Alloy wheels explained!
What is PCD? What is offset? How do you stance a car? If you’re scratching your head trying to figure out the answer to any of these questions then you’re in the right place! With over 40 years of motorsport experience, our own dedicated Wheels, Tyres and Exhausts Team and an on-site Fitting Centre, we’re the experts when it comes to new rims.
If you’re browsing alloy wheels on our site, you might be feeling a bit bewildered by all the different fitment options available. Unfortunately, there isn’t really a simple way to show you the different options available, but we’ve put the information on our site for those of you who know what you’re doing! If you’d like to know more about wheel and tyre fitments though, read on! Of course if you just want to know what wheels will fit on your car, either give the experts in our Wheels, Tyres and Exhausts Department a shout and they will sort you out, or have a look online. There’s an online owners’ club for just about every model of car – and almost all of them will be able to tell you what a common wheel fitment for your car would be. For instance, if you wanted to know what offset 8.5 inch wide wheels will need to fit correctly on a VW Scirocco mk3 then you could check out this thread over at Scirocconet. Turns out that the standard fitment is ET42 on that particular car – and there is some room for adjustment with 8.5 inch wide wheels. Obviously you should err on the side of skepticism when getting info from a forum, but you’ll generally be able to get the gist of what’s going on – especially if you check out a few different sources.
Changing wheel size
How to read a car tyre
How to calculate rolling radius
PCD/number of holes
How is a car tyre fitted?
Tyre load index table
Tyre speed rating table
When picking out a set of wheels, the first thing that most people will think about (after the style) is the size. Wheel sizes have been getting bigger and bigger recently and what was considered large just a few years ago is nowadays just average – or even small (remember kids, it’s not the size of the boat …)! Of course, bigger isn’t always better (just keep telling yourself that ok?) and it’s quite possible to downsize rims too if that’s your style. Below, we look at the pros and cons of each of these choices.
Big rims are the classic way to improve your car’s looks. They feature on everything from touring cars to tarmac-spec rally cars and you’ll rarely find a high performance road car that doesn’t feature some enlarged hoops (which sounds a bit painful when you put it like that!). “How big can I go” is a question that we get asked a lot here at Demon Tweeks – and it’s one that we never mind answering! Basically, if you want to keep things reasonably sensible, you’re going to want to stay within a similar rolling radius to the wheels and tyres your car has fitted as standard. All this means is that the circumference of the outside of your tyre would be roughly the same as it is as standard. In other words, you’re effectively just changing the diameter of the rim and making the tyre height smaller to compensate for this. If you make your rolling radius larger than it is as standard, your speedo will under-read – so you don’t want to introduce too much error here if you value your licence! Another effect of making the rolling radius larger would be that your cars acceleration would be dulled slightly because of the gearing change this would introduce. Your top speed would probably be increased a bit though.
Finally, making the rolling radius larger will make the car sit higher unless you lower its suspension at the same time. Check out what performance suspension options are available for your car on our main site if you’re worried about this. If you would like to learn more about suspension in general, then you might want to read our guide on how to make your car handle better too. If you want to learn how to calculate your rolling radius, then read on!
Smaller rims have never been as popular as large ones, but there is a lot to be said for them in the right situation. Whereas larger rims will generally be shod with skinny low profile tyres which don’t compress very much through corners, a smaller rim can use a higher profile if you want – meaning increased comfort. Most people who fit smaller aftermarket rims are after a very specific effect though – and often the suspension on these cars will be low enough to slam them properly into the weeds. Euro Style Golfs are a classic example of this.
If you make your rolling radius smaller, you’re going to run into the opposite problems to those you would if you made it larger. Obviously it will make your car lower – which may cause ground clearance issues (especially if you already have lowered suspension), but it will also make your speedo over-read! This perhaps isn’t as serious as the opposite problem (because you’ll think you’re going faster than you actually are), but is still far from ideal! The biggest problem with fitting smaller rims is brake clearance. Especially on sportier cars or cars with brake upgrades you may find that smaller rims won’t actually fit over the brakes your car has fitted. You could make them smaller, but that would be pretty extreme – and not what we’d generally call a performance modification!
