Ok
this is a continuation of this topic pub1.ezboard.com/fultimat...1321.topic
about brakes.
OK...I'm sorry, but what they have there is WRONG!
www.kvrperformance.com/newpage4.htm
www.kvrperformance.com/newpage5.htm
CROSS-DRILLED ROTOR OVERVIEW
You buy cross-drilled or slotted rotors for performance right? Well
they say, "Cross Drilled Discs will last up to twice as long as O.E.M.
rotors (depending on your braking style)." That little parenthetical
statement should give you a clue! Most people that are going to buy
cross-drilled rotors are going to buy them because they drive harder,
demand more out of their braking system, hence a more aggressive braking
style. I warped my Brembro cross-drilled rotors in about 2 years....
and I was in school...my car sat most of the time.
I agree with these two statements, within a certain extent "40% Better
Cooling, 20% better stopping; Improved Wet Braking"
This statement is very misleading, "Lower Brake Temperature Reduces
Rotor Warpage” Yes your normal operating brake temperature may
be lower, and yes your rotors may cool a little faster, however lower
operating temperature and cooling speed aren't major players in warpage.
Actually, cooling something faster will warp it faster then a gradual
cool down.
Again...very misleading, "Less Brake Fade and Longer Life" Yes they
reduce brake fade, by keeping operating temperatures down. However brake
fade is more attributed to the compound material on the brake pad itself.
Same rotor with different pad material (ex. organic, and semi-metallic
or carbon metallic) will exhibit totally different temperature range,
braking performance, and life expectancy. The reason OEM brakes fade
like hell is the pad compound, it's organic. I could nuke a set of organic
pads in no time flat...cause their level of heat tolerance is so much
lower than a semi or carbon metallic pad. However they are quieter,
and since Joe Public doesn't give a shit about how their car performs,
they just want it to go from point A to B, it don't matter. However
for the few that want better performance you need to step up to a better
pad.
GAS SLOTTED ROTOR OVERVIEW
Ok before I look at what they said, one thing should be noted. Look
at how one sided their two pages are for cross-drilled rotors...hmmmm.
Ok first thing they say "Enhanced Initial Bite" Sure this gives you
may get a stronger "BITE" from the slot, BUT that is NOT what it is
there to DO!! The slots on the rotors are to vent the build up of gasses
produced in the interface between the rotor and the brake pad material!
That is really all they're meant to do. Some brake pads have a slot
down the middle, this slot also helps release those gasses. Main thing
is, if you have gasses between the rotor and the pad, you loose braking
performance, you do not have as solid a direct contact between the two.
"Intended For Track Use" I can say the same for cross-drilled, this
statement is TOTALLY OPINION and should not be on a page comparing slotted
and cross-drilled!
I'll agree with this "Better Cleaning of Pad Friction Material" Yes
over time the pad material will get a glazed surface on it. The slots
will help wipe this glaze away, or keep it from forming. However...this
is trivial.
Again....more misleading information, "Slotted discs offer cleaning
of the friction material (brake pads), but do little in terms of additional
heat dissipation. Slotted brake discs do not cool better than cross-drilled
discs or even standard discs. The face grooves will slice the brake
pad material allowing the pad to bite harder into the disc, therefore
causing an increase in disc temperatures. This is recommended for competition
vehicles to bring pads and disc temperatures up to optimal operating
ranges. (Race cars warm-up engines, tires and brakes for the best possible
performance)."
The thing they are pushing in this paragraph is how much better the
cooling is with cross-drilled rotors, and that slotted add heat. The
main thing I find very misleading is, with ANY rotor-pad combo, you
will have heat generation from driving...even if you DO NOT touch the
brakes AT ALL. Rotors drag against the pads. You will get heat build
up. Your rotors and brakes are designed to work better with a little
heat in them. Ever drive you car first thing in the morning on a cold
day. The brakes aren't too good when they are cold, so why do you need
to excessively cool these rotors that are going to generate heat anyway?
(Exception is organic pads...they work decent when cold, but better
when a little warmer)
MAJOR POINTS MISSED BY THESE TWO ARTICLES
www.kvrperformance.com/newpage4.htm
www.kvrperformance.com/newpage5.htm
There are several key points that they failed to mention, rotor warpage
and the main cause of it. The main cause of warpage in any material,
object, whatever is heating it beyond a certain point where plastic
deformation occurs, now there’s really no force being put on the
rotor that it can’t handle, our force is heat, so think of heat
being a force and there is a curve that this material will follow on
when it is heated and cooled. Think of warpage when the material is
pushed past a certain point along this curve.
