Wheels for a duster offset and backspace

[BOXHEADgumby]

Okay guys I have been reading and I don’t think I’m fully comprehending how back spacing and offset work.
I’m working with a 74 duster with a 8.25 rear with disc brakes that I took off a Jeep. The car is BBP.
I’ve put on tubular upper control arms stock steering rods.
In the back Hotchkis leafs and sway bar in stock location and Hotchkis shocks all around.
I’m attaching a link for some wheels that I’m Curious if the specs would work ? If I’m understanding all of this correctly.
Motegi MR138 Satin Black Wheels MR13877012738

 

[leblanc]

Personally I don't like big oversized wheels on a 70's automobile, but I find 15'' to be fine since that is what I run on my 76 Dart Sport, 15x7 in front and 15x8 on back. I like the classic look better on these cars but then again I am probably much older than you. By the way the Demon, Dart Sport and Duster are essentially all the same vehicle, the Duster was first, then the Demon and then the D. S. replaced the Demon name due to religious beliefs.

 

[383Scampman]

There is always a problem when you put parts on that came from another vehicle . Because most of us don't have first hand knowledge of your setup we can't make informed advise . There are general parameters that we all use to get fitment . I suggest you get set up rims . Start with 15"x7" or 15"x8" rim with a backspace of about 4"-4 1/2" and go from there . It's easier to use spacers if you need to move the rim outside than to move it inside . Just be sure the car is at ride height .

 

[diymirage]

Seems like a lot of backspacing, and a real ugly wheel

 

[BOXHEADgumby]

Seems like a lot of backspacing, and a real ugly wheel

Haha yeah the wheel wasn’t what I was looking at but more so the specs on it.

 

[Garys72Duster]

Just lay a steel ruler across the face of the hub where the back of wheel mounts. Then measure the distance from the ruler to your leaf spring. Deduct 1", and that is your max back space. My Dart measures 5.5" there, deduct 1", I get 4.5" max backspace. The tire will take up 1/2", then 1/2 " clearance to the leaf. That's how I understand back space.

 

[DionR]

I like 15” wheels, too, but I think 17” and 18” give a more purposeful look to the car, 18” wheels are the only way to get some real rubber under the car without fender flairs, and they don’t make modern tires in anything smaller than 17”. Not everybody wants modern radials (i.e. designed before EFI was a thought) but if you do and you want a 275 or so tire on the front in a stock fender, 18” wheels are pretty much your only option. And I am excluding drag radials from the definition of modern only because they really aren’t intended to do more than go in a straight line.

There is a huge thread on here about running 17” wheels, might give a search for that.

Based on the OP’s mod’s, sounds like he is looking for something better than 1980’s hockey pucks.

Best I remember, a 17” wheel is about maxed out at right around 5.5” of backspacing as a general rule. The issue is usually the tie rod end rubbing on the wheel, and sometimes the upper balljoint. Most are going to 18” wheels to avoid that interference, at which time you can run more like 6” of backspacing and a 9” or 10” wheel without hitting anything.

There are better people than myself to help, that is just what I remember. Look up 72bluNblu, he runs a 285 tire on the front of his car.

 

[72bluNblu]

Sorry I'm late, looks like the big wheel haters are already trying to force their archaic nonsense on somebody else because they still haven't figured out how lousy their BFG T/A's really are.

What size tires are you looking to run? A '74 Duster can actually run a ton of tire with the upgrade to 17" rims. Up to 255/40/17 in the front, and as much as 275/40 or 275/45/17 out back if you want. 255's out back would be pretty easy, the 275's will be tight and might take a little work on the quarter lip depending on body tolerances.

Not sure what your exact plans are for the car, but it based on the set up you've shared so far, it sounds like you're setting it up for better handling. If that's the case, you have a lot of options if you go to a 17" rim. In the back the 8.25" axle is narrower than an A body 8 3/4, so be careful who you listen to for backspace. What works on a BBP or even SBP 8 3/4 will be different for an 8.25", even with the Jeep disks in the rear adding a little to the track width. Are you still running factory disks in the front?

