Wheel Offset Questions - 8.5" and 9.0" widths

[DeepH20]

I've been trying to work through the math of proper offset for aftermarket wheels considering the below. Note I have the +2.5 inch Eibach springs.

OEM: 7.5" Wheel, 265mm / 14.33" wide KO2 Tire, +55 offset (the 17" steelies)

Desired Tire: KO3 LT315/70R17, 12.4" wide

Trying to solve for optimal offset for an 8.5" and a 9.0" wheel. I'm ok with some extra poke, main priorities obviously being clearance and minimizing any additional stress.

 

[anand]

https://www.willtheyfit.com/ <--- will help you with this

 

[DeepH20]

https://www.willtheyfit.com/ <--- will help you with this

That is hands down the best online calculator I've seen. Thanks for sending. Any idea how much clearance is there for movement toward the suspension? from recall it seems like there is quite a bit of space.

Moving from the 7.5 to the 9.0, and +55 offset to +50, it appears to move 14mm closer to the suspension (I assume that would be the springs) and poke of plus 24mm, so roughly an inch. That seems very doable.

 

[C-Mack]

Keep in mind changing offsets can impact your steering scrub radius. If you can, keep the changes as close to stock as possible as excessive changes can lead to even worse steering. You want to keep the new wheel centerline and steering axis center line as close to zero as possible to minimize handling issues.

If the wheel centerline is further out past the steering axis centerline your wheels move in a greater arc when turning the steering wheel. It’s this scrubbing effect and extra leverage which can in some cases make the steering feel less stable. It’s like trying to hold a heavy object still with your arms extended versus holding the same object up against your body which is much easier to control the mass.

Image below illustrates it well even though it shows a strut suspension but it’s the relationship between the steering axis and wheel centerline which is important as the same principle applies for live axles with steering knuckles. It’s certainly possible to fit wider wheels assuming the wheel width, center lines and offsets are all balanced or changed minimally to the point steering is largely unaffected.

By all means, fit the wheels you want but keeping these variables in mind when choosing a wheel will help keep your rig running as best you can.

 

[Zimm]

I've been trying to work through the math of proper offset for aftermarket wheels considering the below. Note I have the +2.5 inch Eibach springs.

OEM: 7.5" Wheel, 265mm / 14.33" wide KO2 Tire, +55 offset (the 17" steelies)

Desired Tire: KO3 LT315/70R17, 12.4" wide

Trying to solve for optimal offset for an 8.5" and a 9.0" wheel. I'm ok with some extra poke, main priorities obviously being clearance and minimizing any additional stress.

Optimal offset will be determined by what poke you want, not the opposite.

Both the offset and the rim width will affect this.

stock is 7.5x55. making it 7.5x30 moves the tire out 25mm or 1" nominal poke vs stock. now an 8x30 is not the same, as the center line has shifted .25" or 12.7mm. so that gets added to both sides and the pole is now about 1.25"

What you need to do is determine if poke or internal tire interference is most important and work your way inward from there.

Keep in mind why tires interfere.

solid axles tuck, meaning as the axle rotates on its pivot point, the tire will tilt inward at the top. which is good and an advantage over IFS, depending on the arm length.

Another factor is the scrub radius. I have no idea what the kingpin center line is to the hub mounting surface. As such it'll be tough to determine how close the outer edges of the tire will be to interference points when in full lock.

Sitting around here somewhere I did a sketch to rehoop the stock steelies where I drew out multiple BS with multiple hoop widths, and I'm down to 7.5-8" hoops, set to 25-30mm BS. On a Toyota I'm running 9" Interco Birddogs for 35-37" tires, but they have a real aggressive lip design that permits single digit air downs, and stock or aftermarket steel hoops won't be that way, so I'm sticking to minimum widths (to -.5") for the tire size I'm wanting. Why do you think you need an 8.5-9.0 hoop width?

 

[DeepH20]

The BFG recommended rim width range is 8.0 to 11.0 for the KO3 LT315/70R17. I air down to 10 PSI and am switching to beadlocks. I could go down to 8s... manufacturer is recommending 9.0 to 9.5 for the tire. The offset range options for the wheels is +50 to -50.

If I'm doing the math right, I think i'm leaning toward
8.5" with +40 = +27.7 mm poke, -2.3 mm inset
or
9.0" with +35 = +34.0 mm poke, +4 mm inset

Does that sound right or am I missing something. I think the 27 to 34mm of additional poke is fine with the 35s. This is admittedly the deepest dive I've ever made into this subject... I don;t want to get it wrong.

 

[Zimm]

I believe the AO aluminum wheels are poking out about 1-1/4” past stock, no one is reporting any issues with nominal 35’s and, I think they look really good from the pictures. If I were to get aluminum I would likely get those for 35’s. I would go back thru the tire threads and cross what you’re planning with what products people have said were successful to get an idea where you stand.

I’m not sure if any of the 35’s were 315’s though. The experiments seem to lean towards skinny. I would focus on the tread width when comparing, not the cross section.

 

[nuclearmonkey]

I'm sure it's frowned upon, but I'm running 35x12.50 on stock steelies. No issue so far. To be fair, I've been too lazy to run a chalk test.

 

[DeepH20]

One last question on this. If I keep the change in backspace/inset the same as stock (150.3mm), and running a 35x12.5 in either case, is there any disadvantage to running a 9.0" wheel vs an 8.5" wheel? the numbers come out as below. Note I have a slight preference for the 9" but also some weight advantage to the 8.5. Pasting comparison Anyone running 9.0s?

below:

 

[Rocky]

I too will be running a 315/70R17 tire on the quartermaster but with a 30 offset 8” wide wheel. If your plans are to run mostly down fire-roads and pavement go with a 9 inch wheel. If offroad will be a priority then stick with a 8-8.5 inch width wheel. There will be less chance of damaging the outside rim on rocks and such with skinnier rims. Running a skinnier width wheel will also provide more inside clearance to the suspension parts.

BTW… A skinnier rim will affect the overall tire height and width to a small degree. The manufacturer specs don’t mean a recommended width but they just split the difference between minimum to maximum width wheels.
Example: 315/70R17 BFG Ko3 on a 8’’ wheel - 34.5” tall and 12.2” wide
315/70R17 BFG Ko3 on a 10’’ wheel - 34.3” tall and 12.6”. wide