Rear end length.. Is there a science to it?

I will be doing spring relocation and mini tubs in the Duster which has a 7 1/4 rear in it. I have an 8 3/4 housing that I will size to fit whatever calculation I come up with - which I haven't.

I'm wondering if I should pick my wheels first then calculate backwards to arrive at the proper housing length.

wheel width + 2" for tire bulge - backspacing = housing length ??

Pretty much.
My fabricator buddies start with the wheel & tire combo.
That way they're not too tucked in & no rubbing.
Backing plates add a little.

Yeah, you definately want to pick wheels as available backspacing varies between wheels offered on the market. Assuming you want as wide a wheel as possible, 8 inch? 9 inch? Maybe even 10’s on a Duster?

I've been through the wheel deal with tires and what back spacing to use with 1/2 inch clearance on both the front and back side of the tires and wheels
Are you going to leave the stock wheel wells and outer 1/4 panels stock? What year Duster?
Answer these questions and we can talk more with better, more accurate, answers
The stock 8 3/4 housings for the Duster and other A bodies is right at 51 1/2 inches wide from the outside of the wheel bearing flange that the brake drum fits against to the other side, how wide is yours now ?

I just sold a friend an 8 3/4 housing that came from my early A body , but I am under the impression that the housing width is the same on the later A's.
It was narrowed 2 " per side and I had 295-55 X 15 Mickey's in my '65 Dart with a 15 X 10 with 5 /12 " back space. Springs moved to the frame rails and mini tubbed.
That was as big as I could go.
In his '71 Scamp , with the same treatment......3 " inboard springs and mini tubbed ( He used the U.S. Car Tool kit ) , he has 315 - 60 X 15's under it with room to spare , using 15 X 10 Rocket wheels with the 5 " back space.

Mount your tires on your wheels . Put
Them under the car and measure between them . That’s your total housing (plus brakes) width . That’s what I did and it fits looks perfect

^^^This is the easiest way of doing it,i did it the hard way ^^^measure the rear-end inside the frame rails and bought the rim/tires

How much wheel & tire is the OP planning on ?
My Duster (see avatar & below) runs 13 x31s on 15x12 wheels, 4.0" BS.
Front & rear of wheel openings trimmed back, upper lip just rolls under, bulge in outer wheelhouse removed.
No rubbing, but I'm not sure a 14x32 would fit with adequate clearance.
Stock wheel openings are limited to about 29" tire diameter; 1st point of sheetmetal contact is the forward part of the wheel opening.
Don't know if this (off-throttle) photo helps...

This info helps a lot !

Guys, what I have is a B body housing that I will send to have narrowed and axles made for.

I am using the US car tool spring relocation kit

I am going to split the rear wheel house and modify the radius on the outside wheel well for clearance.

I would like to get a 10 or 11 “ wide wheel in there and probably stay with 15” diameter rim to help combat wheel shock. I don’t see me using anything taller than a 30” tire.

I have a better side shot if it helps you for trimming the front of the opening.
The rear of the opening is likely to be OK.
Did my '72, recommend a 1/4" tape layout to get the shape looking right.
You can lose or keep the rounded flange end.
Keeping it will mean welding or panel bonding adhesive.

I had a duster before that had 28x10 tire and I can remember having to massage that front wheel arch. I also remember the tire just barely scrubbing the spring which was dangerous.

I recently had to buy a new rear end housing to fit the new wheels that I bought for my Duster. My new wheels had 3 inches more backspacing than the old wheels so the new housing had to be 6 inches wider. I'll end up selling my old rear end along with the matching wheels and tires to someone with a Duster that wants the whole setup.

This is what I did. After having the tires mounted I made a fixture out of 1 1/2" angle, basically a tire rack to set the tires in so they would stay square with each other. Then used a floor jack on each end to raise them into the wheel wells. After centering them up I measured the width I needed minus the rotor thickness.

I had my old pump gas Duster converted to ladder bars with coil overs and had the rear wheels stretch five inches, 3 inches to the front and two to the rear. I was lucky at that time to have several other junky Duster to cut the rear 1/4 panels up to use as donor panels to the new car, their are five different angles on Duster rear 1/4 panels over the tires
I used a 1964 Dodge B body 8 3/4 housing and had it narrowed down 50 1/2 inches bearing flange to bearing flange and put a Strange Engineering 35 spline spool and axles, I reused the stock 10x 1 3/4 wide drum brakes and had the drums drilled out to the larger 4 1/2 pattern so they would work on the new axles. I did run into some hassle on the axles not fitting them drum properly and I had to have the O.D. of the axle flanges cut down to fit those drums
I ran both 10 and 12 inch wide rims with either 4.5 or 5.5 back spacing with no issues at all, full 38 inch wide wheel tubs to the inner sub frame also

Measure Measure Measure, there is a good calculator on Dr. Diffs page. Something to watch out for, my wilwood disc brakes added almost 1 inch to the track width because the aluminum hats were .500 thick. I didn't even consider that originally and luckily caught that before I submited measurments.

