GT4 and Brakes

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

  1. Scaff

    Scaff Staff Emeritus

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    Sorry, but you have either not read or not understood the 'Physics of racing' series; weight is not a critical factor in the overall stopping distance.

    If you believe it is then explain why a Range Rover has a quicker stopping time than an Elise?

    Lossing weight has many, many advantages when it comes to cars, but braking is not one of them.

    I have provided links to detailed explinations of why this occurs, either you do not believe the racer (and Physics Phd) who wrote them or you have an alternative theory.

    The Following is from the Physics of racing and can be applied to braking and acceleration as they discuss maximum levels of grip available during straight line travel. The second you start looking at accel/decel during cornering it gets far more complex.

    "Finally, by Newton's second law again, the acceleration of the tire due to the force applied to it is We can now combine the expressions above to discover a fascinating fact: The maximum acceleration a tire can take is , 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 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 being constant is only approximately true, because changes slightly as tire load varies, but this is a second-order effect (covered in a later article)."

    Nothing theoretical about it, almost any modern braking system (including drum brakes) can exert sufficent force to make the tyres lock. You are quite right about why better brakee setups are an advantage, as they allow the driver to exploit the system to its full, with better feel and modulation availiable, with low levels of fade.
     
  2. Crayola

    Crayola

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    Yer, I saw this coming, but I figured that the fact on my bike the pad slips and the tire doesnt makes it comparable to the tire slipping as opposed to the pads. As for the people who think better brake systems stop quicker: Im getting a new bike with disk brakes in 2 weeks, Ill do the test again however I expect in this situation it will make a large difference because push bike brakes are competely different.
     
  3. Crayola

    Crayola

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    From a physics standpoint, all else being equal a lighter car will stop quicker however you may not be maximising the grip of the tires unless they are still getting adequate weight applied to them. This why light cars need stiff set ups.
     
  4. SP33D

    SP33D

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    Way too much jibberish on this post! Admin please close.
     
  5. Scaff

    Scaff Staff Emeritus

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    This is taken directly from my first post

    I'd did not say that old brakes would not make a difference, I am talking about (as I said) correctly working braking systems.

    You are right that non or poorly functioning brakes will have an affect, but that is not the point of this thread.
     
  6. Scaff

    Scaff Staff Emeritus

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    Or you could try and understand it!
     
  7. MR_GT

    MR_GT

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    Because a Range Rover has different tires than an Elise :)

    I think the link is interesting but it leaves out some critical points. Yes "total tire grip" determines the distance a car can stop (because the tires are the only part contacting the road), but the "total tire grip" is not constant...it becomes variable depending on many factors (tire pressure, weight of the car, location of weight on the car, etc.)...the main factor is weight.

    For example take 2 identical cars (say Ford GT) with identical tires (brand new, same size, same tire pressure) and same setup...add 1000KG to the 2nd car...the link you provided is saying that these 2 cars will have the same braking distance. Simply it's not true at all.

    Another example...if you watch racing such as F1 on TV...an F1 car with barely any fuel onboard will have a much shorter braking distance than one with a full fuel tank. This is because fuel is heavy. Tire grip is strongly affected by total weight...and accerlation, cornering speed, as well as braking are all affected by total weight.
     
  8. Scaff

    Scaff Staff Emeritus

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    Yes and why its the weight distribution and transfer that affects the stopping distance and not the weight itself.
     
  9. SP33D

    SP33D

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    You make no points!.. What a complete waste of time reading this post!
     
  10. Crayola

    Crayola

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    Its the transfer of weight though, not how much it weighs, think of it this way if you need to apply 200kg to each of the front tires to achieve maximum grip, any more than that would overload the tires. Therefore a light car that can still apply 200kg will stop much better than a heavier car applying the same amount of force because Impulse Ft= mv-mu, to reduce t (time) with the same force you wont less initial momentum.
     
  11. Scaff

    Scaff Staff Emeritus

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    The braking distance overall is not shorter, however due to the lower weight the car can carry more speed into the corner and will be more stable through the corner.

    This gives the impresion of a shorter braking distance, but it is not the case.

    Please read up on transient weight transfer and grip levels through corners.

    Please note that this is not a subject that I look at for fun, but rather as part of my job.

    But right now as its Sunday morning I'm off to spend some time with my kids.
     
