Cast vs Forge Alloy Wheels
The conventional wisdom in wheel manufacturing is that forged wheels are lighter and stronger, and cast wheels are heavier and softer. Naturally, the more intricate production processes involved with forged wheels also often make them substantially more expensive than their cast counterparts. Yet recent advances in production technology have greatly increased the strength and reduced the weight of cast wheels.
Forging uses intense heat and pressure to transform a solid slug of alloy material into the final shape of a wheel. Forged aluminium is about 300 per cent stronger than cast aluminium, yet less material is needed to produce the same "cast alloy wheel", which results in a lighter product. Because of the basic limitations inherent in forging, most forged wheels are two or three piece units. In two-piece construction, a centre is forged and welded or bolted into a spun or stamped outer rim. In a three-piece wheel, the centre is bolted to an inner and an outer rim half. This stands as an advantage of being easily customisable for a variety of widths and offsets.
Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel. There are two methods used. One, system is known as gravity casting... whereby the molten material is poured into a mold and allowed to cool. These molds are usually made by machining a piece of material on CNC machine equipments to produce a wheel that only requires minor finishing (like drilling or possibly trimming of some excess metal) to be considered complete. The other and better system used is the low pressure or negative pressure casting. Here instead of pouring the molten material into the mould, the molten alloy is drawn up into the mould using a high-pressure vacuum. This eliminates much of the trapped air found in gravity casting process, producing a stronger wheel that is less porous than a gravity-cast one.
Two European companies are pio-neering advanced production technologies for cast wheels. BBS of Germany and O.Z. Racing of Italy are both making an impact on the industry through their innovative use of technology and development of new lighter, stronger alloys.
O.Z., largely considered a premium niche brand in North America, is a leading producer of professional racing wheels, as well as a supplier to European tuners such as Hamann and TechArt and an original-equipment supplier to Lamborghini. In an effort to increase its presence and popularity in the sport compact market, the company introduced the Superleggera model last year, using a combination of proven production technology and new processes.
Though the wheel is produced using conventional low-pressure casting or gravity casting, depending on diameter, the thin-spoke design is derived from O.Z.'s Formula One wheel designs, aided by the application of finite-element analysis and computer modeling. The wheel uses a proprietary titanium-enriched aluminum alloy for added strength and is lightweight. The final production step before painting is a shot-peening treatment to relieve surface stress and increase strength. The wheel is priced competitively with other conventional cast wheels, and in the 18x8-inch size, the wheel weighs a scant 18.5 pounds, ranking it among the lightest 18-inch wheels available to the general public.
BBS, a leading original-equipment supplier to many European automakers, also does approximately 50 percent of its business in aftermarket wheels. For its high-performance wheels the company employs a patented production process known as flow-forming. The wheel casting, or the outer rim section in the case of a two-piece wheel, is heated to 572 degrees Fahrenheit, and the rims are formed and compressed by a special rolling mill. This allows a denser, thinner rim, increasing strength and decreasing weight. Like O.Z., BBS employs finite-element analysis to determine the correct proportions of compounds in the alloy and uses X-rays to check for hairline cracks before shipping.
BBS has also pioneered a new finishing process to drastically cut toxic waste at its German plant. The new paint finish, available on selected wheels, offers a chrome-like finish, with the durability and cleaning ease of a conventional clear-coat wheel. This innovation is expected to reduce projected toxic waste output by 130 tons a year.
Though they may not claim the bragging rights of forged or billet, modern-day cast wheels are hardly slouches when it comes to state-of-the-art production technology. With weight and strength properties that rival forged wheels, high-performance cast wheels present demanding enthusiasts with more affordable, high-quality wheel options than ever before.
http://www.motegiracing.com/techzone/wheeltechdetail.asp?filename=wheeltech5
What are the advantages of lightweight alloy rim?
A car's suspension system has to coupe with both sprung & unsprung masses. Sprung mass basically includes the chasis, engine, passengers etc. Unsprung mass includes the suspension components themselves, brakes, driveshafts as well as the rims & tyres. The dampers do most of the work of controlling the oscillations of these two group of masses & the heavier the component the more work the dampers have to do. It therefore follows that a weight reduction would translate to positive benefits in terms of ride & handling.
Alloy Rims - The Alloy Sports rims for the same diameter are lighter than steel rims by as much as 5 kilograms on each corner. With less unsprung weight at each corner, the suspension system is able to work better at handling undulations on the tarmac. Less weight means better acceleration as well due to the lower inertia. Most people would upsize the rims when changing to alloy rims and this will negate the weight savings as bigger rims and wider tyres are heavier.
Here's a website that explain on tyres & wheels, quite technical but good explaination.
http://www.carbibles.com/tyre_bible.html
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