This is not meant to be a perfectly comprehensive write-up.
Instead, use this information as an informal, helpful guide,
but always remember: how you use this information is YOUR responsibility.
Golden Rule of Braking: You’ve Got to Compromise
most important aspect of braking performance is that compromises must
be made -- period. To improve on one area of performance, you must
trade off in another (or spend more money). This being true, it becomes
obvious there is no “best brake setup” out today. There is merely the
best setup for your application and needs.
said, let’s delve into the different factors that affect
the end of the article, you’ll also find a list of recommended
products in each of the above categories so you can choose
the right brake system upgrades for your needs.
PADS: The most important piece of the puzzle
many people overlook them, brake pads are the single most important
part of any braking system. Even with the most advanced big brake kit
available today, if you have a substandard pad for the application, the
whole system will end up not performing as well as a stock braking
system and a well chosen pad. The most important thing to look for when
choosing brake pads is one that will remain consistent over the
temperature range the pad is being used. Unfortunately, because
everyone uses their vehicle differently, this temperature range can
vary quite a bit. Hopefully, this guide will explain and outline what
you need to know to pick the right pads for your application.
preamble to brake pads: It’s hot, DAMN hot!
is the process by which the kinetic energy of the car is converted
to heat energy through the friction between the brake pads
and rotors, so heat production is unavoidable, and there is
no way to change the amount of heat produced by a brake system
by changing brake components. You can, however, reduce overall
temperature of the components through several means, including
cooling and larger components.
happens when a pad gets too hot:
constantly adding more heat, the temperatures of your rotor
and pad interface can rise to dangerous levels. If you exceed
the fade critical temperature of a brake pad (exceed its effective
temperature operating range), which is commonly termed “brake
the amount of braking force it can provide will steadily diminish as
the temperature increases. This will feel like your car wants to not
stop even with very hard pedal pressure. So the harder you use them to
stop, the more they “go away.” It is a vicious cycle, and needless to
say, quite dangerous.
during street driving: no problem
a general rule of thumb, any conditions that you see outside of a track
or road course environment will have the maximum temperatures below
600-800*F, so most street performance pads will be more than capable of
not fading even under “spirited” street driving. Going down a mountain
or steep canyon road is the only time on the street when your brakes
might get used near as much as on a track, so always be careful in
these situations if you don’t know how your pads are going to react.
important to realize, however, is that if you’re driving
your car almost exclusively on the street, you should base
your choice on a set of pads on the following:
lower temperature (street driving) performance
brake dust characteristics
initial bite & release characteristics
heat situations call for high heat brake pads – but
don’t use them on the street!
the other side of the temperature coin, pad compounds that exhibit
excellent high temperature performance usually do this by having very
low coefficients of friction at low temperatures. What this means for
the driver is that a brake pad that doesn’t fade until 1500*F might
seem to take an eternity to stop a car on the first application of
brakes on a cold day. It will seem as if after a few hard stops the
brakes all of a sudden “wake up” when the pads have gotten into their
optimal temperature range.
this isn’t so much a problem on the track, and the extra
temperature before critical fade more than offsets this shortcoming,
it would be suicide to run a set of pads on the street that
didn’t bite until 300-400*F.
Bite and Release Characteristics: How a Pad Feels
important aspect of the consistency of brake pads is how the brakes
“feel” to the driver. More accurately, the initial bite and the release
characteristics play a large role in how most people perceive the
effectiveness of their braking system. Initial bite refers to how the
brake pad will feel upon first application of the brakes. If a pad
bites very hard, this can cause ABS to engage, and make modulation of
the brakes difficult for the rest of the braking episode.
pad with a high level of initial bite does tend to feel better
in street driving though, where instant responsiveness is
more desirable than huge coefficients of friction and high
other hand, release characteristics refer to how smoothly a pad
compound will “let go” of the rotors when you ease off the brake pedal.
It is almost always better to have a pad that has smooth release
characteristics. Any abrupt inputs can cause a loss of traction when
close to the limits of adhesion of the tires. Thankfully, most street
pads have excellent release characteristics, but the skilled driver can
still detect the subtle variance in feel between different pads all the
way from first application of the brakes, to letting go of them.
What they are & how they work
are what applies the force to the brake pads that ultimately stop the
car. The biggest difference between calipers is the number of pistons
that exert this force to the brake pads and the size brake pad they can
fit. Common configurations for BMW big brake kits are aluminum calipers
with 2 and 4 pistons. Almost all stock BMW calipers are single piston.