Those letters and numbers on your tyres aren’t just for show you know! Learning what they all mean makes things a lot easier when shopping for tyres and finding rims that will fit well on your car. Below, we go through each one in turn – using the standard layout shown in the image above. If your tyre doesn’t have information in this layout on its sidewall, it’s likely that you’re looking at something a bit out of the ordinary! In that case, either speak to the guys in our Wheels, Tyres and Exhausts Department, or try Googling your tyre size to see how it’s laid out!
Section width (225)
The section width is the width of the tyre’s tread in millimetres. In this example the tyre is 225 mm wide.
Aspect ratio (40)
The aspect ratio is the height of the sidewalls of the tyre expressed as a percentage of the section width. In this example the height of the sidewalls is equivalent to 40% of the tyre’s width (225 mm) which is 90 mm.
‘Z’ indicates that the tyre can safely operate at speeds higher than 150 mph. This is in addition to its normal speed rating (see below). Your tyre will not have this ‘Z’ if it’s rated for a speed lower than 150 mph.
‘R’ simply stands for ‘radial’ – indicating that this tyre has a radial (rather than cross-ply) construction. You should never mix radial and cross-ply tyres – although just about every consumer tyre you can buy nowadays is radial.
Rim diameter (18)
The rim diameter is the size of wheel that the tyre will fit onto – measured in inches. In this case, the tyre is designed to fit on an 18 inch wheel.
Load index (92)
‘92’ refers to the tyre’s load index – which tells you how much weight it can safely carry. This is really important, so you need to ensure that any new tyres you purchase have a load index the same as or better than what is specified for your vehicle. In order to find out the load index in kg, you need to look up the number on your tyre in a tyre load index table. The bigger the number, the higher the rating.
Speed rating (W)
‘W’ refers to the tyre’s speed rating – which tells you how fast the tyre can safely travel. This is very important for safety. In order to find out what your speed rating actually is, you need to look this letter up in a tyre speed rating table.
Although these wheels are different sizes, it’s possible to keep the overall size the same by using different tyre profiles
Calculating your rolling radius isn’t so bad as long as you do it in stages. The main problem is that for reasons of convention, wheel height is measured in inches whereas tyre profile (height) is expressed as a percentage of the section width – which is measured in millimetres! Below, we’ve written out the steps you need to go through if you want to calculate your rolling radius:
Find section width
This will be printed on the side of your tyre and will typically be three digits long. In the photo above, the section width is ‘225’ – meaning that the tyres are 225 mm wide.
Find aspect ratio
This will generally be the next figure in the size printed on the side of your tyre. In the photo above, the aspect ratio is ‘40’ – meaning that the sidewall height is equal to 40% of the section width.
Find rim size
This will generally be the next number in your tyre size – and probably comes after an ‘R’ (which stands for ‘radial’). In the photo above the rim size is ‘18’ – meaning that the tyre is designed to be mounted on an 18 inch rim.
Calculate sidewall height
For this, you need your section width and aspect ratio figures. Take your aspect ratio (in this case 40) and add a zero and decimal point before it (0.40 in this case). Multiply your section width by this figure to give the sidewall height and make a note of the result. For the photo above, the sidewall height is 90 mm.
225 X 0.40 = 90
Convert rim size to mm
We already have the rim size, but it’s measured in inches rather than mm. To convert inches to mm you just need to multiply by the magic number (25.4).
For the photo above, the rim size in mm is 457.2 mm.
18 x 25.4 = 457.2
Divide rim size in mm by two
We’re interested in the radius of the wheelset – not the diameter – so we only need half of the rim size. Divide the mm figure you just calculated by two.
For the photo above, half of the rim size in mm is 228.6 mm.
457 / 2 = 228.6
Add this figure to the sidewall height
Now that we know half the height of the rim in mm, we can add it to the sidewall height to find the overall rolling radius. We only need to worry about the sidewall height once because we’re calculating the radius of the wheelset – not the diameter.
