Big Brakes

[Larry C Leung]

To simplify things, the bigger brakes can increase the RATE of the
conversion of kinetic energy to heat, so POTENTIALLY, bigger brakes can
stop a car faster. BUT, the limit is the actual frictional force that can
be created at the tire/road interface. Alex was not speaking of locked
wheel braking, in which the heat generated is mostly between the tire and
the road, and then little brakes will do just fine, but of threshold
braking, where there is essentially a static (but highly stressed)
surface interface between tire and road. 

This is the theoretical limit to the stopping rate of ANY braking system,
i.e. ANY system CAPABLE of locking up the wheels should theoretically be
able to produce the same stopping force by modulation to the traction
threshold. Better systems, by being stiffer and therefore more
mechanically predictable, are easier to hold at this limit by either
driver or ABS system.

Now Bigger Brakes can dissapate the heat faster (not really, it just gets
spread around more) hence they can keep up the RATE of braking longer
without fade.

Generally, this wouldn't make a significant difference from one stop,
unless from VERY high speeds (say 100 to 120 MPH or more), however high
speed track or spirited driving with heavy braking in rapid succession,
this becomes more important. Remember the equation for Impulse, which is
the measure of the ability (in this case) to change speed, the bigger the
impulse, the greater the change, the shorter the stopping distance:

I = F(t) where F = force, t = time, I = Impulse

IF a smaller system is able to maintain F (that is, not fade ) compared
to the bigger units, then a bigger unit will not shorten stopping times,
nor distance. If there is a difference in Fade resistance (due to the
inablility to get rid of enough heat to prevent : a) pad outgassing,
which shouldn't occur if the pads are properly bedded, and if it DOES
occur, there will be essentially no useful pad left) 
and/or b: fluid boiling, gasses are compressible, so force at the brake
pedal just goes into compressing the gas, not into forcing the pads
against the rotors (or shoes/drums), THEN the bigger brakes win. 

It takes fairly severe circumstances to reach this condition, generally
full track speeds (i.e., speeds over 100 MPH (160 KPH)) or sprited
driving in heavy traffic (similar speeds, lots of slowing for rolling
chicanes) to produce such conditions in a typical street car. Bigger
brakes are a help in these circumstances, otherwise they are mostly for
show. Incidentally, even the fastest Autocross courses rarely produce
significant fade for most streetable cars. They can't accelerate fast
enough in the short chutes to get enough speed to get enough heat to
produce fade. If you've picked up the Grassroots Motorsports brake tech
issue, they give hints on pad materials that CAN avoid problem a: for a
stock system for most peoples typical track days. Not pads you'd want on
the street mind you, but doable. 

Finally, the comment on 13" brakes on an F1 car. Keep in mind, by the
rules, F1 cars must be at a minimum of 800 kg (or is that 850 kg?) or
about 1760 to 1870 lbs. They don't NEED as much force to produce the same
stopping distances as our cars, AND they access to brake materials that
can handle MUCH higher temps than our systems, and you can bet that they
always run AT the weight minimum, not a gram more!

LL - NY

On Sun, 2 Sep 2001 10:41:01 -0500 Unka Bart <gatorojo at earthlink.net>
writes:
>Holas, Amigas y Amigos
>
>A minor contribution from DEBUNKMAN (aka, yer kindly ol' Unka Bart) on 
>this
>subject.
>
>>-----Original Message-----
>>Reply-To: "Alexander van Gerbig" <Audi_80 at msn.com>
>>Subject: Re: Big Brakes
>>Date: Fri, 31 Aug 2001 03:28:18 -0400
>>
>>    It's all about locking those tires up.
>
>No offense, Alex ol' Chum, but it ain't about that at all.  It's all 
>about
>changing kinetic energy to heat, and bigger brakes are a dandy way to 
>do
>that.  Locking those tires up is the worst way to stop that there is,
>unless one includes the option of not (seriously) braking at all.
>
>In fact, unless the laws of thermodymanics (like nearly everything 
>else in
>the world) have changed dramatically since I studied them at COL 
>Thayer's
>Institute for Wayward Youth in my Cow year in 1959; in a moving 
>automoble,
>the "braking force" varies as a function of the coefficient of 
>friction
>(all other things being equal) between the contact patch of the tire 
>and
>the road.  And the simple, if counter intuitive, fact is that the
>coefficient of friction between a rolling tire and the pavement is
>significantly greater than the coefficient of friction for the same 
>tire,
>when sliding.  Optimum breaking is achieved when the tires are *NOT* 
>locked
>up, but are on the threshold of being so.
>
>Please note the "all other things being equal" caveat.  A larger 
>contact
>patch helps increase braking force  as does a larger heat sink 
>(bigger
>brakes) into which to dump additional heat (more thermodymanic 
>engergy
>converted from kinetic energy).  Either will improve both stopping 
>time and
>distance, both will improve things even more.
>
>Being a (62 year old) newly (34 days) married man means that I don't 
>read
>the digests as often or in as much detail as I otter, but this is the 
>first
>time I've had a chance to jump in on a thread in quite a while.  And 
>having
>a much younger, Red-Headed bride means both that Pharmaceutical sales 
>are
>up in direct relation to the decline in my time available for
>internet-related (or any other non-spousal) activities.
>
>BTW, she and I hope to see as many of you as we can at the Audi corral 
>at
>Road Atlanta for the Petit LeMans in October.  Look for the big ol'
>Ex-Grayhound, aka, the dirt-boat "Gatorojo."
>
>Bart
>
>
>