In the front, you have a ton of options. Your limiting factors will be the fenders on the outside, and the outer tie rod on the inside because of the 17" rim diameter. That limits backspace to about 5.6" on most rim designs. Assuming you're using the factory BBP, 73+ disks you can use any of the following-

17x7 with anywhere from 4.25" to 5.6" of backspace, or +8 to +38mm of offset (so those Motegi's will fit on the front). Tire is limited to a 225/45/17 or a 225/50/17, the first will be kinda short (25"), the second a little tall (25.9")
17x7.5, 4.75" to 5.6" of backspace (+10 to +35 for offset), with a 235/45/17. Or, you can run a 245/45/17 on that rim also, but you'll want minimum 5" of backspace for that (+18 offset)
17x8, 5" to 5.6" backspace (+12 to +30) with a 245/45/17. You can also get a 255/45/17 on that rim, it will be tall at 26.1", but should work if you stick to 5.25" to 5.6" of backspace (+20 to +30 offset)

In the back the 8.25 is the same width at a BBP 7.25. I was able to get a set of 275/40/17's on mine with a 17x9" rim and about 4.75" of backspace, but it was super tight on the springs in the factory location. You'd have a little more room though, because those disks widen the track width some (about 5/16" per side). Those Motogi's won't fit at all, too much offset. So...

17x8 with 4.25 to 4.5" of backspace (-6 to 0 offset), 245/45/17's would fit pretty easy. 255/45/17's would fit too.

17x9", 4.5 to 4.75" backspace (-12 to -6 offset), 255/45/17's. That rim is a better fit for the 255's, less sidewall squeeze. You may need a small wheel spacer on the inside, but with the disks maybe not.

17x9, 4.75" of backspace (-6 offset), 275/40/17's. This is a tight fit, so tailoring with a spacer or trimming/rolling the quarter lip may be needed depending on your ride height. They'll fit, but I wouldn't suggest it unless you're willing to do some trimming, especially since you're sitting fairly low with Hotchkis springs in the back.

You'll notice the offsets/backspacing is very different from the front. The track width front to rear is very different with BBP disks up front and an 8.25" out back, it will make wheel choices more limited.

I like 15” wheels, too, but I think 17” and 18” give a more purposeful look to the car, 18” wheels are the only way to get some real rubber under the car without fender flairs, and they don’t make modern tires in anything smaller than 17”. Not everybody wants modern radials (i.e. designed before EFI was a thought) but if you do and you want a 275 or so tire on the front in a stock fender, 18” wheels are pretty much your only option. And I am excluding drag radials from the definition of modern only because they really aren’t intended to do more than go in a straight line.

There is a huge thread on here about running 17” wheels, might give a search for that.

Based on the OP’s mod’s, sounds like he is looking for something better than 1980’s hockey pucks.

Best I remember, a 17” wheel is about maxed out at right around 5.5” of backspacing as a general rule. The issue is usually the tie rod end rubbing on the wheel, and sometimes the upper balljoint. Most are going to 18” wheels to avoid that interference, at which time you can run more like 6” of backspacing and a 9” or 10” wheel without hitting anything.

There are better people than myself to help, that is just what I remember. Look up 72bluNblu, he runs a 285 tire on the front of his car.

Got me while I was still typing my response!

Everything for 17's is above, 18's are definitely easier up front because of the tie rod deal. I run 275/35/18's up front on 18x9's, 285's would take 18x10's and some more push on my front fender for sure. 275 up front is the limit with relatively stock bodywork

 

[DionR]

Just to make sure I am clear, most of what I said above is focused on the front of the car.

After re-reading the original post, I realize I didn’t help with the rear.

Don’t remember what numbers work on a stock 8.25” axle, but pretty sure 5.5” of backspacing is too much in the rear. The Jeep disk brakes will add a little as they are thicker than the drums, but only 1/8-1/4”.

 

[DionR]

Sorry I'm late, looks like the big wheel haters are already trying to force their archaic nonsense on somebody else because they still haven't figured out how lousy their BFG T/A's really are.

What size tires are you looking to run? A '74 Duster can actually run a ton of tire with the upgrade to 17" rims. Up to 255/40/17 in the front, and as much as 275/40 or 275/45/17 out back if you want. 255's out back would be pretty easy, the 275's will be tight and might take a little work on the quarter lip depending on body tolerances.

Not sure what your exact plans are for the car, but it based on the set up you've shared so far, it sounds like you're setting it up for better handling. If that's the case, you have a lot of options if you go to a 17" rim. In the back the 8.25" axle is narrower than an A body 8 3/4, so be careful who you listen to for backspace. What works on a BBP or even SBP 8 3/4 will be different for an 8.25", even with the Jeep disks in the rear adding a little to the track width. Are you still running factory disks in the front?