For the photo above, the overall rolling radius is 318.6 mm.
228.6 + 90 = 318.6
If you do this for both your existing and new setups, you will be able to see the difference (if any) that they will give in terms of rolling radius. As long as your old and new measurements are within about 2.5% of each other, you shouldn’t run into any problems.
So, now that you know all about a wheelset’s vertical dimension, what about the horizontal? A set of wide tyres will look great (especially from behind the vehicle), but go too wide and you could risk messing up your car’s handling or fuel consumption – or the wheelset simply not fitting under your arches! So this section explains everything you should know about tyre width.
Tyre width is a bit more complicated than just knowing your rim width and matching a tyre to it. Because of the way a tyre mounts on the rim, you can usually oversize or undersize them a bit – which can be helpful in certain situations. The thing is, the amount you can do this varies with the profile of the tyre – because there will be more or less sidewall to stretch and accommodate any size difference. Anyway, to keep things as simple as possible, we’ve included a link to a table below for guidance. We do stress the word guidance. Sometimes there will be other factors at play when choosing tyres that we can’t account for here – so if you’re at all unsure, then please give our Wheels, Tyres and Exhausts Team a shout and they will give you a definitive answer.
One of our best-selling performance tyre brands is Yokohama – and on their main site they’ve included a handy tyre size table for width. Check it out here. Whilst you might be able to physically mount a given tyre on a rim width other than the ones shown in this table, Yokohama don’t recommend it.
PCD sounds a bit like it’s going to be complicated, doesn’t it. It doesn’t get any better when you explain the acronym either – Pitch Circle Diameter. Still a few blank faces! Well luckily PCD isn’t actually complicated at all. All it is is the diameter of the circle made by the lugholes on your wheels. Not your ears! The holes that your studs/bolts go through to hold the wheel on. The diagram below shows this visually. The number of holes is pretty self-explanatory really! Quite simply, this is one thing that you’d better get right when shopping for new wheels – or they won’t fit at all! Generally, PCD will be expressed as a single number, followed by an ‘x’ symbol and then another, larger number. An example would be 5×100. This simply means that the wheel has five lug holes spaced around a circle 100 mm in diameter. So 4×108 would mean four holes spaced around a circle 108 mm in diameter.
Offset. The Big Daddy. The King of Sting. The Master of Disaster. The Grand Poobah of complicated wheel-related things. But when you actually stop to think about it, offset ain’t so tough. It’s actually pretty simple (if a bit confusing)! Check out the diagram below and read on:
Ok, so, first things first. Offset is measured in ET. But what does ET mean? Well it’s nothing to do with small brown aliens who like telephones, that’s for sure. In this case, ET actually stands for ‘Einpresstiefe’, which is German for ‘insertion depth’ (and which effectively just means ‘offset’ in English).
Now for the confusing part. Every wheel has an offset number (this is usually stamped into it somewhere) such as ET15 or ET45. This refers to the distance in mm between the axle pad of the wheel (the flat plane that mounts against the hub) and the imaginary centre line of the wheel. In simple terms, wheels with a lower offset will stick out more than those with a neutral or positive offset. No, we didn’t get that the wrong way round – told you it was confusing!
Conversely, if you were to fit a set of wheels with an offset much higher than your standard rims, you would find that the wheels would be closer to the hubs – possibly fouling on your suspension components, wheelarches, etc. Clearly, offset is something that you don’t want to mess up!
Unless you’re after a stance look – where owners will often use crazy offsets to fit very large wheels under their arches and achieve a specific look – you’re probably going to want to stay fairly close to whatever offset your factory wheels have. When speccing new wheels, it’s important to remember that offset is measured from the centre line of the wheel – so any increase in the width of the wheel will be divided equally between the inside and outside. Say for instance that your car’s standard wheels have an offset of ET35 and that all four of them are 8 inches wide. You decide to buy a new set of wheels – with the same offset (ET35), but 10 inches wide – increasing their width by 2 inches. This would mean that the wheels would stick out towards the arches 1 inch more – but they would also stick in towards your suspension by a further 1 inch.
