Help me confirm my measurements and back spacing thought

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70Roadkill

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i have a 1970 Dart Swinger, 8 3/4 rear axle, dr diff big bolt pattern axles, and I installed a 3/4" offset kit.

When I measure from the drum surface to the outer wheelhouse lip it's about 4 7/8" and from the the drum surface to the spring I get 7".

My plan is to is to get a 15x8 rim and put on a set of 275 60 15 tires.

I'm thinking I need a 5" offset to make it work. Can someone chime in to make sure my math is correct.

Thanks
 
Deleted incorrect information.

Thanks for the correction.
 
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i have a 1970 Dart Swinger, 8 3/4 rear axle, dr diff big bolt pattern axles, and I installed a 3/4" offset kit.

When I measure from the drum surface to the outer wheelhouse lip it's about 4 7/8" and from the the drum surface to the spring I get 7".

My plan is to is to get a 15x8 rim and put on a set of 275 60 15 tires.

I'm thinking I need a 5" offset to make it work. Can someone chime in to make sure my math is correct.

Thanks

So a 275/60/15 has an 11" section width. Your spring to quarter measurement is 11 7/8". You need a 1/2" to the quarter, which leaves 3/8" to the springs. That's pretty much the minimum clearance you can have and make it work, so, they'll fit but your backspace will have to be perfect.

So, let's get your backspace perfect. A 15x8" is rim is 9" wide outside to outside. The section width is 11", so, 1" of tire hangs over on each side. You've got 7" to the springs, so subtract your 3/8" of clearance, then subtract the inch of rubber, and you've got 5 5/8". That's what your backspace needs to be to clear the springs. Check the math on the front- 3-3/8" front space, plus that other inch of tire, and you get 4 3/8". That leaves your 1/2" to the quarters (which are probably uncut right?).

So, if your measurements are right you should want a 15x8" with about 5 5/8" backspace. Maybe a bit more because it's easier to slide in a thin spacer to get the tires off the springs than it is to cut the quarter lips back. It will be super tight if you don't do any trimming on the quarter lip, but if the math matches the actual section width it should work.
 
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72bluNblu thanks for the info, I first thought 5.5 bs but forgot about the rim being 9" then the 1" overhang I was going off 8" with 1.5 tire overhang. I'll look for some 5.5 bs and a shim and see how they stuff up in there. Swap meet coming up this weekend keep my eye out for some.
 
72bluNblu thanks for the info, I first thought 5.5 bs but forgot about the rim being 9" then the 1" overhang I was going off 8" with 1.5 tire overhang. I'll look for some 5.5 bs and a shim and see how they stuff up in there. Swap meet coming up this weekend keep my eye out for some.

5.5" and a spacer won't work. If you want to use a spacer you'll need more, not less backspace. I'd be looking for a 6" backspace honestly, I know you won't find a 5 5/8" unless you have them custom made and that 1/8" will be important to the quarter lip.
 
5.5" and a spacer won't work. If you want to use a spacer you'll need more, not less backspace. I'd be looking for a 6" backspace honestly, I know you won't find a 5 5/8" unless you have them custom made and that 1/8" will be important to the quarter lip.

I'm sitting here still trying to figure this out. So I drew it out. In my mind it looks like a 5.5 backspace will put the tire on the spring and a 6" would be worse .i attached a picture and drew it out. Take a peek and let me know if my thinking is wrong.

IMG_4757.JPG
 
Doing the math looks like I need a 5" backspace, am I crazy? I updated the drawing with 5" backspace.

IMG_4758.JPG
 
i have a 1970 Dart Swinger, 8 3/4 rear axle, dr diff big bolt pattern axles, and I installed a 3/4" offset kit.

When I measure from the drum surface to the outer wheelhouse lip it's about 4 7/8" and from the the drum surface to the spring I get 7".

My plan is to is to get a 15x8 rim and put on a set of 275 60 15 tires.

I'm thinking I need a 5" offset to make it work. Can someone chime in to make sure my math is correct.

Thanks

Just taking a few steps back on this....

Is the rear a small bolt A-body rear you just put dr diff axles in and moved the spring in?

Do you know the outside drum face to outside drum face distance?

Trying to determine what basic axle housing you have.

Did you mini tub your 70 Dart? You should hit the inner wheel well before the spring.
 
It has dr diff axles and drum set up.

Using a level the springs and almost even to the inner fender well.

3/4" relocation kit.
 
So it's an original A-body width rear end. Just big bolt axles installed?

The inner wheel well curves in. When the car leans to the side, the top of the tire tilts in relative to the body.

