A good ol' fashioned forum mystery

[R2 SPD2]

Alright sleuths, I need your help. Starting with the clues:

@BigBronco1 posted in 2021 side profile shots of tire poke with standard fender flares for 17x9 +30 wheels with 285/70s on them.
https://www.bronco6g.com/forum/thre...lo-rtr-wheels-on-my-2021-bronco.18735/page-12

@Dez posted six months later similarly angled shots of tire poke with standard fender flares for 17x8 +18 wheels with the exact same 285/70 tires on them.
https://www.bronco6g.com/forum/thre...-wheels-with-sasquatch-package-fenders.35310/

If you do the math, these two setups should have the exact same poke, all else being equal. However, the pictures appear to show a pretty clear variance between them.

Could it be an imperceptible camera angle difference? Inaccurate manufacturer specs? Undisclosed wheel spacers? The truth is out there.

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[Smocaine]

Your math isn't mathing. The offset decides the tire center. 12mm less offset will mean 12mm more tire poke, regardless of wheel width. The tire width changes some at the bead, but nearly none at the tread.

 

[LVS]

It may just be the slight difference in angle. Only way to be sure would be to get a tape measure and measure the overall width, and confirm tire pressure is the same.

 

[R2 SPD2]

Your math isn't mathing. The offset decides the tire center. 12mm less offset will mean 12mm more tire poke, regardless of wheel width. The tire width changes some at the bead, but nearly none at the tread.

Wheel width definitely impacts the position of the outer and inner faces of the wheel in combination with the offset. Your comment did make me realize I made an error in the post, though: the +18 wheels were actually listed as 8" wide, which is what makes the maths math.

 

[Ducati1098]

You can tell from the pictures alone that one is taken more to the right side of the vehicle.

 

[Smocaine]

Wheel width definitely impacts the position of the outer and inner faces of the wheel in combination with the offset. Your comment did make me realize I made an error in the post, though: the +18 wheels were actually listed as 8" wide, which is what makes the maths math.

I almost replied admitting defeat, but I will die on any hill and the calculators got my back.
Two +30s of differing widths will have the same tire center, just like two +18s of differing width will have the same tire center. This affects the shape of the tire, but as I said, mostly at the bead. The tread (and the bulge underneath) deforms significantly less.
Since we know the tires are identical, and both wheels are within measurement range, the poke of the tire itself will be almost entirely decided by the offset.
The +18 looks half an inch further out because it is.
e: you may be confusing offset with backspacing, two very similar concepts
e2: prepared these visual aids to assist:
Wheels of matching offset and differing width: wheel faces moved apart, identical maximum tire poke.

Wheels of differing offset and matching width: wheel faces move together, different tire poke.

Wheels of differing offset and differing width: wheel faces are different distances apart, but regardless the center (and tire) moves outboard by the amount of the offset difference, different tire poke.

Notice that the outboard wheel face is indeed identical in this example - where you think your math maths - but what you're asking about is the tire face, which moves significantly, and is decided by the center of the wheel faces, which has moved out 12mm.

 

[Willub]

Wheel width definitely impacts the position of the outer and inner faces of the wheel in combination with the offset. Your comment did make me realize I made an error in the post, though: the +18 wheels were actually listed as 8" wide, which is what makes the maths math.

You’re pushing the center of the tire out by 12mm. The tire is the same width in both scenarios. It’s that simple.

 

[j_marinelli]

You can tell from the pictures alone that one is taken more to the right side of the vehicle.

This.

You would be surprised at how the slightest change of angle of a picture taken can change the dynamic of what the picture is showing.

 

[Willub]

This.

You would be surprised at how the slightest change of angle of a picture taken can change the dynamic of what the picture is showing.

Actually sticking out an additional 12mm also helps.

 

[Valhalla]

12 mm ones front ones rear camera angle and a box of crackers.

 

[R2 SPD2]

Notice that the outboard wheel face is indeed identical in this example - where you think your math maths - but what you're asking about is the tire face, which moves significantly, and is decided by the center of the wheel faces, which has moved out 12mm.

This! That's a great calculator to also take the tire into account and then spit out good visuals. Thank you.

So, despite the matching outboard face of the wheel, it is the tire's width and center that drives the outcome. The narrower wheel also contributes to the effect by way of a larger sidewall angle. Despite the tread technically being flattened (wider) on the wider wheel, its sidewall will have a steeper angle, lining it up even closer to the face of the wheel.

Case closed! Thank you for the responses and insight.

 

[Squatch]

I've never seen so much hullabaloo over a half inch of poke.

 

[R2 SPD2]

Cheap thrills on a Friday night.

Just wait until next week when we get into back pressure and exhaust scavenging.

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