Re: brakes, bigger is better?

[John Karasaki <johkar@teleport.com>]

I did NOT author this.  Rather, with all the brake bench engineering that
has gone on lately on the list, I thought this was a totally different view
that hasn't been discussed...


>X-From_: racing-admin@porschelist.org  Tue Jun 16 12:40:45 1998
>Date: Tue, 16 Jun 1998 12:42:27 -0700
>From: Derrek Khajavi <dkhajavi@flash.net>
>Organization: Huntley Racing
>To: "Racing" <racing@porschelist.org>
>CC: Racing <racing@porschelist.org>
>Subject: Re: brakes, bigger is better?
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>Anyone who puts a three decker wing on their stock 1.7L 914 because it looks
>faster should hit 'next' message.  I have caught a lot of static on this
product
>because it goes away from the conventional theory on brake upgrades even
though
>it has been proven in countless hours of testing in everything from Semi's
to my
>humble 951 race car.  If you can lock up all four wheels on the track, are
not
>able to push the pedal to the floor when the brakes are cool, and like the
>modulation of the brake pedal you do not need 'big reds'!  The reasons people
>change to big reds are plentiful but only one is actually good the rest
are bad.
>The good thing is that the greater mass of the rotor and caliper/pad help to
>dissipate heat (actually a lot of heat).  Big reds are phenomenal brakes.
They
>also add significant amounts of sprung weight and obviously expense.  The
problem
>is heat not insufficient braking force.  What are you going to to lock up the
>brakes more, faster, harder?  When your stock brakes build up heat they
transfer
>it to your brake fluid.  Depending on how much you spent on that fluid
determines
>when it will begin to boil.  When it boils small bubbles form which
displace the
>fluid and give you the dreaded soft pedal or if not remedied the feeling
you get
>at the end of the strait when you feel the pedal hit the floor (hope there is
>plenty of run-off!).  We (Huntley Racing) did not invent this but we did
>re-engineer it to work in Porsches.  We call it a brake re-circulator.
What we
>do is convert the bleed nipples on your calipers to return lines and
install our
>recirculator near the master cylinder.  Every time you press the brake
pedal the
>brakes act as normal.  When you relece the pedal that action pulls the brake
>fluid in your calipers out to the master cylinder.  The fluid that is being
>heated up in the caliper never has the opportunity to get hot enough to boil.
>Also any bubbles in the system are 'auto-bled' as well.  Now if you did not
>listen to my first line you are probably thinking about how cool the 'big
reds'
>look and how uncool the stokers look, well I can sell you the recirculator
for
>$500 and a can of red spray paint for $1500 to make up the difference of
what it
>will cost you to buy the 'big reds'.
>
>Derrek Khajavi
>Huntley Racing
>(619) 287-9300
>
>
>
>
>wrote:
>
>>  Walker Aumann <walkera@gg.caltech.edu> wrote:
>>
>> >From what I understand, the point of larger brakes is that they can
>> >handle more heat.  They will also likely cause the pedal to be more
>> >sensitive, but the main reason behind getting larger is to take care of
>> >the extra heat generated as you start going faster.  For street and
>> >autocrosses, you shouldn't need to worry about larger brakes as long as
>> >you can reliably lock up the wheels.  For track use, the brake system
>> >tends to build up a certain amount of heat each lap, and the amount of
>> >heat it can dissipate each lap depends on a number of factors
>>
>> [snipped for brevity]
>>
>> I am following this thread with great interest, as I need to improve the
>> braking on my early 911S.  I ran it in 4 autocrosses and never had any
>> problem, but in my first time trial I managed to overheat the brakes after
>> about the third 20-minute practice session of continuous lapping.  The
>> pedal started to grab lower and the brakes had less stopping power, so I am
>> assuming that I reached the heat limit of both the pads and the fluid about
>> the same time.  I drove off the course and back to the pits without
>> incident and bled the brakes twice, but couldn't get the pedal all the way
>> back to it's original height/firmness.  I still had some brakes, and
>> managed to finish the timed runs by being very ginger on them, but needless
>> to say, it did not help my confidence in the car.
>>
>> Surprisingly, after driving it home and bleeding the brakes again the next
>> day, the pedal was back to it's original feel/height (which was a bit too
>> low to begin with, admittedly).  I have the stock "M" cast iron calipers
>> with fairly new (1500 mi.) OEM "street" pads and new stock rubber hoses.  I
>> had just upgraded the master cylinder to the later dual-circuit model and
>> changed the fluid to Ate Super Blue a month before the event, to try to
>> gain a little pedal height and firmness. Although this helped a bit and it
>> was adequate for autocross, I was concerned that a TT would over-stress the
>> brakes, and it did, even with the improvements.
>>
>> I can't afford to go whole-hog with a major brake upgrade at this time
>> (like putting later S or SC brakes on it), so what should I do to improve
>> what I've got?  My first thoughts are to get more cooling to the rotors, by
>> ducting air from the front directly to them.  Also, I probably should have
>> changed the fluid the day before the race, not a month before, perhaps?
>> Would fresh fluid have prevented the fade I experienced?
>>
>> Another lower-budget improvement I am considering is to rebuild the caliper
>> pistons, as I think I had one on the left front dragging a little, which
>> may have contributed to heat buildup. To get more pedal height to begin
>> with, I guess I have to replace the rotors, which are a bit worn (but
>> smooth, still), and perhaps go to a different compound pad? There's no
>> other adjustment possible, other than the pedal free play at the MC, which
>> is set at the 1mm spec, right?  The book says the brakes are
>> "self-adjusting".  The seal actually rotates and pulls the piston away from
>> the rotor when hydraulic pressure is released, eh?
>>
>> I would appreciate any input on this from the racers who have already
BTDT...
>> Thanks in advance,
>> Tom Tweed
>> La Jolla, CA
>> (fearlessly crossposting to two lists on the new PorscheList server)
>>
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