Back Spacing, which way?

Looking for some help from the wheel experts.

I am working with a 1972 Plymouth Scamp. I just installed an “A” body 8 3/4, Doctor Diff large bolt pattern axles, Drum brakes, 15 x 7 Rallye wheels with a 4.25 back space and mounted 255 60 R15 Cooper Cobra GT tires that have a section width of 10.2”

The inside bulge of the tire is 1” away from the leaf springs on both driver and passenger side and the outside bulge of the tire is within an 1/8” of rubbing the lip and would rub the inner fender when hitting bumps in the road and I would rather not cut the lip up to make it not rub. I have looked through the wheel tire combos thread and lots of people use the 4 ¼ backspace but mine are just too close for me.

I work swing shift and spend 3 to 4 hours in the mornings working on the car before work and another couple hours when I get home from work and I’m trying to figure out wheel offset. My brain is foggy and I can argue with myself both ways on offset. One voice says I need a 4” offset and the other voice says I need a 4.5” offset to move the tire inboard more. I have spent some time on google and for the life of me I keep going in a circle.

Wheel Vintiques offers the Rallye wheels in three offsets. A 15x7 wheel in 4 ¼ (which is too close), 4 1/2 and in a 15x8 wheel with an offset of 4 5/8” but do not offer a 4” offset.

Can somebody point me in the right direction that I need to go?

snowmann said:

Looking for some help from the wheel experts.

I am working with a 1972 Plymouth Scamp. I just installed an “A” body 8 3/4, Doctor Diff large bolt pattern axles, Drum brakes, 15 x 7 Rallye wheels with a 4.25 back space and mounted 255 60 R15 Cooper Cobra GT tires that have a section width of 10.2”

The inside bulge of the tire is 1” away from the leaf springs on both driver and passenger side and the outside bulge of the tire is within an 1/8” of rubbing the lip and would rub the inner fender when hitting bumps in the road and I would rather not cut the lip up to make it not rub. I have looked through the wheel tire combos thread and lots of people use the 4 ¼ backspace but mine are just too close for me.

I work swing shift and spend 3 to 4 hours in the mornings working on the car before work and another couple hours when I get home from work and I’m trying to figure out wheel offset. My brain is foggy and I can argue with myself both ways on offset. One voice says I need a 4” offset and the other voice says I need a 4.5” offset to move the tire inboard more. I have spent some time on google and for the life of me I keep going in a circle.

Wheel Vintiques offers the Rallye wheels in three offsets. A 15x7 wheel in 4 ¼ (which is too close), 4 1/2 and in a 15x8 wheel with an offset of 4 5/8” but do not offer a 4” offset.

Can somebody point me in the right direction that I need to go?

Click to expand...

Is your car ,freshly painted ? There might be a way ,without damaging the paint ,to be annoying .Can you take some times ,& get some pictures...? HAVE a '72 scamp , shown up with the same ,as I got it (your combination) After 40+ years , all cars are different .I honestly : Would post pictures of the outer fender to tire , clearance ... Inner fender to factory rear unibody rail clearance ,and accurate knowledge of rear leaf spring ,and leaf spring mounting points. (For instance : I mounted a set of 3/4 " inch offset hangars on mine,made some room .. We don't know what ,you actually have .Take some pictures, would make your life less stressful here.

snowmann said:

Looking for some help from the wheel experts.

I am working with a 1972 Plymouth Scamp. I just installed an “A” body 8 3/4, Doctor Diff large bolt pattern axles, Drum brakes, 15 x 7 Rallye wheels with a 4.25 back space and mounted 255 60 R15 Cooper Cobra GT tires that have a section width of 10.2”

The inside bulge of the tire is 1” away from the leaf springs on both driver and passenger side and the outside bulge of the tire is within an 1/8” of rubbing the lip and would rub the inner fender when hitting bumps in the road and I would rather not cut the lip up to make it not rub. I have looked through the wheel tire combos thread and lots of people use the 4 ¼ backspace but mine are just too close for me.

