But no need to be scared, my goal is to provide a little insight as to what you're getting yourself into, and along the way I hope to teach you proper terminology and provide basic -and hopefully- over-simplified visuals for your understanding.
Let's start with the basics:
Wheels...Not 'Rims', and Why it's All About Them
Here's a wheel:
Wheels come in a series of measurements, typically by height (say, 18 inches) and width (8 inches). But more often than not they come in different fitments depending on the make and model of the car. We can classify them with the following terms:
Square: all four wheels are the same size (18x8 front, 18x8 rear)
Staggered: all four wheels are the same height, but two are different widths (18x8 front, 18x10 rear)
Double Staggered: two wheels are a different height and width than the other two (17x8.5 front, 18x9.5 rear)
We might also know that they come with various lug patterns, which are measured both in inches and millimeters. If we measure the distance as follows, like so:
we can determine the lug pattern of our wheels through simple conversion:
Lug pattern is essentially the specs of your hub (where the wheel mounts on your car). In order for the wheel to marry to the hub, you need to find a wheel with a matching lug pattern, or a means of cross-matching the wheel and hub together.
Lets take it a step further. Here's an image of a wheel broken down:
Wheels are manufactured either through a forging or casting process. Forged wheels have more dexterity at a molecular level, where-as cast wheels -although formed in a solid mold- tend to lack tactile strength (by comparison).
There are also various types of wheels, but we can generally classify them into simple groups:
OEM: Original Equipment Manufacturer; the factory wheels that came with a car
1 Piece: All the components are one solid piece
2 Piece: The face and the barrel are normally one solid piece, the outer lip is separate
3 Piece: The inner barrel, outer barrel and wheel face are three separate pieces
Aftermarket Cast Replica: a often a 1 piece wheel designed to look like a another 1 or 3 piece wheel
What's important to remember is that wheels take impact -- a lot of impact. If the wheels on your car were manufactured with poor materials, if the casting process was stream-lined for sake of sales, or to cut costs - well, there's a chance they're going to fail.
Replica wheels are notorious in this light:
But, bare in mind that wheel failure is not always a bad thing. High performance wheels are designed to give out at certain points. If the wheel were too dexterous, there's a chance of damage being transferred to the suspension, body or otherwise. Thus, one true sign of a well manufactured wheel is its ability to take impact and still hold air:
Offset is a Game of Millimeters, Not Inches!
Now that we're familiar on general wheel measurements, lets take a look at wheel offsets! Normally, stamped on the inside of the wheel (either in the barrel of the wheel, or recessed in the spokes) is the wheel's offset, or 'ET':
For the time being, let's just consider 'ET' to be synonymous with wheel offset.
ET or Wheel Offset is the measurement of the wheel in relation to its mounting point, in millimeters. It's important to know, because ET specifies the wheel's design specifications, and helps us determine wheel application.
Odds are, if you're looking at buying wheels, you'll inevitably see the following:
"FS: 18x8 all around, 5x114.3, ET35, $$$$ and etc.". These are all crucial details in determining whether or not the setup will work.
The Center Line is basically what it implies: it's the approximate center of the width of the wheel, and where the hub and measuring point of the wheel are constructed in relation to it. If you're confused, it's ok - referencing the Center Line is mostly left to determining clearance for brakes and independent suspensions, as with a positive offset.
Most wheels with a positive offset come from an older generation of manufacturers. Wheels with a positive offset naturally came with a lip, and the face of the wheel was recessed towards the centerline.
Imagine a wheel hanging on the side of a car. Although the barrel is stationary, you're given the ability to push the face of the wheel forward and back. If you push the face of the wheel towards the car, what's the first thing it might come into contact with? Odds are it's the brakes or suspension.
The most common offset measurement is the negative offset, which will be explained below.
Learn to Invert Your Frame of Thought
(This was the most difficult part for me to learn, and in light of that fact, I don't expect you to get it right away either.)
Recall this image:
We can identify the markings; the fact that it states "ET 35". However, what it should actually say is "ET-35". This is because most wheels now-a-days have more of a negative offset than positive.
Another way to think about it would be:
If you designate the mounting point of the wheel as 0 (or close to it), then +1mm would be away from it (or 1mm away from the car), and -1mm would be towards it (or -1mm towards the car) (but the majority of the time, it depends on where the mounting point lies). Therefore, ET35 actually translates to -35mm on a negative offset wheel.
