71 Demon 15 x 10 Backspace Question

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Six Barrel Bill

Six Barrel Bill

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Hey everyone, I have researched on this forum about backspace. There is a lot of info but not finding exactly what I'm after.

For those running 295/50R15 on a 15x10 wheel in a Demon/Duster with only a 1/2" spring relocation kit (no mini tub), what wheel offset are you running?

We are building my daughters 71 Demon with an 8 3/4. We have moved the springs in 1/2" using Dr Diff spring location kit and perches to match. We are also using Dr Diff BB axles. When it was at the body shop last year, we had them remove the lip on the upper 1/4 panels. She is also running the Mopar Performance super stock springs and shocks so it will sit higher than factory.

The tire she is running on the rear is 295/50R15 Cooper Cobra's and she insists that this car has to have Magnum 500's. We will be running the 15 x 10's (overall wheel with is 11") and they only come in 2 different backspace options: 5" and 5.5".

I have the rear end mocked up with the brake backing plates, axles installed and drum on. With a straight edge on the front of the drum, the measurement to the spring is ~ 6 13/16.

If we go with the 5" backspace, that gives us ~ 1 7/8 to the springs but moves the wheel out further. (Note: The pic illustrates the overall with of the wheel at 11")
5 inches.jpg


If we go with the 5.5" backspace, that gives us ~ 1 3/8 to the springs and moves the wheel in closer which we prefer. (Note: The pic illustrates the overall with of the wheel at 11")
5_5 inches.jpg


I guess the part I don't know is how much we can expect the tire (295/50R15) to bulge on the 15 x 10 wheel. Sounds like we will have to run @24 psi as well.

Will the 5.5" backspace give enough room for tire bulge if we only have ~ 1 3/8" from the wheel to the springs?
 
I don't have the correct answer for you, the only Duster/demon I ever messed with was mini tubbed with springs under the frame.
So, I can only say what I would do... I would ALWAYS buy the wheel with the most backspace. If it rubs the springs, spacers fix that (billet only please). If you don't have enough backspace and it rubs the fender, how do you fix that?
Edit: I have fine-tuned tire location with spacers on three different cars. Custom made hub centric 3/8 spacers on a eleven second car, 1/2" billet I made for a nine second car, and 1 1/2" on a former eight second car that has been slowed down to ten ohs voluntarily. Sometimes wheels aren't offered in exactly what is necessary.
 
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Looks like 295/50x15 section width is supposed to be 11.6" (also saw a place that said a 295/55x15 was 12.2, not sure about that) and remember, your 15x10 wheel is really 11" wide, but backspace is measured from inside edge, so your 5.5 backspace mounts in the exact middle of the wheel.
Your sidewall bulge looks to be slightly more than 3/4" on each side. 1.6"divided by two.
 
Six Barrel Bill said:
the measurement to the spring is ~ 6 13/16.
Click to expand...
You need minimum 3/8 for clearance from the bulge to the body to allow for sway in the corners.
my 295Coopers on 15x10 are a lil over 12.6 across the bulges; so 6.3 from the centerline adding the 3/8 = 6.68 So you are cutting it pretty fine with a Zero offset wheel, which is 5.5 backspace
So all in all I think 33IMP is right on target

Yes I run 24psi for long tread life, cuz honestly 10s are almost too narrow for 295s. At 24psi, the car will roll over the sidewalls during modest cornering, making the back steer the car further into the turn than you expect. Watch for it cuz it is a lil unsettling till you get used to it.
FYI
For straightline action 275s are adequate up to say 350 hp. I only run the 295s to get sideways rubber, so that if I get into trouble in a corner, I just lift off the gas and let the tires couple,and then I'm not in the weeds again. I hated that,lol.
 
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"Tire size calculator" says 8"-10.5" for a 295/50, so a ten is about right. I've got more tire than that on an 8.5. But then again, I don't care about cornering on that car..... it's a 10.5x28 slick.
 
After considering everything, I'm going to take your advice @33IMP . We will order up the wheel with the 5.5 backspace and if we have to adjust a little bit to get it away from the springs we'll do that with spacers.

I did plan a little bit ahead when we ordered the axles from Dr Diff and did not have the wheel studs installed just in case of this issue with needing a wheel spacer and possibly longer studs. Who knows, it might just work out without spacers. It's not like she'll be doing a road course or anything, this is strictly street duty.

If we're successful, this is what it should look like. Here's a pic when we were mocking up a few things.
PXL_20220129_193228512.MP.jpg
 
I heartily endorse quality long studs for ANY application using spacers. All the cars I mentioned have at least three inch studs, front and rear, with open end lugs. My aftermarket axles use screw in studs, press in for the front, and for stock axles.
 
