Wheel math (backspacing and width)

At the moment I'm running a 255/60/15 tire with 15x8" wheels with 5.5 backspacing in my 72 demon which has a 68 coronet bbody 8 3/4 rear installed and I'm thinking of switching wheels. I already know (found out the hard way) that 15x8 w/4.5 bs and same size tire sticks out too far.

I found some wheels that I like locally but the rears are 15x7 with 4.5 backspacing. I'm trying to picture in my head if these wheels will put the tire/wheel in the same location as the current wheel/tire setup I have, or if they will stick out too far like the original wheels I used.

Any ideas?

That 7" wheel will stick "out" the same amount your current 8" wheels do.

Just to be clear, the 15x7 4.5bs wheels will put the tire/rim in the same location as the 15x8 5.5bs wheels? Thanks

The 7" wheels, with the same backspacing as the 8" wheels will "stick in" the same amount - 4.5". Since they are 1" narrower, the outside should stick out 1" less. It should be an improvement for your problem.

If you can borrow one of the new wheels for fitup, you can check for sure.

The only thing you will change is potentially pull in the section width about .2" per side.

The wheel outer lip will be in same place.

ok great. I was having trouble picturing it in my head but now I got it

Could someone demonstrate the math on this? The way I look at it, the 15 X 7 rims should result in the tire sticking out 1/2 inch more than his current 15 X 8 rims. Here is my reasoning: a 15 X 8 rim which is perfectly centered on the hub has a 4.5" BS. This is because the backspace is measured from the rim edge, which is 9" wide on a 8" rim (8" is the inside distance where the tires seats). Therefore, his 5.5" BS rim has the wheel centerline moved in towards the springs by 1".

Now, the new 15 X 7 rims. A perfectly centered rim should have a 4" BS, because again of the 1" difference between the outside width of the rim and the inside width of the rim. If the 15 X 7 rim has an actual BS of 4.5", that means the rim centerline is only moved 0.5" in towards the spring.

If the tire section width is the same, and the tire is centered on the rim (always is), then the outside edge of the tire section width will stick out 0.5" more.

Granted the more narrow rim will actually decrease the section width some, but my guess is this will not totally account for the offset change.

Is there a flaw in this reasoning?

Dave

Factory 15x7" Mopar rims SHOULD have a 4.25" backspacing - any OE ones I've measured anyway.

Math:

Inner wheel lip = axle flange minus backspace (4.5")

outer wheel lip = inner wheel lip positon + wheel lip x 2 + wheel width

Aluminum wheels have about a 1/2" lip and steel wheels about 1/4" lip. Let's assume steel wheels and use 1/4".

With 8" wheels, 4.5 BS and axle flange = 0:

inner lip = axle flange - 4.5"= -4.5"

outer lip = -4.5+.25+.25+8=4.00"

With 7" wheels and 4.5 BS

inner lip = -4.5

outer lip - -4.5+.25+.25+7=3.00"

So the "outer lip" of the 7" wheel with same 4.5" backspace would stick out 1" less.

There are two things to consider, wheel backspace and offset backspace, To find the offset, start by determining the wheels' centerline. To obtain that, measure the wheel's overall width from one outside edge to the other. This is not always the same as the width given in the manufacturer specs.

For example, a width of 15x8 inches refers to the distance between the inner flange and outer flange. Next, divide the width in inches by 2 to find the centerline. If the actual width of our example wheel is 9 inches, its centerline would be at 4.50 inches. Then, subtract the centerline to get offset 9 - 4.5 = 4.5.

1: Measure the room in your wheel wells relative to the wheel hub.
2: Backspace = The distance from back of the wheel to the wheel's mounting space, in inches.
3: Centerline = The wheel width divided by 2, in inches.
4: Offset = centerline - backspace, which will be positive or negative, in inches.
5: Offset in Millimeters = Offset x 25.4 (they use mm in their literature)

So...measure your wheel, then measure the new one and you will know exactly how it will fit. In your post you say you are going from a 15x8 with 5.5 backspace to a 15x7 with 4.5 backspace, if those numbers are correct you will only gain half inch.

Great site to play with the values

http://www.rimsntires.com/rt_specs.jsp

Yes, but in considering interference, the outer wheel lip is typically not important. The outer side wall of the tire is. So if the same size tire is put on the smaller rim, with the offsets described, then I still think the outer tire sidewall will stick out 0.5" more than his old solution.

You said this much more clearly than I did! In your final statement, you said he would gain 0.5". By "gain" do you mean stick out further from the wheelwell?

Dave

Very cool website! I put in the numbers under discussion, and it did confirm that the outside sidewall will stick out 0.5" more than the old package.

I believe you would gain 1" in the wheel lip. It would not stick out as much. The tire bulge would be bigger with the narrower wheel so the gain in outer sidewall would be less. I agree with the estimate that the gain in sidewall would be about .5". It would stick out .5" less.

Another way to think about is that the same backspace with a narrower wheel moves the center of the wheel inwards so the tire doesn't stick out as much - even accounting for the sidewall bulge.

Well unfortunately I was too slow and they got were sold...but it looks like this thread is still a great source of information...thanks everyone