Also cyclic loading of a material will also cause failure, or warpage
in our case. Heating up and cooling of a rotor is our cyclic loading.
Again, the more rapidly you do this, the greater your chances are for
warpage. So if you go drive like a crazy person and then park your car
the rotors will cool faster then if you drove nicely around for a couple
miles to slowly bring them down in temperature.
That brings me to my next point, When you drive your car hard, and park
it, the part of the rotor which is exposed to air cools at a different
rate then the part that is touching the pad material. The pad material
will retain heat a lot longer then the air, so you will get a spot on
the rotor where the pad sat that will be warped. A friend of mine who
works in motor sports, Indycar to be exact, told me that during a practice
session they would go through rotors, i.e. warp them faster then they
would if they were in a race. The main reason, like I said above, is
the cyclic cooling of the rotors from really hot to cold, and the “hot
spot” from the brake pad.
The main point or problem with what they say about warpage has to do
with something they talk about, heat dissipation. They say that they
have increased heat dissipation with cross-drilled rotors. This is partially
true. At lower to medium temperatures the holes in the rotor allow more
surface area, and as any good mechanical engineer or thermal scientist
knows, you can achieve better cooling with more surface area. The holes
in the rotor sort of act like the fins in your radiator, they aid in
cooling the rotor. However those holes are a double-edged sword. They
actually do more damage then they are supposed to help. When you cross
drill a rotor, you take material out of it. Well guess what, that material
is what helps that rotor maintain a certain level of performance before
it warps. The more material you have the better heat distribution you
get through the object. Think of it as I said before, a curve that this
rotor follows when it is heated and cooled, and there is a certain plateau
or ceiling where warpage occurs. When you remove material from the rotor,
you decrease this ceiling. So it actually takes less heat to warp the
rotor then it did before you started drilling holes in it. So if you
look at the cooling by the holes in comparison to the loss of heat dissipation
or heat tolerance that the rotor can handle, you have a balance scale.
What is more important? Well In my book, I would want that ceiling to
be as high as it could be to ward off the effects of warpage. Who cares
if you rotor runs slightly cooler for normal use, and may cool slightly
faster. The main thing they were saying was that cross-drilled reduces
brake fade. This is 1/10 correct, better pads reduce brake fade.
Another thing, heat flows through the rotor as it heats up. With a normal
rotor this heat is evenly dispersed and expelled. With the holes in
cross-drilled rotors you get an interruption in this heat flow through
the rotor. You again get hot spots or points where the material is discontinuous,
and with these discontinuous points you get added stresses or higher
temperatures at these discontinuities. So if you would look at a thermal
image of a normal rotor you would see an evenly spaced out heat flow
in the rotor. With cross-drilled you will see cold and hot spots in
certain areas of the rotor. Temperature is a cyclic force and over time,
with these hot spots occurring around the holes you end up with cracks,
where the material has failed.
One last thing, there is actually a formula to calculate how much material
can safely be removed from a rotor without compromising it’s ability
to dissipate the heat generated in it. So with larger rotors you can
have more or the same amount of holes and it don’t matter, there
is plenty of material there. But with smaller rotors there is less material
so you either need fewer holes to maintain a safe amount of material
to dissipate the heat or you will lower the “ceiling” at
which the rotor will warp.
CONCLUSION
Ok to sum everything up. Cross-drilled rotors are good for fast heat
dissipation and reduction in braking gasses, however they are prone
to warpage because of their less amount of material, and lower peak
temperature tolerance. They are good for racing applications where you
need very fast cooling from high-speed stops, and where they don’t
care about the longevity of the rotors. They are not practical if you
want to get more life out of your rotor. Slotted rotors main advantage
is that they help get rid of the braking gasses between the rotor and
pad. They are good for mild to medium racing applications and for the
performance minded street driver. The longevity will be greater then
that of cross-drilled, yet may be a little less then stock. There are
also high performance rotors that offer a combination of both slotting
and cross-drilling.
I guess now you may be asking what gives him the authority or background
to be saying all this. Well first off I have gone through the hassles
of cross-drilled rotors myself, and had the down sides of cross-drilling
happen to me. Secondly I have researched and found NON-BIAS articles
and information on the two types of rotors, as well as consulted several
automotive professionals on the topic. Next I am a mechanical engineering
major at Penn State, and have had courses in the areas I have talked
about, the stresses, cyclic loading, temperature, discontinuities, and
heat transfer. So I’m not just talking out of my ass on these
things. And finally, like other performance-minded drivers I too want
to get the most out of my car, and have looked into the different possibilities
and options.
If anyone has any questions comments, feel free to email me.