I have a 68 Dart that has the same wheel lip as your car (same wheel lip molding). I've got 245/60/15 15x7 rim with 4.25 backspace and small bolt axles. It barely touches on the outside and inner wheel well top and the springs. Springs stock location.
 
So it's an original A-body width rear end. Just big bolt axles installed?

The inner wheel well curves in. When the car leans to the side, the top of the tire tilts in relative to the body.

I have a 68 Dart that has the same wheel lip as your car (same wheel lip molding). I've got 245/60/15 15x7 rim with 4.25 backspace and small bolt axles. It barely touches on the outside and inner wheel well top and the springs. Springs stock location.

Right, and a 15x7 with a 4.25" backspace has the same centerline as a 15x8 with a 4.75" backspace. But he has BBP axles so add 5/16" for the increase in track width. And he has a 3/4" offset so add another 1/2" (yes, I know I added a 1/2" but even the 3/4" offset only offsets the front hanger a 1/2") . Now the backspace is at 5-9/16".

I run my duster with a 1/2" offset and about 5/16" of clearance to the spring, it just barely touched the inner wheel well when I was cross-axling speed bumps before I added the rear sway bar. Inner tub is the same on both of those cars. He should be able to manage 5 5/8" with the measurement he has. I wouldn't run the spring clearance that tight on a 275/60/15, my 295/35/18's have like 2.5" less sidewall to flex.
 
Right, and a 15x7 with a 4.25" backspace has the same centerline as a 15x8 with a 4.75" backspace. But he has BBP axles so add 5/16" for the increase in track width. And he has a 3/4" offset so add another 1/2" (yes, I know I added a 1/2" but even the 3/4" offset only offsets the front hanger a 1/2") . Now the backspace is at 5-9/16".

I run my duster with a 1/2" offset and about 5/16" of clearance to the spring, it just barely touched the inner wheel well when I was cross-axling speed bumps before I added the rear sway bar. Inner tub is the same on both of those cars. He should be able to manage 5 5/8" with the measurement he has. I wouldn't run the spring clearance that tight on a 275/60/15, my 295/35/18's have like 2.5" less sidewall to flex.

I thought later dusters had more room than the earlier darts. That 70 Dart does not have the room as my 2nd gen Barracuda.

?I thought 73-76 duster/dart sport's have more room inside than earlier barracudas and darts?

Are your tubs untouched in your Duster? Outer lip rolled?
 
I thought later dusters had more room than the earlier darts. That 70 Dart does not have the room as my 2nd gen Barracuda.

?I thought 73-76 duster/dart sport's have more room inside than earlier barracudas and darts?

Are your tubs untouched in your Duster? Outer lip rolled?

They do, all of the Dusters/Demons/Dart Sports have about an inch more room than any of the Darts/Scamps/Valiants. Barracuda's are close to the same as the sport body cars in the rear, there's about an extra inch compared to the Darts. But the extra inch is in the outer wheelhouse, so only the measurement to the quarter is different. The inner wheelhouses, framerails and spring locations are the same on all of the 67-76 A-bodies. Some of the 67-69 Darts seem to run a little tighter for clearance, but again it's all to the quarters, all the inner measurements are the same. My '71 Dart has 12" from the inner wheelhouse to the quarter flange, which is within an 1/8" of the OP. So pretty darn close considering what we're measuring. My Duster had 13" inner wheelhouse to quarter flange before I started cutting.

On my Duster the quarter flange is cut back to the spot welds with the outer wheel house, which added roughly a 1/2" of clearance on my car. And I have a 1/2" spring offset to get 295's in there, but the wheelhouses are untouched. So for determining the clearance on the springs and inner wheelhouse I can still use the measurements from my Duster, both the OP and I are using an offset spring kit. Just can't use the outer measurements because I've got 13.5" of wheelhouse to work with now. I actually get pretty similar spring to wheel mount measurements on my Duster if I correct for the B-body rear axle. My car as it sits with the 68-70 B-body 8 3/4 has 8" from the springs to the wheel mount, and the B body rear axle is about 1.15" wider per side than the A-body 8 3/4 with BBP axles. So that would put my distance at 6.85" compared to the OP's 7". An 1/8" could easily be a measurement error or just factory tolerances. I know I'm using a tape measure and a square for those measurements so they're not perfect.
 