In the front, you have a ton of options. Your limiting factors will be the fenders on the outside, and the outer tie rod on the inside because of the 17" rim diameter. That limits backspace to about 5.6" on most rim designs. Assuming you're using the factory BBP, 73+ disks you can use any of the following-

17x7 with anywhere from 4.25" to 5.6" of backspace, or +8 to +38mm of offset (so those Motegi's will fit on the front). Tire is limited to a 225/45/17 or a 225/50/17, the first will be kinda short (25"), the second a little tall (25.9")
17x7.5, 4.75" to 5.6" of backspace (+10 to +35 for offset), with a 235/45/17. Or, you can run a 245/45/17 on that rim also, but you'll want minimum 5" of backspace for that (+18 offset)
17x8, 5" to 5.6" backspace (+12 to +30) with a 245/45/17. You can also get a 255/45/17 on that rim, it will be tall at 26.1", but should work if you stick to 5.25" to 5.6" of backspace (+20 to +30 offset)

In the back the 8.25 is the same width at a BBP 7.25. I was able to get a set of 275/40/17's on mine with a 17x9" rim and about 4.75" of backspace, but it was super tight on the springs in the factory location. You'd have a little more room though, because those disks widen the track width some (about 5/16" per side). So...

17x8 with 4.25 to 4.5" of backspace (-6 to 0 offset), 245/45/17's would fit pretty easy. 255/45/17's would fit too.

17x9", 4.5 to 4.75" backspace (-12 to -6 offset), 255/45/17's. That rim is a better fit for the 255's, less sidewall squeeze. You may need a small wheel spacer on the inside, but with the disks maybe not.

17x9, 4.75" of backspace (-6 offset), 275/40/17's. This is a tight fit, so tailoring with a spacer or trimming/rolling the quarter lip may be needed depending on your ride height. They'll fit, but I wouldn't suggest it unless you're willing to do some trimming, especially since you're sitting fairly low with Hotchkis springs in the back.



Got me while I was still typing my response!

Everything for 17's is above, 18's are definitely easier up front because of the tie rod deal. I run 275/35/18's up front on 18x9's, 285's would take 18x10's and some more push on my front fender for sure. 275 up front is the limit with relatively stock bodywork

Dang, I knew I should have just waited. I was surprised you hadn’t already answered this.

Felt completely out of my element, but figured I would take up the challenge and do the best I could.

 

[72bluNblu]

Dang, I knew I should have just waited. I was surprised you hadn’t already answered this.

Felt completely out of my element, but figured I would take up the challenge and do the best I could.

No worries! You were right on with your info, and if nothing else at least you weren't just slamming the OP like the usual suspects because he offended your delicate sensibilities by suggesting something other than 50 year old technology.

Believe it or not, I'm trying to spend less time on here and work more on my own projects

 

[roccodart440]

Just lay a steel ruler across the face of the hub where the back of wheel mounts. Then measure the distance from the ruler to your leaf spring. Deduct 1", and that is your max back space.

Sorry I'm late, looks like the big wheel haters are already trying to force their archaic nonsense on somebody else because they still haven't figured out how lousy their BFG T/A's really are.

at least you weren't just slamming the OP like the usual suspects because he offended your delicate sensibilities by suggesting something other than 50 year old technology.

 

[DionR]

No worries! You were right on with your info, and if nothing else at least you weren't just slamming the OP like the usual suspects because he offended your delicate sensibilities by suggesting something other than 50 year old technology.

Funny thing is, I kind of live with the hate. My brother (who got me into Mopars when I was in high school) has a '70 Barracuda Grand Coupe and loves 15" wheels and BFG's. I ran 16" wheels on the rear of my Duster for a couple of summers and he used to call them wagon wheels. Can't wait to see what he says about the 17" wheels and 245/45's it will roll out on this summer.

Believe it or not, I'm trying to spend less time on here and work more on my own projects

Just make sure you check in once in a while so we know the next step in the evolution.

 

[03JeepJeep]

Blu is always down to help and fights the good fight with small wheelers haha

I end up with 18x8.5 5.1 bs with a 225/45 in the front and 18x9.5 5.3 bs 255/40 in the rear.

Im planning some new tires for the summer, I’ve got room for a 245 in front and 275 in rear.

