Re: Big Red Stuff 2

[Luis Marques <marques@rtis.ray.com>]

From: "Ken Keith" <ken.keith@impulsedata.net>
>
> So an increase in clamping force has no effect?  Assuming the 
> proportioning is correctly set.

I think we need to differentiate between "clamping force" and "clamping
ability".  The former refering to how hard the caliper squeezes the
rotors (piston size, etc.) and the later refering to how resistant it is
to flexing.

Having more "clamping force" won't do you much good except that you
won't have to press the brake pedal as hard, but it will go further
down.  Like switching to a smaller master cylinder.  The brake pedal is
lighter with more assist, but less feedback.  You still will only be
able to stop as hard as your tires will let you until the brakes 
overheat.  Brake distance might actually suffer if the rear brakes are
not working as hard as they should.

Now, "clamping ability" or the caliper's resistance to flex is very
important and you can never have enough caliper stiffness.  Standard
Brembo calipers (which so many people praise) flexed so much under
extreme braking that Porsche had them custom make the Big Reds with 50%
bigger bolts to take care of the problem.  Otherwise they would flex
under hard braking and give you a spongy pedal feel.  Kind of like
rubber brake hoses.  Also, your pads would wear unevenly and heat the
rotor unevenly.  OTOH, if your calipers don't have any significant flex
under the use that you give them, improvements will be marginal.

> I would have thought that it would depend on what the limiting factor
> is.  If the brakes can't dissipate the heat fast enough, then yes I
> agree.  But what if the caliper's clamping force/pad material/rotor
> material or surface doesn't generate enough the heat fast enough to
> slow down the car fast enough?

Right.  Ussually the limiting factor is thermal management.  You will be
able to stop once just as well with any kind of brakes - tires will be
the limiting factor.  But as the brakes get close to overheating
performance will degrade.  The heat you generate is a strict function of
how fast you are slowing down and for how long.  If heat is not being
generated fast enough then you need to press harder on the brake pedal
or (assuming you are standing on the brakes and the car still doesn't
slow down) get better pads with a higher coefficient of friction and/or
better fade characteristics.

> I thought that an increase in clamping force was also beneficial to 
> braking, but in a different way than big rotors, which is also a
> little  bit different than the benefits of slotting/cross-drilling.  
> No?

Being able to squeeze the rotors without flexing is very beneficial.  It
gives you great pedal feel and heats the rotors evenly.  But it's
completely useless if your rotors overheat and warp and/or your pads
fade.

> And if we're talking simply heat transfer, wouldn't a larger caliper
> or more piston surface area increase the amount of heat sink available
> to pull heat away from the rotor/pad contact area, and dissipate it
> into the air, and brake fluid?

Calipers most definitely help cool the brakes, but that is a secondary
function.  The primary functions are to squeeze the rotor and to isolate
the brake fluid from the heat.  Rotors (and wheels) take care of the
bulk of the cooling.  Brake fluid should never be called upon to cool
the brakes.  I guess my main point is that you may get slightly better
pedal feel and your brakes may be a little more resistant to fade, but
more likely it won't be any kind of night or day difference.  If you
want to make significant improvements to your brake system, then you
need to look at the whole package: tires, fluid, calipers, rotors, and
pads.  HTH

Luis Marques
'87 4kcsq