WHEEL rims manufactured from metals other than steel have been around for a very long time. In the 1920s, for example, Bugattis were already using aluminium to save weight at the time when many car wheels were still being made from wood!
It took quite a while before more affordable sportscars started using alloys to convey sportier appearances than the rather plain pressed-steel wheels, even if they were treated and painted. As for the other sporty solution – spoked wire wheels – these were complex, time consuming to lace and prone to corrosion.
Commonly referred to as “mag” wheels because early castings were initially being manufactured from magnesium, these days aluminium is the preferred metal because magnesium, which sits above aluminium on the galvanic table, is more susceptible to corrosion and cracking, especially when exposed to salty air or water. Magnesium also has less ductility and therefore it’s more brittle than aluminium alloys. Nevertheless, continuous improvements in the tricky art of alloying various elements with the lightweight metal has meant that strength and toughness replaced brittleness, which meant that mass (and therefore unsprung weight) was reduced. Fast track to the 21st century and we see that today very few cars still utilise steel rims.
Apart from reduced mass, other benefits include aluminium’s inherent thermal conductivity that provides increased heat dissipation from braking energy, making these types of rims particularly useful for racing. Of course, given the many shapes that are easily designed to have more than sufficient strength, there’s an abundance of styles and finishes to choose from. Some manufacturers favour classic designs that are reminiscent of wire spoked rims.
Casting vs. forging
The two methods used for production are casting and forging. Because it improves the grain structure in metals, forged wheels are stronger than cast wheels. It is, however, a more expensive process. Rotary forging, which uses heat, pressure and rotation to increase strength as well as save weight, is sometimes employed.
Low pressure or high pressure die casting are the more common methods of production, however. Getting the right mix of alloys involves some interesting metallurgical sciences and the most common elements used to improve the strength of aluminium are silicon, magnesium, copper, vanadium, zinc and tin. Getting the percentages of each alloying element just right adds to the complexity of the process.
Some factors to remember before buying a set of wheels
DIAMETER
First is the most obvious – wheel size. This should match your existing rim unless, of course, you want to go bigger. If this is your aim, get some expert advice first. Your tyres will probably need a smaller aspect ratio (lower profile) to keep the overall tyre diameter the same. Be prepared to put up with a much firmer ride.
WIDTH
Apart from the appearance of alloys, increased width of tyres is what buyers most would want to increase to give their vehicles that “race car” look and improve grip.
PCD
Next is the PCD (pitch circle diameter) of the bolts. Naturally your studs or bolts must fit perfectly through the holes of the new wheels, so as to match the original rims’ specification. This figure is the distance from the centre of wheel to the line that runs through the centre of all the studs.
SPIGOT
Another aspect that cannot be changed is size of the spigot hole. This locates the rim on the hub. If the hole is bigger, you will need a spacer to take up the gap. The wheel studs are supposed to locate the wheel, not take all the stresses of static and dynamic loading.
OFFSET
This is the distance between the mounting point of the wheel to the hub and the centre line of the tyre. An increased offset has a similar effect to adding spacers between the hub and the wheel. Be mindful that bending stresses will increase and you might cause your tyres to foul your vehicle’s wheel arches.
Where to begin
If you’re in the market for a fresh set of metal, start your search here:
www.twt.co.za
www.autostyle.co.za
www.xtrememotorsport.co.za
www.mintys.com
Interesting local flavour
TSW was a South African company started by Eddie Keizan. The name stood for Tiger Sports Wheels. After listing on the JSE, the company went global, later purchasing the German company ATS wheels. Manufacturing has since expanded to Germany, USA and Poland. The company was split up after the turn of the century and has had various owners. Talk about wheeling and dealing!