cross drilled rotors [Round Two]

[jason@scirocco.org (16V Jason)]

At 09:06 PM 3/23/2002, Brian McGarvey wrote:

>In our cars and most other cars out there we experience brake fade because
>the fluid in our brake systems becomes very hot. THis inturn heats up the
>rubber elements in our systems. Agruing about which brake master clyn to
>use for what pressure required is pointless at this point.


Uh, Brian, all that is true and all, but the largest contributing factor to 
fade is indeed not stretch in the brake lines due to heat, it's the loss of 
the brake pad's coefficient of friction at high temperatures.


>So now here is when you want to upgrade to Stainless steel braided lines.
>basically all you are doing is removing the baloons at each wheel.

Yes, which will likely stiffen up the pedal -- but roast the pads, and 
it'll be stiff but useless.


>Nw the little problem of effieciency(sp) comes in.
>For the same amount of work (moved fluid) having a longer lever arm
>provides more torque.

That is very true -- but that brings up a great problem, which is the 
reason I'm jumping in here.

So, we are all agreed that having a larger rotor with a caliper positioned 
further out from the center of the hub would, all other things being equal, 
provide more braking per unit of pressure applied to the pad.  The two 
brakes will generate the same amount of heat by the way, regardless of how 
far the caliper is from the center of rotation, since they're both stopping 
the same mass.  We're also agreed that, again all other things being equal, 
the larger rotor and caliper would be able to absorb and dissipate this 
heat better than the smaller.  Agreed.

But look what you just did -- you increased the braking on the front end of 
your car for every pedal pressure.  You helped to eliminate brake fade on 
that axle as well helping ensure that this increase in braking force 
continues even at high temperatures.

But you didn't do shit to the rear axle, did you?

So what you did was tilt the car's front/rear brake bias further towards 
the front.  You just *increased* your braking distance.  Say before your 
brakes were proportioned "ideally", meaning that you were able to easily 
hold all 4 wheels at impending lockup.  Now, you'll be able to keep the 
fronts at impending lockup, but not your rears.  Regardless of how badly 
your brakes were proportioned in the first place (unless you had a 
rear-bias, which you don't or you'd be dead), you have just increased the 
maximum braking distance of your car.  Sorry, that 11" brake conversion 
just cost you your insurance deductible.

OH!  Wait!!  Something else just popped up!  Since we all know that you 
will get your best braking at impending lockup (hence the reason 
ABS-equipped cars that keep all 4 wheels at/near impending lockup under any 
conditions stop more quickly almost all the time), we know that we have to 
modulate our brakes to keep them on the edge.  But since the pressure 
exerted on your brake pedal is now amplified more, modulation becomes even 
more difficult:  A smaller difference in pressure on the pedal now defines 
the difference between impending lockup (100% braking ability) and full 
lockup (only about 80% of full braking ability).

So not only did you increase your theoretical minimum stopping distances, 
you also made it harder to reach that theoretical minimum.

And at the same time, you increased unsprung and unspring-reciprocating 
weight.  That means your car is now slower, handles worse, and doesn't 
brake as well.  COOL!

:)
Jason



modulation



>DOnt belive me. What about your front wheel axle
>nuts. while brakeing does turn KE into heat there are more effienect ways
>to do it so you get less heat. (you thermo ppl can help here). (my
>connection is startign to suck again so typos will start up again). SO if
>you still have the problem of you want to stop quicker and your tires are
>NOT sliding if you are sliding.. get BETTER tires or stop pressing on the
>brakes so hard. as the tire starts to slide your coefficent of friction
>goes waaaay doen its very inefficeint. a sliding stop takes as much as 50%
>longer to stop in as a non sliding stop all things being equal.. thats why
>new cars and some motorcycles have ABS brakes. . increasing diameter adn
>the swept area remaining constant will result in less heat or equal heat
>generated. Increasing the swept area
>will integrate the heat over a larger area overall reducing the average
>temperature.
>
>Holes and slots. <-- in some extreme conditions they may actually help.
>but getting larger dia disks with holes or slots is still better than
>smaller ones. (more swept area lewer temp not less heat).
>
>tired of hearing this argument... rage for days.
>basically slots and drilled rotors unless you are RACING for long periods
>of time or in extreme conditions (mud, snow, rain, desert) it deos do
>shit. So there.
>no vented vs. not vented is an easy one. more Surface area  means more
>convection. means lower temperature NOT lower heat. you basically dOuble
>the surface area and increas the mass of the rotor so yes that is a good
>upgrade. but if you drive like my wife or an old lady ... dont bother.
>unless you wanna look cool.....or they come on the car already installed
>by the PO... <-- my case!
>
>brian
>2.0L Topless bunny
>Brakes.
>Stock booster -- bad sticky brake pedal syndrome manual return often
>required
>Stock master clyn new
>10.1's in the front zimmerman xdrills PO of 16v bought them
>drums in the rear
>one new wheel clyn in the rear.
>
>
>On Sat, 23 Mar 2002, Brett Van Sprewenburg wrote:
>
> > >Bigger rotors do give the caliper much more leverage to work against
> > >the spinning wheel. You can easily set up your own experiment with a
> > >spinning bike wheel and see what I mean. What do you think is easier
> > >to grab and stop, near the hub or near the rim? Also the rotor and
> > >pad material used in F1 is so exotic we cant even begin to relate a
> > >tenth of the physics going on there and apply it to our street cars.
> >
> > I'm going to scan the article now, as I just can't stand this
> > anymore..The physics of
> > turning kinetic energy into heat works exactly the same between our
> > cars and an F1 car.  You are of course correct that
> > they can use exotic materials to increase their heat handling and
> > transfer...you are also correct that it's easier to stop
> > something spinning near the rim, then near the center.  Here's how to
> > generate the same effect with a smaller rotor:
> >
> > Press On The Brake Pedal Harder.  No kidding. :)  In other words
> > create higher clamping load, which may generate more
> > heat, but then we're back into the heat issues previous discussed.
> >
> > Once you reach the point of tire lock up (and if you cannot, then you
> > DO need brake work) you will NOT generate any
> > more braking force, no matter how far 'out' your caliper is away from
> > the hub.  Just physics.
> >
> > >
> > >While watching some Tarmac rally last year I remember in a tech
> > >sidebar they were talking about putting the largest brakes they
> > >could fit under the wheel in order to cope with the extra braking
> > >needed. I put more stake in a WRC engineer than a guy racing most
> > >anything in the US let alone a Saturn.
> > >
> > >Jasin
> > >one cents worth
> >
> > That 'guy' racing a Saturn is (as I wrote) an engineer creating
> > braking systems for the Robert
> > Bosch corporation (we've all probably heard of them) and I only threw
> > out the fact he was
> > also a racer as a point to his practical experience.
> >
> > Yes, this is info on how braking actually works can be suprising and
> > offensive, as it upsets
> > several beliefs and notions we (including myself) have had...
> >
> > ==Brett
> >
> >  \/  '84 Scirocco (ITB racer 2B) | "Hot VW's, take two home. They're small"
> > \/\/ '88 Scirocco 16v (Show), '92 Passat 16v (Winter+) | - brett@netacc.net
> >
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