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How to Read and Understand Wheel Size and Offset
#1
I found this thread on the my.is forums a couple days ago and thought it was some pretty useful and easy to understand information that could some people on the Squad who are looking at getting some aftermarket wheels. ***All credit for this thread belongs to PhillyB on the my.is forum.***

PhillyB Wrote:Im not going to say everything in here is exact or even in the right terms but to my know how it is i did not do much research further into this rather this is everything off my head and hope it helps put things into perspective and will guide others in the right direction.

[Image: wheeldiagram.jpg]

PCD

The term PCD stands for (pitch circle diameter) and is the diameter of a circle drawn through the center of your wheels bolt holes. PCD is measured in millimeters and also indicates the number of studs or bolts the wheel will have.


OFFSET

Every car requires a unique offset. This is where the outside of the wheel needs to be in relation to the body-line of the vehicle, realistically you can go 5-7mm outside these recommendations, but always check with vehicle and wheel manufacturer' if you are unsure, as there are often other factors that need to be considered.


One thing to remember for wheel offset is everything is in MM So if you had a wheel width of 8 Inches you would multiply it by 25.4 for the Inch to Millimeter Conversion

So i will use this next picture to explain it as simple as i can. I will use a default size.

Default Wheel Specs:
Wheel Diameter= 18 inch
Wheel Width= 8 inch aka 203.2mm (8 x 25.4)
Offset= +30mm

So center line of the wheel is found by dividing the wheel width by two which would be 8/2 having center-line at 4" inward.

Center-line is also Zero offset. So if your wheel was 8 Inches wide and had a Zero offset you would have a 4 Inch lip or 4 Inches of concave.

That being said your wheel of 8 Inches wide with a +30mm offset would be looked at as the following.

203.2mm (8 Inches)
101.6mm (Center-line 8/2 = 4" Center-line 4x25.4)

Now that you know Center-line known on this wheel as 101.6mm you can take your +30 which is 30mm added to the center-line going towards the outside of the wheel away from the brake. A negative offset would pull towards the brake side.

+30mm lets you know that your hub is sitting at 30mm plus of the center-line of the wheel.

so knowing your center is 4" that converts to 101.6 add the 30mm it would be 131.6mm.

203.2mm - 131.6mm = 71.6mm which converts back to 2.8.. inches of lip/concave.

131.6 is your back spacing which converts to 5.18... inches.

5.18..." + 2.8..." = 7.98.." as shown above 8 Inch wide wheel.

Sum it up again:
203.2 is overall width in MM
101.6 is the center line of the wheel in MM
+30mm is the Offset of the wheel.

Add the offset and centerline to get the 131.6 which is your back spacing. Take the overall width in mm and minus the backspacing you will have your offset or inset as shown above.

[Image: wheeldiagram2.jpg]
[Image: measuringbackspacingandcalculatingo.jpg]

**Disclaimer spacers are to be used at your own risk. Do not purchase a cheap set of spacers and if i may recommend use the bolt on spacers which give you new stud mounting points rather then using slip on spacers

Bolt on spacers shown below.

With that being said Lets talk offset with spacer applications:

Im going to use one of my friends Twestman's car as an example.

Twestman's setup.

[Image: IMG_0006.jpg]

His setup is as follows:
Enkei RPF1 18x9+35, 18x10+38

Yet to reach the fitment he wanted he leaned towards spacers.

The way a spacer works is it adds to the offset of the wheel. Does not physically change any properties of the wheel yet does add to the backspacing.

Example of an 18 inch Diameter wheel with a 9 inch width and a +35mm offset.

[Image: 18x935example.jpg]

Now with the application of the spacer you have moved Outer Position of the wheel 10mm out and now have earned yourself 10mm of Inner Clearance.

***Shown in the picture is a 15mm spacer yet same theory.

[Image: WheelSpacerExample.jpg]

With the addition of the spacer you can see how it pushed the wheel outward by using a spacer to achieve a 18x9 +25.

[Image: 18x93510mmexample.jpg]

An example of how this works:

Remember that 25.4mm is equal to 1 full inch.

