Monday, September 12, 2011

The iPad vs. the bPad

A Google search on “iPad” returns 1.3 billion results. A search on “brake pad” returns only 17 million results. Our collective knowledge of brake pads then is almost 1,000 times less than our knowledge of the iPad. This in spite of the fact that the iPad has been around for less than two years and the brake pad has been around for decades.

What’s In a bPad?
It’s easy to select an iPad. There is only one. Pricing differs only by storage capacity but the features are all the same. It is the exact opposite with the bPad. There is a myriad of manufacturers and a variety of applications (we are strictly talking brake applications here. For other types of applications, check this iPad MadTV skit). Most brake pads sold today are semi-metallic. They are made primarily of chopped steel wool mixed with resin. You can often tell the quality of a semi-metallic brake pad by how smooth it is. High quality pads are made of more finely chopped metal pieces, which help with dispersing heat and resisting fade. Assuming similar metallic content, a cheap brake pad will be rough to the touch and could even leave splinters on your hand when you run your fingers across it. And unlike the iPad, you want your bPad thick, so it can dissipate heat better. Brake pads create the friction that helps slow down the car (the tires are what stops it). But why do the other components of the brake system matter?

Brake system diagram courtesy of HowStuffWorks.com 

Rotors: Size Matters!
The size of the rotors determines to a large degree how well the heat created under braking is dissipated. Bigger is better. And remember what Archimedes said: “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” Because pads on larger rotors make friction with points of the rotor further away from the axis of rotation, larger rotors also provide a mechanical advantage that helps stop their rotation more easily. Rotors can be solid, cross-drilled and/or slotted. Cross-drilled and slotted rotors allow gases from the pads to evacuate faster under heavy braking, which allows more solid contact between the pad and the rotor. The drawback of these rotors is that they have slightly reduced overall mass available for dissipating heat. Drilled rotors may also develop hairline cracks over time. Slotted rotors may grate the pads faster and reduce their life.

A New Design: Floating Rotors 
A newer design commonly known as “floating rotors” works best. These rotors are typically cast in two pieces – the rotor and the carrier. Having two separate sections of the rotor helps keep heat generated in the rotor off of the carrier. This improves cooling and also reduces flex, which ultimately results in better stopping ability  under extreme braking conditions. All else equal, for track driving it is more important to have the right bPad than a fancy rotor. A solid rotor will work just fine with pads designed to operate at a high temperature range.

Calipers & Brake Lines 
Steel brake lines (as opposed to rubber) help improve brake pedal feel and the ability to modulate the brakes. However, they are not a critical component of the brake system’s ability to dissipate heat. The pistons in the caliper push the pads against the rotor. Most cars have a single piston per caliper. The number of pistons in the caliper is not a decisive factor for braking ability either (we can probably argue this till dawn and I’ll admit that multi-piston calipers at least look cool).

Bake Fluid Is Very Important!
Brake fluid, on the other hand, is a decisive factor! Once the rotors are too hot, the brake pads transfer heat into the calipers. This in turn can overheat the brake fluid, which enters the caliper to hydraulically compress the pads against the rotor, to a point where it starts to boil. As this happens, bubbles start to form in the brake fluid. Air is compressible, and fluid isn’t; so you can sink your left foot all the way down without stopping, if there are bubbles in the brake lines. Brake fluid is hygroscopic, meaning it absorbs moisture over time. The higher the percentage of moisture in the brake fluid, the lower the boiling point. This is why changing the brake fluid is one of the most important things you can do to  get your car ready for track driving. Some clubs now use a tool to measure the percentage of moisture in the brake fluid during all track event tech inspections.

Your First Track Date
So you‘ve got a hot track date coming up and would like to know how to dress for the occasion. (We are still talking driving here.) The stock pads in modern sports cars are typically very good on the street. They may even be fine at the track for someone new to performance driving. As long as the brake pads have near-full thickness, just change the brake fluid and use your factory pads on your first track day. Keep in mind that for bigger and heavier cars, the stock pads will have to do more work to stop it. Monitor braking feel and pad thickness throughout the day to make sure you’ll able to catch it if they start wearing out and/or overheating too fast. If you like the driving event (who doesn’t?), start thinking about bringing better pads as spares for your next event. Do this at least for the front, which does most of the braking work.

Pick a Pad To Match Your Needs. 
What pads you pick will depend on how often you plan to track the car, how demanding the specific tracks are on the brakes, how advanced your driving abilities are, and how comfortable you are changing your own pads. There is no year-round street pad that will work well at faster tracks, especially for a quick driver. Conversely, there is no dedicated track pad that will work well on the street. The most popular street/track pad is probably the Hawk HP Plus, which will work okay for a while under heavy braking at the track, but it will squeal louder than a school bus under light braking on the street. Aggressive street-only pads like the Hawk HPS (S is for Street), will fade badly in most fast cars at the track. Popular track pads like the Performance Friction 97 and 01 compounds do not provide good cold bite on the street. Also, the brake dust from most track pad is corrosive and will bake into the wheels if left to dry up on them after rain.

I’m a Track Pad Junkie
Well, it’s a good sign you’re still reading and didn’t “hit the brakes” on this write-up earlier. I trust that you have dedicated track pads and a strong preference for certain brands and compounds. Personally, I use the new Hawk DTC60 for my '08 Z4 M Coupe and PFC97 for my '01 325i (both with stock brakes). You probably bring your brake cooling ducts to every hot date and win more often than Charlie Sheen (Winner, Winner, Brake Pad Dinner). So tell everyone that there is no substitute for a dedicated track pad and that everyone planning on going to track events regularly should use them. And don’t forget to put new tiger’s blood in your brake fluid reservoir, warlocks.

Marketing the bPad
The tagline for the iPad is: “Thinner. Lighter. Faster. FaceTime.”
The tagline for the bPad is: “Thicker. Rock-solid. Slower. Less FaceTime With the Wall.”

That says it all!

9/15/11 update: Eagle-eyed reader Jon Cowen brought to my attention that it is incorrect to say "Brake fluid is hydroscopic". I corrected it to read "Brake fluid is hygroscopic". Thanks, Jon! You'll be first on my list to hire as a QA Manager when I can afford it!

2 comments:

  1. Brake fluid is Hygroscopic, not hydroscopic.

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  2. Thanks for correcting, Jon. I had no clue and had to go look it up:

    Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment through either absorption or adsorption.

    The similar-sounding but unrelated word hydroscopic is sometimes used in error for hygroscopic. A hydroscope is an optical device used for making observations deep under water.

    Good to see someone reading the write-up so carefully!

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