more weight to a car won't develop any more grip (for this extra component of grip), since all of the holes in the road have been filled with rubber. This extra component of tyre grip in relation to Weight must follow an asymtopic relationship. The first weight applied to the tyre yeild lots more grip, but extra weight yeilds less and less grip until almost no extra grip is gained from extra weight.

    So I conclude that the formula for tyre grip is somthing like this

    Grip = kM + (1 - 1/(kM Squared)), where M is the mass of the car, and k is a constant.

    This explains why a lighter car stops faster than a heavier car.

    We can learn an interesting lesson in breaking preformance and weight distibution across the tyres from this.

    The best breaking performance for a car should be when the weight of the car is equally distibuted over each of it's four wheels durring breaking (and not when most of the weight is over the front wheels, as I wrongly posted before). This is because, when transfering weight between an equally loaded set of 4 wheels, the wheel that gains weight gains less extra grip, than the grip lost by the wheel which has lost weight on it.

    Under heavy breaking there is a large weight transfer from the rear wheels to the front wheels. So to gain the best breaking performance it's neccessary counteract this shift by loading more weight into the back wheels than the front when the car is static.

    So in conclusion the best breaking preformance should be acheived by cars with low weight, large tyres and more weight over the back wheels than the front wheels.

    This is the case with Elise and with Porches (porches have the engine at the back giving static weight to the rear wheels.)

    If you look at the table of stopping distances you will notice that the Porches all have very low stopping distances, which is consistent with this theory.

    I therefore conclude that the stopping distance in GT4p for the Elise, is incorrect as Buggyboy noticed at the start of this thread, since it has both very low weight and has considerably more weight over the back wheels than the front, and should stop faster than most other cars.

    I hope this helps and I have made plain what I'm trying to say here. If you have any questions, or if I've explained poorly, or if you disagree, please ask me to clariffy.


    EDIT: Buggy Boy - 'Mid engined cars are supposed to stop better, not worse!!'#

    I now think that mid engined cars are should corner better under neutral throtle than other cars but will break worse than a rear engined car.

    EDIT 2: Changed the grip formula to somthing more realistic. Changed wording a little to make reading easier. Corrected spelling mistakes. Lesson learnt : don't make a technical post after consuming a bottle of wine. :)
     
  6. TMM

    TMM

    Messages:
    1,203
    Location:
    England
    I do so heartily agree. When I bought my new car I got ABS added as I wrecked the bottom engine of my last car taking it over a full kerb reservation. It was wet and I was going to fast to stop, if I had tried a handbrake turn I would of ended up on my roof!!! So I thought ABS would be good, however it scared the crap out of me in the snow 1 year cause it really didn't help me stop, mananged to drive out of it though. So now Brussel's is making it law that ABS must be fitted to all road cars, great there goes the fun of driving again.
     
  7. okoj

    okoj (Banned)

    Messages:
    275
    Did you use the DFP for your tests?
     
  8. Rapidone

    Rapidone

    Messages:
    75

    Well making a blanked resolution like this is an easy sollution, that will probably save lives, and is therefore considered justified. I have to agree with the politicians, however, it would be nice to see a more sophisticated set of rules here. Drivers that are able to display advanced driving skills, (In the UK there is the advanced licence test that can be taken) should be allowed to use a car without ABS.
     
  9. yeti

    yeti

    Messages:
    3,165
    The amount of cretins I see behind the wheel of 1 (or more) tonne killing machines that need all the help they can get far outweigh the need for the odd master to be able to control his own braking!!

    Anyway.... if you're that good a driver and you've got ABS you'll just never use it!!
    (You used in general term... I'm not picking on You in particular!)

    C.
     
  10. userone

    userone

    Messages:
    32
    Yeah, ABS is no help on gravel or snow, as it's the build up of gravel/snow under the locking wheels that stops you. The other problem for performance driving is that it will often cut in early if the brakes get hot. All in all it's safer with it, but I'd like the option!
     
  11. Buggy Boy

    Buggy Boy

    Messages:
    660
    Yes I was wrong there. This is more consistent with the graph I wrote about : more weight on the front axle will increase the grip there, but not as much as it's decreased at the same time on the rear axle. Best is an even distribution over both axles.
    btw, I didn't want to keep anybody from sleeping by starting this thread, but thanks for your reflections :)


    No, they were done with the Dual Shock - analog sticks.


