Technology : Using Ceramics, Brakes Are Light but Cost Is Heavy June 18, 2006
Ceramic brakes are lighter and withstand heat better than those made of iron, but they can cost as much as a new car.
CHECKING boxes on an option list can quickly run up the price of a new car, but few add-ons are in the league of the ceramic brakes on a Ferrari F430: at $16,808, they cost about the same amount as a nicely equipped Honda Civic.
Skip to next paragraph RelatedStopping Short(June 18, 2006)
A cutaway of Audi’s ceramic brake disc, which is lightweight but very expensive.
The high-tech brakes, an option on the $172,505 F430 but standard on some other Ferraris, are an upgrade for iron discs that already deliver impressive stopping power. Only those drivers who spend weekends on the racetrack are likely to notice a big difference in performance.
Car-savvy pedestrians may take note of the owner’s selection, though. Instead of a metallic gleam visible through the spokes of the alloy wheels, they will see discs that look like patio stones.
Porsche was the first automaker to use ceramic brakes on a production car; in 2001, it offered discs made of a novel ceramic composite material to reduce the weight of a special sport model. Several Porsches now offer them as an option; on the Cayman sports car they cost $8,150.
Brake discs, also called rotors, of similar ceramic material are optional on the Audi S8 (although not yet in the United States). The $210,000 Bentley Continental GT Diamond Series will also be equipped with ceramic discs.
Why replace metal discs, which have served well for many years and are easy to make? Using a ceramic composite takes advantage of a material with outstanding hardness (and potentially long life) and an ability to retain its strength and shape at temperatures that would melt conventional iron brake material into a glowing puddle.
Simple single-ingredient ceramics tend to be brittle like dinnerware, though some types work well in turbochargers or as bearings for jet engines. To make ceramics that are tough enough for a brake disc, the material is manufactured as a composite: strands of carbon fiber, which are highly resistant to stretching, are embedded in the material, using a process developed by the Mitsubishi Chemical Company.
Production begins with a disc-shaped “preform” of carbon fibers, essentially a bundle of woven cloth in the approximate shape of the finished disc. The preform is saturated with a liquid polymer containing carbon and silicon. It is then heated to convert the polymer into silicon carbide, an extremely hard ceramic. The finished surface looks like stone.
Today, ceramic brakes are of interest for their performance advantage ? maintaining their stopping power even when extremely hot. But because ceramic discs will last four times as long as iron ones, according to automakers, their use could increase.
More important, ceramic discs weigh about half as much as iron discs ? a valuable benefit for handling and acceleration.
There are other good reasons to seek lighter, more durable brake disc materials. Since the introduction of CAFE ? the federally mandated corporate average fuel economy standards that an automaker’s fleet of models must collectively meet? there has been strong pressure to reduce the weight of automobiles.
As a vehicle accelerates, its rotating parts require more energy to accelerate than nonrotating parts like seats or engine blocks. This is because they gain energy from both their accelerating forward motion and from their increasingly rapid rotation. This gives brake discs a special importance in fuel economy.
Because so much driving is stop-and-go, and because it takes more fuel to accelerate a heavy car than a light one, reducing weight can help automakers meet the CAFE standards. Heavy iron brake discs are a favorite target of weight-conscious auto engineers.
When a car is braked, friction between the disc and the pads that grip it converts the kinetic energy of forward motion to heat. The heat is absorbed mainly by the discs, and eventually dissipated to the surrounding air. The higher the speed, the more kinetic energy there is, and the hotter the brakes become.
As discs have been made lighter, their average operating temperature has risen, leading to more rapid pad and disc wear. Braking force increases with disc diameter, so any attempt to remove weight by reducing disc diameter also lengthens stopping distances. Carmakers sometimes compensate by installing higher-friction pads ? which in turn may wear more rapidly.
Discs from the pre-CAFE era included extra material that allowed worn discs to be machined one or more times and re-used. Today’s lighter discs have little extra; often, they must be replaced when worn.
- Posted in : Uncategorized
- Author : arnold
Comments»
no comments yet - be the first?