I went from a drum brake 8.75 to a disc brake Dana. I knew I needed to narrow it just a small amount to help with tire clearance so I did. But I did it wrong, my rear end setup guy fixed it for a very fair price, but he fixed it wrong and had to do it again. It is all good now but I think at one point each of us was frustrated with the other guy haha.

Aw man that sounds like my luck lol. However, I did buy a barracuda that was tubbed with narrowed rear end. I had no wheels ! I tried to speculate tire bulge with wheel width and then a smoking deal on some wheels that I thought would work came up on racingjunk so I got them! They fit great ! Got lucky !

I have no idea how a 29 would fit. I had an uncut 70 Duster. A true 88 inch 28 would fit, but i had to actually sort pairs to not rub in the front. An 88 1/4, 88 3/8 bias would grow enough to rub at half track(9 sec car)
87 7/8, true 88 would be ok. Think a 29 is a 90 circumference, correct? No way would have fit. And these were Hoosiers, they run smaller than Mickeys. Tried mounting a 28 by 10.5 MT slick, couldn't bolt it up even without air in the tire

I’ll have to cross that bridge when i get to it. My ‘71 duster had 28x10 MT slick and it went right on - just rubbed the spring a little.

I’m in the midst of this exercise right now. I decided to do as described above and actually mock up the wheels and tires under the car. I’m building around The M/T 29.5 x 10.5 ProBracket Radial on a 10” Wheel under a 67 Plymouth B-Body. I bought the wheels and tires first, as I didn’t trust the advertised dimensions. So I mounted them up and mocked it all up, looking for 5/8” clearance to the wheel lips. I’m also moving the axle rearward 3/4” to make room for the PBR’s 30” ht at the leading edge of the wheel well. I’m also Mini-tubbing and moving the leaf springs into the frame at the same time.

In general, I think physical dimensions are the way to do it.

from my tire and wheel experience over the years, i have found tire sidewall bulge can be anywhere from 1-2" per side, depending on the tire manufacturer and how big [or small] a tire tread width you mount on your wheel of choice.
in my opinion, a guy should, IF possible, try to borrow a tire/wheel combo you may be considering to use as a mock up so you can see if it will work in your application.
if that is not possible, [as said before] MEASURE, MEASURE,MEASURE !
then, figure a tad on the wide side for your sidewall bulge so you don't get any unwanted surprises.

What did you go with for rear and gear choice ?

On the rear wheels, I want the center/hub of the wheel centered up in the wheel regardless of total width needed.

Why?
Do you want the wheel back spacing to be in the center of the wheel width exactly, IE 10 inch wide wheel with 5.0 back spacing?
Centered or negative offset makes them harder to put on and take off usually, the wider the wheels and tires the worst the problems
Been down that road more than once due to NHRA rules in NHRA stock classes years ago

That’s what I think too bc I believe it would be more stable but I do see these super narrow rear ends with super deep outside spacing but to me that doesn’t make sense.

High offset wheels benefit you by lightening the total weight of the rear end as an assembly in the car. If you have partial skirted wheel openings like a Dart, you really do not want long axles. Trying to get a big tire out with a high offset wheel is bad enough without adding to the drama. Wheel offset is of no concern with strength. All the fastest cars have the wheels offset. Heck, look at any vehicle with dual wheels. They are made for heavy loading.

I will always make the rear as narrow as I can, i.e.use the deepest offset wheel for the width I am using or may end up using. 1) Axles are shorter, less likely to twist or flex, likewise for the housing 2) Easier to get the wheels and tires on and off 3) The entire rear assembly is lighter. If I can save weight for the same money, I'm doing it.

If I'm fabricating spring mounts or suspension mounts using big tires, I will cheat the housing forward a bit 1) Weight distribution 2) If I need to trim the wheel opening, I only need to fool with the front of it.

I agree.

My rear end is moved back 1", it would be interesting to see how much the weight changes by moving the rear 1" either side of the factory location.

A factory A body 8.75 is 52 5/8"

The shorter number is from that garbage chart that is wrong. They are not 51.40"

AS mentioned measure to get it right with the wheel/tire package you want to run. You must have enough stick out beyond the leaf springs for a U bolt to grab before the housing end is in the way

That's why I said "I", it's just my preference. Assuming a 10" wide wheel with a 1" wide wheel center/hub, it would be 4.5" per side or wheel half. I don't size the rear end width or wheel stud length so I can't get the tires off in the first place, and don't mind if I need to let the rear end hang by using jack stands on the frame.

The roughly 1 lb. per axle saved making the rear end width 4" shorter isn't going show you much at all, because it's so close to center, there's more to be gained from the wheel itself and even better yet the tire. Some Pro Stock guys have a tire burn in car that's used to shave the tire OD's down, losing 4 lbs. at a 16"+ radius from center, now we're talking.

Fully aware of what's involved regarding NHRA Stock and Super Stock stuff, been there, done that, and doing that.