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General Brake Design Theory Question

Discussion in 'Technical Q&A' started by DoubleD33, Aug 12, 2014.

  1. Fast_ian

    Fast_ian Two Time F1 World Champ

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    To a certain degree, that's true.

    But, there's a *lot* more to it than that..... If not, we'd all still be using the 6" drum brakes I was unfortunate enough to grow up with..... Trust me, they don't stop my ass anywhere near as well as some huge disks. ;)

    Cheers,
    Ian
     
  2. finnerty

    finnerty F1 World Champ
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    Biggest advantage of going to larger (diameter) rotors (and concurrently increasing caliper clamp point distance from the axle) is the increase in the torque authority (just as using a breaker bar does on fastener) against wheel rotation --- i.e. greater stopping power.

    Unless you are going to track your car a lot, forget wasting money trying to "upgrade" stock brakes on any modern, high performance sports car (Ferraris in particular) ---- for general road use, you are not going to improve significantly vs. money spent over what any of these cars leave the factory with, IMO.
     
  3. fatbillybob

    fatbillybob F1 World Champ
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    There is a lot of truth here. Even in racing converted streetcars you gotta ask yourself if the money spent is worth the few tenths. Of course it is.... When people modify brake they always go bigger. Bigger more pistons in calipers and or bigger diameter or thicker rotors and stainless steel braided brake lines. Interestingly, it is hard to prove stopping distances actually improve over solitary tests. But actually they do and they don't. In my pee brain I think tires stop cars not brakes. Tires are the limit of braking as they roll to a stop at the verge of lock-up. ABS does this better and more consistently than most humans. When ABS kicks in that's the limit of braking. If your stock system gets into ABS that is the limit of the tire on the ground. If a giant brake caliper gets you into ABS the same limits have been reached. So it makes sense that stopping distances are mostly unchanged. So where is the positive change? Well there is this short period of 1/4 second or less from application to brakes actually working. At 100mph that 1/4 second is 36 feet. So in a race if you can control 36 ft in the braking zone you have out braked your opponent by nearly 2 car lengths. Big brakes allow you to modulate brakes better because their capacity is higher. Also big brakes can absorb and throw off the heat generated over and over again with consistency over the life of a brake pad with no or minimal pad taper that a stock set-up just was not designed to handle. At best a stock system is designed to handle a romp in the canyons or a 20 min trackday session at 6/10ths with a rest. Big brakes are designed to be beaten on over and over again all day long everyday until you have no pad left. As an example racing in SCCA T1 corvettes we used to crack stock rotors on the 3rd trackday and wear out pads to an taper in 2 days while braking performance diminished by the minute while on track. Then Stoptech Brakes designed/developed their T1 racing brake kit on my corvette. Now rotors last me over 1 year of racing and pads last me 1/2 a season all while providing the same consistent controlled braking from the first day on new rotors and new pads to the last day on old pads and old rotors. That is what a well engineered brake system will do for you.
     
  4. Far Out

    Far Out F1 Veteran

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    #29 Far Out, Aug 15, 2014
    Last edited by a moderator: Sep 7, 2017
    Every ABS software has a boatload of tuning parameters, these aren't one-size-fits-it-all algorithms. If you're a manufacturer and buy an ABS/ESP system from one of the usual suspects (Bosch, Conti etc.), they'll charge you a six to seven figure sum for setting it up for a specific car model.


    Attached you find a slip/friction coefficient curve for a tire, first one I found in a quick Google search (for the record, I take offense in the inadequate axis labeling). The friction coefficient, i.e. the force the tire can take, first rises with slip until a maximum is reached, and then decreases again (=tire starts to lock). The number I know is that the maximum is typically at 10% slip, but 7% sounds good too.
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  5. Fast_ian

    Fast_ian Two Time F1 World Champ

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    Fair enough, thanks for the insight. I thought, mistakenly it would seem, that at least ABS was pretty much a 'utility' thing these days. (ESP is a different can of worms of course). You seem to be implying that changing, eg, tire compound or even wheel weight could somehow mess it up?

    As I said, I thought it was pretty 'simple' engineering these days; wheel sensor(s) detect lock up and the computer cuts the pressure in the circuit. If I'm running 'sticky' tires that lock up may come later, or it's wet, sooner, but the ABS doesn't give a damn, surely?

    I know you've commented before about 'inadequate' labeling..... Offends your sensibilities! (Mine too come to that - Sloppy engineering ;))

    Anyway, it makes sense, but I would still think it can vary fairly significantly based on quite a few variables; tire compound, vehicle weight and initial speed all pop into mind.

    Cheers,
    Ian
     
  6. Todd308TR

    Todd308TR F1 Veteran
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    Who said we were talking about old drum brakes?
     
  7. Fast_ian

    Fast_ian Two Time F1 World Champ

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    Well, we weren't, but my point stands; there's many more variables in the equation than just tires and road surface.

    Cheers,
    Ian
     
  8. don_xvi

    don_xvi F1 Rookie

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    #33 don_xvi, Aug 17, 2014
    Last edited: Aug 17, 2014
    Fatbillybob,
    The dreaded "ice mode" wherein the driver of an R-compound tired street car feels that the ABS is isolating them out comes from the differences in the mu-slip curves (see plot above provided by Far Out) of street vs race tires.

    Racers are generally aware that race tires work at higher slip angles (and longitudinal slip %) than street tires. The peak of that curve may be at 7% or 8% for a street tire and higher (let's call it 10%+) for a race tire. This means that the race tire isn't even developing really good deceleration force before it hits the slip level at which the ABS is tuned to try and recover the wheel by dumping brake pressure.

    Because it's hard to come back after you cross over the peak of that mu-slip curve, the system has to dump a lot of pressure, fast, to recover when it thinks it has gone over that hump (programmed at 7%). Combine that with other inputs that are often incongruous, and the ABS is prone to not work well at all, to the extent that it may actually fault out due to loss of confidence in the inputs.

    Like any job, it's a lot harder than it looks from the outside...


    * These are my own thoughts and do not reflect the official position of my employer. *
     
  9. don_xvi

    don_xvi F1 Rookie

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    Yes. More than once the "big tire/wheel" package on a production vehicle has required unique tuning. Once you replace the tires with something different, there's no guarantee that these slip control systems will still meet the performance they left the factory with, but within the range of a similar replacement tire, the expectation is that systems are robust enough to still provide adequate performance.

    Making a large change in tire characteristics, like large diameter/weight increases or drastic compound changes can lead to significant performance degradation that cannot be predicted by the manufacturer.

    * These are my own thoughts and do not reflect the official position of my employer. *
     
  10. andyww

    andyww F1 Rookie

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    Thats how aircraft brakes work. Although there are more than 3 discs used.
     
  11. Fast_ian

    Fast_ian Two Time F1 World Champ

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    Fair enough. "Learn something everyday!" Thanks.

    My simple mind figured they "simply" detected the wheel speed reach zero (while there was still vehicle motion of course!) and reduced the brake pressure.

    Good to know they're (a lot more) sophisticated than that.

    Again, thanks for the insights guys,
    Cheers,
    Ian
     
  12. forgeahead

    forgeahead F1 Rookie
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    Isn't somebody working on non friction magnetic braking systems?
     
  13. DoubleD33

    DoubleD33 Formula 3
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    A lot of good info here. I did not realize all of this went into the design of a braking system. Thanks I have truly learned something.
     
  14. 412fan

    412fan Karting

    Aug 1, 2005
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    Those already exist (eddy current brakes) and are used on some trains, for instance.
     

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