 

[BOXHEADgumby]

Thank you for the advice ! You are correct I am working to improve and modernize the handling of the car, the improvements I’ve made have already made a large change in the handling, it no longer feels as sloppy or like a boat on the short drives I’ve taken it on so far. One thing I am still wrapping my head around that I’ll put into my words is backspacing is basically how far back into the wheel the center point of the wheel diameter sits and offset would be essentially where the barrel of the wheel sits in relation to the centerline of the wheel? If that makes sense or are they the same thing ?

 

[03JeepJeep]

From what I understand offset measures the distance between the wheel center and the mounting surface and backspace measures the distance between the mounting face and the inside edge of the wheel.

 

[DionR]

...backspacing is basically how far back into the wheel the center point of the wheel diameter sits and offset would be essentially where the barrel of the wheel sits in relation to the centerline of the wheel? If that makes sense.

It is all related to the mounting surface of the wheel.

Correction - see Blu’s post below...

Back spacing is a little difficult to calculate because it is between mounting surface of the wheel and the outside of the lip of the wheel, while the wheel width is actually measure from inside to inside of the lip. And because nobody lists the thickness of the lip, I just use 1/2" as an assumption.

The other measurement is offset which relates to how far the mounting surface is from the centerline of the wheel barrel. Positive being away from the centerline of the car and negative towards the centerline of the car.

So, your 7" wide rim with 38mm of positive offset should calculate out to 5.5" backspacing assuming 1/2" for the wheel lip:

3.5" (half the rim width) + 1.50" (offset in inches) + 1/2" (lip thickness) = 5.5"

In the case of a negative offset, subtract the offset rather than add it.

 

[72bluNblu]

From what I understand offset measures the distance between the wheel center and the mounting surface and backspace measures the distance between the mounting face and the inside edge of the wheel.

It is all related to the mounting surface of the wheel.

"Backspacing is defined as the distance from the hub mounting surface to the inside lip of the wheel (measured in inches). More backspacing puts the tire in closer to the center of the truck."

Back spacing is a little difficult to calculate because it is between mounting surface of the wheel and the outside of the lip of the wheel, while the wheel width is actually measure from inside to inside of the lip. And because nobody lists the thickness of the lip, I just use 1/2" as an assumption.

The other measurement is offset which relates to how far the mounting surface is from the centerline of the wheel barrel. Positive being away from the centerline of the car and negative towards the centerline of the car.

So, your 7" wide rim with 38mm of positive offset should calculate out to 5.5" backspacing assuming 1/2" for the wheel lip:

3.5" (half the rim width) + 1.50" (offset in inches) + 1/2" (lip thickness) = 5.5"

In the case of a negative offset, subtract the offset rather than add it.

Here's a link to someplace to read about it - https://www.discounttiredirect.com/learn/offset-backspace

Backspace is NOT measured to the inside lip of the wheel. It's measured from the outside of the lip to the mounting pad. Yes, I realize discount tire direct says otherwise, but they've got it wrong.

Forgeline actually makes wheels (really expensive ones at that!), here's their explanation of backspace and offset.



And if that's not good enough for you, here's a few more

https://blog.tirerack.com/blog/turk-t/measuring-backspacing
https://static.summitracing.com/global/images/instructions/crw-35-580547500.pdf
https://www.jegs.com/images/wheelmeasureguide.pdf
Offset Explained | WheelPros

Not sure what they're smoking over at discount tire direct. Maybe they're thinking they'll sell more rims if everyone put their rims a ~1/2" too close to their springs?

Not enough you say? Here's one of the wheels from my Challenger
Vision American Muscle 141 Legend 5 Series Gunmetal Wheels 141H7965GM0

17x9, 0 offset, 5" backspace.

ET 0, 17x9, cast into the rim...

And look at that, outside lip to mounting surface, 5".

 

[DionR]

Backspace is NOT measured to the inside lip of the wheel. It's measured from the outside of the lip to the mounting pad. Yes, I realize discount tire direct says otherwise, but they've got it wrong.

Well crud, guess that’s what I get for cutting and pasting from the internet. Thanks for correcting that.

At least I showed it correctly in my sample calculation.

Corrected my post above.

 

[72bluNblu]

Well crud, guess that’s what I get for cutting and pasting from the internet. Thanks for correcting that.

At least I showed it correctly in my sample calculation.

Corrected my post above.

No worries! There's bad information out there. Discount Tire should definitely know better, even as just a retailer.

