How to measure Wheel offset.

[75wagon]

Here is a good read on how to calculate and understand wheel offset and it's measurement.
https://www.rsracing.com/tech-wheel.html


I've done some measuring and here is what I have come up with.

Note:
Wheel: Style/type
Size: Rim Diameter and width
FS: Front spacing
BS: Back spacing
Offset : Formula: FS + BS divided by 2, then minus FS = offset (+ is positive, - is negative)




Wheel: Cheviot Hustlers (same as on my wagon)
Size: 13 x 6 inches
FS: 85mm
BS: 95mm
Offset : +5mm

Even though this rim is a 13 x 6 inch rim, the actual sizes are different. The 13 x 6 inch measurement is taken between the bead seats, They don't include the outer flanges.

6 inches from inner to outer bead seat (150mm)


7 inches overall including the outer flanges (180mm)


Overall Diameter 14 1/3 inches (365mm)


To measure Front spacing, measure through the centre bore from the mounting pad


To measure the Rear spacing, measure from the mounting pad


Other Rims I have laying around that I have measured.


Wheel: Standard Galant Rims
Size: 13 x 4.5 inches
FS: 35mm
BS: 110mm
Offset : +37.5mm



Wheel: Standard fitment steel GH Sigma rims
Size: 14 x 5 inches
FS: 55mm
BS: 110mm
Offset : +27.5mm



Wheel: Superlites
Size: 14 x 6 inches
FS: 65mm
BS: 115mm
Offset : +25mm



Wheel: Bathurst Mk1
Size: 13 x 6 inches
FS: 70mm
BS: 110mm
Offset : +20mm



Wheel: Standard fitment FWD Colt rims
Size: 13x 5 inches
FS: 35mm
BS: 120mm
Offset : +42.5mm


If you want to add sizing for more rims, copy and paste this to put up the sizes.

Code: Select all

[color=#FF8000][b]Wheel:[/b][/color]
[color=#FF8000]Size:[/color]
[color=#FF8000]FS:[/color] 
[color=#FF8000]BS:[/color]
[color=#FF8000]Offset :[/color] 

There is also another factor to wheel offset, and that is strut choice. Eg: If you have a modified Galant or Lancer, fitting Sigma/Scorpian struts changes your front wheel track.
For differences read this topic Strut offset differences

Dave...

[lancersr20]

There may be a couple of other considerations to think about when talking about tyre fitment:

Scrub radius (front) - this is basically an imaginary line from the top of your strut through your lower outer ball joint (when veiwed from the front of your car) and to the ground. Now find the centre of your tyre where it is touching the ground. The gap between the two is called "scrub radius". Typically, you may have 10 - 20mm with standard tyres from the factory. If you fit wider wheels with more +ve offset, you will increase this scrub radius to maybe 30mm. What this does is makes steering at low speeds heavier and also increases the feeling of "bump steer".

Scrub radius (rear) - with a diff rear end, drop a vertical line from the axle bearing to the ground. As above, the difference between the two is "scrub radius" or for the rear, this is the amount of torque that you are applying to the axle/bearing. Offseting to the outside "a ridiculous amount" may lead to snapping axles at high cornering loads (circuit racing). Under normal use, this will probably not be a problem and you don't feel "scrub radius" in the rear of a diffed car !

Bump steer (steering) - this is when one wheel goes into bump or rebound and causes a change in "toe in / toe out" ..... making the car "dart" to the left or right

Toe in /toe out (front) - this is quite technical and is "all in the geometry/pick up points" of your suspension. Basically your car will change "toe" as the suspension bumps and rebounds. Racing cars set up their suspension geometry to have as close to "zero change" as possible through 6" of travel. Factory geometry is not always "great" . These "toe changes" cause your tyres to "wear" on the inside or outside . So when you change pivit points of your suspension, you will effect these "toe changes" (usually for the worse) .... you will generally make it "bump steer" more !

So in a nutshell - be wary of just widening the wheels to the outside so that it "looks cool". It may have some negative effects on handling and/or safety as well. The best rule would be to keep the same factory offset but widen wheel the same to both the inside and outside (if possible) .... or at least don't change the offset tooooo much more than 10 - 20mm.

If this has over complicated things - feel free to delete or move this to another discussion - maybe "suspension geometry" !

Lindsay

[75wagon]

I just received my rims that I bought from Osaka Japan (6 week wait) through JDM Wheels.com.au





I was originally told they were 15 x 6.5 +9 offset, but on getting them here, they are actually 2 sets of pairs of the same rim, not a matching set.
Using this formula in this topic, I have measured them to find out what they really are.

Wheel: Volk / Rays Engineering
Front
Size: 15 x 6 3/8 inches
FS: 88mm
BS: 101mm
Offset : +6.5

Rear
Size: 15 x 6 1/2
FS: 90mm
BS: 105mm
Offset : +7.5mm

Dave...

[smithdavidjames]

Just for further reading this site links with thread http://www.carbibles.com/tyre_bible.html
Has some good calculators for rolling radius etc and tyre fitments.

David