GT4 and Brakes

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

  1. sukerkin

    sukerkin

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    Excellent work Scaff. I'm always happy to read informed, well researched, contributions from those who quite clearly know what they're talking about.

    You also helped steer (yeah, automotive pun :D) some to more complete understanding of your posts with deft politeness.

    Plus, you kept your cool when those who hadn't understood or couldn't be bothered to learn decided to clump their 'bovver boots' all over the thread.

    Anyhoo, I don't want to embarass you with over-praise, so I'll shut up ... kudos :tup:
     
  2. Mr Whippy

    Mr Whippy

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    Assuming that your existing brakes can lock the tyres at any speed, then yes this is true.

    However, at speeds over 100mph, many cars brakes on everyday cars just can't brake hard enough on the disc to lock the wheels.

    Thats good in a way, as locking up at over 100mph is bad I expect.

    Either way, yeah, sticky tyres are a better investment, as they help in acceleration, braking, turning and on-limit behaviour.

    However, making an everyday car good for competition use, better brakes are needed for decellerating from higher speeds over prolonged duration. Bigger discs and pads mean longer life per unit of dispensed energy assuming compound and material stay the same.


    Still, bigger brakes weigh more, and add alot to unsprung mass. Not a great start.

    Light cars are the way to go.

    Dave
     
  3. ferret-turbo

    ferret-turbo

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    damm ...just submitted a thread about this, didn't see this one....
    how do you actually know when your wheels have locked when going ina straight line ? I can never hear tyre squealing or anything :( ??
     
  4. SuperChris

    SuperChris

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    From what speed? In my case I'm not going faster than 50-60 km/h

    I am familiar with the point you're making about people not braking hard enough.
    That's the reason Mercedes, among others, developed braking assist. But I'm braking as hard as I can.
    I do some low-level sunday racing, and spend some time on a track so testing off public roads is no problem.
    I'm also aware of the shorter stopping distance you get if braking close to the ABS-threshold.
    I prefer (in real-life) to race a car with ABS disabled.

    Anyway, the point of the post was to show that, in my case (straight-line braking at low speed) the tires "outperform" the brakes.
    And a brake upgrade might be worth it in terms of stopping distance.
    I would benefit from better feel and modulation anyway.
    If it was the other way around (ABS kicking in all the time) I would upgrade tires instead.

    Good discussion! Me like!
     
  5. Scaff

    Scaff Staff Emeritus

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    50mph to 100mph, in 10mph increments; make you feel sick after a while.

    EBA (electronic braking assistance) was exactly what I was testing, it a pain when you don't back off from the ABS. Had to learn to brake the wrong way!
     
  6. SlipZtrEm

    SlipZtrEm Staff Emeritus

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    Excellent post Scaff, it's true that no matter how effective one's suspension and brakes may be, the tires are still the only connection between car and road.

    I've been impressed with the improved braking in GT4 (it was way too strong in GT3), but I think most of the complaints in regards to the GT/Zonda/et al is just that they take a fair bit longer than other, comparable cars. I've been testing braking times from 60-0 and 100-0 in the game in various cars (Test Course -> hit those speeds -> brake -> pause as soon as indicator hits 0mph), and while I've seen the M5 do stops repeatedly in 2.32 and 3.75, respectively, the Ford GT struggles to manage 2.52 and 4.05. Plus, taking them around the 'Ring, I like to use the first turn as an indicator of how good the brakes work.

    Speaking of which, I also think the complaints might centre around the brake indicator in the game. I know it should not be trusted as the defacto "okay, this is where you should start braking" final word, but most cars are able to take a particular circuit fine just by following it. Some won't be able to slow down sufficiently when the warning finally comes on... the GT and Zonda are the two prime examples.

    Anyways, just my two cents.
     
  7. daggoth

    daggoth

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    I know I know. I didn't meant to say that discs don't have advantajes over drums! It's just that on a perfect scenario: ( In just a one-off braking test with equal cars, one with drums, one with discs ) the whole brake issue disappears! Provided that the two systems can create the same braking force :)

    Of course discs have many vantages over drums.... you don't see a racecar with drums :D

    I'm not a smart guy, but I'm not that dumb as well ;)
     
  8. Badana Kid

    Badana Kid

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    Braking is just something for the people to believe.
     
  9. nasanu

    nasanu (Banned)

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    Scaff the brembo stuff you posted backs up the theory that better brakes will stop you faster with the key words being at speed.