Remember, ET35 implies the distance from the inner face of the wheel to the inner lip. Therefore, the inner lip of the wheel is -35mm from the inner face.
Here's an example of how we measure a negative offset:
While we can modify wheel characteristics through full out customization, we can also take the easy route by adding millimeters through the use of adapters and spacers.
Utilize All the Space!
Adapters take a lot of criticism from wheel enthusiasts. They claim everything from unnecessary load and weight causing wheel bearing failure to uneven tire wear -- some of which is true, but most of the time it's exaggerated. If they come from a reputable source who manufacturers them properly, you can avoid encountering these issues entirely.
What are adapters?
They're a solid piece of metal that mimics a wheel and hub. On the inner side, they have the car's factory bolt pattern. On the outer, they have the bolt pattern of the wheel. This enables us to cross lug patterns from 5x100 to 5x114.3 and so on.
There are essentially two different kinds: Hub-centric adapters, which are designed to marry to the hub of the wheel, and Non-hub-centric adapters, which -although less expensive- are not hub-specific, and can cause vibrations at high speeds and essentially all the problems listed above.
Spacers are the same idea and feature the same details. The only exception is that the come with the factory bolt pattern in mind. Their purpose? To space the wheel further out from the car.
Both adapters and spacers are useful technology in this light. But the thing we need to consider is that while they have their own bolt pattern fitment, they also have their own offsets/ ET as well.
Protip: don't ever try to "space out an adapter", whereby placing a spacer on either side of an adapter. I will laugh, and you'll end up in someone's youtube fail compilation.
The minimum offset for an adapter is 15mm, but they range as high as 25mm, 30mm and so on. Consequently, that also means that the adapter must be at least 15mm long. Spacers on the other hand can be as small as 2mm and range as high as their counterpart.
What to consider when you're applying an adapter or spacer is how they're going to affect the overall offset of your wheels.
For example, let's say the spacer shown above is 10mm long. We know the overall offset of our wheel is ET35. Since we're adding mass to the hub of the wheel, we take millimeters away from the offset. Therefore, the new distance from the spacer/ adapter to the inner lip of the wheel determines our new offset: ET25. We can streamline the process with simple math:
However, apart from measuring wheel ET, there's another type of offset to consider:
How Will It Sit?
Final ET, or how far the outer lip sits in relation to the car's fender.
Now that we know how to identify wheel offset, we have to determine how the wheel will look or "sit" on the car.
A good rule of thumb is that the lower the car gets, the further inward the wheel sits. It's not a lot, (no more than a few millimeters) but just enough to require the use of spacers/ adapters. And wheels will naturally camber roughly 1 degrees as well.
This is vitally important, because two cars can have the same offset, but depending on their ride height, the lip of one wheel might poke out, the lip of the other might tuck in the fender well.
ET1-10 might require the car be completely on the ground in order for the wheel fitment to work! It's that reason why the final ET will range based on ride height.
These are actually the Kerscher KCS wheels on my car. On the right, we were sitting at final ET28, on the left: final ET32. When we lowered the car a couple more millimeters we gained a few degrees of camber. It took a full afternoon with the fender roller before we acquired the desired look.
This is why we call it a game of millimeters!
However, most aftermarket manufacturers and enthusiasts have figured what specifications work best. We relate that to Natural ET.
Natural ET is the measurement which results in the outer lip of your car is sitting relatively flush with the fender.
We can say that the Audi featured above might've come with 16x7" ET45 wheels. However, trial and error proved that the proper fitment for the car was actually 18x8, ET35 (not just for sake of appearance, but performance). Inevitably, the guessing portion is virtually eliminated for most wheel fitments.
We still have to take ride height into account, however.
Assuming we lower the car to the extent that the wheels are nearly tucking inside the fender, provided a little wheel gap. The solution would be to add a 5mm spacer and achieve ET30. Essentially the look we're getting is the same as above, just predominately lower.
And that's basically the essentials. I hope to add any additional info I can think of, but for now it's back to work!
If you have any questions, or would like to see anything featured, feel free to throw a comment or two below.
Thanks for reading!