    Samberto, thanks for sharing your track day experience and your ideas. As I understand you were the guy whose RUF is recorded by PD, I thought you live in Europe. Do you live part-time in Australia or do you travel there especially for these track days??
     
  12. RyosukeFCDS

    RyosukeFCDS

    Messages:
    147
    They're all production cars, with production car specs. So...

    The reason cars don't simply rely on their weight to determine stopping distances, is,

    1. Gear ratios (presuming you manually downshifted, which would be technically wrong, you should drop it into neutral then brake hard)
    2. Size of brake rotors
    3. Ventilated discs/not
    4. Brake rotor composition
    5. Size of brake calipers
    6. Brake pads
    7. Brake master cylinder/etc parts
    8. ABS/TCS controls
    9. Limited slip settings
    10. TYRES, manufacturers use a specific tyre on each production model which polyphony would've taken into account.
    11. Aerodynamics
    12. (Only for a few cars like some of the porches and McLarens), the rear wing/ air brake.
    13. WEIGHT DISTRIBUTION
    14. Tyre size, wider tyres more grip etc.


    Probably more things but that's all i can be bothered typing.
    So there's so many factors instead of just, weight, so there, problem solved.
     
  13. Sambert

    Sambert Premium

    Messages:
    1,304
    ---
     
    Last edited: Jul 14, 2018
  14. k2racer

    k2racer

    Messages:
    350
    I am in need for a giggle (will loose my job on friday, serious), link please...?
     
  15. Sambert

    Sambert Premium

    Messages:
    1,304
    Sorry about your job…be sure to enjoy some GT on your short holiday. :)
    Here’s the link: http://www.m5board.com/vbulletin/showthread.php?t=43370
    Note: this is the very same car that was tested by PD Europe for GT4. Enjoy
     
  16. okoj

    okoj (Banned)

    Messages:
    275
    As you can turn off all driving assists when using the Driving Force Pro you may find that would give more accurate results.
     
  17. yeti

    yeti

    Messages:
    3,165
    Since when is using Engine braking wrong?!?!

    You're technically meant to work your way down thru the gears in sequence to provide the best stopping distances.

    What do you think you're meant to do if your brakes fail? Neutral and handbrake?!

    No you jam it in as low a gear as possible and keep going down gears until your in first... this will ensure you are going quite slow... and hopefully the handbrake can stop you from there.

    C.
     
  18. okoj

    okoj (Banned)

    Messages:
    275
    and if not a tree or signpost will do the trick ;)
     
  19. RyosukeFCDS

    RyosukeFCDS

    Messages:
    147
    For benchmark brake testing you're not meant to have it in gear.

    Of course you use engine braking though to slow down normally
    :)
     
  20. Buggy Boy

    Buggy Boy

    Messages:
    660
    All I can say on this is that everybody agrees with you, it's all been said in this thread before.

    But you wouldn't succeed an exam with this conclusion ;)



    Sorry Samberto, I should have done a bit more research myself ...




    AFAIK, the only difference is that with the DFP, you can switch steering assistance off. This is discussed in the thread GT4 Driving Physics. Very important imo, but without influence on braking. For that, you'd have to shut braking assistance off, which sadly isn't possible in any GT-version untill now.



    Hey! Check your quote : I didn't say that! :sick:
     
  21. GarySheehan

    GarySheehan

    Messages:
    7
  22. GarySheehan

    GarySheehan

    Messages:
    7
    C,

    You don't really use engine braking on the track, unless you are already experiencing a braking problem. Slowing the car is done with the brakes.

    The absolute best stopping distance would be accomplished with the car in neutral, putting all concentration on braking.

    Drivers row through the gears as they brake to establish a rhythm with engine rpm vs. roadspeed, so that it is easier to shift into the right gear at the right time. Skipping gears can be faster around the track, but you run the risk of not matching engine rpm to road speed as accurately during your single downshift.

