The wheels on your car play a vital role, but they have a bigger impact on performance than you might think. Find out why with the Fast Car lightweight alloy wheel guide rammed full of the best tips, tricks and advice…
The existence of the wheel goes back as far as the Stone Age, and although the basic principle of the wheel’s purpose in life hasn’t changed, it’s fair to say that it has seen a few upgrades over the years. We’ll skip the first 6000 years of the wheel’s history, because the style of wheels we were bolting to our cars as little as ten years ago is nowadays largely unrecognisable.
Gone are the days of the TSW Venom, the Wolfrace Voodoo and the Momo Arrow. In their place is found a huge selection of aftermarket wheel options that can often make choosing a new set a bit of a minefield. In this guide, we investigate the different types of performance wheel on the market, the various different fitment options to consider and importantly, what to look for in order to maximise performance.
TYPES OF WHEEL
Alloy wheels are now fitted to virtually all new cars, which wasn’t always the case. They’re now available to the masses and it’s certainly a buyer’s market – the price of alloy wheels has fallen at the lower end, but if you’re looking for premium wheels then you can expect to pay some serious money. You tend to get what you pay for with wheels, and in terms of manufacturing they’re typically either cast (made from molten metal) or forged (shaped from a single block of metal).
Forged wheels are lighter and much stronger than cast wheels, but unsurprisingly, they’re also more expensive. The use of exotic materials such as magnesium and carbon fibre is also an option if budgets are considerable, but they do have their disadvantages – in some cases they’ve been known to crack, or worse, shatter into pieces. For practical use, we’d recommend you stick to a high-quality forged aluminium wheel wherever you can. Most of these wheels are constructed using a single piece of aluminium alloy, but some are constructed from multiple components.
Known as a ‘split rim’, this type of wheel uses three main components – an inner rim, an outer rim and separate spokes. The three are then bolted together around the perimeter of the wheel. The use of split rims originated in the ’70s, when tyre sidewalls weren’t flexible enough to stretch onto the wheel without cracking. By using multiple components and bolting the wheel together, the tyre was saved from cracking. This isn’t a problem with modern tyres, so the advantage of split rims is now questionable. One plus point with split rims is that it means individual components of the wheel can be changed without having to replace the whole wheel, so if you drive into a kerb then you can get away with replacing just the outer rim.
This is one of the most important aspects to consider before buying a new set of wheels. Too often, we see cars where the owner got the fitment horrendously wrong, so hopefully this should make your life a little easier in terms of understanding the fundamentals. The best advice is to do some homework and ask around, but this should clear up some of the common mistakes.
WHEEL BOLT PATTERNS
This is the most basic criteria – if you don’t get this right, then your new wheels won’t even bolt onto your car. Find out the facts – are there three, four or five studs (or nuts) holding each wheel on? Secondly, you need to know the diameter of the circle that the centre of these studs lie on, in millimetres. This is called the PCD (pitch circle diameter) or, in some cases, BCD (bolt circle diameter) and is a critical dimension you need to know when ordering wheels.
Different car manufacturers use different bolt patterns, and they’ll be denoted by numbers such as ‘4×100’ or ‘5×114.3’. The first digit refers to the number of studs, and the second to the diameter of the circle they reside on. Get these two right first before you do anything else!
Gone are the days when squeezing 18-inch wheels onto your hot hatch made you a hometown hero. Over the years, cars have gotten bigger and fatter, and now it’s not uncommon for 18s or 19s to be an optional extra at the dealership. Bear in mind that if you’re not planning to upgrade your brakes, having huge wheels can often leave the standard brakes a little exposed, not to mention the fact that larger wheels are likely to make your car slower.
Do some research into the optimum size of wheel for your car, and don’t forget to look into tyre prices, too – you’d be surprised how much they can change for that extra inch in diameter. Performance should be one of the main criteria to determine your wheel size, which we cover in more detail later.
WHEEL CENTRE BORE
Like the bolt pattern, this is an important factor to consider. The centre bore is the hole in the centre of the wheel that fits onto your car’s hub, and it should always be a tight fit to the hub, so as to centre the wheel properly. Some wheels use plastic ‘hubcentric’ rings to reduce the diameter of the centre bore to fit the hub, but a common mistake is to ignore centre bore entirely.
If there’s a gap around the hub, it can cause unwanted vibration as you’re relying on the nuts to centre the wheel, which they don’t always do reliably. Different car manufacturers use different centre bore diameters, so make sure you either order the correct size for your car or that your new wheels come with the correct plastic spacers.
Once you’ve established that your wheels will physically bolt to the car and that they’re centred on your hub properly, then offset is the next area to consider. It’s also another area where people often get it badly wrong. Offset (or ET as it’s referred to) is the measurement taken from an imaginary line drawn through the centreline of your wheel, to the flat mounting surface that bolts to the hub. It’s measured in millimetres and the easiest way to remember it is that the larger the offset, the further the wheel will sit into the car.
The lower the offset, the more the wheel will stick out from the arches. Bear in mind that manufacturers choose offset to offer the optimum geometry and handling characteristics and that straying too far from the standard offset can have an adverse affect on both handling and performance. Don’t forget also that the lower the offset, the more stress it puts on your wheel bearings. Low offset can also lead to problems with clearance on full steering lock due to the larger arc that the wheel follows.
This is the important bit, and unfortunately it’s often overlooked when people buy new wheels. All the components on your car that aren’t supported by the suspension (i.e. the wheels, tyres, brakes, hubs etc.) make up the ‘unsprung’ weight. From a performance perspective, it’s important to reduce the unsprung weight as much as possible, as it allows the wheel to follow the contours of the road more closely, and sharp changes in direction are much easier to accomplish.
Wheels have the added importance of being a component that rotates, and therefore possess inertia. This means that every time you slow down or accelerate, you’re changing the rotational velocity of the wheel, and changing the inertia. The heavier the wheel, the harder it is to make these changes. To paint the picture, imagine a truck wheel rotating at the same speed as a carbon-fibre road bike wheel. The truck wheel would take a much larger opposing force exerted on it to stop. So, how does this relate to you and your car? Well, the bottom line is that you want to fit the lightest wheels you can find.
Fitting lightweight wheels is a far more effective way of adding performance than saving the same amount of weight from elsewhere on the car, as there is a multiplying factor when saving unsprung, rotational weight. Some people say it’s double, and others up to ten times the effectiveness of weight saved from the bodywork. In terms of sizing, choosing your wheels from a performance perspective should really be driven by factors such as how much brake clearance you need, and what width of wheel allows you to fit the optimum width of tyre for maximum grip, without fouling the arches. The larger your wheels, the slower your car accelerates, and larger wheels are mostly used for cosmetic purposes on modern cars.
If you’re fitting 378mm six-pot brakes to your track car, then it’s a different matter, as you’ll need large wheels for clearance, but from a pure performance perspective, then smaller is nearly always better. Take a look at the Nissan R35 GT-R – all the fastest examples have ditched the standard 20in wheels in favour of lightweight 18s. In the real world, wheels often represent a compromise between performance and looks, but if you’re serious about performance, then an 18in wheel is generally considered the limit in most cases. Also spare a thought for tyres, certainly if you’re planning on fitting track tyres, as at sizes above 18in the choice is somewhat reduced and the prices get scary.
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Words Rich White