A brake pad if the friction surface against the rotor (disk), it consists of a steel backing plate (this is what the caliper presses against) with e a friction surface bonded to it (some of the older style or even more economical type are riveted on) the friction surface can consist of many compounds.
The first disk brake use was in England in the 1890’s and patented by Fredrick William Lanchester and used on his automobiles in 1902, although a better technology as a whole the limited availability of metals and varying compound research was a hindrance. The Lanchester automobile used primarily only copper braking compound which wore out very quickly. Disk brakes did not become mainstream until approx. 50 years later, but it was not an easy transition. The first U.S. made vehicle to premier disk brakes was the Chrysler Crown Imperial built in 1949 (derived from aviation braking systems); however it’s usually incorrectly acclaimed to the 1950 Crosley Hot Shot. Disk brakes in their early years suffered from much criticism due to lack of research: dust, salt or other elements from the road surface would contaminant the braking surface and pads rendering them useless, ironically drum conversions were popular on the Crosleys Goodyear developed brakes. Chryslers Braking system was much more efficient and utilized advanced technology that assisted in braking when pressure was applied, although efficient and “grabby” they came at a very high cost. Reliable caliper type braking systems were first introduced in the UK on the 1953 Jaguar C-Type (racing car) developed by Dunlop and then on the 1955 Citroen DS with power assisted disk brakes arriving to the masses. In the U.S. disc brakes did not become standard until the mid-60’s and were still optional on most vehicles at that point.
originally the compound contained Asbestos (due to its flame retardation) this compound has been found to have many adverse health risks associated, thus is no longer in use. I will note that Asbestos is still found in some braking components manufactured today but extremely rare in mainstream automotive industry.
Now the typical brake pad will have an organic compound (basically saying its asbestos free), there are also semi metallic compounds which improve braking and heat dissipation (these will have small specks of metallic origin embedded into the friction surface)
There are also ceramic and cremetalic compounds, ceramic known for its high temp qualities.
And now with the evolution of the pads compounds we are seeing carbon and ceramic mixes which have trickled down from automotive racing into every day vehicles.
Brake pads/disks as opposed to shoes/drum allow for much better heat dissipation (heat is generated at a very high rate due to the transfer of energy) heat is the enemy of brakes and causes what is known in the industry as brake fade or glazing, this is when the heat causes the surfaces to lose their friction properties. Again different brake pad compounds assist in this further, as do ventilated disks, drilled rotors and the like.
With new technology in braking and pad composition comes many advantages and choices. Smei-metalic improves braking by better dissipating heat and causing less dust, and less wear on both the pad and rotor, ceramic is further better at doing so.
There are now many options available to the public, ranging from the average consumer to performance minded, some pads are engineered to function better at higher temperatures, thus they are not suitable for daily commutes but are highly effective in a track vehicle that builds up heat on the brakes quickly.
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