Offset can be changed by adding a set of wheel spacers – but obviously you can only space a wheel in one direction! So spacers can be used to add negative offset (wheel further out) but not to add positive offset (wheel further in). More information on spacers is available in the section on centre bore below.
When considering any modifications to your road vehicle, you always need to check that they will be road legal. In the UK, the MOT test manual (as of February 2014) contains no mention of a tyre protruding from a vehicle’s bodywork being a cause for failure. With this said, such a setup may still present a safety issue, and there have been cases of people being prosecuted for this. Check out these threads over at Pistonheads and LR4x4 for more info. Regulations may vary if you’re outside the UK – so it’s always best to check before making a purchase.
If you’re happy with calculating all this for yourself then great – but it might be worth making a call to our Wheels, Tyres and Exhausts Department before pulling the trigger. These guys have seen it all before, and will be able to walk you through whatever it is you’re after. This is also an area where finding out the experiences of other enthusiasts can really pay off. As we mention at the beginning of this guide, it’s always worth checking on owners’ forums for your make and model to see what works and what doesn’t. You might even get some ideas!
The centre bore of a wheel is something fairly self-explanatory, but not everyone realises just how important it is. On a hub-centric fitment (which describes the majority of cars), the wheel studs or bolts do not bear the weight of the vehicle itself. Rather they’re simply there to hold the wheel on. The weight of the vehicle is held up by the spigot at the centre of the hub – which protrudes out and fits into the centre bore of the wheel. On most OEM (Original Equipment Manufacturer) wheels, this bore will be a close fit – meaning that it simply slips over the spigot. But on most aftermarket wheels, this bore is oversized to allow for a range of fitments to be accommodated. It is vital that in such cases a suitable spigot ring is fitted to increase the diameter of the spigot and ensure a tight fit. Quite simply, without one of these simple devices fitted, you run a real risk of your wheels falling off!
It’s also important to consider the spigot when fitting wheel spacers to a car with a hubcentric fitment. If enough of the spigot does not stick out past the spacer then there will be nothing to take the weight of your car! In these cases, hubcentric wheel spacers are available, which include their own spigot. Check out the section of our site on vehicle-specific wheel spacers for more info – but do give our Wheels, Tyres and Exhausts Team a shout if you get stuck. If you’ve spaced the wheel far enough for the spacer to go out past the original spigot, then it’s likely that your standard wheel bolts/studs are not going to be long enough either. Luckily, we sell a full range of longer bolts and studs – which can be found under Wheel Nuts and Bolts on our main site. This is of obvious importance for safety!
Going from running steel wheels to a set of alloys? You’re going to need new wheel bolts/nuts then (depending on what your car has fitted)! The type of fixings you use on steel wheels are not suitable for use with alloy wheels – because they’re much harder. This means that they can eventually chew through the bit of the alloy responsible for holding them to your hubs! Not good at all! As before, check out the section of our site on Wheel Nuts and Bolts to see what we stock.
Alloy wheels are a considerable investment, so if your car doesn’t already have any, a set of locking wheel nuts/bolts are a very worthwhile investment. Instead of having a standard head that a normal socket will go around, locking fixings have a unique pattern – meaning that a ‘key’ is required in order to remove them. This will give you peace of mind the next time you park your motor somewhere a bit … dubious, so it’s well worthwhile.
If you’re fitting a set of lightweight alloys with the aim of reducing the unsprung weight of your vehicle, then it’s worth also considering your wheel fixings. We sell lightweight wheel nuts and bolts that can save you a few extra all-important grams. If you’re not familiar with the reasons that unsprung weight is so important then check out this discussion over at Pistonheads, which does a pretty good job of explaining the main factors at play.
We’re not going to go into too much detail here about suspension mods – given that we’ve already written a guide on how to improve your car’s handling – but you can’t really talk about wheels without considering this issue. The main issues here occur if you change the rolling radius of your wheelset. If you’ve gone bigger, then you’ll have increased the ride height of your car – and a smaller rolling radius will have had the opposite effect. Not only can this make your car look strange (especially in the case of an increased ride height), but you can run into issues with tyre/wheelarch contact or even parts of your car scraping on the floor in extreme cases!