I work swing shift and spend 3 to 4 hours in the mornings working on the car before work and another couple hours when I get home from work and I’m trying to figure out wheel offset. My brain is foggy and I can argue with myself both ways on offset. One voice says I need a 4” offset and the other voice says I need a 4.5” offset to move the tire inboard more. I have spent some time on google and for the life of me I keep going in a circle.

Wheel Vintiques offers the Rallye wheels in three offsets. A 15x7 wheel in 4 ¼ (which is too close), 4 1/2 and in a 15x8 wheel with an offset of 4 5/8” but do not offer a 4” offset..

Click to expand...

I'm no wheel expert, but I have been doing a lot of research into what I need if when I convert my '67 to the big bolt pattern.

First question,.... Which drum brakes are you using? According to conversations I had with Cass at Doctor Diff, if you go to the big bolt pattern and use the 10x2.5" drums. it will increase track width by .3" per side, which makes sense with what you are seeing. He recommended going with 15x7 wheels with 4.5" back spacing. That will move the wheel 1/4" towards your leaf springs (from where you currently are), which from what you are saying, should put you at 3/4" from your leafs and 3/8" from your fender edge.

Remember that back spacing and offset are not the same thing. Backspacing is a measurement from back edge of rim to the mounting face. Offset is a measurement from the mounting face to the center of the wheel. I was a little confused when you said Wheel Vintiques offers a 15x8 wheel with a 4 5/8" offset. To my understanding, that would make your mounting face outside of the wheel width (center of 8" wheel is 4" from either side of wheel). I think you may have meant 4 5/8" backspacing.

From what you are describing, and assuming you are staying with a 15x7 wheel, I would look for a wheel with a backspacing somewhere between 4 1/2 and 4 3/4" back spacing. 4 1/2" back spacing would give you the spacing I mentioned above. 4 3/4" should set you at 1/2" from leafs and 5/8" from fender lip. To me it sounds like a 15x7 with 4 5/8" backspacing would pretty much center you in the wheel opening as it would move the wheel 3/8", and should put you 5/8 from leafs and 1/2" from fender lip. Sounds pretty centered to me.

I don't know enough about all this to know how much going to the 15x8" with the 4 5/8" backspacing would affect your section width (using the same tire). I know if won't make a full 1" (1/2" in each direction) difference, but I'm not confident enough to say how much it would affect it. Seems like the wider wheel width with that backspacing would push the tire out towards your fender again. I start getting confused once you start changing wheel widths AND backspacing

To simplify, it sounds like you want to move the tire in about 1/2" to center the tire in the space available, so your new backspacing should be around 4 3/4" with the 8" wheel. That is equal to a positive "offset" of 1/2" with the 8" wheel.

Don't forget, for measuring backspacing the 8" wheel is actually 9" lip-to-lip. Nearly every wheel made is 1" wider lip-to-lip than the wheel's bead width (8" here).

Abodybomber said:

Is your car ,freshly painted ? There might be a way ,without damaging the paint ,to be annoying .Can you take some times ,& get some pictures...? HAVE a '72 scamp , shown up with the same ,as I got it (your combination) After 40+ years , all cars are different .I honestly : Would post pictures of the outer fender to tire , clearance ... Inner fender to factory rear unibody rail clearance ,and accurate knowledge of rear leaf spring ,and leaf spring mounting points. (For instance : I mounted a set of 3/4 " inch offset hangars on mine,made some room .. We don't know what ,you actually have .Take some pictures, would make your life less stressful here.

Click to expand...

No fresh paint, just not interested in cutting sheet metal to make it fit. It’s using factory spring mounting locations. Moving leaf springs inward won’t help the outer tire clearance issues.

I can fix the issue with wheel offset but I'm trying to understand which way to go with it

You're mixing the terms offset and backspacing.

Those are two different things. And it's makes things confusing.

It sounds to me with your 15x7 situation, you want the tire inward 1/2. That would be 3/4" outside and 1/2" inside clearance by your information.

autoxcuda said:

You mixing the terms offset and backspacing.

Those are two different things. And it's make my things confusing.

Click to expand...

At 2am, and after a long day I was trying to understand backspacing and offset and it was confusing to me too.

Frnknsteen said:

I'm no wheel expert, but I have been doing a lot of research into what I need if when I convert my '67 to the big bolt pattern.

