Often I get customers inquiring, "Do you keep mag wheels?" What they generally mean or refer to are 'cast light-alloy wheels'. No doubt alloys have great advantages over pressed steel wheels or wire-spoke ones but most alloys are bought or fitted simply to impress the neighbours. There are two types of alloy wheels available, one the more common aluminium alloy, and the other is expensive magnesium alloy wheel. A typical aluminium alloy has a density of about one-third that of steel. Cast magnesium alloys are even lighter, having a density slightly less than one-quarter that of steel. A cast magnesium alloy rim gives greater improvement in the specific strength over typical steel pressed rim when considering the tensile strength per unit density, yet weighs considerably less. For example a wheel of size 5.5X13, a popular size used for rally cars would weigh around 4.2kg as against 6.0 to 6.7kg for the same size pressed steel wheel.

Rally wheels are designed to withstand very high shock loads. Since tyres are often ripped apart by jagged rocks, these wheels have to withstand such brutal treatment and therefore rims with thickness of 6.4mm are used. Most rims used for circuit racing do not suffer such high impact loads but might occasionally clip a curb or two. This is why in nearly all circuit racing single-seater cars, be it Cart, Formula 3 or Formula 1, wheels used are expected to survive with a rim thickness of no more than 2mm. Most Formula 1 racing car rims consist of two deep pressings in magnesium alloy ZM21. A disc of honeycomb foil mounted on the central magnesium alloy hub attachment, is used as a spacer element. A tubular tension strap envelopes the central honeycomb spacer. The complete assembly is bonded together by a high-temperature adhesive. This sandwich construction of the wheel disc, in which a honeycomb spacer is used to separate two stressed outer skins, is an aerospace technique; hence such alloy rims are highly expensive.

The cheaper and more common varieties seen in the market are usually aluminium cast alloy wheels. The weight saving compared with a steel wheel varies from about 30-50 percent, the saving being greater for wider wheels. Most of these wheels are given a stove-enamelled finish to improve appearance and for corrosion protection. It was for the lightness that alloys were first used, during the 1924 French Grand Prix. Bugatti discovered that alloy metals could provide superior strength and significant weight reduction and thus the use of alloys in racing began. There are basically three ways in which most automobile alloy wheels are constructed and their performance will depend on the technique employed in manufacturing. The three types commonly used are cast, billet and forged. I would like to explain these processes and how they make the end product better or worse.

Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel. Here again there are two methods used, one whereby the molten material is poured into a mould and allowed to cool. These moulds 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. This system is known as gravity casting.

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.

Manufacture of Billet Wheels using 'Spin' Technology

Billet wheels are machined from a solid chunk of extruded aluminium alloy. This stock aluminium is then sliced up into sections, which are machined down into either complete wheels or just wheel centres. Since the piece of stock used is generally extruded, the grain, which is very similar to the fibers within a single strand of wire, runs through the stock. This grain structure, which is not present in a cast wheel, gives the final product a backbone or makes these wheels even stronger without adding additional weight. Of course, billet wheels are extremely expensive to produce as much of the original material is wasted as well as a lot of time is spent in machining the original stock down to a finished wheel, which again adds to the cost of the finished product. Thus entire wheels forged from a single billet are so rare as to be almost non-existent and are usually seen only on show cars. Billet centres on multi-piece wheels are however quite common.

Unlike casting or machining, forging uses intense heat and pressure to transform a slug of alloy material into the final shape of a wheel. This the third type of construction, which is used and is normally better than the billet type. As the original grain structure of the stock material is forced away from the centre of the wheel towards its outer edge it serves to strengthen the forged wheel's spokes as they run along the spokes, while the grain in a billet wheel simply runs through the spokes. Secondly a forged aluminium is about 300 per cent stronger than cast aluminium, yet less material is needed to produce the same, 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.

A month back I had an opportunity to visit the Hindalco Industries Limited's plant at Silvassa where they are manufacturing the 'Aura' brand alloy rims. The system in which they ensured that all their products met with high international quality standards recommended by their collaborators Stahlschmidt & Maiworm (S&M), Germany was impressive.

High-quality and fairly strong alloy wheel are manufactured by using low pressure die casting with state-of-the-art technology. Each wheel is subjected to a 100 per cent X-Ray check for minute pinholes and leaks and those that pass these stringent tests are sent for painting and finishing. I appreciated the open plant where no area was off-bound for us including the tests for ensuring optimum hardness and tensile strength. Finished rims are checked in batches for additional mechanical tests that cover radial, cornering and impact fatigue tests. Uniform paint thickness, scratch resistance test as well as a combination of environmental tests for resistance to salinity and humidity are ensured. Each Aura rim meets ARAI (Automotive Research Association of India) standards and is marked for their correct PCD, diameter, hub hole and offset, ensuring that they truly match the model of the vehicle specified.

Are alloys really worth the expenditure? Yes. My preferred choice is forged type of alloy rim as their multi-piece type construction allows for custom offsets, along with in-built strength and lighter weight construction. But such wheels are a rare buy in off-the-shelf market conditions and are frightfully expensive. It is not just the looks and cost one has to ensure while buying an alloy rim. For example while buying a forged wheel, one should check how close to net the forging is - the closer the forging is to the final product, generally the stronger the wheel. Secondly in imported wheels you should also ensure availability of rims, as in case you lose a rim by way of damage will the dealer supply just one piece or will you have to buy an entire set? Pressed cast alloy wheels like Aura are not only a good buy but also pocket friendly. Alloy wheels do reduce the unsprung weight of the car, which is one of the most critical factors affecting a vehicle's road holding ability. They also improve acceleration and braking as well as significantly reduce the wheel/tyre deflection in cornering, especially when used with low profile tyres. Finally they are excellent conductors of heat and thus dissipate the heat generated by the brake drums.

My general fear of alloy wheels is over cheap imitations of either forged or cast alloy wheels available from the general market at exorbitant rates causing innumerable problems as two of my case studies show.

CASE STUDY
We received a Uno with complaint of a wheel wobble. The car had an upsize tyre of size 165/65-R-13 fitted on alloy wheels. The balancing of the wheels was correct but yet the wobble persisted. On close inspection we noticed that since a Uno hub has a wheel-positioning stub, the alloy wheel had no such hole, so the wheel did not sit correctly in the hub. Secondly the PCD of a Uno is slightly different from the normal hubs which too gave way to a play on the wheels causing this wheel wobble. The correction was carried out in an unsatisfactory and a bit of non-technical manner to suit the requirement of the customer which though I am giving out, I would generally not recommend.

We removed the positioning studs and grinded the hub hole off to suit the PCD. This removed the wobble but left me with a type of distaste, as how we too must sometimes bend to a customer's whims to try and satisfy him.

There is not much to be said in the second case has as the picture speaks a thousand words. A cheap alloy rim which had the centre hub hole area cast with iron and not alloy, retained the heat generated by the brake drums instead of dissipating it, resulting in overheating of the tyre. Thus when the tyre failed due to heat failure, the direct impact was on the centre of the well base, which cracked in two.