Now, a common misconception is that the more number of pistons your
calipers have, the better and faster you will stop. This is largely
calipers: more is, well, sorta better.
main advantage of a multi-piston caliper is that they squeeze the brake
pads more evenly than a caliper with a lesser number of pistons. This
means you use the brake pad area more effectively, and it also
contributes to more even wear of the pad. But if your pad fades, you
aren’t going to stop quickly no matter how many pistons your caliper
has. This is why BMW chose to save costs by going with a single piston
caliper. There are definite advantages to simply putting on a
multi-piston caliper, but they cost much more to implement.
an aftermarket caliper, such as in a big brake kit, the required brake
pad is usually larger than the stock pad. This results in more swept
area, increasing total braking force just due to a larger pad. A larger
pad is just another perk to running a larger brake system on your car.
Big, iron heatsinks
rotors are the workhorses of your braking system. They are
literally giant heatsinks that store thermal energy generated
during braking and then dissipate it into the surrounding
airstream. There are three main characteristics of brake rotors
that impact their performance:
Size: The bigger the rotor, the bigger the lever
arm of the braking force acting on the wheel hub. Think
of the braking force as constant for any given pedal pressure,
but by spacing it out farther from the hub you gain more
braking torque. This is same concept as using a bigger ratchet
to loosen a stubborn bolt.
Thermal mass is simply a measure of how much heat energy
the rotor can hold for any given temperature. Since the
friction surface on most rotors are made of the same material,
cast iron, the only way to increase its thermal capacity
is to increase the mass of the rotor. This usually gives
you a bigger diameter rotor, which we have already learned is a good thing.
The reason why more thermal mass is almost always better
for braking is that it gives you the ability to scrub off
more speed without overheating all your components. With
a very large rotor, you can stop repeatedly from high speeds
without worrying about pad fade that would leave the same
pad roasting away on a smaller rotor.
rotors on passenger cars are solid, meaning the friction
surface of the rotor is a solid cast piece. On many performance
vehicles, such as most late model BMW’s, rotors are
vented. That is, cooling vanes are cast into the edge of
the rotor, passing air throughout the rotor. This results
in faster heat dissipation and greater fade resistance.
Fortunately, it’s likely this is a performance advantage
built right in to your BMW.
Surface: How Cross-Drilling & Slotting Affect Performance
are a couple of other features to rotors than can have an impact on
braking performance, specifically adjustments made to the friction
surface of a rotor that affect how the rotor interacts with your brake
Friction Surface: This is far and away the most
common type of rotor found on a modern vehicle, and it’s
what comes stock on most BMW’s (some higher performance
models such as the M3 Competition Package come with cross-drilled
rotors which we’ll discuss in a moment). Essentially,
this type of rotor simply has a smooth, flat friction surface
for the pad to press against. This is the most efficient
rotor type for street driving because it provides the greatest
swept area for the pad/rotor surface.
This is the most common type of “performance rotor” you’ll see on the
market. Quite simply, small, beveled holes are cast into the rotor
surface. This does a couple of things: 1) As pads heat up under heavy
braking, they release gasses which can build up between the pad and
rotor, reducing braking performance. By placing holes on the rotor
surface, this allows the gas to dissipate, keeping performance
consistent. 2) As pads heat up, they may reach fade-critical
temperature, causing the pads to essentially “melt.” The small holes on
a cross-drilled rotor continually swipe clean the surface of the pad,
ensuring a fresher, cooler surface is in contact with the rotor and
thereby reducing fade.
find cross-drilled rotors on most modern sports cars such as the M3
Competition Package, the new M5 & M6, and most modern Porsches.
They’re also extremely popular as an aftermarket upgrade due to their
purchasing cross-drilled rotors, be sure that you purchase rotors that
have had the holes cast into the rotor rather than drilled into a blank
rotor. Why? The drilling process introduces small stress cracks and
flaws in the rotor that can lead to catastrophic rotor failure (such as
the rotor breaking apart at speed).
on the same principals as a cross-drilled rotor, these rotors use a
series of evenly spaced continuous grooves from the inner to outer edge
of the friction surface of the rotor to “swipe” the rotor. They afford
the same performance benefits as slotted rotors, but also have the
advantage of greater surface area. This means more pad is in contact
with the rotor at any time, producing a greater level of friction and
braking force. Due to this difference, slotted rotors are generally
accepted as being better performers, though not as aesthetically
pleasing as cross-drilled rotors.
