Can someone explain wheel offset?
Thread Starter
Rennlist Member
Joined: Feb 2013
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From: Summit Point, probably
Can someone explain wheel offset?
I am looking for new wheels for my new addition and have been perusing the classifieds here and other places and I have no idea how to determine if certain wheels will work on my car (2002 Targa).
Stock sizes were 17" and 18" with tirerack and other places reporting optional 19" but that is apparently dependent on offset. (And of course bolt pattern, but that one I understand.)
Anyone have a primer on this stuff, or even a good link to one?
Stock sizes were 17" and 18" with tirerack and other places reporting optional 19" but that is apparently dependent on offset. (And of course bolt pattern, but that one I understand.)
Anyone have a primer on this stuff, or even a good link to one?
Instructor
Joined: Dec 2011
Posts: 223
Likes: 2
From: Driftwood TX
Not sure what you want to know... but follow the link to an offset calculator:
Offset Calculator
Hope that helps
Offset Calculator
Hope that helps
Thread Starter
Rennlist Member
Joined: Feb 2013
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From: Summit Point, probably
I guess, all else equal, what affect does changing the offset have? The linked calculator indicates that increasing offset moves the wheel in toward the car and decreasing offset moves the wheel away from the car. (Keeping the wheel width the same.) Is that right?
How do I determine whether a certain width and offset will fit on a certain car? If I go from a 17*9 wheel to a 18*10 wheel, what offset ranges do I need to be within? etc.
I can't be the only one who has no idea about this stuff.
How do I determine whether a certain width and offset will fit on a certain car? If I go from a 17*9 wheel to a 18*10 wheel, what offset ranges do I need to be within? etc.
I can't be the only one who has no idea about this stuff.
Instructor
Joined: Dec 2011
Posts: 223
Likes: 2
From: Driftwood TX
I guess, all else equal, what affect does changing the offset have? The linked calculator indicates that increasing offset moves the wheel in toward the car and decreasing offset moves the wheel away from the car. (Keeping the wheel width the same.) Is that right?
How do I determine whether a certain width and offset will fit on a certain car? If I go from a 17*9 wheel to a 18*10 wheel, what offset ranges do I need to be within? etc.
I can't be the only one who has no idea about this stuff.
How do I determine whether a certain width and offset will fit on a certain car? If I go from a 17*9 wheel to a 18*10 wheel, what offset ranges do I need to be within? etc.
I can't be the only one who has no idea about this stuff.
Press calculate and it will show how much inner clearance and outer wheel difference you will have... then check your current setup clearances on your car. if you have the adequate room you will be good. if not adjust the offset until you are within spec, then find wheels with those specs.
Rennlist Member
Joined: Dec 2011
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From: Gig Harbor, WA
Offset is where the hub mounting surface is in relation to the center of the wheel. Positive offsets indicate the hub mount is closer to the outside edge of the wheel, while negative offsets indicate the hub mount is closer to the inside edge of the wheel.
How a wheel fits with X offset is a factor of the width of the wheel. An 18x9 et30 wheel will not fit the same as an 18x10 et30 wheel (offset is identified by the letters "et", which comes from the German word "einpresstiefe" which translates to "insertion depth"). While the offset is the same, the 18x10 et30 wheel is ~26mm (1 inch) wider than the 18x9 et30 wheel, so each side of the wheel will stick out 12.5mm more than the 18x9 wheel. So, if you wanted the 18x9 wheel to fit the car like the 18x10 wheel, you would need to lower the offset 12.5mm, so the offset would need to be et17.5.
Does that help at all?
Here is something I made awhile back that might help visualize it as well:
How a wheel fits with X offset is a factor of the width of the wheel. An 18x9 et30 wheel will not fit the same as an 18x10 et30 wheel (offset is identified by the letters "et", which comes from the German word "einpresstiefe" which translates to "insertion depth"). While the offset is the same, the 18x10 et30 wheel is ~26mm (1 inch) wider than the 18x9 et30 wheel, so each side of the wheel will stick out 12.5mm more than the 18x9 wheel. So, if you wanted the 18x9 wheel to fit the car like the 18x10 wheel, you would need to lower the offset 12.5mm, so the offset would need to be et17.5.
Does that help at all?
Here is something I made awhile back that might help visualize it as well:
Intermediate
Joined: Feb 2013
Posts: 49
Likes: 0
From: Baltimore, MD
Offset will vary depend on the thickness of the wheel as you can see in the picture. Offset is the area between the mounting pad of the wheel and the center line of the wheel. The centerline would change based on thickness of the wheel.
What this means is a 8.5" wheel with a offset of 50 will sit much differently than a 9.5" wheel with the same offset, for two reasons. One being the wheel is wider, second being the offset as it determines how that extra width will sit, either towards the fender or towards the strut.
Here is a article written by the guys who made the above picture explaining Offset. They are currently making me a spacer setup.
http://42draftdesigns.com/tech/offset.html
Thread Starter
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From: Summit Point, probably
All of that is really helpful. Thanks for the summaries. Next step:
How do I translate knowing the wheel diameter, width and offset to knowing whether the wheel will work with a specific model of car?
How do I translate knowing the wheel diameter, width and offset to knowing whether the wheel will work with a specific model of car?
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Rennlist Member
Joined: Jul 2012
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From: US
For example, on an 18"x10" rear wheel of arbitrary offset, you might mount multiple different tires which have differences in width and diameter including but not limited to:
- 265/35-18*
- 265/40-18
- 285/35-18*
- 295/30-18
And there are also considerations beyond 'will it work'... Understeer/oversteer can be affected by changing front/rear tire width radio and large changes in wheel position through offset can effect handling.
Your best bet is to post up what you are thinking about for wheels/tires, on what car, and for what kind of driving and get some feedback from folks who might have a similar setup.
This diagram shows how tire selection is equally important to offset for proper clearanceL
Also note that 'offset' and 'backspace' are both used to define the position of the wheel once mounted. Offset is from the centerline of the wheel, regardless of wheel width. Backspace is the inside clearance, taking the wheel width into account.
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Rennlist Member
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From: US
Off topic, but I've always found the odd mix of measurements used in tires & wheels a bit strange...
Wheel diameters in inches
Wheel width in inches
Wheel offset in metric
Bolt patterns in metric or inches
Tire inner diameter in inches
Tire width in metric, occasionally inches
Tire diameter as a calculated value of percent of the metric width (x2) plus the inches diameter, unless width is in inches, then overall diameter in inches as well.
Clear as mud?
Wheel diameters in inches
Wheel width in inches
Wheel offset in metric
Bolt patterns in metric or inches
Tire inner diameter in inches
Tire width in metric, occasionally inches
Tire diameter as a calculated value of percent of the metric width (x2) plus the inches diameter, unless width is in inches, then overall diameter in inches as well.
Clear as mud?
Thread Starter
Rennlist Member
Joined: Feb 2013
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From: Summit Point, probably
Found this doing some random googling in my quest for new wheels. http://www.wheeldynamics.net/appguide.html