    I highly doubt that at 200km/ph normal road car brakes would have the force to lock the brakes instantly. This is where fiting better brakes will make the difference. 60-0 tests will tell you nothing about stopping from a much faster speed.

    I am a skeptic about weight not being an issue as well. I have my TV remote sitting here with me on my desk. I can hit it with a tissue and and the impact wont make it move, yet if I hit it at the same speed with a pack of cds it moves. But in each case it had the same traction and contact patch with my desk. Sure this is starting something to move rather than stopping something, and I have not slept in 24hrs, but I can't see why this would not translate into tyres.

    Thinking along the same lines, I could stop a matchbox car from rolling with ease, but what if that car weighed 2 ton? I really don't think it would be as easy to stop it, do you? If its not easier to stop that means it requires more force to stop it. In the case of tyres more traction. Do tyres really produce more traction the more you add weight to a car?
     
  10. Scaff

    Scaff Staff Emeritus

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    Very valid point and putting aside contact patch size, lets look at what the Ford GT and Zonda (also the Elise from earlier) have in common. Drivetrain layout, they are all mid-engined and as such will have very different weight transfer characteristics to a FR car such as the M5 you tested they against.

    Its all to do with weight transfer and brake bias (again see the StopTech details), and MR and RR cars are generally the most difficult to set-up effectively due to the lower level of weight transfered to front. This in theory should be offset by their ability to carry greater speed through corners (but thats another subject).


    To a degree you are right that it will depend on the car, but I can assure you that a lot of road cars are capable of locking the wheels at 100 mph+, just make sure you try it away from public roads. The problem they have is not the initial braking force, but maintaining it with out fade.

    In regard to your TV remote and matchbox cars, both unfortunatly lack air-filled rubber tyres, which are an important part of the equation.

    It can be a hard one to get your head round, but this may help. Its not a true comparison, just one of those things that helps.

    If I take my mobile phone (Nokia 6310) and a Bic pen (both are handy at my desk) and drop them from the same height of five feet, they will both hit the ground at the same time. This is despite being very different in mass and size. Its a nother good example of weight not playing a part, and in a situation when a lot of people think it should.

    Get some sleep (I know how you feel) and have a look over the Physics of Racing stuff again.



    Its also worth remembering that it is often very difficult to compare braking distance between cars (both in GT4 and real life) due to the huge number of variables that exist in the tyres themselves. Contact patch size being about the simplest of the lot.

    Pressure, ambient temp, tyre temp, road temp, sizewall size, profile, compound, air mixture in the tyre, track surface materials, etc; all these have to be factored into the equation.


    Edited to just bring this back to my reason for starting the thread and a hope that all of the above (and my thanks to all who have contirbuted so far) helps to show that I do not believe that PD 'messed up' on the braking performance of certain cars.

    From my experience of the cars I've driven in GT4 (and its not all 700 - yet) I have nothing by praise for the improvements in this area; its not perfect but for the PS2 it is damn impressive.
     
  11. Scaff

    Scaff Staff Emeritus

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    Many thanks. :)
     
  12. Giancarlo

    Giancarlo Premium

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    Braking works fine for me after just switching to manual.
     
  13. mrchicken&fries

    mrchicken&fries

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    I think u probally need to adjust the distance the brakes travel or maybe its somethingelse I cant believe your wheels wont lock up.
     
  14. Natural Newbie

    Natural Newbie

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    Great thread!

    It's always nice to freshen up on some physics.

    On the brake article you linked to...

    I saw that #34 saturn and I thought, wow that car looks oddly familiar. Then I look at the track, ok... Curbs look like Mid-Ohio... Ok he's at Mid-Ohio going through the keyhole/hairpin. Walker on the roof... HOLY CRAP!!! I have raced against that fine gentleman many times! Small world!

    But anyways, I love the engineering/physics aspects of cars, and I probably understand them all to well. You begin to get very anal about the cars you drive and you know their weak points all to well. Then you get pissed driving them because you know EXACTLY why your loosing time at point X on the track. Ignorance is bliss, maybe not... Back to racing!
     
  15. SuperChris

    SuperChris

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    I also suspect my brakes need a checkup. The thing is though, it stops VERY well!
    Compared it to other somewhat similar cars where ABS kicks in all the time, and my 206 stops just as good. Sometimes better (not by much, as this is done at 60 km/h) and certainly not worse.

    I'm puzzled. And I'm not a newbie at this. I know I should be able to lock the wheels.

    Anyway, the car is a blast to drive!
     
