There are varying types of wheels to be found on a car these days, mostly under the banner of steel or alloy wheels. Within this you can then have one-piece or composite (2 & 3 piece) wheels.
Steel disc wheels, still found on the majority of cars sold, are inexpensive, durable, heavy and flexible. If you’ve ever wondered why hubcaps fly off cars in old movies, and will still do it today on any high speed cornering, it’s because the wheels flex and pop the covers off. We recommend you don’t try this at the last corner on the way home. Steel wheels have their place but today they should be considered unacceptable for any performance use. Besides that, they just don’t look as good now do they?
Alloy wheels are lighter than steel which improves the steering and speed of the car. Lighter wheels are easier for the suspension to control, improving ride and handling, and are easier to accelerate and decelerate. Alloy wheels are also better heat conductors thereby improving heat dissipation from the brakes, which reduces the chances of brake failure (which generally occurs under more demanding driving conditions).
Alloy wheels are generally considered better looking than hubcaps (come out of the woodwork you hubcap lovers and send us an email espousing their finer points) though they are more expensive.
We’ll let you decide.
There are generally two types of alloy wheels; magnesium (or “Mag”) and aluminium, though sometimes a mixture of both has been used. The more common aluminium wheel of today is often mistakenly called a mag wheel. Not so.
Be aware that alloy wheels are prone to galvanic corrosion if appropriate preventative measures are not taken (see “Wheels – Maintenance & Care” at Top)
This type of wheel first came about in the 1950’s and was modelled on aircraft technology (with the stipulation that the car should remain on the road). They are most often used on racing tracks these days as they are lighter, giving better performance. However, they have been banned in some UK motorsport as they have been known to catch fire. Exciting stuff. Don’t panic if you have mag wheels though as this tends to occur when a tyre blows and you keep driving on the rim. If you blow a tyre, stop immediately.
Some variants of mag wheels have a low corrosion resistance though they are rather expensive.
For the die-hard magnesium fans amongst us, never fear. Magnesium is set to revolutionise the engine industry in the years ahead, replacing aluminium engine blocks. Aluminium is around 66% lighter than steel, whereas magnesium is about 75% lighter. Check out the CSIRO website for more information Click Here
They are NOT called Al. Just seeing if you are still paying attention. Most modern day alloy wheels are made from an aluminium alloy and are done so from one of several processes; casting, forging and billet.
Casting is done by pouring molten metal into a mould shaped more or less like the finished wheel, and letting it cool. Some polishing and finishing is then done to complete the wheel. There is a slight variant to this called pressure casting where the metal is pumped into the mould creating a less porous structure. The vast majority of alloy wheels are cast and provide many years of good service (ie they will go round and round).
Forging is an interesting process. Basically a chunk of aluminium is heated slightly so that it is soft and pliable, and is then slammed with hundreds of tons of force to press it into the rough shape of a wheel. In doing this the structure of the metal is changed whereby the crystals, or grains, change from a non-directional structure to a multi-directional high-integrity structure.
We could bore you to tears with the exact science of this so instead we’ll let 2 pictures speak 2,000 words. Fig A is the structure of the alloy in a cast. Notice how there are different sized crystals in different areas, depending on how it is poured. In Fig B the alloy has been smashed so hard that the crystals have been “coerced” into a uniform structure. Imagine a bed sheet of average quality with varying thread counts across its width and breadth. It’s strong but in time will break down. Now think of the finest Egyptian sheet with high density, uniform threads.
The forged “blank” is then placed in a spin-forging machine to spin out and forge the rim section.
Ultimately this process creates a wheel that has little to no porosity (preventing corrosion and cracking) and is about the strongest aluminium possible. The diagram below illustrates the forging process.
Billet is the process of forcing metal into a hallow cylinder which is then pressed out through an opening of a particular shape. Think back to when you pressed Play-Doh and it came out in little tubes, cylinders, stars, squares etc. The metal tube is then cut into discs and these are then machined using CNC (Computer Numerically Controlled) millers to carve out the finished product.
The advantage of billet over casting is that it uses a solid piece of metal rather than liquid metal poured into shape, which can be affected by many variables including such things as humidity and the presence of foreign objects (depending on manufacturing standards).
The billet process can be rather expensive as each item requires much more time to be handled and cut.
Wheels can be created as a single piece structure or made up of two and three pieces.
One-piece wheels are usually much cheaper to produce as they require no additional assembly. The initial setting up of tools/machines is more expensive but the manufacturing process is much quicker. It is often lighter as well as there are no extra bolts for the connection of the pieces.
Composite wheel’s main advantage come from providing exact fitment and are of course easy to repair. For those of you who have not read our “Wheels – maintenance & Care” section and have hit a gutter, with a composite wheel you can remove the outer rim and have it repaired or replaced. Composite wheels tend to be a bit heavier due to the extra bolts etc required for connecting the pieces, and they take more money and time to build as expensive humans are required to assemble the pieces.