Disc Brake Pads – Construction, Selection and Break-In

Wednesday, October 29th, 2008 Tech Articles No Comments

From: The Ducati Garage Tech articles.

Most of you will have to replace your bike’s brake pads at least once during your lifetime — unless your idea of riding is pushing your bike back-and-forth in your living room, Flintstone-style.

As you wander into your local dealer or thumb through the pages of your favorite catalog, you’ll find many different brake pad manufacturers, such as Ferodo, DP, Galfer, and EBC. The problem is selecting which pad best suits your riding style and conditions. Other than the a few lines of marketing blurb on the back of their packaging, there’s not a lot of information available on how to differentiate pads from one another.

Construction

Your brake pads started life in the hands of a friction material formulator, most likely a chemist by degree. Chemists select the composition of a brake pad by choosing from a fixed list of compounds that fall into 4 categories.

Fibers, such as fiberglass, kevlar, arimid, stainless steel, and aluminum maintain the heat stability of the pad. These fibers have various binding strengths and can be organic or metallic. Friction Modifiers such as graphite adjust the friction level and fine tune the performance characteristics of the pad at specific cold and hot temperatures. Fillers take up dead space in the pad. These are generally organic materials with some low frictional effect such as sawdust. Finally, Resins are used to hold the elements of the pad together so they don’t crumble apart.

Every motorcycle disc brake pad that I’ve ever seen has been manufactured using an Integral Molding (IM) process. Bascially, the pad formulation (sort of like a cake mix) is pressed in a mold against the steel backing plate using adhesives and mechanical locking.

Most high performance pads are positive molded. This means that only one pad is pressed in the mold at a time. Some pad manufacturers who use mass production equipment will flash mold their pads. This means that the mold creates a whole tray of pads, all pressed at the same time. In general a positive mold pad is more durable and has a high sheer strength because the full force of the manufacturing press is applied to one pad at a time. If you’re unsure of what method your favorite brake pad manufacture uses, ASK THEM.

After the pad has been molded is molded into a puck and bonded to the steel backing plate, it is usually cured in an industrial oven to slowly remove the moisture that remains in the pad compound. Without this slow curing step, your pad would basically be “rapid cured” the first time you applied your brakes. The problem with that is the rapidly escaping moisture would seek an immediate escape route out of the pad compound, creating small fissures that would lead shortly to cracking and chunking.

Finally, the steel backing plate of the pad is painted and a series of edge codes are printed on the back of the pad.

The edge codes are the seemingly random combination of letters and numbers that appear on the back of the pad. These codes usually refer back to the registered friction formulation, and more importantly, show a consumer how the pad ranked in a Chase Test.

The Chase Test

If you’ve barely stayed awake reading the previous paragraphs, then grab a cup of java and read on because here’s where the good stuff is!

The Chase Test, better known as the SAE J866A test procedure, provides a uniform means of identification that may be used to describe the initial frictional characteristic of any brake lining.

The Chase Test is used to assign a two character code (e.g. EE, FF, GG, HH, etc) to a specific friction formulation. These characters represent the coefficient of friction when a 1″ square piece of friction material is subjected to varying conditions of load, temperature, pressure and rubbing speed on a test apparatus known as the Chase machine.

The coefficient of friction measured by the Chase test describes the relationship between the two forces acting on the friction material. A clamping force is exerted on the friction material, resulting in a frictional or resistance force. A low coefficient of friction means that very little of the clamping force is transferred into resistance force. On the other hand, a high coefficient of friction means that given the same level of clamping force, a higher resistance force is generated by the brake pad.

For example, a pad that carries an HH code has a normal coefficient of friction of 0.55 or higher, and a hot coefficient of friction of 0.55 or higher.

The first letter of the code represents the normal friction coefficient. This is defined as the the average of four test data points measured at 200, 250, 300 and 400 degrees Farenheit.

The second letter of the code represents the hot friction coefficient based on a fade and recovery test. We all should know what brake fade is. If you’ve ever had to use the front brake extensively and found that its effectiveness quickly diminished, that’s fade. Recovery is basically the period where the brakes are gradually cooling off.

The hot friction coefficient is defined as the average of 10 data points located at 400 and 300F. on the first recovery cycle of the pad; 450, 500, 550, 600 and 650F. on the second fade cycle; and 500, 400, and 300F on the second recovery cycle.

The range of friction coefficients assigned to each code letter are as follows: C = less than 0.15. D= 0.15 to 0.25. E= 0.25 to 0.35. F= 0.35 to 0.45. G= 0.45 to 0.55, and H= over 0.55.

If H is the highest coefficient of friction, then why aren’t all pads manufactured and rated to a HH specification? Well, while a HH pad might be ideal for high performance or track day riding, it could be too “grabby” or aggressive for those of you who spend your time commuting. Similarly, some “racing” brake pads are unsuitable for the casual Sunday canyon ride because they never reach their ideal operating temperature, much like racing tires, in these relatively gentle conditions.
So how do you select the right set of pads for your bike? This may seem silly but just contact the manufacturer and ask them for a little advice. With your newfound knowledge on brake pad construction plus an honest assessment of your riding style and conditions, any qualified brake technician should be able to steer you in the right direction. If you can’t decide between brands, try asking other Duc riders on the Ducati Index “Help Needed” message board. If no one else on the board has heard of the brand you’ve selected, you may not want to be the guinea pig for an unknown product.

Of the brands mentioned in this story, I’ve personally used DP and EBC products in the past and have been happy with their performance and wear characteristics. My riding conditions are weekend canyon rides and an occasional open track day, so the street compounds from both companies worked fine for me.

Disc Pad Installation and Break-In

Here’s a quick plug for proper maintenance of your brake system. Bleed you brakes as frequently as you can (at least once per year.) Thoroughly clean the brake caliper, dust seals, guide pins, etc with a can of automotive brake clean solution after removing your old pads. On higher mileage bikes, remember to use a micrometer to measure the thickness of your rotors. You may think a set of new pads are all you need, but your rotors might just be one ride away from the junk bin.

Once you’ve decided on an appropriate disc brake pad for your riding style and have them installed on your bike, you’ll want to bed the pads in by making a series of gentle, controlled stops from a speed of 40-50mph. Find a nice open stretch of road. The first few stops may seem a little scary as you squeeze the brakes and get poor stopping results. Don’t be alarmed. This is merely the result of the last amounts of residual moisture and adhesives working their way out of the pad and into the atmosphere. After a few more controlled stops, you should feel a dramatic increase in braking effectiveness.

Resist the urge to immediately go out and hammer on the brakes with high speed, tire smoking stops. This will likely lead to pad glazing, a condition where the resins in the pad crystalize on the friction surface, resulting in poor stopping performance and excessive noise. If this occurs, the pads are useless. Throw them away, buy another set and start the whole process over again.

Source > The Ducati Garage Many thanks to Greg for putting all this together.

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