BackSpace question with a B-Body 8 ¾” in an A-Body

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Look at the articles in the tech archives on Tim's red Valiant. We sat his car up to use Mustang 17x8 (or 17x9) rims on four corners. It had a B-body rear end under it and special hubs up front.

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Here’s the deal, I want to go to a 17” x 8” wheel probably running a 235/50-17 which are 26.3” tall all around on my 1971 Duster. This size tire allows me to run modern maximum performance summer tires while still retaining enough sidewall height to look correct. Ideally I would like to run the same size wheel with the same offset (BackSpace) all around as well. I may consider larger rear tires for the track if traction is a big problem.

Since I’m also upgrading the front suspension to 73-76 spindles with 11 ¾” discs with Gen I Viper calipers that will push the front wheels out requiring a wheel with a lot of positive BackSpace; it seems that the narrow A-Body 8 ¾” rear is not wide enough and will require a much deeper negative BackSpace to push the wheels back out into the quarter Panels. If that’s the case, I can’t rotate the tires front to rear and would have to carry a generic spare tire like a 205/70-15 on a steel wheel assuming that would clear my front brakes.

Question:
Would a 8 ¾” B-Body rear that is 3” wider overall push the rear wheels back out into the quarter panel enough so I can safely run the same positive BackSpace wheels front and rear? What BackSpace am I looking for (5.5” I’m guessing)?

Help!

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The 68-70 B-body rear that is 3" overall wider will be 1.5" wider on each side. That's compared to a original A-body rear with thinner 5x4 axle flanges.

I run thin axle flanges on a re-drilled original 5x4 axles. I have found 4.5 backspacing was max with a 245 wide 50 series tire on an 8" rim. More than that, the tire will rub the leaf. Much less than that the tire will run the outer fender lip and will keep you from running a 255 or 265 wider tire on an 8" rim.

So if you run 5.5" backspace rear tires, you need a rear that sticks out on each side only 1" more than an original A-body rear. Or 2" more total drum to drum width.

To me, that would be a 65-67 B-body rear that is 59 1/2" drum to drum. That's 2 1/8" wider or 1 1/16" on each side wider.

Also, do you plan to run rear disks? Typical rear disk have a thicker flange plate than a drum. Would be nice to know that difference now and account for it.

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The 68-70 B-body rear that is 3" overall wider will be 1.5" wider on each side. That's compared to a original A-body rear with thinner 5x4 axle flanges.

I run thin axle flanges on a re-drilled original 5x4 axles. I have found 4.5 backspacing was max with a 245 wide 50 series tire on an 8" rim. More than that, the tire will rub the leaf. Much less than that the tire will run the outer fender lip and will keep you from running a 255 or 265 wider tire on an 8" rim.

So if you run 5.5" backspace rear tires, you need a rear that sticks out on each side only 1" more than an original A-body rear. Or 2" more total drum to drum width.

To me, that would be a 65-67 B-body rear that is 59 1/2" drum to drum. That's 2 1/8" wider or 1 1/16" on each side wider.

Also, do you plan to run rear disks? Typical rear disk have a thicker flange plate than a drum. Would be nice to know that difference now and account for it.

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I plan on running Discs all around.

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Do you know the thickness of the rear disk at the axle flange portion?

Just went in garage and measured a stock A-body drum at around 0.110"