Rim/Wheel Offsets

Gene FJ20DEBT
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Rim/Wheel Offsets

Post by Gene FJ20DEBT »

iv noticed in a few rim discussions that people ask” will this rim fit my gem”, “will that rim fit my gem” etc.. to which they get the reply “if the offset is right”

By the number of times its been asked im assuming a few people are unsure on what a rim offset actually is… so heres a basic rundown and a few diagrams…

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As seen in the diagrams, the offset is the distance from the centre line to the surface the hub mounts to…

For example.. 7” wide rims are a common after market size for a Gemini. This is approx 175mm
With 7” (175mm) wide rims for Geminis an offset of ET38 is commonly used… the 38 stands for 38mm meaning the hubs mounting surface is 38mm outwards of the centre of the wheel... there for, of the 175mm wide rim.. there is 125.5mm of rim inward of the hub surface and 49.5 outward of it…

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So if u were to have ET18 (18mm) rims, then this would put 145.5mm of the rim inward (instead of the 125.5mm). Meaning the whole rim itself is now 20mm close to the shock tower (meaning it would well and truly be rubbing on the shocks)
So u would need to fit a 20mm spacer between the rim and the hub of the car to space the rim out the extra 20mm effectively making the offset ET38 (38mm)

NOTE: spacers are not street legal

If you were to have ET58 (58mm) then this would mean that 69.5mm of the rim would be outward of the hub mounting surface (instead of the 49.5mm)... this means that the whole rim itself s now sitting 20mm further outward of the car.. More then likely hanging way out side the guards/wheel arches

So rim size doesn’t mater, (within reason) as long as the rim width is spaced correctly between the shock and the guards/wheel arches
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OTD 079
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Re: Rim/Wheel Offsets

Post by OTD 079 »

Hey Gene,
I think you have the calculations for the inward and outward values wrong.

With a high offset, greater chance of rubbing on the shock
With a low offset, greater chance of rubbing on the outer wheel arch

Say you have a 7in wide rim - 177.8mm

I get the following values:

Offset ET38 gives
126.9mm inward (commonly called backspace)
50.9mm outwards

With an offset of ET18
106.9mm inwards (commonly called backspace)
70.9mm outwards

Cheers,
Matthew
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Post by Tassie ZZ/Z »

If I put 10mm spacers on an et38 offset, the offset thus becomes et28 moving the wheel toward the guard.
antus
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Post by antus »

So, the offset is measured from the center, and the center nor the point where the wheels bolt on move for different rim sizes, so that makes gemini offset ET38 for any size rim, to keep it center? Ive been thinking about this all day and that seem about right (after coming to different conclusions about 4 times) heh.
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Rodeobob
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Post by Rodeobob »

Yep Gene is wrong.

antus wrote:So, the offset is measured from the center, and the center nor the point where the wheels bolt on move for different rim sizes, so that makes gemini offset ET38 for any size rim, to keep it center? Ive been thinking about this all day and that seem about right (after coming to different conclusions about 4 times) heh.

And your wrong too.

Offset is relative to rim size. What works on a 6" rim wont work on anything but a 6" rim.

If you go to a wider rim you have more rim either side of the center line.

To borrow matthews calculations
Say you have a 7in wide rim - 177.8mm

I get the following values:

Offset ET38 gives
126.9mm inward (commonly called backspace)
50.9mm outwards

With an offset of ET18
106.9mm inwards (commonly called backspace)
70.9mm outwards

Cheers,
Matthew
8" would be 200mm

So ET38 would be
138mm backspace
62mm dish

ET 18
118mm backspace
82mm dish


If my memory is right you have 4 1/2" allowable back space on a Gemini.
With the 7" rim at 127mm thats just over 5" so its close. With the 8" rim with the same 38 offset the back space is 138mm thats half an inch too much. That half inch extra is half the extra width of the rim. Thats half either side of the center line.


Do you follow now????


Cheers, Bob.
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Gene FJ20DEBT
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Post by Gene FJ20DEBT »

WTF :roll: yeah i just re-read what i wrote, i got it backwards.. my bad.. dunno how i managed that :lol: :?
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mygemibaby
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Post by mygemibaby »

hey i know this is gunna be a strange question everyone but im getin an slx and someone told me that ford escort m2 mags will fit is there anyone that can confirm that or not

Thanks heaps if you can
:D
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Post by zep »

The amount of wheel going INTO the car from the wheel centreline is POSITIVE.

The amount of wheel moving OUT of the car from the centreline is NEGATIVE.

The drawing below shows the POSITIVE offset, along with dimensions ‘A’ ‘B’ ‘C’ and ‘D’

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The measurement from the wheel mounting face to the outside of the car STATE C DIMENSION

The measurement to check the brake caliper protrusion forwards of the wheel mounting face STATE D DIMENSION
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Post by Gemi Coupe »

mygemibaby wrote:hey i know this is gunna be a strange question everyone but im getin an slx and someone told me that ford escort m2 mags will fit is there anyone that can confirm that or not

Thanks heaps if you can
:D
No, they are a different stud pattern and different offset, no chance in hell
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Post by Crazytim »

so in each common size say from 13 to 17 what is the most perfect off set and width for each? the charts and figures may as well be written in swahili, i still dont get it... can any1 help? thanx heaps!! :twisted:
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Post by boony_124 »

approx. what is the max positive offset to run on a gem?

Just as a guide so some and me.
Gene FJ20DEBT
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Post by Gene FJ20DEBT »

depends how wide the wheels are
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RedGemCoupe
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Post by RedGemCoupe »

Simply put, it is not legal in Queensland to make the track of the vehicle narrower. A more positive offset reduces the track. You can however increase the track by up to 26mm.
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Post by MGOTD »

RedGemCoupe wrote:Simply put, it is not legal in Queensland to make the track of the vehicle narrower. A more positive offset reduces the track. You can however increase the track by up to 26mm.




off set has nothing to do with changing track width, rim width and tyre width are what changes that.

Cheers
Matt
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RedGemCoupe
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Post by RedGemCoupe »

Track is measured at ground level from the centre of the tyre on one side to the centre of the corresponding tyre on the opposite side of the vehicle. Changing the offset changes the relationship between the mounting surface and the centreline of the wheel. A more positive offset means the mounting surface of the wheel is further from the centre of the vehicle.
Cheers
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