  16. Grip

    Grip

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    okay, so you say bigger calipers and discs wont stop you quicker, is that a correct state ment?To busy to read this many posts.
     
  17. Scaff

    Scaff Staff Emeritus

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    That is correct, bigger calipers and discs will not stop you quicker.

    If you do have time it is worth reading through the posts, however if time is short start with the first post and work your way throughthe rest when you have time.

    In particular the StopTech white paper on the first post is worth a look.
     
  18. Geobaldi

    Geobaldi

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    I don't intend this to be an in depth study in the physics of negative-acceleration (braking to non-physicists), but as a simple high-school level simplification and general explanation.

    The weight of a car is relatively unrelated to the distance it will take to brake a car. To counter the energy generated by the momentum of a mass at speed, heat must be generated. In essence, a larger car will generate more heat at the brake caliper/rotor interface, and at the tire/road contact.

    I -believe- it is this heat that might make larger cars tend to have longer stopping distances. From what I understand, there are temperature ranges for optimal stopping, and once this is exceeded, the braking system's capacity to stop your car is diminished?

    The energy, momentum and pressure generated by stopping has a very small effect on the coefficient of friction of a substance, in this case, say vulcanized rubber. It becomes the case that the heat generated by friction starts melting the tire long before the tire will slip (sudden loss of friction) due to weight.
     
  19. Div is back

    Div is back

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    I've notice something while racing on Fuji.

    When I'm in a slipstream and I brake, my braking distance is greatly increased and I frequently overshoot corners with the average braking point.

    To me it's logical, drag makes your car slower and it's what stop your car in a sense, it makes it easier, without drag you have no opposite force to your car so it's harder to brake.



    Nobody answered this..
     
  20. daggoth

    daggoth

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    Nobody answered because you made your point without leaving any edges to be polished :)

    I think no one can disagree with your statement ;)
     
  21. Scaff

    Scaff Staff Emeritus

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    Well said and its exactally why brake fade occurs and sufficent cooling of the braking system is so critical.

    Without sufficent cooling to keep the braking system within its optimal operating temp range you will experience brake fade and a reduction in braking ability.

    Larger pads and calipers will help to ofset some of this problem, but additional cooling is the best way to go.
     
  22. Scaff

    Scaff Staff Emeritus

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    A valid point, but if you are still using the average braking point this may be too late for the additional speed you have gained through drafting.

    Just a thought.
     
  23. Giancarlo

    Giancarlo Premium

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    You're right. Racing Medium tires make the 80s RUF drive like a dream.
     
  24. gisleDK

    gisleDK

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    So what you are saying is a 2000kg car and a 1000kg car decelerating with 1 G will demand the same friction from the tires?
    That's really something
     
  25. Crayola

    Crayola

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    No, but the car that ways 2000kg will amost certainly apply more weight and thereby increase the amount of friction produced by the tires. 1000kg is a big difference though.
     
  26. Scaff

    Scaff Staff Emeritus

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    No, but not because of the weight, that would not affect the straight line braking distance.

    Two cars with a weight difference of 1000 kilos would have very different weight distribution and transfer characteristics and centre of gravities. These factors do effect the stopping distances, but not to the degree that people believe. The single biggest factor is the tyre. Thats why a Range Rover is capable of stopping quicker than an Elise.

    Stoping times and distances between cars are a lot less different than people think.

    The links below are to the results of last years Autocar 0-100-0 mph tests, if you look at the 100-0 mph figures you will see that the difference in times is far less that you might imagine. Particularly given the very diverse nature of cars, and one of the slowest of the lot, the bike put in for comparison.

    0-100-0 mph part 1

    0-100-0 mph part 2

    Setting the bike aside the slowest car from 100-0 was the Westfield XTR4 at 5.66secs. The fastest was the Caterham R500 Evo at 3.6secs, a difference of 2.06 seconds. However 84% of the cars tested were able to brake from 100mph to zero in between 4 and 5 seconds.

    Lets compare that to the 0-100mph times, the quickest was the Ford Focus Rallycross at 4.74secs, the slowest the Pug 206 GTi 180 at 19.66secs. Thats a difference of 14.92secs.

    Now given the range of cars, the differences in weight, engine layout, braking systems the difference in braking performance from a set speed is very small.

    The one area of commonality is tyres, generally those cars with a larger contact patch stopped quicker (also aided by a small degree by weight transfer, etc).

    If the above should tell you anything its that the best way to improve stopping distances is to fit better tyres.


    Back to the original question, and if the two cars you mentioned had the same weight distribution and transfer characteristics and COG, then the difference in braking distance between the two would be almost none.

