Technologue Clean wheels, clean air

Frank Markus Technologue

Clean Wheels, Clean Air: Eliminating brake dust for health and (automotive) beauty

ook out, the emissions police may soon be coming for

Lyour tires and brakes. Now that most noxious fumes and hazardous particulate matter have been cleansed from exhaust pipes, global regulatory agencies that monitor those nasty 2.5–10.0-micron particles that can lodge in your lungs and irritate your eyes, nose, and throat are zeroing in on brake dust and tire emissions.

Not even electric vehicles and other “clean” vehicles can escape these measures. But non-exhaust PM emissions will prove devilishly difficult to measure, let alone regulate. Happily, there’s already a solution to the brake dust problem that pays handsome aesthetic dividends: clean wheels and mirror-finish rotors.

Our fuel economy and emissions-measuring partners Emissions Analytics first drew my attention to this problem in a report last year, in which its testing revealed that a popular family hatchback running on brand-new, correctly inflated name-brand tires had emitted 5.8 grams/ kilometer of particles. That’s more than 1,000 times the EU’s 4.5-milligrams/kilometer of allowable tailpipe particulate emissions.

As a result, buyers gravitating toward larger, heavier trucks and SUVs are making our environment worse in ways besides poor fuel economy. Meanwhile, electric vehicles’ heavy battery packs exacerbate tire wear, even as regenerative braking helps reduce the brake dust they generate.

Research by U.K. government air quality experts found that non-exhaust particle emissions contribute to well over half of particle pollution from road transport. Closer to home, the California Air Resources Board awarded the University of California Riverside a research contract to investigate the impact of brake dust, tire wear, and road dust resuspension on near-roadway environments over a two-year period ending in mid-2021.

Future legislation aimed at curbing tire wear certainly threatens to impact every tire parameter that motoring enthusiasts hold dear, from rolling resistance to lateral grip. But a solution to the brake wear problem has already been commercialized, if not “democratized. ” The concept first broke cover as standard equipment on the Porsche Cayenne Turbo under the moniker Porsche Surface Control Brakes (PSCB), and just months ago Brembo rolled out a very similar concept under the name “Greentive. ”

In each case, a typical gray cast-iron rotor undergoes a thermal spray coating process known as high-velocity oxygen fuel coating. HVOF involves combusting a fluid fuel and oxygen, then forcing the gas they produce out through a nozzle at supersonic speeds. A powder—in this case, tungsten carbide—gets injected into this stream, partially melts without altering its chemistry, and gets deposited on the workpiece, as a dense, 0.1mm-thick, low-porosity coating with extremely high bond strength.

In each case, the result is a mirrorlike finish on the rotor that will never rust and that greatly extends the life of both the pads and the rotor. Curiously, Porsche pegs the pad and rotor wear improvements at 90 and 30 percent, respectively, while Brembo reverses those—pad wear is reduced by 30 percent, rotor wear by 90 (when the Brembo logo embossed in the rotor disappears, it’s time for new rotors). The Greentive project was undertaken as part of the European LOWBRASYS initiative to develop a low-environmental-impact brake system, which resulted in a claimed reduction in particulate emissions of 50 percent. Naturally, these mirror-finished rotors require special pads. Brembo has yet to divulge much information on its Greentive pad chemistry, but it’ll likely match Porsche’s closely. They’re described as mildly “adhesive, ” impregnated with microscopic hard particles capable of briefly penetrating the tungsten carbide on a microscopic scale. The low rotor porosity means the entire pad contacts the rotor, with none of the grooves, pits, or voids that limit friction on standard rotors.

How does this technology affect braking performance? Porsche says PSCB maintains its stopping power even at high temperatures, and it creates so little dust that the automaker identifies the package with white calipers. Hence, whatever wheels one chooses, they remain clean as they frame rotors that always look like mirrors.

That HVOF coating process is spendy, so even legislative coercion is unlikely to force this technology down to entry-level cars. Porsche charges $3,490 for PSCB on Cayenne models. (For reference, Porsche’s ceramic-composite brakes cost $9,080.) I expect Greentive brake pricing to align with PSCB, appearing on higher-end vehicles first. Q

Porsche developed PSCB in conjunction with Bosch subsidiary Buderus Guss. Its process employs a galvanized layer underlying the tungsten-carbide finish. Brembo employs a different, as yet undisclosed preparatory layer.