Of all the products we test at SCC, tires and brake pads are among the hardest to evaluate. Both require extensive knowledge of the product as well as multiple tests to give us enough information to impart the complete picture. In the case of brake pads, stopping distance alone tells just a small portion of the story.
Other factors like noise, bite, modulation, dust, wear and overall feel are much harder to quantify, and are all interrelated characteristics. Making a quiet, dust-free, race pad that lasts thousands of miles is impossible. One feature often comes at the cost of the other, making the ideal performance pad hard to choose. In most cases, picking the right pad is more about seat time, application and preference.
Endless Racing, one of the big Japanese aftermarket performance brake parts manufacturers, has released the SS-H brake pad, specifically formulated for applications that use an OEM Brembo system, like the Mitsubishi Evo, Subaru STI and Nissan Skyline GT-R. While the company already has the SS S-sport pad for aggressive street use, the SS-H was reformulated to reduce noise while maintaining the same performance and feel.
Brake squeal is primarily the result of pad compound and rotor/pad shape. The noise is the rotor and pad excited to a combination of the natural frequencies of each part, creating an audible resonance. This is why brake squeal is often speed-dependent. The cause is abrasion, one of the friction mechanisms of braking. Abrasion is mechanical friction caused by the pad scraping across the microscopic ridges of the brake rotor, like scrubbing coarse sand paper across a brick (the brick being the pad and the sandpaper the rotor's surface). Every time the brick catches the edge of a grain of sand and then releases, energy is released in the form of heat and vibration. If this happens at a rate that coincides with the natural frequency of the pad or rotor, the energy compounds and resonates, causing squeal.
Squeal is also temperature-dependent, because pad materials act differently with temperature. As brake temps rise under hard driving, a layer of pad material transfers onto the rotor. So now the pad is not interfacing with the rotor's jagged iron surface, but a thin layer of pad material. This transfer layer is where the majority of brake friction comes from. Instead of abrasion, adhesion of the pad material on itself provides the best brake performance.
On everyday street pads made to work on a cold winter start or in wet-weather traffic, it doesn't take a lot of heat to create a transfer layer on the rotor. But in the case of most performance brake pads that operate at high temps, it takes a lot more heat. Until that temperature is reached, the brakes rely mostly on abrasion, which has the effect of scuffing off the previous transfer layer from the last time the car was driven. This is why stock brakes work in the cold and don't squeal, while performance compounds are typically horrible and loud when cold. At the extreme, this is why race compounds are extremely abrasive at street temperatures.
Since rotor shape and geometry are primarily constrained by size, manufacturing and cooling needs, dealing with squeal is usually handled through either changing pad geometry (in terms of slotting) or, more commonly, through pad compound. Changing the compound not only changes how abrasive (and screechy) a pad can be, but also at what temperatures the material transfers to reduce abrasion (as well as the density of the pad, which will change its natural frequency). From a performance perspective, most pads will sacrifice cold braking, dust and noise for high friction and better temperature tolerance properties of a semi-metallic pad. Ferrous materials contribute more to squeal than organic/ceramic compounds.
By Arthur Guo
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