LINES: A key element for achieving consistent pedal feel
brake lines are responsible for transferring pedal pressure to your
calipers, which in turn squeeze your pads and provide braking force.
Most stock brake systems, and all of them in a BMW, use rubber brake
lines. Unfortunately, rubber brake lines have a big problem: spongy
pedal feel. You see, as braking force (or brake system temperature)
increases, the pressure your brake lines see as they force your
calipers to squeeze down increases. Because rubber is a flexible
material, this pressure causes the brake lines to expand and bulge
outward, leading to reduced braking force and a “spongy” pedal feel.
switching to a set of stainless steel braided brake lines,
you can eliminate this entirely and achieve a consistent firm
pedal. How? Well, because stainless steel lines exhibit almost
zero flex or bulge under pressure, the highest possible braking
force is maintained, and so is pedal feel.
One of the most overlooked components in your braking system
fluid is the medium through which the braking force is transmitted
from your brake pedal to the brake calipers. Ok, so how do
we improve it? By keeping it from failing completely as a
result of overheating, of course! Heat is a byproduct of braking,
and we’ve already learned how it affects the other components
in the system, so, as with the other pieces in the puzzle,
heat in your brake fluid is bad! (Noticing a trend here?)
Heat transmission to your brake fluid is almost always unavoidable,
so you must choose a good brake fluid for your application.
point is most important
performance yard stick for brake fluid is simply the temperature
at which it boils. When brake fluid boils, the brake pedal
will go to the floor since the vapor boiling brake fluid produces
is compressible, while brake fluid is incompressible. The
end result of boiling brake fluid is no braking force and,
most likely, you hitting something.
fluid unfortunately absorbs water as well; when the fluid
absorbs water its boiling point usually drops quite drastically.
This process of the fluid absorbing water is unavoidable.
Therefore, brake fluids have a dry and a wet boiling point
listed. It should be obvious at this point that frequently
bleeding your brake system to replace old brake fluid with
fresh, dry fluid is cheap insurance for your braking system.
have found ATE Superblue and Type200 (only the color differs)
brake fluid to be very good fluid for the price, and should
stand up to track driving as well as any form of street driving
just fine. If you manage to boil either of these fluids, something
else is wrong in your braking system.
SYTEMS: A simple, easy way to improve braking life & performance
down that hotheaded braking system…
pads, nicer rotors can all cope with the heat, but push the
car harder and you’re bound to hit the “thermal
wall” and start getting pad fade. The simplest solution
to this is to simply get rid of the heat more efficiently,
and the easiest way to do this is by ducting air to the front
rotors. The most effective way is to fit a backing plate to
the rear of the rotor and run a duct to it. This creates a
steady source of cool air that flows through the vented rotor.
course, good cooling can only go so far when you are really
pushing your brakes, and only really applies to track driving.
On the street, a pad that has excellent cold response, good
initial bite and uses a compound that offers low dust, low
noise and a reasonable temperature range is ideal. Compromises
must be made in each of these areas to improve in others,
though; there is no such thing as the “best” when
it comes to brakes, with one possible exception, although
this particular solution’s compromise is cost…
BRAKING HOLY GRAIL: Big Brake Kits
brake kits come in for the really demanding drivers. They
increased thermal mass (usually HUGE rotors)
distributed clamping force (multi-piston caliper)
larger brake pad (more swept area)
larger rotor for more brake torque
or slotted rotors for reduced fade
steel brake lines for great pedal feel
with a good big brake kit, you get every possible upgrade
you’d want in a brake system.
stock system on most BMWs is quite good, but there is always room for
improvement, and a big brake system truly is the ultimate in braking
performance, virtually eliminating fade and drastically reducing
braking distances. Plus nothing beats the look of huge rotors and
calipers underneath your wheels!
Right for My Application?
we’ve discussed all the aspects of brake systems at
length and talked about what types of upgrades you should
choose. Let’s get more specific and discuss some specific
products and items that will fit YOUR needs:
|Street, low dust
|Street, light track, autocross,
HP Plus, Axxis
|Aggressive street use,
autocross, medium track use, moderate dust
HP Plus, Cobalt
|Heavy track / race use,
Racing Pads, Cobalt
Steel Brake Line Recommendations
|Street, Track, or Race
|Street, Track, or Race
SuperBlue / Typ200
(same fluid, different color)
Brake Kit Recommendations
|Street, Track, or Race
Big Brake Kit