We sell a full range of suspension tuning products that can help to offset these issues – as well as making your car look and handle great. Whilst a set of lowering springs or a lowered suspension kit will get your car closer to the deck, for the ultimate in performance and adjustability what you really want is a set of coilovers. Coilovers will allow you to adjust your car’s ride height to suit your wheels and personal preference – allowing you to get the perfect setup. For more information on performance suspension, check out the article we link to in the previous paragraph!
Unlike, say, a bicycle tyre, car tyres are not something that most of us would be wanting to fit ourselves in the shed! Balancing car wheels correctly requires (expensive) specialist equipment, and unless you fancy doing this four times on your brand new rims, we recommend that you get them machine-fitted too! Read on if you want to find out how it’s actually done.
Rather than yoink the tyres on with a piece of angle iron, professionals like our technicians use a special tyre-fitting machine. The bead of the tyre is lubricated with soap to make it go on easier, and the inside of the wheel is gripped by the machine’s specifically-designed jaws (which won’t damage anything). A new valve is pulled through the hole in the rim from the outside, and then the tyre is placed over the wheel. The tyre machine turns the wheel slowly, and it has an arm – which pushes the bead of the tyre over the rim without actually touching the metal. This ensures that your wheels won’t get damaged by the machine. Another arm of the machine is then used to press the second bead of the tyre over the rim. All that’s left then is to use a high pressure air pump to ‘pop’ the tyre onto the rim correctly. If you’ve ever stood next to a fitting bay and been surprised by what sounded like gunfire, then it was probably a tyre being popped onto a rim (either that or you should probably change tyre fitters!).
Next up, it’s time to balance the wheel. This takes account for any imbalances in the wheel and tyre combo – and ensures that your wheels won’t vibrate at high speed. The wheel is bolted on to a balancing machine through its centre bore and spun at a high rpm (like in the photo above). A computer in the machine is able to pick up on any imbalances in the wheel from this, and indicates to the technician where weight needs to be applied. We use adhesive weights which stick to the inside of the rim – so they’re hardly visible from outside. See above for an example of this – you can hardly even see the weights from the inside of the wheel!
When the balancing is done, the wheels are ready to be bolted on to the car. After they’re on, any remaining soap is cleaned off, the tyre pressures are double-checked, and the fixings are torqued up. You’re ready to roll!
You’d be surprised at how many people will get an expensive set of wheels and tyres fitted, but won’t spend another few quid to make their setup look extra special. Something as simple and cheap as a set of wheel nut/bolt covers, alloy tyre valves or flush fit valves will make all the difference and really set your shiny new rims off. Speaking of which – want to keep your wheels looking as good as they did when you first had them fitted? We present to you the pro-detailer’s secret weapon – Poorboy’s Wheel Sealant. Apply a coat or three of this to your wheels once you’ve had them fitted and repeat every few times you wash them. We promise that you’ll be amazed at how easily they clean up after that! It smells lovely too – which is always a bonus.
Don’t forget that your brakes may well be more visible after you’ve had new rims fitted (unless you’ve gone for Ronal R10 Turbos!). This means that you might want to consider making them look a bit better! The cheapest and simplest way to do this is to spruce them up with some brake caliper paint, but if you want to go all out then you could consider some performance brake discs or even a big brake kit!
Finally, whilst all four of your tyres are off the car, why not take the opportunity to have a tyre pressure monitoring system (TPMS) fitted? These things really do save lives (as well as cars, expensive wheels and Armco!) and are well worth the small extra investment – especially if you do a lot of miles on the motorway. If you’ve never experienced a high speed blowout, then lucky you! Let’s keep it that way!
We hope that you’ve found this guide helpful, and that you now know everything you need to about choosing a new set of wheels! Is there anything you can think of that we missed? Also, if you’re a fan of our guides, what would you like us to cover next? Until next time, take it easy!
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