  12. Crayola

    Crayola

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    I'll say it again because people mightent see it now: If you need 200kg (this particular number is irrelevant) of force on each front tire to attaint maximum grip, any less or more weight will only be detrimental to stopping. The reason a lighter car will stop better, provided it can still transfer enough weight to the front is because impulse Ft=mv-mu. Basically, to reduce the time when the force is the same you need to decrease initial momentum which is why either going slower or weighing less will decrease braking distance.
     
  13. Scaff

    Scaff Staff Emeritus

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    I'm sorry but the only person who has not made a point in this thread is you.

    If you don't like the thread PM a mod or report me, otherwise how about leaving a good discussion thread that does make valid points in relation to GT4 .

    Everyone else here is involved in an adult discussion.
     
  14. SP33D

    SP33D

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    Blah, blah, blah.
     
  15. t1210s

    t1210s

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    why do the best stopping cars (f1) use carbon fibre brakes running at about 700 degrees(temprature) and not crappy drum brakes if it is only tire surface contact that matters since they run on slicks the type of brakes or brake compound should not matter
     
  16. Scaff

    Scaff Staff Emeritus

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    Quite simple because they have to slow from 180 mph+ repeatedly for over an hour with no fade, while providing the driver with excellent feedback, feel and modulation.

    Find me a drum braking system that will do that?
     
  17. t1210s

    t1210s

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    but you said the brakes dont matter
    so if they only had to stop once because the tires have maximum surface contact the type of brakes they use wont have an affect on the stopping
    so i get two f1 cars fit one with drums and leave the other standard your saying that the stopping distance will be the same
     
  18. Scaff

    Scaff Staff Emeritus

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    I said ther type of brakes do not matter in terms of the overall stopping distance, because they don't. If the brakes can exert sufficent force to exceed the tyres grip limit and lock the wheels then the type of brakes do not affect the stopping distance.

    Brake feel, fade resistance and modulation are a totally different subject. Please read my first post again, particularly the extract from the StopTech white paper which explains this.

    Remember that is a quite directly from a company who manufacturer race spec braking components, you may also want to look at a thread in the GT4P forum, which covers this subject. A race driver called Gary provided this very useful information.

    I understand that this can be a right pain to get your head around, and the first time it was explained to me (by a Renault race technician) my reaction was one of doubt.

    However the more you look at it the more sense it makes.
     
  19. Scaff

    Scaff Staff Emeritus

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    As a number of you seem to want to doubt what has been said about discs and pads affecting stopping distanes, and StopTech do not seem to hold much weight.

    How about Brembo, sure you will all agree that they know a fair bit about brakes, so here is an quote from their FAQ's

    http://www.buybrakes.com/brembo/faq.html#q6

    "Where can I find test data on stopping distances?
    At the speeds that stopping distance is generally measured from (60 to 70mph), the test is primarily testing the tire's grip on the pavement. As delivered from the manufacturer, nearly all vehicles are able to engage the ABS or lock the wheels at these speeds. Therefore, an increase in braking power will do nothing to stop the vehicle in a shorter distance. For this reason, we do not record stopping distances at this time. The Brembo systems will show their greatest advantages when braking from higher speeds, or when tasked with repeated heavy braking. The increased braking torque provides for maximum deceleration at speed, and the ability to absorb and quickly dissipate the intense heat generated during repeated braking insures that the braking system will perform at the same high level each time."

    "Is the biggest brake disc the best answer for my vehicle?
    Not necessarily. There are many factors to take into account when choosing a performance brake system. First and foremost, a brake system must be designed based on the vehicle parameters, and the type of use it will experience. Brake systems are designed to operate best within a prescribed temperature range. While modern high performance friction materials broaden this temperature range from relatively cold pad temperatures to the high temperatures experienced in performance driving situations, the use of a disc that is too large will limit disc temperatures to the extreme lower end of this range. Not only will the disc temperature not reach the optimum range, but it will also be heavier than necessary. Below you will find an explanation of how reducing the weight of the braking system further enhances performance."



    Not a single claim that Brembo braking systems will stop you quicker, just that they will function to the same level despite high temps, from repeated high speed braking. While providing greater feel and control for the driver.

    It short (and again) as long as the brakes are sufficent to lock the wheels, increasing the size of discs, type of pad material or the calipers will not decrease you stopping distance. However it may provide you with increased fade-resistance, better feel, etc.
     