    You're using a brake system failure as justification for using engine braking when the brakes are operating properly. Two completely different scenarios. Downshifting on the track is to engage the proper gear needed to provide the best acceleration out of the upcoming corner, not to slow the car down.

    Gary
    Sheehan Motor Racing
    www.teamSMR.com
     
  23. Buggy Boy

    Buggy Boy

    Messages:
    660
    Hi Gary,
    Thanks for your input. It's good to see someone with a technical and racing background posting. What's your opinion about the subject of the thread (modelled performances in GT4 Prologue)?

    btw : your second link is not accessible without permission
     
  24. GarySheehan

    GarySheehan

    Messages:
    7
    Thanks on the second link. It's been updated.

    A significantly lighter car should always stop shorter than a much heavier car, given the same tire, similar F/R weight bias and optimized brake bias for the platform.

    Gary
    Sheehan Motor Racing
    www.teamSMR.com
     
  25. Segnit

    Segnit

    Messages:
    9
    Guys, not to get off topic but this is an excellent thread with really valuable information. Buggy Boy, your research in GT4 Prologue was great. Because of it all of us got to learn more indepth about the authenticity of the game, not to mention got valuable insight on braking from various posters. Rapidone especially did some terrific writing and explaining. Keep it up guys. btw i am linking this thread to my "Racing Education" thread in GT4 forums.
     
  26. Buggy Boy

    Buggy Boy

    Messages:
    660
    GarySheehan, I think you made a good summary for a one-time stop, which is discussed in this thread.

    Segnit, thanks!
    ---
    Tonight I was driving the Daihatsu Mira TR-XX, which is 700 kg. This made me think of including it into the test, as its weight is almost the same. The Lotus is even a bit lighter ... (670 kg). I had to redo it however, as the Mira couldn't get to 150 km/hr, the speed at which the test was done :)

    So I did an emergency stop with the Mira, and with the Elise 190, both at 125 km/hr, further with the same conditions as in post#1.

    Guess what :banghead:

    - Mira : 4 lines and 1 car length
    - Lotus : 4 lines and 2 car lengths ...
     
  27. Scaff

    Scaff Staff Emeritus

    Messages:
    23,224
    Location:
    United Kingdom
    The reason you were able to drive out of it was the ABS.

    Its a common misconseption that ABS stops you quicker, the principal function of ABS systems is to stop the wheels locking under heavy/emergency braking. This allows the car to still be steered while braking force is maintained, alowing you to drive out of potential accidents.

    Another thing that should be remembered is that stopping or manouvering on ice or deep water is that ABS will not make the slightest bit of difference (for or against).

    I found that out to my cost earlier this year when I hit a deep puddle of water while rounding a slight bend, as the car hit the puddle (at about 45mph) it began to aquaplane, bythe time grip returned the front of the car was in a grass verge. A 180 spin later and the back end hit the verge and the car ground to a halt looking a lot worse for it.

    The leason I learnt that day was that no matter what the set up of the car, if you misjudge the situation or the conditions change quickly you can't always do a lot about it.
     
  28. rx7guy

    rx7guy

    Messages:
    38
    I wanna know sumthing.
    The mazda3 sp23 recorded the best ever braking distance from 100-0km/h i think about 32metres, by autocar in new zealand, until it was just beaten recently by the evo 8.
    how is this? it doesnt have very high performance tyres or especially good brakes, they are standard mazda brakes.

    And also why is it so hard 2 wheelspin even in the mclaren with the aids all turned off?
     
  29. Scaff

    Scaff Staff Emeritus

    Messages:
    23,224
    Location:
    United Kingdom
    The F1 is fitted with racing tyres which will increase grip levels, however in a 2nd gearhair pin if you floor the throttle on exit the back tyres light up quite nicely (as will the M5).

    Try it on the first corner at Fuji, slam the throttle to the floor before the front wheels have straightened completely and watch the back step right out.
     
  30. Buggy Boy

    Buggy Boy

    Messages:
    660
    Sounds interesting. Do you have a link to this for more information?
    As e.g. a Formula 1 car stops faster from 100-0km/h then 32 metres, there will probably some standards that compared cars have to meet. If I see the examples you mention it could be a comparison between standard sedans on standard tyres.