How to upgrade your wheels like an Engineer
The good ol' wheel upgrade is probably one of the simplest modifications you can make to your pride-and-joy. It can be a relatively cheap way of transforming the look and performance of your car. This is usually how 'it' starts: You buy some wheels, then you want stickier rubber, then you need better brake pads... Eventually, you've gained a turbo kit, a stripped interior and some divorce papers.
However, if you're still adamant about venturing down this road (you should be), there's a lot to consider. Let's start with the basics.
By "basics", I literally mean "making sure your wheels will fit on your car". The first thing you need to know is that size isn't everything (so I'm told).
A larger diameter wheel may look more visually appealing, but you need to make sure the wheel will actually fit on your car. The same can be said for the width of the wheel. Lastly, the offset makes a big difference too (the offset of a wheel is the distance the mounting surface is from the centre of the wheel). Use a website such as www.willtheyfit.com to assess how wild you can go.
How Size Affects Performance
Wheels are available in a huge variety of rim diameters and widths. Your choices here make a significant impact on the way your car will perform.
Consider your wheels & tyres as the final piece of the drivetrain puzzle: the power produced by your engine ultimately has to turn these wheels to put power to the tarmac. Ignoring mass for a second, increasing the diameter of your wheel & tyre combo makes the final 'gear' of your drivetrain system bigger. This actually means your car will be slower to accelerate!
The good news is that your top speed will increase. It's worth noting that these changes are fairly minimal. My quick (and boring) maths estimates a Honda S2000 will creep from 158mph to 164mph at the top of 5th gear if you increase your wheel/tyre size by an inch. VTEC, yo.
Increasing the wheel size will also give you a slightly larger contact patch where the tyre touches the road, but this is even more minimal than the gearing differences discussed above. One thing that could surprise you is the fact that your speedo will be incorrect. Speedometers are always calibrated using the OEM wheels & tyres - the speedo in the S2000 mentioned above will actually read about 4% lower than its actual speed when wearing its new shoes.
Fair warning - the constabulary will not accept this as an excuse for speeding.
Increasing the width of your wheel & tyre setup has a completely different impact to the dynamics of your car. Increasing the tyre width significantly increases the contact patch between tyre and tarmac. In very general terms, this means more grip at the expense of marginally slower acceleration.
Mass and Manufacturing Methods
Reducing the mass of your wheels is the golden ticket for increasing performance. Assuming the same dimensions, a lighter wheel improves pretty much every other performance aspect there is to consider.
Not only does this reduce the mass of the vehicle overall, it reduces the rotating mass, meaning the engine doesn't need to work quite as hard to turn the wheels. Reducing the unsprung mass (mass which moves up and down with the suspension) also means the suspension can 'react' quicker, giving an improved handling response.
With this in mind, let's take a look at how wheels can be made and which is the best choice for you.
Cast wheels are most common these days. This process usually involves heating an aluminium alloy to 700+°C until molten. The molten aluminium is then poured into a wheel-shaped mould and allowed to cool. The end result is an inexpensive wheel which can look aesthetically pleasing. The downside is that this process significantly weakens the material, meaning the engineer needs to use more of it - this adds mass.
Forged wheels are, in my humble opinion, the most badass method of wheel manufacture. To cut a long story short, a big lump of aluminium alloy is forced into wheel-shaped submission by a herculean pressing machine. The material may be heated slightly to make the process easier, but for the most part it's a process of brute force.
This process maintains the strength of the original material, meaning the design can accept less mass to achieve the same strength as a cast wheel. However, the cost of a forged wheel is significantly higher than a cast equivalent.
A split rim is a more old-school approach that solved a lot of the problems of the time. Forging an entire wheel in one piece used to be a ridiculously difficult (expensive) process (a nice set of classic Porsche Fuchs forged alloys can fetch ~£3000!). Splitting the wheel into two-or more parts made the manufacturing process much easier.
The outer face was forged and bolted to an easily-made rim. This also meant that a dented rim could be swapped in a jiffy at a fraction of the cost of replacing an entire wheel. These days, the manual process of bolting the pieces together makes this manufacturing method a rarity. Although often lighter than a cast equivalent, modern manufacturing technology has rendered single-piece forged alloys the modifier's choice for mass reduction and strength.
This article only scratches the surface of alloy wheels. There's many more options and much more detail to explore. Leave a comment below if you have any specific questions you'd like answered or if you'd like a new feature on a completely different part.