On 2009-06-24, wrote:
Ben C? sniped:

[...]

Holes through the disk increase turbulence. Greater turbulence
increases heat, mass, and momentum transfer. Correlations for
turbulent transfer typically include the effect of surface
roughness -- sharp edged holes are the equivalent of a very rough
surface. Even though there may be no net flow through the holes,
there will be enough flow in and out of them to cause the air
inside the holes to quickly approach the conditions just outside.

Improved heat transfer through increased turbulence may well help
to cool the disk.

Also, the contents of the holes provide a bulk flow of air past
the *pad*. Without holes, when not braking, there is only a
narrow space for air to flow past the pad, which makes convection
cooling very slow. When braking there is no space for air to flow
and cooling can occur only by conduction, to the disk or to the
caliper. With holes there is significant flow of air past the
pad, whether braking or not. The air inside the holes will be
well-mixed, and quickly approach equilibrium with the surface of
the pad. Once past the pad, air in the holes will be exchanged
with the free stream of air flowing past the disk, thus cooling
the pad.

The above is just intuition on my part, I have neither calculated
nor experimentally verified any of it, nor do I have any
experience designing brakes.

This is beginning to sound like a "stress relieving" thread. If
the effects are so scientifically definable, why are most disks of
disk brakes not cross-drilled?

Most road cars in normal use don't have such big problems with brake
cooling, and the holes have side-effects the normal user doesn't
want like faster-wearing pads and possibly cracked disks in what a
normal user would consider a low mileage.

What means: "Such a problem" on roads whose legal limit is 70mph?
These cars are not racing, at least not legally.

Actually in my experience you get brake fade from repeatedly braking
hard into tight corners without long straights in between for them to
cool down.

In a road car, fast driving on twisty roads such as when doing a road
rally can overheat the brakes. Speed probably never goes above 70.

I think your claim of effectiveness of the puny suggestions of
aerodynamic devices on the rear of production sedans, or even sports
cars, needs some supporting data.

I don't have any data, but you can google for it. Here's a
link for example:

http://www.automobilereviews.com/audi/tt_2000.html

Note that there is no speed limit on many German autobahns-- you only
got this aero problem at about 125mph according to that article.

[...]
I think you will find that civilian sedans formerly crashed from brake
fade as trucks, still equipped with drum brakes still do judging from
warning road signs before steep descents in North America and Europe.
What you present is a compendium of auto enthusiast magazine lore.

Sure, drum brakes were nearly always worse. But disk brakes do still
fade (I've experienced it myself).

[...]
Some of those spoilers do do something even for road cars. Don't
know if you get the Audi TT in America, but a few of them spun off
the autobahn while changing lanes at 100mph because the back end of
the car experienced aerodynamic lift. The problem was solved by
adding a small spoiler.

Like the Porsche trunk lid that is automatically raised slightly at
speeds above about 70mph to affect an aerodynamic appearance.

Yeah, I never understood that. You want the picnic table deployed when
you're parked, not at 70+mph.

It's also a good way of advertising to plod that you're speeding.

You may not have noticed, but that part of the car is swathed in eddy
currents which deposit dust and tire spray on the car, for which it
has a windshield wiper.

If you believe in the merit of spoilers on sedans, you are susceptible
to much folk lore.

No, I do think most of them are cosmetic actually.