Rear Wheel Size and Backspacing

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69valiant21

69valiant21

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I have a set of cheap rims I bought from summit to get my 69 Valiant on the road several years back. I cannot find the receipt anywhere and I cannot remember the dimensions of the rims. So I am sort of taking the lazy way out and posting here instead of measuring.

I want to buy a set of wheels and put a set of slicks on them for the track but I am unsure of the backspacing and max rim width. Its a 69 Valiant with no mods for larger tires. I believe my current rims are 15x8? The rears are 1" wider than the fronts, that I do know.

26 x 8.5 Hoosiers will fit, just need rims. Any help is appreciated.
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8 3/4 rear?

An 8" wide rim is really about all you need with the stock spring locations. Your maximum section width is about 10", so, a 245 is usually all you can do. Some cars can manage a 255, but that usually requires some trimming. Backspace for an 8" wide rim on an A-body 8 3/4 should be ~5" for BBP axles, ~4.75" for SBP axles to maximize tire width. Those are approximate of course, it's always best to measure your own car because body tolerances can easily vary as much as a 1/4", even side to side on the same car.
 
yes, 8 3/4. It was SBP but I converted to BBP. I will just have to measure what I have now, like you said.
 
I would suspect it would be fairly close to my '71 Dart with that info. Mine measures 6" from the spring to the wheel mounting face (2.5" wide drums), and 5" from the wheel mount to the quarter lip. But every car is a little different. Once you measure, don't forget that backspace doesn't include the outer lip of the rim, it's measured from the mounting surface to the inside lip of the rim where the tire actually mounts. And of course that the tire will probably hang out over the edge of the rim, which is why the section width is important to know.
 
What 72 says is correct. I ended up with 5.375" backspacing for a 8" wide wheel and a 245 tire with an 8-3/4 and LBP axles. Since you have a wheel on the car, that will help you figure your center of the wheel well and what backspacing you need. Crawl under with a tape measure and a straight edge and go to town. If you have to error, error the inside so you can space out, as in the rim you want only comes in certain back spacings, unless your ordering custom.
 
Got home from work and looked at what I am running. 255/60s with no trimming or anything. The tires are about an inch from the springs and it was hard to tell how far from the fender but it looks as long as I don't get a taller tire I will be fine. Also measured that I'm running an 8" rim and about a 4.5" back spacing. Those are rough numbers with the tire mounted.

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Don't lower the car any. With those numbers I would be looking for an 8" wide rim and 5" of backspace.

I bet if you put a straight edge on the outside bulge of the tire it will hit the quarter lip. Not a problem with that diameter tire and ride height, but if you lower the car it'll interfere.
 
I did put a straight edge from the bulge to the fender and they do not interfere, but extremely close. And I love the stance so that won't be an issue :)

But you think I could squeeze another 1/2" of backspacing? That'll leave me just under a half inch between the tire and spring.
 
69valiant21 said:
I did put a straight edge from the bulge to the fender and they do not interfere, but extremely close. And I love the stance so that won't be an issue :)

But you think I could squeeze another 1/2" of backspacing? That'll leave me just under a half inch between the tire and spring.
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1/2" to the springs should be plenty. The tire to spring clearance really only has to account for sidewall flex. Well, unless maybe not all your spring clamps are tight and the leafs shift around. I run less than a 1/2" on my Duster, but it has 18" rims so the sidewalls don't move much.

The clearance to the quarter lip on the other hand has to be at least a 1/2", and more like 3/4" if you have softer springs with no rear sway bar. If the quarter lip is low enough on the tire anyway. If you run the quarter lips above the tops of the tire, it doesn't matter nearly as much. If at all, depending on how stiff the rear springs are.
 
Ya, I think for me, the important thing to check would be the full travel of the suspension being smaller than the distance from the lip edge to the side wall contact point....if that makes sense. Since the lip is so far above the tire.
 
Hoosier states that tire you mention has a section width (bulge) of 10.3 with a stiff sidewall and Copper says your 255/60/15 has a section width of 10.2" so if what you have clears nicely on the street then I would just buy what you have. If you plan to lower the rear then add to your backspacing.
Your lucky Plymouth didn't use the 67-69 Dart rear quarters......
 
dano said:
Hoosier states that tire you mention has a section width (bulge) of 10.3 with a stiff sidewall and Copper says your 255/60/15 has a section width of 10.2" so if what you have clears nicely on the street then I would just buy what you have. If you plan to lower the rear then add to your backspacing.
Your lucky Plymouth didn't use the 67-69 Dart rear quarters......
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Ya, I was reading some of the other threads on here and I saw some comments that 255 is a squeeze for 67-69 A bodies (close to that range) then I look at what I have and scratch my head thinking "wheres the squeeze? is there something Im missing?" but I guess I lucked out.
 
Haha. Yeah the wheel openings are taller on the Valiants. The quarter skin is in the same place, but on a Dart the whole top half of the wheel is covered because the opening is lower. So regardless of ride height you have to clear it. Still, if you lowered your car you would have to contend with the clearance to the quarter.

Happens a lot with the Dusters, they have more room to begin with but I've heard about all kinds of giant tires that allegedly fit, only to see a picture with the tires hanging 2" outside the quarter and the car jacked up like a 4x4. True, they don't rub, but they don't "fit" either. Which is why I give my recommendations based on the tire actually fitting inside the wheel well.
 
OK! So, I have changed direction. I am going to run Mickey Thompson ET Street radials. BUT I want to run 275/50s. The section width is 1" wider than the 255/60s I run now. SO that will leave me 1/4" to the springs (new heavy duty springs from Mancinis so they shouldn't sway at all). And the tires are and inch shorter so I will be an additional 1/2" away from the fender. So the fenders shouldn't be a problem. Tell me Im crazy, or tell me they will fit.....

Pics are my current 255/60s:
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You're crazy.

A 1/4" isn't really enough to the springs with 15" rims. It's not the springs moving that you have to worry about so much, it's the deflection of the tire. The taller the sidewall, the more deflection there will be. To get away with a 1/4" to the springs I think you'd need sidewalls like I've got with my 18's. And that still might not be enough. A 275/50/15 has a 5.4" tall sidewall. My 295/35/18's have 4" sidewall. That's a lot of real estate.

And for fun, autoxcuda has 245/50/15's on his Barracuda. That's a 4.8" tall sidewall (.6" shorter than the 275's you're planning). Check out the sidewall deflection on his tires. Just watch the first 1:30. Around 55 seconds and 1:18 you'll see some really impressive sidewall movement. Think that's more than a 1/4"? I know it is.


And, your tires will not clear your quarters. No, that's not a problem if the suspension doesn't travel enough for the tire to encounter the quarter, as yours probably doesn't raised up like it is. But, if somehow you did compress your suspension enough, your would get interference.
 
Those look much wider than 245s...or is it just an optical illusion?
 
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69valiant21 said:
Those look much wider than 245s...or is it just an optical illusion?
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As far as I know autoxcuda is still running the same tires, so, 245/50/15's. Probably has to do with the angle. And, they're only a 24.6" tall tire, so, they may look wider because they're so short.

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