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Old 02-22-2010, 12:15 AM   #1
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Default calculation for backspacing -> ET

I'm purchasing some Diamond Racing wheels for my track car. Going with a 13x8 or 13x9. I haven't decided yet.

Anyways, the available backspace is 1 to 5''. On a 13x8, what would equal a +0 offset... as well what would a +0 be on a 13x9.

Is there a chart out there or something?

Going to be running the Ver 1 flares and 235/40 tires.
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Old 02-22-2010, 11:34 AM   #2
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Dude, do a little searching.

But, 0 offset makes backspace 1/2 wheel width. Positive offset moves the wheel toward the inside of the wheel well.

That tire is going to be pretty short, flares may not be absolutely necessary unless you go really low.
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Old 02-22-2010, 11:48 AM   #3
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Old 02-22-2010, 12:50 PM   #4
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I have searched. And your calculation is wrong, webby. +0 does not equal half of the width. Don't know where you got that from... if it were that simple I would have figured it out already. And don't take my post count for lack of searching, I may be new on this forum but I know how things work from others.

According to the calculator, 13x8 4.5'' backspace is +0. And 13x9 5'' backspace is +0.
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Old 02-22-2010, 01:02 PM   #5
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Backspacing = (wheel width/2)+(offset/25.4)

therefore I get 13x8 +0 = 4" backspacing and 13x9 +0 = 4.5" backspacing
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Old 02-22-2010, 02:02 PM   #6
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Can't argue with the math... And that equation is correct .
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Old 02-22-2010, 05:20 PM   #7
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Quote:
Originally Posted by falcon View Post
I have searched. And your calculation is wrong, webby. +0 does not equal half of the width. Don't know where you got that from... if it were that simple I would have figured it out already. And don't take my post count for lack of searching, I may be new on this forum but I know how things work from others.

According to the calculator, 13x8 4.5'' backspace is +0. And 13x9 5'' backspace is +0.
No Colonel Sanders, *you're* wrong!

It really is that simple
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Old 02-22-2010, 05:22 PM   #8
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Quote:
Originally Posted by Braineack View Post
Backspacing = (wheel width/2)+(offset/25.4)

therefore I get 13x8 +0 = 4" backspacing and 13x9 +0 = 4.5" backspacing
Then why did you post a calculator that is incorrect?
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Old 02-22-2010, 05:22 PM   #9
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The calculator is taking the bead of the rim into account. It has the rim width at the inside of the beads and the backspacing to the outside of the bead. That is why the calculator and braineack are getting different answers. Not sure which one is technically right
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Old 02-22-2010, 05:25 PM   #10
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If you just think about it its pretty obvious, lol
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Old 02-22-2010, 05:26 PM   #11
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still to be fair, the bead shouldn't add .5"

but, at least you do know there's a .5" backspace difference between the two regardless of the discrepancy.
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Old 02-22-2010, 05:36 PM   #12
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I was thinking .5" was a pretty thick bead too. I've always seen the calculation you used
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Old 02-22-2010, 06:09 PM   #13
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I wish manufacturers that insist on using backspace as a wheel descriptor would also list exactly the distance between the inner tire bead surface and the most inboard edge of the wheel I don’t think all wheel manufactures have the same beed thickness.

Wheel width is measured between the inside bead surfaces. Backspace is measured from a different surface entirely.

A 10 inch wide wheel with a 5” backspace does not equate to 0 offset. It ends up ends up something like -10 to -15mm depending on wheel manufacturer and how they shape the bead lip and it leaves some question as to where the tire ends up relative to the hub mounting surface.

Bob
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Old 02-22-2010, 06:18 PM   #14
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I was reading on some of the autocross forums, and some people have found that the 215 V710 fits under the stock fenders (rolled) with a 13x9 5.5'' backspace. Anyone here run that by chance?
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Old 02-22-2010, 11:09 PM   #15
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Quote:
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I was reading on some of the autocross forums, and some people have found that the 215 V710 fits under the stock fenders (rolled) with a 13x9 5.5'' backspace. Anyone here run that by chance?
No.

I swear I'm not following you to continue being a douche. But, what are you trying to do? How low are you?

Take a look at the 255/40-13 Hoosier A6 on the same wheel. The overall diameter is smaller, but the tread width is WAAAAY bigger with the A6. You will be a man among Kumho boys with the A6.

FWIW, I was on a 15X8 +30 (around the same backspace), with a 225/45-15 NT-01. It scraped some on an unrolled lip, but that tire is a full 2" total diameter taller than the A6. IMO, if you roll the lip and have a fair amount of camber, you will be ok with the 255/40-13 A6, the tiny girlie man Kumho will clear with no problems.

Also, think about the effective gearing of the 13" wheels. Unless you have a tall final drive, the 13" wheel setups are not appropriate (on the drive end) for a FI car. For FI cars, stick with the 225/45-15 or better the 275/35-15 combos.

Sit on those 215 Kumhos and meet me at the autocross. You will be buying me beers and polishing my car, I will be your Daddy.
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Old 02-22-2010, 11:25 PM   #16
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It's not for AutoX. It's for sprints and hillclimb. The A6 255 is out of my budget for this year. I am just getting into doing real competition, and this year is basically going to be a shakedown year.

My "mentor" you could say (also ran a turbo miata in our series for a few years) ran the 215 kuhmos on 13x8 panas and was up in the front when it came to times. I figured since I'm buying some diamonds, go up 1 inch and it should help a bit with contact patch and leave room for when I have the funds to go to the 255 A6's.

I am also going to have a set of 15x8's with 225 Nittos for track days/hillclimbs. The 13's will only be used at our track, since it is VERY short and not high speed. He found he was faster on the 13's than the 15's.

Again... autocross is how I got into this stuff, but it is mainly for fun now (I am an exec memeber of our club and help organize them) and I am putting my focus to the road course/hillclimb for this year. Sorry if my last post was unclear of that... I was just quoting the "autoX'' forum thing in regards to wheel fitment and nothing else.
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Old 02-23-2010, 12:11 AM   #17
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I was previously looking in to DR wheels and bookmarked some stuff=
Hoosier A6 255/40/13 -13"x9" with 5.25" backspacing, a 1/4" spacer on the front to bring it to 5.5" on a 1990 Miata.
( original thread:Hoosier A6 255-40-13?)
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calculation for backspacing -> ET-hoosier56.jpg  
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Old 02-23-2010, 12:19 AM   #18
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Wouldn't adding a 1/4'' spacer make it 5'' backspacing?
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