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  #1  
Old 03-04-2012, 07:30 PM
NewBird13 NewBird13 is offline
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Default Cragar wheels

Hey Everyone
I am doing the scarebird disc brake conversion and switching to a Cragar rim while I am at it. I am still a little confused on back spacing and offset. I am looking to get 15x7 cragar ss rims. Does anyone know the proper backspacing or offset I need? I have found a good deal on a set that says the offset is +6mm and a backspacing of 4.250 in. Would these work?
Thanks in advance. Your knowledge is always appreciated.
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  #2  
Old 03-05-2012, 05:48 PM
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c4clewis c4clewis is offline
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I believe 4.25" offset is "standard" for our cars. I put 16x8 with 4.25" wheels on and they fit well (tight in the back and I have to drop the rear axle to get them on and off). Do some reading here:
https://www.rsracing.com/tech-wheel.html
and here:
http://www.fourwheeler.com/techartic...tte/index.html

for some good info on wheel measurements.
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Old 03-05-2012, 06:19 PM
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I'm running 15" Cragar's on my Scarebird retrofit. Fact is, just about ANY 15" wheel has no problem with calipers. It's when you run 14" (as some of our members do) that you must be careful about wheel choice.
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Old 03-06-2012, 12:13 AM
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Back spacing and offset are two ways of measuring the same thing, namely where the mounting surface of the wheel is relative to the width.

Back spacing is just what it implies - the amount of 'space' in the 'back' of the wheel. If you were to measure from the mounting surface (where the wheel meets the hub face) to the lip, this is the back space. Lay the wheel on its face and place a board across it. Now measure from the mounting surface to the board.

Offset is harder to measure. Offset is how far away from the actual center of the wheel width the mounting surface is. Say you have an 8" wide wheel, zero offset means the mounting surface is in the center of the 8" width, so 4" to the inside, 4" to the outside.

Now, since wheel width is measured inside the wheel lip, backspacing and offset aren't directly related. What do I mean? Take my wheels, 17" diameter, 8" wide, zero offset. You'd think that would mean 4" of backspace, right? Nope. The 8" is inside the wheel lips and with my aluminum wheels, the lip is ~1/4". So, 8" wide / 2 = 4" then add 1/4" for the inner wheel lip and you have about 4.25" of back space.

Wheel manufacturers and car designers prefer offset since it's a more precise measurement and it's what they are concerned about - where the load is being applied. However, when you have a wheel in front of you and you need to know if it'll fit, back space is a lot easier to measure.

I'm not sure what our original wheels measure for either of these. 3.5" of back space and 5.5" wide sounds about right, but I'm not sure.
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Old 03-06-2012, 01:38 AM
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Quote:
Originally Posted by dgs View Post
...I'm not sure what our original wheels measure for either of these. 3.5" of back space and 5.5" wide sounds about right...
And try to find a new one... we used to get these for fifteen bucks each, new.

The Mustang wheel I keep showing I believe is a 14" x 6" with 3-1/2" back spacing. This wheel works beautifully on all Thunderbirds. I know Ray and Howard are using them. Ray found them at his local bone yard but they have no more.

Ray had them blasted and powder coated, and they really look good. They have the four 'nubs' for dog dish caps, huge Granada hub holes, and they look very 'stock'.

If you find more of these wheels at your local bone yard, grab 'em. - Dave
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