Article: 78031 of rec.autos.tech
From: jet@sun.soe.clarkson.edu (Jeff Thompson)
Newsgroups: rec.autos.tech
Subject: Re: Cross-drilled rotors
Date: 19 May 1995 13:13:51 GMT
Organization: Clarkson University
Lines: 47

Matthew E. O'Toole (motcm@ix.netcom.com) wrote:
: If drilled properly (like swiss cheese), the extra surface area does
: increase cooling.  Also, the reduced mass has less total heat capacity,
: and so will cool quicker as well as heat up to racing-brake-pad
: temperature more easily.
: The "grinding in" you mention is partially true: the holes (or slots)
: are there to evacuate a layer of gas that builds up between the pad and
: rotor.  Giving this gas someplace to go allows better contact and
: friction between pad and rotor.

   The reasons that rotors are cross drilled are (in order of
importance)

   1.  More leading edge area.  Since the pad deforms microscopically
	   when the brakes are applied, a series of holes will increase
	   the effective friction coefficient.  *Nifty home experiment*
	   Run your hand down the front of your refrigerator, then run
	   your hand down the front of a cheese grater.  That's how 
	   cross-drilling works.  More leading edge area is also why
	   some brake pads come with a slot machined into them.

   2.  Reduction of unsprung mass.   Cross-drilling can reduce the
	   wieght of a rotor significantly, which leads to less un-
	   sprung mass.  Less unsprung mass leads to quicker reaction
	   to handling transients because of the reduced inertia
	   of the parts.

   3.  Evacuation of water in wet conditions.  It is much easier to
	   push water into the holes than it is to wait for the steam 
	   to vent itself out.

   4.  Better cooling.  In most serious performance applications you
	   will not have to worry about your brakes getting hot enough.
	   You will have to worry about the cooling though.  This is not
	   a big consideration in cross-drilling because the same thing
	   can be acccomplished with bigger vented rotors or additional
	   air ducting.  The IROC Daytonas have special ducting and a
	   water injection system that sprays water on the brakes to 
	   cool them.  Alternately, you can go with carbon-metallic 
	   rotors and pads and not worry about it at all ;)

   5.  Evacuation of gasses produced by braking.  The amount of gas
	   produced when braking is really negligible compared to the
	   other factors.

Hope that clears things up,
Jeff	jet@sun.soe.clarkson.edu


Article: 78040 of rec.autos.tech
From: motcm@ix.netcom.com (Matthew E. O'Toole)
Newsgroups: rec.autos.tech
Subject: Re: Cross-drilled rotors
Date: 21 May 1995 07:15:49 GMT
Organization: Netcom
Lines: 82

In <3pi5if$h62@library.erc.clarkson.edu> jet@sun.soe.clarkson.edu (Jeff
Thompson) writes: 
>
>Matthew E. O'Toole (motcm@ix.netcom.com) wrote:
>: If drilled properly (like swiss cheese), the extra surface area does
>: increase cooling.  Also, the reduced mass has less total heat
capacity,
>: and so will cool quicker as well as heat up to racing-brake-pad
>: temperature more easily.
>: The "grinding in" you mention is partially true: the holes (or
slots)
>: are there to evacuate a layer of gas that builds up between the pad
and
>: rotor.  Giving this gas someplace to go allows better contact and
>: friction between pad and rotor.
>
>   The reasons that rotors are cross drilled are (in order of
>importance)
>
>   1.  More leading edge area.  Since the pad deforms microscopically
>	   when the brakes are applied, a series of holes will increase
>	   the effective friction coefficient.  *Nifty home experiment*
>	   Run your hand down the front of your refrigerator, then run
>	   your hand down the front of a cheese grater.  That's how 
>	   cross-drilling works.  More leading edge area is also why
>	   some brake pads come with a slot machined into them.

The Ferodo people told me (10 years ago!) that outgassing was a much
bigger deal than the "cheese grater effect", which does exist.

>   2.  Reduction of unsprung mass.   Cross-drilling can reduce the
>	   wieght of a rotor significantly, which leads to less un-
>	   sprung mass.  Less unsprung mass leads to quicker reaction
>	   to handling transients because of the reduced inertia
>	   of the parts.

Absolutely!  Especially on a race car, where the brake rotor is by far
the heaviest single component of the unsprung mass (next to the wheel
and tire).

>   3.  Evacuation of water in wet conditions.  It is much easier to
>	   push water into the holes than it is to wait for the steam 
>	   to vent itself out.

That falls into the outgassing category, I think.

>   4.  Better cooling.  In most serious performance applications you
>	   will not have to worry about your brakes getting hot enough.

Not always true!  Autocrossing is one common example where a critical
braking area (in terms of lap times) appears at a time when brakes are
too cold for the pad material being used to work properly.  Other
similar situations do occur in road racing occasionally.  Sometimes you
give up a little everywhere else, so things are working well at a
certain critical corner.  I've run into the brake thing in IMSA GTU
endurance racing.

>	   You will have to worry about the cooling though.  This is not
>	   a big consideration in cross-drilling because the same thing
>	   can be acccomplished with bigger vented rotors or additional
>	   air ducting.  The IROC Daytonas have special ducting and a
>	   water injection system that sprays water on the brakes to 
>	   cool them.  Alternately, you can go with carbon-metallic 
>	   rotors and pads and not worry about it at all ;)

If you really have money to spend, carbon-carbon rotors are the state
of the art.  That's right, carbon fibers in a carbon matrix!  Really
light, too.

>   5.  Evacuation of gasses produced by braking.  The amount of gas
>	   produced when braking is really negligible compared to the
>	   other factors.

Not according to the Ferodo guy!  He said this really mattered, even on
a street driven vehicle.

Cheers!
Matt O.

>Hope that clears things up,
>Jeff	jet@sun.soe.clarkson.edu