The Golden Rule of Braking: You’ve Got to Compromise

The 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.

That said, let’s delve into the different factors that affect brake performance:

• Pads
• Calipers
• Rotors
• Brake Lines
• Fluids
• Brake Cooling Systems

At 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.

BRAKE PADS: The most important piece of the puzzle

While 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.

A preamble to brake pads: It’s hot, DAMN hot!

Braking 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.

What happens when a pad gets too hot:

By 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 fade,” 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.

Heat during street driving: no problem

As 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.

What’s 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
• noise
• brake dust characteristics
• initial bite & release characteristics

High heat situations call for high heat brake pads – but don’t use them on the street!

On 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.

While 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.

Initial Bite and Release Characteristics: How a Pad Feels

Another 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.

A 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 fade temperatures.

The 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.

CALIPERS: What they are & how they work

Calipers 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 false.

Multi-pistion calipers: more is, well, sorta better.

The 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.

With 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.

ROTORS: Big, iron heatsinks

Brake 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:

• Rotor 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: 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.
  
  
• Venting: Most 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.

Rotor Surface: How Cross-Drilling & Slotting Affect Performance

There 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 pads:

• Blank 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.
  
  
• Cross-Drilled Rotors: 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.
  
  You’ll 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 excellent looks.
  
  When 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).
  
  
• Slotted Rotors: Based 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.

BRAKE LINES: A key element for achieving consistent pedal feel

Your 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.

By 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.

FLUID: One of the most overlooked components in your braking system

Brake 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.

Boiling point is most important

The 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.

Water absorption

Brake 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.

We 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.

COOLING SYTEMS: A simple, easy way to improve braking life & performance

Cooling down that hotheaded braking system…

Better 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.

Of 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…

THE BRAKING HOLY GRAIL: Big Brake Kits

Big brake kits come in for the really demanding drivers. They offer:

• increased thermal mass (usually HUGE rotors)
• more distributed clamping force (multi-piston caliper)
• a larger brake pad (more swept area)
• a larger rotor for more brake torque
• cross-drilled or slotted rotors for reduced fade
• stainless steel brake lines for great pedal feel

Essentially, with a good big brake kit, you get every possible upgrade you’d want in a brake system.

The 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!

What’s Right for My Application?

So 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:

Rotor Recommendations
Application	Recommended Rotors
Good blank replacement rotor for street and light to heavy track /race use	OEM BMW Blank Replacement Rotors
Cross-Drilled replacement rotor for street and light to medium track use	Zimmerman Cross-Drilled Brake Rotors
Slotted replacement rotor for street and light to heavy track / race use	Power Slot Slotted Rotors

Stainless Steel Brake Line Recommendations
Application	Recommended Lines
Street, Track, or Race	StopTech Brake Lines

Brake Fluid Recommendations
Application	Recommended Pads Link
Street, Track, or Race	Ate SuperBlue / Typ200 (same fluid, different color)

Big Brake Kit Recommendations
Application	Recommended Pads Link
Street, Track, or Race	Brembo Big Brake Kit