Rim/Wheel Offsets
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- Supreme Overlord
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Rim/Wheel Offsets
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…
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…
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
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…
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…
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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Re: Rim/Wheel Offsets
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
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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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.
- Rodeobob
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Yep Gene is wrong.
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
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.
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
8" would be 200mmSay 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
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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- Supreme Overlord
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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’
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
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’
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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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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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
Cheers