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Alloy Wheels 101

Many new models trundling out of car showrooms these days sit proudly on alloy wheels, which are usually measured in inches (only two other things are habitually measured in inches these days, with the other two being display/TV/computer monitor screens and a gentleman’s 11th finger).  These alloys look very pretty but do they have any other advantages other than simple aesthetics?

Alloy wheels are often contrasted with steel wheels.  Here, the pedantic geek in me has to stand up to tell you that, technically speaking, steel is an alloy of iron and carbon (and other bits, such as chromium, vanadium, boron, tungsten, titanium and other obscure elements on the period table).  It’s probably one of the most common alloys, though it’s not the oldest: that honour goes to bronze (an alloy of tin and copper) and electrum (an alloy of gold and silver that can occur naturally).  There are lots of alloys that have been used since ancient times, and the ability to create them is one of the earliest metalworking technologies out there*. 

To be more precise, alloy wheels are made from alloys of aluminium or magnesium.  This is why you’ll hear some people referring to mag wheels or mag-alloy wheels; mag is an abbreviation of “magnesium alloy”.  This term probably dates back to the 1960s, which is when these wheels, previously only available to the car racing community, hit the market.

Steel wheels have their benefits, such as being cheaper and being easier to bang back into shape after a serious ding.  However, they’re usually only fitted to cheaper cars and entry-level variants (if at all), and will never be found on any luxury vehicle worth its leather seats.  So why do they use them? 

The metals used to make alloy wheels tend to be a lot lighter, but they still have the strength needed to stand up to the rigours of driving.  Getting the weight down is important to car designers (the weight of the vehicle, that is, not the designers) for a number of reasons. Firstly, lowering the unsprung weight of the vehicle makes things easier for the suspension, which, in turn, makes the car handle a lot better.  So that’s definitely a good reason for fitting a car with alloy wheels.  Being lighter also improves the fuel efficiency of the vehicles they’re fitted to because the lighter something is, the less energy it takes to move it.  Needing less force to get moving also means that acceleration gets better. The reverse is true as well: objects that don’t weight as much are easier to stop and/or slow down.

Having less weight also means that a vehicle can have bigger wheels without adding extra kilos, and the general thinking is that if it’s measured in inches, bigger is better.**  

However, having less weight is not the only advantage.  The aluminium and magnesium alloys have better ability to conduct heat away from the brakes, meaning that the brakes perform better.  If you’ve got an aluminium frying pan and a cast iron or steel skillet in your kitchen, you can see this easily.  If you get them both up to the same temperature then whip them off the heat, the aluminium pan will cool down more quickly than the steel one (have your oven mitts handy).  However, because of the greater strength of the aluminium or magnesium alloy, the wheels can be made with an open design – you know, those pretty stars and spokes.  Yes, these are a lot more aesthetically pleasing than a plain old steel wheel but this sort of design isn’t just beautiful but functional as well.  The open design allows the aluminium or magnesium alloy to release some of the heat generated by braking to the air, and the more surface area it’s got, the more heat it will lose.

The main ways of making alloy wheels are forging and casting.  Forging involves heating up the metal or alloy, rolling it, hammering it and generally mashing it about.  This process of heating, etc. makes the alloy grow stronger (I can see a nice little metaphor for a life lesson in there).  However, it’s a long and complicated process, and is more costly than casting.  Casting is where molten metal is poured into a mould, where it hardens.  Cast alloy wheels are cheaper and easier to produce en masse, but they aren’t quite as tough as forged alloy wheels.

Of course, these days, there is a new kid on the wheel block: carbon fibre.  Carbon fibre is even lighter than aluminium or magnesium alloys while still being super tough (diamond is pure carbon, remember).  Carbon is also better able to withstand bumps without forming microcracks, meaning that it’s tougher in the long run.  However, carbon fibre is a lot more expensive.  Will we see carbon fibre becoming more common (and cheaper) as time goes by?  I suspect we will, especially as EVs weigh a lot more than ICE vehicles, and thus cause more wear and tear on our roads, so trimming the weight down will be important (there’s also part of me that wonders if carbon fibre could be a way to sequester carbon, ultimately leading less carbon dioxide in the atmosphere, but this part is probably wrong).  Anyway, in EVs, regenerative braking transforms a lot of the kinetic energy lost during braking into electrical potential energy rather than heat energy, so there’s no need for open wheel designs that dissipate more heat. Instead, the designers can go for aerodynamics for even better efficiency (and look even cooler).  It will be interesting to see what they come up with.

* Could somebody please inform the writers of Amazon’s The Rings of Power of this fact?

** This may be true of wheels and screens, but speaking as a straight woman, it’s not true of the third.  Seriously, size really doesn’t matter.