 

[QMopar]

When I bought my Dart Sport, it already had 18s: 225/40-18 in the front and 245/35-18 out back. I have no idea what backspacing is on these wheels (is there an easy way to tell?). I'm thinking I'd like to run all four of the same size wheels/tires so I can rotate tires and only carry 1 size spare. Ideally I'd probably like 17's but I read that 18's give you more clearance for front suspension items. Could anyone share their experience with one size for all 4 wheels/tires? Thanks

 

[famous bob]

When I bought my Dart Sport, it already had 18s: 225/40-18 in the front and 245/35-18 out back. I have no idea what backspacing is on these wheels (is there an easy way to tell?). I'm thinking I'd like to run all four of the same size wheels/tires so I can rotate tires and only carry 1 size spare. Ideally I'd probably like 17's but I read that 18's give you more clearance for front suspension items. Could anyone share their experience with one size for all 4 wheels/tires? ThanksView attachment 1715288300

I still don't understand why a person would worry about offset, the back space is what really matters for wheel fitment ,if the back space aint right , it wont work if ur pushing the limit on wheel/tire size.
I have to have a 3 1/2" back space with my rear end length, and tire / wheel combination, I could care less about the offset in mm measurements.???????

 

[MRGTX]

I still don't understand why a person would worry about offset, the back space is what really matters for wheel fitment ,if the back space aint right , it wont work if ur pushing the limit on wheel/tire size.
I have to have a 3 1/2" back space with my rear end length, and tire / wheel combination, I could care less about the offset in mm measurements.???????

I'm not going to tell you what you should care about but take a few minutes to try out this calculator and see if it changes your mind.

Custom rims, wheel tire packages

 

[DionR]

I still don't understand why a person would worry about offset, the back space is what really matters for wheel fitment ,if the back space aint right , it wont work if ur pushing the limit on wheel/tire size.
I have to have a 3 1/2" back space with my rear end length, and tire / wheel combination, I could care less about the offset in mm measurements.???????

The problem is, backspacing doesn't take into account the tire. Just because the wheel fits, doesn't mean it will with a tire on it.

With offset, I can get a fair estimate of how much room I need just by taking half the tire section width and adding/subtracting the offset. Pretty easy to figure out.

Backspacing really only comes into play when trying to figure out clearances for things like suspension parts. So, backspacing is pretty useless on the rear axle, but can be hugely important on the front suspension.

So, both are important, just depends on what you need to know.

 

[72bluNblu]

When I bought my Dart Sport, it already had 18s: 225/40-18 in the front and 245/35-18 out back. I have no idea what backspacing is on these wheels (is there an easy way to tell?). I'm thinking I'd like to run all four of the same size wheels/tires so I can rotate tires and only carry 1 size spare. Ideally I'd probably like 17's but I read that 18's give you more clearance for front suspension items. Could anyone share their experience with one size for all 4 wheels/tires? ThanksView attachment 1715288300

Easiest way to determine the backspace is shown in the video I posted above, pull the wheels and measure from the outside of the lip on the wheel on the backside to the mounting surface. The rims may also be stamped or marked on the inside, but that will probably be an offset number.

The problem is that with an A-body the backspacing needed in the front and in the back isn’t the same. The wider the tire you’d like to use, the more that’s true. So, to use the same rims front and rear limits your width.

The other issue is the rear axles aren’t all the same width. A-body 8 3/4’s are wider than A-body 7.25 and 8.25 rear axles. So, what rear axle do you have?

I’m also assuming you have the 5x4.5” bolt pattern since you have 18’s already.

I still don't understand why a person would worry about offset, the back space is what really matters for wheel fitment ,if the back space aint right , it wont work if ur pushing the limit on wheel/tire size.
I have to have a 3 1/2" back space with my rear end length, and tire / wheel combination, I could care less about the offset in mm measurements.???????

That’s because you don’t understand what offset is telling you. Offset references the centerline of the rim. More importantly, it doesn’t change when you change the width of the wheel.

So, if you did your homework and centered your wheels in the wheel wells, but want to change the size of your rims or tires, you just get the same offset rim. For example, you have 17x8’s and want to get 17x9’s and you have the same clearance at the spring and the quarter. If you have 0 offset 17x8’s, you just get 0 offset 17x9’s, the wheel stays centered. Those rims don’t have the same backspace, the 17x8’s are a 4.5” backspace, the 17x9’s have a 5” backspace, but the offset is the same.

Offset just uses the centerline of the rim as it’s reference point, backspace uses the outside of the lip on the back of the rim. Both are really useful in determining what fits your car, but offset it especially helpful if you’re comparing different width wheels.