If you take 4 7/8" + 7" that gives you 11 7/8" total fender lip to spring spacing, at the most. Your 275 tires are about 11" wide at the sidewalls. Not including a margin of error, sidewall flex from cornering, side-to-side body tolerances, etc... So you have 7/8" total clearance, or 7/16" on each side at the most. Did you measure both left and right sides of the car? They are usually different. It doesn't matter what backspace wheel you put on it, as far as I'm concerned a 275 tire just isn't going to fit without some kind of body surgery.

{By the way, by my figuring, the theoretical backspacing for your car should be 5 9/16" (or 1 1/16" offset) with the 8" wheel. Same as what 72bluNblu came up with.}
 
Thank a for all the feedback, this weekend there is a pretty good swap meet near by, probably not much mopar but a lot of ford stuff so same bolt pattern. Hopefully a set or 2 there with tires I can take a level across and measure to see the dimensions and maybe pick up a set.
 
Are you talking steel, or aluminum wheels?
Quite often aluminum wheels have wider outside measurements 'cause the edge of the rim is thicker.
Won't affect the section width, 'cause the rim width is the same but may confuse you when comparing back spacing of two different wheels. Given the choice always get the bigger back spacing, as others have said.
Remember measure,measure,then measure again!
And don't forget to bring your own straight edge(s) and tape(s)!
 
Are you talking steel, or aluminum wheels?
Quite often aluminum wheels have wider outside measurements 'cause the edge of the rim is thicker.
Won't affect the section width, 'cause the rim width is the same but may confuse you when comparing back spacing of two different wheels. Given the choice always get the bigger back spacing, as others have said.
Remember measure,measure,then measure again!
And don't forget to bring your own straight edge(s) and tape(s)!

Might be off 1/16" or so wheel to wheel. If it's more than that they aren't adhering to the industry standards.
 
If you take 4 7/8" + 7" that gives you 11 7/8" total fender lip to spring spacing, at the most. Your 275 tires are about 11" wide at the sidewalls. Not including a margin of error, sidewall flex from cornering, side-to-side body tolerances, etc... So you have 7/8" total clearance, or 7/16" on each side at the most. Did you measure both left and right sides of the car? They are usually different. It doesn't matter what backspace wheel you put on it, as far as I'm concerned a 275 tire just isn't going to fit without some kind of body surgery.

{By the way, by my figuring, the theoretical backspacing for your car should be 5 9/16" (or 1 1/16" offset) with the 8" wheel. Same as what 72bluNblu came up with.}

Yeah an even split won't work, but you can run the springs a little tighter. 1/2" at the quarter and 3/8" to the springs. It has to be perfect though, because if you miss either of those measurements it will probably rub. And good point about adding the reminder about the sides not being equal, because they usually aren't and even an 1/8" difference in this case would mean trimming something. That's why I would get more backspace than required, so I could adjust the final backspace with a thin spacer.

Most of the Darts that run 275's do have some work done to the quarter lip, because realistically none of these cars are perfect, and although the math works the actual execution is obviously what matters. And with a 275/60/15 you need more space for sidewall flex than you can get away with if you're running 17's or 18's because the shorter sidewall doesn't allow as much movement. I run 17's and 18's so I may be selling the sidewall flex on the 275/60/15's a little short.
 
Thank a for all the feedback, this weekend there is a pretty good swap meet near by, probably not much mopar but a lot of ford stuff so same bolt pattern. Hopefully a set or 2 there with tires I can take a level across and measure to see the dimensions and maybe pick up a set.
Best way is to beg, borrow, or steal a set of wheels/tires that are a little narrower than you want, with a similar wheel offset. Mount them on the car and measure/observe what you have for clearances. Measure the tire and the wheel too. Now you have known measurements. Which are a very good place to start calculating what you might need to do to make something bigger fit.
 
I can peel a set of 14" off my mustang only a 205 or 215 tire but can get some additional ideas for measurement and figment. The measurements I gave you are for the short side, the opposite side has an additional 1/8 to 3/16 to play with. I noticed last night the rear spring mounts the driver side is closer to the rail and the pass side is 1/4" further away from the rail. I guess the tolerances were a little looser. Lol. I also shimmed the driver side front mount back 3/16" may need to go 1/8" more but what I had at the moment.
 
Bolt a flat to the hub and take some measurements. See if it supports your findings.

upload_2017-7-18_12-14-42.png
 
I found a 14x5 1/2" mopar rim just to give me an idea of spacing. It has 3 3/4" backspace and it leaves me 3 1/4" to the spring. So with a 5 1/2" backspace and the 1" for the tire will leave 3/8" to the spring. If I understand this all. Using a level on the front of the rim and measureing to the front lip leaves me 2".
 
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