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In the automotive industry, alloy wheels are wheels that are made from an alloy of aluminium or magnesium. Alloys are mixtures of a metal and other elements. They generally provide greater strength over pure metals, which are usually much softer and more ductile. Alloys of aluminium or magnesium are typically lighter for the same strength, provide better heat conduction, and often produce improved cosmetic appearance over steel wheels. Although steel, the most common material used in wheel production, is an alloy of iron and carbon, the term "alloy wheel" is usually reserved for wheels made from nonferrous metal alloys.
The earliest light-alloy wheels were made of magnesium alloys. Although they lost favor on common vehicles, they remained popular through the 1960s, albeit in very limited numbers. In the mid-to-late 1960s, aluminium-casting refinements allowed the manufacture of safer wheels that were not as brittle. Until this time, most aluminium wheels suffered from low ductility, usually ranging from 2–3% elongation. Because light-alloy wheels at the time were often made of magnesium (often referred to as "mags"), these early wheel failures were later attributed to magnesium's low ductility, when in many instances these wheels were poorly cast aluminium alloy wheels. Once these aluminium casting improvements were more widely adopted, the aluminium wheel took the place of magnesium as low cost, high-performance wheels for motorsports.
Better heat conduction and a more open wheel design can help dissipate heat from the brakes, which improves braking performance in more demanding driving conditions and reduces the chance of diminished brake performance or even failure due to overheating.
Alloy wheels are also purchased for cosmetic purposes although the cheaper alloys used are usually not corrosion-resistant. Alloys allow the use of attractive bare-metal finishes, but these need to be sealed with paint or . Even if so protected the wheels in use will eventually start to corrode after 3 to 5 years but refurbishment is now widely available at a cost. The manufacturing processes also allow intricate, bold designs. In contrast, steel wheels are usually pressed from sheet metal, and then welded together (often leaving unsightly bumps) and must be painted to avoid corrosion and/or hidden with wheel covers/hub caps.
Alloy wheels are prone to galvanic corrosion, which can cause the tires to leak air if appropriate preventive measures are not taken. Also, alloy wheels are more difficult to repair than steel wheels when bent, but their higher price usually makes repairs cheaper than replacement.
Alloy wheels are more expensive to produce than standard steel wheels, and thus are often not included as standard equipment, instead being marketed as optional add-ons or as part of a more expensive trim package. However, alloy wheels have become considerably more common since 2000, now being offered on economy and subcompact cars, compared to a decade earlier where alloy wheels were often not factory options on inexpensive vehicles. Alloy wheels have long been included as standard equipment on higher-priced luxury or sports cars, with larger-sized or "exclusive" alloy wheels being options. The high cost of alloy wheels makes them attractive to thieves; to counter this, automakers and dealers often use locking or bolts which require a special key to remove.
Most alloy wheels are manufactured using Metal casting, but some are Forging. Forged wheels are usually lighter, stronger, but much more expensive than cast wheels. There are two types of forged wheels: one piece and modular. Modular forged wheels may feature two- or three-piece design. Typical multi-piece wheels consist of the inner rim base, outer rim lip and wheel center piece with openings for lug nuts. All parts of a modular wheel are held with bolts. BBS RS is one of the most famous three-piece modular forged wheels.
However, pure magnesium wheels are no longer produced, being found only on classic cars. Pure magnesium suffers from many problems. Vintage magnesium rims were very susceptible to pitting, cracking and corrosion. Magnesium in bulk is hard to ignite but pure magnesium wheels can be ignited by a burning tire or by prolonged scraping of the wheel on the road surface following a puncture. Magnesium alloy were later developed to alleviate most of these problems. Automotive Detailing: A Complete Car Care Guide for Auto Enthusiasts and Detailing Professionals By Don Taylor – HP Books 1998 Page 72 In fact, US Federal Aviation Administration has conducted wide-ranging tests over the past decade, and has reached a conclusion that potential flammability of magnesium is no longer deemed to be a concern. Modern surface treatment technologies provide protection from corrosion and significantly extend the average lifecycle of magnesium rims.
When processed using best practice methods, low pressure die casting wheels can offer improvements in ductility over magnesium wheels and any cast aluminium wheels, they remain less ductile than forged magnesium.
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