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These are some brief rundowns of wheel fitment, and all of the aspects that make a wheel fit properly. Also, this should act as a place to ask questions about which wheels will fit, how they will fit, or any other questions about wheels in general.
Alas, the basics.
We'll be using the stock specifications for Detroits as examples throughout. These specs are: 18x7.5 et51.
Offset: The 'et' part of the specs usually given. Distance from the centerline of the wheel to the hub mount (where the bolts mount). The lower the offset is, the more aggressive the wheel fitment will be, and usually the wheel will be more concave, or set in from the lip.
Example: If a wheel had an offset of 51, and another had an offset of 41, the second wheel would be 10mm more aggressive, or poke 10mm more than the first wheel (assuming they have the same width.)
Now here's another picture, which explains why lower offsets push the wheel out further. The red thing is basically the hub, and it's the same length for all 3.
Centerbore: The diameter of the circle in which your wheels mount to your car's hub. Our centerbore is 57.1mm. If wheels have a different centerbore than the one your car has, you can get hub rings, which will allow the wheel to fit snugly around the hub of your vehicle and reduce vibration.
Bolt pattern: How many bolts there are, and how they are spaced apart. Our bolt pattern is 5x112. This means that you have 5 lugs, and the top bolt is 112mm away from the bottom right. Wheel companies make wheels in different bolt patterns to fit different cars.
Adapters: Adapters can allow you to fit wheels with different bolt patterns to your cars hub. They decrease the final offset of your wheels, and the minimum size is 15-20mm thick. Adapters bolt into your hub, then bolt into the given wheel.
Example: We have 5x112 hubs, but if you want to run a wheel that has a 5x114.3 bolt pattern, you need adapters (or wobble bolts, that'll be covered later.)If said wheel has an original offset of 50, then the final offset would be 30 because of the adapter's thickness.
As you can see, you bolt the adapter onto the hub of your car, then bolt the wheels to the adapter.
Spacers: Spacers allow you to push your wheel outwards from the wheel well, by putting a gap between your wheel and the hub of your car. The thickness of the spacer can range from 3mm anywhere to 50mm.
Example: Let's say you have detroits, and you want to make them a little more flush with the fenders of your car. We start with et51. We can use 12mm spacers to decrease the offset of your wheels. The 12mm spacers will make the wheels have a final offset of 39.
The thing hanging off is the spacer.
Note: You will need longer lugnuts if you are using a spacer that is more than 5mm thick.
Stock lug bolts are 28mm! Just add the thickness of your spacer to 28, and that's how long your new bolts should be.
PCD Variation bolts aka Wobble bolts: These babies will allow you to fit wheels that are ~2-3mm different from your hub because they, well, wobble. We can fit 5x110 or 5x114.3 wheels with wobble bolts. They will not work with adapters larger than 3-5mm.
see? they wobble
How widths/offsets affect what the wheel actually does/looks like:
Now, going with our original example of 18x7.5 et51. Let's say we wanted to know what a wider wheel (8.5 inches will do) will fit like. Now you'd think that it would poke 1 more inch than your original wheels. But that, my friend, is not true. Half of the difference goes to the inside, and half to the outside. Your wheel would be pushed out toward the fender by about 12.7mm (1/2 inch).
With offsets, it's more straightforward. The amount the offset decreases, the more you're going to poke; the more the offset increases, the more that the wheel will be pushed inside the wheel well.
Tire sizing: Just going to elaborate on tire sizing. It's really important to know the right tire to put on a wheel when changing a wheel set up. We'll use the stock tire specs, which are 225/45/18 for this example. The first number is 225. This is the width of the tire, in millimeters, horizontally. Now the second number in the specifications is 45. This is called the aspect ratio. It's basically saying that the sidewall of the tire is 45% of the given width. The higher the percentage, the larger sidewall. Lower percentage= smaller sidewall. So our tire would be 225mm wide, and the sidewall would be 101mm high. The 18 is obviously the size of the wheel that the tire is going on. I've included a tire fitment guide from Golfmkv that will aid you in selecting the right tire for your wheel.
Update: Frontspacing
Alright, now everyone wants to know what specs will poke, which will be flush, which will tuck, etc. This all has to do with Frontspacing, which is the space between the hub and the fenders of our car horizontally. From what I can gather, An 8" wheel with an offset of 40 is pretty much perfectly flush with the fender. I've done the math, and the Frontspacing of a flush wheel is 61.6mm. What this means is that anything over this number will poke, anything under will be sunk inside the fender(depending on how much your numbers vary. Here's how you can get the Frontspacing of a certain wheel.
The formula: [1/2(Wheel width(in inches)x25.4 )-Offset(in mm)]
The 25.4 is the conversion for inches to millimeters.
Useful links
MK5 Wheel/Tire fitment guide (applies to mk6's)
http://www.golfmkv.com/forums/showthread.php?t=59761
Everything you need to know about tires!(Thanks to ModShack)
http://www.carbibles.com/tyre_bible.html
Alas, the basics.