You have a 18x8 with a +50mm offset if you were to add the following size spacers you would end up with:

10mm spacer: 18x8 +40
15mm spacer: 18x8 +35
20mm spacer: 18x8 +30
25mm spacer: 18x8 +25

Again your not changing anything physical about the wheel so the face, lip , and concave will remain the same. Yet it will push the wheel out away from the hub to reach the fitment desired.

Many people will worry about brake clearance and still want to run aggressive fitments. Going with a wider wheel will have you lose the inner clearance yet if you were to run a smaller wheel you can go with a even lower offset and be safe on both counts.

18x9 +25 will fit the same fender wise as:

18x8.5 +19 With 12mm more inner clearance
18x8 +12 With 26mm more inner clearance

That being said you have the same aggressive fitment that a 18x9+25 would give you yet you made more clearance for the suspension components. I would make visual drawings if this is needed. If so please send me a pm and i can make it happen. Yet i'm sure that if you use the info given previously you can do the math and see for yourself how it adds up.

Okay here's the big break down to help people out...This one is easier than people make it out to be.



Understanding how to read Disc Sizing



You will see it broken down so many ways as every wheel manufacturer will call it something else. Yet it is still the same no matter what they call it.

i will use an example from SSR's wheel lineup. This is from the SSR SP1's in an 18".

Now these are the easier ones to read and figure out as they have preset offsets to help you out. Which most of the time are the only ones they offer.

[Image: DiscSizing.jpg]

Knowing offset is one thing. Yet the offset number does not always refer to how much lip you will actually have.

Looking above you will see i outlined our wheel size of 18x9.5 in purple. So this is a common wheel we would use for the Rear.. Common for the rear! haha i know people out there will throw it on the front. But this is for the Know how not the How i did.

You can see in that section you have the chance to go from +68 on the top section all the way to -13. All within the 18x9.5 Range.

Yet what you need to know is that the Lip sizes are shown above that. So showing that a +30 offset under the 80mm and the 92mm.

So using the information stated before you can figure out your Lip will be 80mm or 92mm which is both at a +30 offset.

80mm = 80/25.4 = 3.15 so you will have 3 inches of lip on that selection
92mm = 92/25.4 = 3.62 so you will have .62 more lip then the 80mm.

So knowing that you can look under the 92mm section and know that each one of those offsets from +30, +18, +10, 0 will all have the same size lip of 3.62" So the only thing that had changed in the equation is the Disc size.

Under the 92mm, which well stay for the rest of the explanation. You will notice the letters:

SL: Super Low
NR: Normal
MD: Medium Disc
HD: Hyper Disc

This is letting you know how much clearance you will have for your brakes.

[Image: DiscDiagram.gif]

*** NOTE *** Not to the exact but you can get the picture.

If you were to look at the overall picture of the Hyper Disc setup you will see that it includes the length off all 4 into one. It would give you the most wheel clearance for your brakes as possible.

This is for those with a need to clear a huge brake disc. Not all brake kits are needed for this yet there are some.

So i hope this one helps some out there. Till next time.

One thing to keep in mind is that the examples used in this write-up are meant for a Lexus IS so the wheel sizes/offsets discussed wont' match exactly w/ EJ fitment but the information is still very beneficial IMO. I'll see if I can't do a little research and make some examples that would fit our car specifically. Hope this helps you guys bigthumbup
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#2
bigthumbup Thanks for the info Dave! I now somewhat understand it Lol, good info. And1
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#3
nice Thumbup
Don't tread on me.

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#4
So damn good info, thanks Dave!
EJ8Squad member #576 - VSM FTW! Rock

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#5
now it makes more sense
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#6
http://www.willtheyfit.com/

Best website ever for this.
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#7
Also I might add, for any guys with questions feel free to ask the guys running really aggressive offsets with their wheels how they made them fit, and so on and so forth...

I know I wouldn't mind to answer questions for anyone. That is the general point of the forum... is to help other people out in some ways.

on another note

I really like the way that guy explained wheel offset, it's very Webster's Dictionary of him.
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