    Weight is not a significant factor, and distribution and transfer only a second order effect (i.e. it does play a part, but a small one). Tyres are the key to stopping.
     
  27. lazydog

    lazydog

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    The total weight of a vehicle IS a factor when braking (and accelerating). This is a direct result from physical laws. In order to accelerate (or decelerate) a given mass m with an acceleration of a you need to apply a force F:

    F = m * a

    The force F is applied by the brakes of a car (and drag to some extent) which is then transmitted to the ground by the tires of a car. (This creates a counter-reactive Force F2 applied to the tires and brakes which then heat up in the process).

    You see from the forumla above, that if you increase the total weight (mass) of a vehicle, the necessary force F needed to achieve the same deceleration a increases as well!

    If the brake system of a car can provide this increased force F3 = F * x (where x is some factor > 1 ) then ok, your stopping distance will not increase (provided the tires still have enough grip).

    If it cannot apply this higher force, the stopping distance increases because of the increased mass of the car.
     
  28. gisleDK

    gisleDK

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    There would be no problem with having a one and a two tonne car having the same weight distribution and the only difference being that the tires of the heavy car in a 1G deceleration would be holding 1000 kgs more.
    This is really silly.
     
  29. Scaff

    Scaff Staff Emeritus

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    Sorry, but you are wrong.

    If you read the whole thread you will see that this has been discussed and resolved before.

    The physics formula you are using is correct, but its does not fully account for everything involved.

    If you had read my original post and the Physics of Racing it is linked to you would have seen the following.

    By Newton's second law, the weight on the tire depends on the fraction of the car's mass that the tire must support and the acceleration of gravity,[​IMG] . The fraction of the car's mass that the tire must support depends on geometrical factors such as the wheelbase and the height of the center of gravity. It also depends on the acceleration of the car, which completely accounts for weight transfer.

    It is critical to separate the geometrical, or kinematic, aspects of weight transfer from the mass of the car. Imagine two cars with the same geometry but different masses (weights). In a one g braking maneuver, the same fraction of each car's total weight will be transferred to the front. In the example of Part 1 of this series, we calculated a 20%weight transfer during one g braking because the height of the CG was 20%of the wheelbase. This weight transfer will be the same 20%in a 3500 pound, stock Corvette as in a 2200 pound, tube-frame, Trans-Am Corvette so long as the geometry (wheelbase, CG height, etc.) of the two cars is the same. Although the actual weight, in pounds, will be different in the two cases, the fractions of the cars' total weight will be equal.

    Separating kinematics from mass, then, we have for the weight [​IMG] where [​IMG] is the fraction of the car's mass the tire must support and also accounts for weight transfer, m is the car's mass, and g is the acceleration of gravity.

    Finally, by Newton's second law again, the acceleration of the tire due to the force F applied to it is [​IMG] We can now combine the expressions above to discover a fascinating fact:[​IMG] [​IMG] The maximum acceleration a tire can take is [​IMG] , a constant, independent of the mass of the car! While the maximum force a tire can take depends very much on the current vertical load or weight on the tire, the acceleration of that tire does not depend on the current weight. If a tire can take one g before sliding, it can take it on a lightweight car as well as on a heavy car, and it can take it under load as well as when lightly loaded. We hinted at this fact in Part 2, but the analysis above hopefully gives some deeper insight into it. We note that [​IMG] being constant is only approximately true, because [​IMG] changes slightly as tire load varies, but this is a second-order effect (covered in a later article).


    The direct link, with formula is
    http://www.miata.net/sport/Physics/07-Circle.html

    If you are so adement that you are correct, please explain why a 2000kilos Range Rover can stop quicker than a 700kilo Elise?
     
  30. lazydog

    lazydog

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    First because the tires of the Rover can take a much higher force when braking and second because the brakes of the Rover can provide this force.

    I never said a lighter car would always stop quicker than a heavier one, I only wanted to point out that the brake system as well must be able to provide more force the more mass a car has.

    Just take your everyday car and load it up with 4 friends and 200 pounds of luggage in the trunk. You'll see you need to apply more force to the brake pedal in order to stop the car as quickly as it would be empty (only the driver in it). This higher force must also be supported by the brake system. If it can not, then the braking distance will increase.
    (I think in almost all modern cars the brakes are strong enough to support a fully loaded vehicle as well as an empty one - but this might not always be the case).

    Or for another example: Swap the brake systems of the above Lotus and the Rover, which one would stop quicker then?