Thanks for your input guys. So I think we're good to go with no spacers needed.

@33IMP you were dead on, there is 3/4" tire bulge.
PXL_20220226_164408090.MP.jpg


There's about 3/4" between the tire and the spring. Where the band is clamped around the springs, there's about a 1/2" clearance.
PXL_20220226_203251746.jpg
PXL_20220226_203329719.jpg
PXL_20220226_203239707.jpg
 
Six Barrel Bill said:
Thanks for your input guys. So I think we're good to go with no spacers needed.

@33IMP you were dead on, there is 3/4" tire bulge.
View attachment 1715879471

There's about 3/4" between the tire and the spring. Where the band is clamped around the springs, there's about a 1/2" clearance.
View attachment 1715879468 View attachment 1715879469 View attachment 1715879470
Click to expand...
Yep,good to go. I only have 5/16 of an inch from tire bulge to spring. I have no rubbing.
 
These were 8.5”w Centerline with 5: backspace and a 275/60r15 m/t drag radials s/s. They touched…. The rims were 1 3/16” away from the leafs just as you measured.
I’m downsizing to a 8” wheel with 4.5 backspace and 255/60r15 because of it.

10F6FDA2-295D-4CFB-87D7-2E072A70B6DD.jpeg


23FB064E-C982-4C7C-95AF-63A03AC21413.jpeg
 
4mulas said:
These were 8.5”w Centerline with 5: backspace and a 275/60r15 m/t drag radials s/s. They touched…. The rims were 1 3/16” away from the leafs just as you measured.
I’m downsizing to a 8” wheel with 4.5 backspace and 255/60r15 because of it.

View attachment 1715882626

View attachment 1715882627
Click to expand...

The thing that probably saved us was using the Dr. Diff A-Body Offset Spring Hanger/Shackle Kit. It's a super nice kit but you do have to move your spring perches as well.
 
I'm using those 8.5 centerline auto drags too. (Centerline called the backspace 4 7/8") with a bit more tire than that..... but I had to use .300 spacers to get the tires to clear. So, basically 4.5 b.s. Good call on the 4.5s
Are those on your barracuda avatar car? You will have a lot less room than my b-body car!
 
33IMP said:
I'm using those 8.5 centerline auto drags too. (Centerline called the backspace 4 7/8") with a bit more tire than that..... but I had to use .300 spacers to get the tires to clear. So, basically 4.5 b.s. Good call on the 4.5s
Are those on your barracuda avatar car? You will have a lot less room than my b-body car!
Click to expand...

they were actually convo pro’s from the day. The catalogue calls them 5” backspace and I measured a healthy 4 15/16” anyway. This car is a factory 360 sport but with an early A body 8.75 housing and big bolt axle package I put together.

54BB09E8-3BC0-454A-96B8-5A9E8751D8C9.jpeg


58951719-8CE2-4761-81A4-35C2E1010FFA.jpeg


7CC9907C-F9C4-4009-B91B-507BF8FBCB47.jpeg
 
33IMP said:
I'm using those 8.5 centerline auto drags too. (Centerline called the backspace 4 7/8") with a bit more tire than that..... but I had to use .300 spacers to get the tires to clear. So, basically 4.5 b.s. Good call on the 4.5s
Are those on your barracuda avatar car? You will have a lot less room than my b-body car!
Click to expand...
The combo did fit fine on my 69 Belvedere with a Dana 60, well, they were about a 5/8” away from the springs, I’d do hear the odd squeal from rubbing when driving in and out of the alleys, when the axle has the most deflection from off camber etc….
 
33IMP said:
All four of my grandparents were Dutch. I'm ashamed to say that all the Dutch I have is the odd expression my mother used, and now that she's gone, I don't have any.....
Click to expand...
It was a point of pride, with the older generation

They might have spoken their native tongue at home, but as soon as they'd head out the door, they'd speak English



I'm the exact opposite (mostly because all my family still lives in the netherlands)
So when I moved here I decided I was gonna teach my kids Dutch

The oldest is 12 and almost fluent
The 8 year old doesn't take to it to well
And the 4 year old is at a place where she hasn't figured out what words belong to who, so she speaks dutchgrish

My boy made some new friends a while ago and they were in the same situation you are, descendants but only grandma still spoke it
Wouldn't you know it, one day my boy is over at their place and grandma shows up...and they end up having a conversation in Dutch

It made that ladies day
 
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