dmalpas

First, I can not fathom how to use the Width Offset Calculator in Tools here. Not even sure it gives the answer I am seeking,

I am going to 35x12.5x17 tires. I understand the stock Rubicon wheel has a 7.5-inch rim width with 6.25 backspacing. What is the math for determining the correct backspacing for wider 10' rims? Is it 10 - 7.5 = 2.5 then 6.25 - 2.5 = about 4 BS should do it?

Thanks

TheDirtman

max 4.5" bs for a 12.5"x35" tire

Wandell

I don't understand what you mean by "calculating the correct backspacing". The JK is over a decade old and it's pretty much universally agreed that you want a wheel with 4.5 inches of backspacing whether you use a 8, 9, 10 or 12 inch wide aftermarket wheel. If you run a wheel with more backspacing, large tires will more than likely rub the control arms.

TheDirtman

He may be taking tire bulge into his calculations which do affect min or max backspacing needed. On a 10" wide wheel with 4" of bs he should be ok with the size tire listed. Most run 8-9" wheel. A 12" wide wheel will push the ire out farther and could be run with more bs vs a 8" wheel

dmalpas

I hated it when the teacher required me to show my work. It may be pretty much universally agreed that you want a wheel with 4.5 inches of backspacing whether you use a 8, 9, 10 or 12 inch wide aftermarket wheel, but my question was how do you know that? It has to be something more than experimentation? Here it seems to my simple mind that with wider tires I would be putting rubber almost two inches closer to the shocks, so backspacing would need to be reduced. Just asking someone to show the math.

So is Is it 10 - 7.5 = 2.5 then 6.25 - 2.5 = about 4 BS the correct way to determine it?

karls10jk

Click here for description

Simplistic explanation

Jeep801

dmalpas

OK, I give, apparently cannot be pre-determined. Hope the mods will delete the whole thread as worthless.

TheDirtman

In your op you ask about converting wheel bs for a 10" wheel vs 7.5" wheel. You don't state why. I can assume you want the out side lip of the wheel to be = to the factory so the wheel does not stick out? if so you need to add your 2.5" to the 6.25" for a bs of 8.75" but this will stick the wheel too far into the wheel well and you will not be able to turn.

If you want the inner lip of the wheel in the same place as factory you would match the factory bs which is 6.25" It does not change because of wheel width and your wheel's outer lip will stick out 2.5" farther then factory..

When you put wider, taller tires on you have to push the inside lip of the wheel farther out to compensate for the extra width and height of the tire to keep the tire from rubbing on the frame and control arms. However tires are different in size even if they are listed with the same numbers. Some run true to advertised size and some run quite a bit smaller so the tires are always a variable and with a 35"x12.5" tire like you listed will fit without rubbing with a factory width wheel and 4.5" bs.

A wider wheel will give you more inside clearance due to tire bulge affect and you can calculate it by width of tire 10.5"-wheel mounting width 7.5" which comes to 3" so you divide that by 2 and get 1.5" of tire bulge on each side of the wheel. a 12.5" wide tire -10" wide rim gives you 2.5" divided by 2 gives you 1.25" of tire bulge. This gives you .25" of extra clearance with the same back spacing on the wheel.

So not that we know this you can calculate your need wheel back spacing. The factory BS 6.25" -.25" (bulge factor)=6" then subtract half the increase in tire width( 12.5"-10.5"/2=1") and half the tire increase height (35-32/2=1.5") which gives you a wheel bs of 3.5". Again tire height and width will vary from brand to brand and even between tire flavors. Look at the section width and actual height to get your exact numbers. Most tire section width will be in the 11"-11.5" range and height can vary by up to 1.5"