Let's talk backspacing

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'74 Sport

'74 Sport

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Here is a drawing I made of the measurements taken on Aaron's '74 Duster project we are building. This is a stock '69 B-body 8-3/4" rearend (not narrowed and perches not moved in), with American Racing Torque Thrust II 15"x8" rims and 235-60R15 BF Goodrich T/A Radial tires.

The goal is to move the wheel inboard to gain more clearance at the outer lip of the wheel well. You can see that the limiting factor will be the leaf springs. Obviously, relocating the perches the necessary 1/2" on this A-body installation would help, but we aren't going to do that right now. If I understand backspacing and offset correctly, I would say this wheel has 3-3/4" backspacing, as noted in the sketch.

Can I safely assume that leaving 1/2" clearance from the tire's outer wall to the springs is sufficient? If so, that means I could move the wheel inboard 1-3/4", increasing the backspacing on a new wheel to 5.50".

Comments?

Backspacing - Rear Wheel med.jpg
 
I would think you could go with a 4 1/2 back spare with no problems.
I got what I think is a stock A body rear in the Duster and
I run 255/60-15s on 15x8 rims with a 4 1/2 back space.
 
Your method of measurement looks correct. Just make sure you are taking the tire section width into account. Section width is taken by measuring from sidewall to sidewall, or you can get this by referring to the tire manufactur specs, based on a set wheel size.

Because a tires sidewall will bulge when you go around corners I would leave 1 inch between the sidewall and the spring.

The following link may be of some help:
https://www.rsracing.com/tech-wheel.html#backspace
 
Brian,
Perhaps, but I don't have the A-body rear, stock or otherwise, it is a longer one from a B-body. 4-1/2" backspacing would only be 3/4" inboard more than what it is now. That would barely clear the wheel well lip - the tire is currently lined up right under the lip. That's why I was suggesting to move it 1-3/4".
 
Demon 408 said:
Just make sure you are taking the tire section width into account.
Click to expand...
Yep, you can see that we measured from the bulging tire wall to the leaf springs. Thanks for the link, but you may notice that my drawing was basically taken from the final sketch in their article. I have been back and forth there a dozen times double checking my measurements and calculations.

What I need is someone to chime in who has:
'73-'76 Duster or Dart Sport
un-modified late B-body 8-3/4" rear end (ours is '69 Charger)
stock leaf springs
stock hangers
stock shackles
un-modified wheel wells
15" wheels
at least 235 tires

Then you can tell me exactly what I need to know and save me from all this cypherin'. :scratch: :banghead:
 
'74 Sport said:
Then you can tell me exactly what I need to know and save me from all this cypherin'. :scratch: :banghead:
Click to expand...

Sorry, I missed that you measured to the sidewall. I do not have the exact same set up that you have, but you have done some great cypherin already! Like I said in my first response, IMHO, I would increase the sidewall clearance to 1 inch. That translates into a 5 inch backspace. I have a Demon with the same rear (B-body) but my springs are moved in using the offset hangers. My cyphern, for my set up results in a 6 inch back spacing...approximately.
 
i have a b-body rear end in my 67 dart with 15x8 5.5 back spacing but have moved perchs to a-body 43'' spot note that the duster has more outside room then the dart this worked for me
 
I know this thread is pretty old, but could you post some pictures of your setup if you still have it Redhot? I have a 67 Dart that I'm getting a B body rear for and was looking at 15x8 rims and trying to figure out backspacing. I plan to an offset hanger kit and moved perches with 255 tires, but I'm having a hard time picking out rims I like since I don't know what backspace I need and that limits selection quite a bit in most cases.
 
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