  20. t1210s

    t1210s

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    i dont disagree that tires help braking but they are only one part of the system the way your talking is they are the beall and end all of the system plus at high speed they dont have the same affect on stopping as they do at low speed and most of the braking in gt4 is done from high speed so you still need to tune your brake controller
     
  21. MR_GT

    MR_GT

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    Nice thread...it's got me thinking about braking more and more. I hope the guys who make GT4 read our stuff here before they start making GT5
     
  22. Scaff

    Scaff Staff Emeritus

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    NO I have not stated that tyres are the be al and end all of braking, just that they are the determining factor in stopping distances.

    Brake Bias (read the StopTech white paper) is very important in stopping stability, and I have not (and never would) suggested that it can be ignored.

    The original reason I started this thread was to get people thinking about what will shorten stopping distances in high-powered cars, and the only was to be certain of doing this is tyres.

    Get the tyres right and then sort out the cars balance.
     
  23. Scaff

    Scaff Staff Emeritus

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    Thanks, its an interesting subject that I have had the fortune to work on a lot.
     
  24. Da_Ork

    Da_Ork

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    Intresting. The wieght of a car does affect stopping distance its just not necessarily if its heavier it will stop slower. Its how the weight is distributed when you hit the brakes. Such as if you break turning instead of in a straight line most of the weight goes to front outside wheel. Just my two cents worth.
     
  25. daggoth

    daggoth

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    If you have drum brakes that can apply as much force as brake discs, then the brakes get instantly out of the equation.

    This makes every bit of sense but it doesn't seem so linear on your first post! At least I didn't understand it that way the first time I've red it.

    If you put a setence similar to mine right below these two, then people can understand it better, and won't harass you that much by saying that bigger calipers and disks make a difference ;)

    I believe that you made an excellent point here, and this is a great thread! :)
     
  26. 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.
     
  27. SuperChris

    SuperChris

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    At high speeds, a drumbrake would very quickly overheat and fade.

    Very high speeds require more brakingforce to lock the wheels. This is one of the areas where better brakes (bigger discs+pads etc.) comes into the equation. At lower speeds, let's say around 60 mph, every car with discs or dums for that matter will lock the wheels.

    If you can't lock the wheels with your current brakes, upgrade the brakes.
    If you can lock the wheels, get better tires.

    On my downhill push-bike I have hydraulic disc brakes with 8" rotors. The don't allow me to stop any quicker than 6" rotors, but I can modulate much better and I can apply higher braking force with just one finger on the brake-lever instead of squeezing with 3 fingers. Both brake-systems can easily lock the wheels.

    In a car it's also easier to apply just the right amount of pressure to the pedal when you don't have to stand on it with both feet!
     
  28. SuperChris

    SuperChris

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    Iv'e got a Peugeot 206 with 17" wheels and really good tires.

    At 60 km/h on a hot summer day, I can't lock the wheels under full braking! The ABS doesn't kick in. It just stops in what feels like 3 metres! If I upgraded the brakes I might be able to stop quicker, since the wheels aren't locking up with the current brakes. Maybe they're on the edge of locking up, which would be absolutely maximum braking. Then I would need better tires.
     
  29. sn00pie

    sn00pie Staff Emeritus

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    Please note that this is your one and only warning before your GTP privileges are enthusiastically cut by the moderating staff. Please post substantially - or don't post at all.
     
  30. Scaff

    Scaff Staff Emeritus

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    Hard to say why this is occuring, as I have not driven your car.

    I have driven a 206 at MIRA (UK proving ground) and found no problem getting the car to exceed the tyres grip limits under hard braking.

    It is possiable (and no offence implied) that you are not braking hard enough; in reality the vast majority of people do not apply maximum braking pressure even under hard braking and also back off when the ABS kicks in.

    However, as I do not know your driving style or individual car, its hard to be exact.

    If you have access to an area that you can test this away fom public roads (an old air-field for example) then try applying maximum braking force as smoothly and rapidly as you can. You should get the ABS to kick in. Once you can get that to occur, then try braking in the same way, but modulate the pedal to apply force up to (but not beyond) the point the ABS kicks in. You should find that the overall stopping distance is shorter (if its dry).

    Now as I said above do not try this on public roads, and be aware that it will give your pads and discs a hammering.

    If at any time you experience brake pad (brake pedal will feel softer that normal) then stop and let the braking system cool down, also do not apply the handbrake as with your pads and discs this hot you may find that they sieze together with the handbrake on (a common problem at track days)