Help with offset math

[TheOtherMe]

If I change my wheels from the stock 7.5" to an 8" wheel with the same offset, would that move the street side of the rim out 1/4"?

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[Chocolate Thunder]

That’s what wheel calculators are for.

https://tiresize.com/

 

[TheOtherMe]

That’s what wheel calculators are for.

https://tiresize.com/

I've seen that calculator. However, I don't see an option for before and after wheel width. Am I missing it?

 

[azwjowner]

The answer is yes, same offset but 1/2" wider means the wheel keeps its position relative to the hub but grows half an inch in either direction. You have 1/4" less backspacing and the outer edge of the wheel is also 1/4" further out.

 

[TheOtherMe]

The answer is yes, same offset but 1/2" wider means the wheel keeps its position relative to the hub but grows half an inch in either direction. You have 1/4" less backspacing and the outer edge of the wheel is also 1/4" further out.

Perfect. Thank you. I'm looking at aftermarket wheels, but most are either 8" or 8.5". I needed a baseline to calculate the proper offset.

 

[azwjowner]

Perfect. Thank you. I'm looking at aftermarket wheels, but most are either 8" or 8.5". I needed a baseline to calculate the proper offset.

What you really want is this: https://tiresize.com/wheel-offset-calculator/

Play with both the "Offset Calculator" and "Offset Converter" tabs.

 

[TheOtherMe]

What you really want is this: https://tiresize.com/wheel-offset-calculator/

Play with both the "Offset Calculator" and "Offset Converter" tabs.

I've been playing with that to figure out tire sizes. If the offset calculator had a data entry point for wheel width it would be perfect.

 

[azwjowner]

I've been playing with that to figure out tire sizes. If the offset calculator had a data entry point for wheel width it would be perfect.

Wheel width sort of doesn't matter though. Offset determines how far the wheel moves in/out, so you can compare that number already, and your tire is always going to be wider than the wheel, so the edges of the tire are more important. Wheel width is more important relative to the tire width so you aren't too narrow or wide for mounting on the rim, but that's not a fitment issue relative to the vehicle.

 

[Old Dogger]

Yes, your just increasing the wheel width by 1/2”. So you divide that by 2, because it increases equally to the inside and the outside, a 1/4” each.

 

[Swisskidd]

I agree with OP that the wheel width certainly comes into play when considering wheel fitment.
Personally I prefer using backspacing measurements. (the distance from the mounting surface to the inner wheel lip).
The advertised wheel width is measured where the tire sits, not the wheel lips. An advertised 8.5” rim is actually 9.5” wide, when measured lip to lip (overall width). Considering a (hypothetical) 5.5” back spacing on such rim would leave a measurement of 4” from the mounting surface outwards.

(Your factory 7.5” rim is 8.5” wide. It has a 6” back spacing, leaving a 2.5” measurement from hub/rotor outwards.)

Considering the previous example above again, it would stick out 1.5” further than the factory rim, while providing 1/2” more clearance on the inboard side due to reduced back spacing.

Tire bulge is another aspect to consider. Tread width and tire width are not necessarily the same. You can find online tire listings that will give an overall width of a particular tire; mounted on a specific rim size (typically about mid range of recommended rim widths for given tire width). Such dimension minus overall rim width will give you tire bulge for both sides combined. Add half of that to your initial measurement from hub to outer wheel lip.

Hopefully this makes sense.

 

[NewJLU2019]

Info.

 

[Swisskidd]

Info.

Good info. The wheel sizes listed here is advertised width, not measured width.
An important detail that can get you crazy when math doesn’t add up in a particular calculation.

(attached example of measuring back space).

[edit:spelling]

 

[JT1]

I agree with OP that the wheel width certainly comes into play when considering wheel fitment.
Personally I prefer using backspacing measurements. (the distance from the mounting surface to the inner wheel lip).
The advertised wheel width is measured where the tire sits, not the wheel lips. An advertised 8.5” rim is actually 9.5” wide, when measured lip to lip (overall width). Considering a (hypothetical) 5.5” back spacing on such rim would leave a measurement of 4” from the mounting surface outwards.

(Your factory 7.5” rim is 8.5” wide. It has a 6” back spacing, leaving a 2.5” measurement from hub/rotor outwards.)

Considering the previous example above again, it would stick out 1.5” further than the factory rim, while providing 1/2” more clearance on the inboard side due to reduced back spacing.

Tire bulge is another aspect to consider. Tread width and tire width are not necessarily the same. You can find online tire listings that will give an overall width of a particular tire; mounted on a specific rim size (typically about mid range of recommended rim widths for given tire width). Such dimension minus overall rim width will give you tire bulge for both sides combined. Add half of that to your initial measurement from hub to outer wheel lip.

Hopefully this makes sense.

Offset matters way more on a car where there is outer and inner interference concerns.
Offset is 100% useless without wheel width.
Backspace is the only relationship that matters on a jeep because there is no concern (within reason) for the tire to contact the bodywork when turning.. Even the lift manufacturers don't advertise anything except backspacing because clearing the spinning and bouncing bits is all that matters... If you want to offset mount your front lower shock, and want to run a 9" wheel, you just get to deal with the poke.

 

[GATORB8]

I'm not personally a fan of using backspace. In the majority of our builds tire sidewall is the controlling clearance point. While wheel width does impact section width to a degree (industry standard is 0.2" in overall tire section width change per 1/2" change in wheel width) that really needs to be added/subtracted regardless of whether you locate the tire with BS of Offset.

Since offset directly locates the centerline of the tire, it's a good static locating metric. Backspace is dependent on width, so you can't locate the tire without knowing the width.

Just for an example of why this can be frustrating, let's look at the difference between using backspace and offset. For ease of calculation, let's use a baseline of a 8.5" wheel with 0 offset /4.75" backspace.

Change to 10" width, but maintain 0 offset:

Change to 10" width, but maintain 4.75" backspace:

In both cases, the change of wheel width would impact the section width the same. Using a BFG KO2 37x12.50R17, their data shows a 13" section width on a 10" wheel, so you'd want to consider that in your use of the tiresize.com offset comparison if you are comparing changing tire sizes with the wheel change. Using the industry standard, the section width on the 8.5" wheel should be closer to 0.6" thinner, or just over a 1/4" more clearance either side with the 8.5. I don't put a ton of weight in the standard, but at least gives you an idea.

 

[hoch]

Are there aftermarket 8-8.5” rims with stock offset of 44.5mm? I haven’t seen anything higher than 25mm.
Not sure why you’d want to stick with stock offset either way. You’ll run into clearance issues if you go with bigger tires.

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