We'll be using the stock specifications for Detroits as examples throughout. These specs are: 18x7.5 et51.
Offset: The 'et' part of the specs usually given. Distance from the centerline of the wheel to the hub mount (where the bolts mount). The lower the offset is, the more aggressive the wheel fitment will be, and usually the wheel will be more concave, or set in from the lip.
Example: If a wheel had an offset of 51, and another had an offset of 41, the second wheel would be 10mm more aggressive, or poke 10mm more than the first wheel (assuming they have the same width.)
Now here's another picture, which explains why lower offsets push the wheel out further. The red thing is basically the hub, and it's the same length for all 3.
Centerbore: The diameter of the circle in which your wheels mount to your car's hub. Our centerbore is 57.1mm. If wheels have a different centerbore than the one your car has, you can get hub rings, which will allow the wheel to fit snugly around the hub of your vehicle and reduce vibration.
Bolt pattern: How many bolts there are, and how they are spaced apart. Our bolt pattern is 5x112. This means that you have 5 lugs, and the top bolt is 112mm away from the bottom right. Wheel companies make wheels in different bolt patterns to fit different cars.
Adapters: Adapters can allow you to fit wheels with different bolt patterns to your cars hub. They decrease the final offset of your wheels, and the minimum size is 15-20mm thick. Adapters bolt into your hub, then bolt into the given wheel.
Example: We have 5x112 hubs, but if you want to run a wheel that has a 5x114.3 bolt pattern, you need adapters (or wobble bolts, that'll be covered later.)If said wheel has an original offset of 50, then the final offset would be 30 because of the adapter's thickness.
As you can see, you bolt the adapter onto the hub of your car, then bolt the wheels to the adapter.
Spacers: Spacers allow you to push your wheel outwards from the wheel well, by putting a gap between your wheel and the hub of your car. The thickness of the spacer can range from 3mm anywhere to 50mm.
Example: Let's say you have detroits, and you want to make them a little more flush with the fenders of your car. We start with et51. We can use 12mm spacers to decrease the offset of your wheels. The 12mm spacers will make the wheels have a final offset of 39.
The thing hanging off is the spacer.
Note: You will need longer lugnuts if you are using a spacer that is more than 5mm thick.
Stock lug bolts are 28mm! Just add the thickness of your spacer to 28, and that's how long your new bolts should be.
PCD Variation bolts aka Wobble bolts: These babies will allow you to fit wheels that are ~2-3mm different from your hub because they, well, wobble. We can fit 5x110 or 5x114.3 wheels with wobble bolts. They will not work with adapters larger than 3-5mm.
see? they wobble
How widths/offsets affect what the wheel actually does/looks like:
Now, going with our original example of 18x7.5 et51. Let's say we wanted to know what a wider wheel (8.5 inches will do) will fit like. Now you'd think that it would poke 1 more inch than your original wheels. But that, my friend, is not true. Half of the difference goes to the inside, and half to the outside. Your wheel would be pushed out toward the fender by about 12.7mm (1/2 inch).
With offsets, it's more straightforward. The amount the offset decreases, the more you're going to poke; the more the offset increases, the more that the wheel will be pushed inside the wheel well.
Tire sizing: Just going to elaborate on tire sizing. It's really important to know the right tire to put on a wheel when changing a wheel set up. We'll use the stock tire specs, which are 225/45/18 for this example. The first number is 225. This is the width of the tire, in millimeters, horizontally. Now the second number in the specifications is 45. This is called the aspect ratio. It's basically saying that the sidewall of the tire is 45% of the given width. The higher the percentage, the larger sidewall. Lower percentage= smaller sidewall. So our tire would be 225mm wide, and the sidewall would be 101mm high. The 18 is obviously the size of the wheel that the tire is going on. I've included a tire fitment guide from Golfmkv that will aid you in selecting the right tire for your wheel.
Update: Frontspacing
Alright, now everyone wants to know what specs will poke, which will be flush, which will tuck, etc. This all has to do with Frontspacing, which is the space between the hub and the fenders of our car horizontally. From what I can gather, An 8" wheel with an offset of 40 is pretty much perfectly flush with the fender. I've done the math, and the Frontspacing of a flush wheel is 61.6mm. What this means is that anything over this number will poke, anything under will be sunk inside the fender(depending on how much your numbers vary. Here's how you can get the Frontspacing of a certain wheel.
The formula: [1/2(Wheel width(in inches)x25.4 )-Offset(in mm)]
The 25.4 is the conversion for inches to millimeters.
Useful links
MK5 Wheel/Tire fitment guide (applies to mk6's)
http://www.golfmkv.com/forums/showthread.php?t=59761
Everything you need to know about tires!(Thanks to ModShack)
http://www.carbibles.com/tyre_bible.html
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