First question,.... Which drum brakes are you using? According to conversations I had with Cass at Doctor Diff, if you go to the big bolt pattern and use the 10x2.5" drums. it will increase track width by .3" per side, which makes sense with what you are seeing. He recommended going with 15x7 wheels with 4.5" back spacing. That will move the wheel 1/4" towards your leaf springs (from where you currently are), which from what you are saying, should put you at 3/4" from your leafs and 3/8" from your fender edge.

Click to expand...

I am using factory drum brakes. I think what Cass was trying to explain was that when switching to BBP axles the axle flange sticks out a bit more than the SBP axles do. I don’t know what that measurement is but the .3” that you mentioned was probably correct. The only thing on top of the axle flange is the thickness of the brake drum hat which is less than an 1/8”. So, the .3” that Cass referenced to may or may not include the hat of the brake drum.

All the posts I read that showed the 4.25 backspace did not mention what axles they were using and I didn’t even think to ask the question so I ordered the 4.25 backspace.

At 2am, and after a long day I was trying to understand backspacing and it was confusing to me too.

I want to stay with the 7” wheel and after your explanation and some sleep I think that clears it up. Using the 4.5 backspace would kick the wheel center section out a ¼" more, thus moving the tire inward to 3/4” from leaf spring and 3/8” from fender lip and that should take care of the issue. Wheel Vintiques doesn't offer a 4.75 backspace and I'm not sure if the trim ring would still fit at that point with a custom wheel being made.

I will order the 4.5" and see what happens. I guess I will find out on the test drive if it rubs at all.

Let us know how it works out, because I will be doing the same thing,... probably next year! Too much money already went out this year and Christmas is coming.

Aw crap, now I see you want to use a 7" wheel. So, here is my revised advice: so you want to move the tire in about 1/2" to center the tire in the space available, so your new backspacing should be around 4 3/4" with the 7" wheel. That is equal to a positive "offset" of 3/4" with the 7" wheel.

Don't forget, for measuring backspacing the 7" wheel is actually 8" lip-to-lip. Nearly every wheel made is 1" wider lip-to-lip than the wheel's bead width (7" here).

Ok, lets get the definitions straight first-

Backspace- On the inside of the rim (car side), the distance measured from the outside lip of the rim to the wheel mounting surface. You can measure this by laying a straight edge across the outer lip and measuring to the wheel mounting surface

Offset- The distance between the centerline of the rim and the wheel mounting surface. Offset can be positive or negative, positive offsets move the wheel mounting surface "out", so, more backspace. Negative offsets move the wheel mounting surface "in", so, less backspace.

Here's an illustration. Nevermind that I modified this for an 18" wheel, it applies to any diameter 9" wheel. This one is a +25mm offset, which is a 6" backspace on a 9" wheel. A zero offset 9" wheel has a 5" backspace.



The different measurements are nice to have, because of how things change relative to the measurement. If you go from a 7" wide wheel to an 8" wide wheel and you want the centerline of the wheel to remain in the same spot, you keep the same offset because the offset measurement is independent of wheel width. But if you use backspace and change the wheel width the backspacing must also change to keep the centerline in the same place.

So, lets say you have a 15x7" with a 4.25" backspace, and replace it with a 15x8" with a 4.5" backspace. You have more backspace, but you actually lost clearance on the outside (fender side), not the inside. You can see that with the offset. A 15x7" with 4.25" of backspace is a +6 offset. A 15x8" with 4.5" of backspace is a 0 offset. So, the center of the rim actually moved a 1/4" toward the fender, even though the backspace is more, because the width of the rim changed too.

Your measurements are pretty helpful. So, with a 15x7" and 4.25" of backspace you have 1" to the springs and 1/8" to the quarter. You need a 1/2" to the quarter, minimum, if you don't want any rubbing. And you only need a 1/2" to the springs, so, you have plenty of room. So you need at least a 15x7" with 4.625" of backspace (4 5/8"). That's a +16 offset. And that's the bad news. The 15x8 with 4 5/8" backspace is only a +3 offset, so, it would actually be WORSE than the 15x7 +6 you have now. And the section width of the tire would probably be wider on the 8" rim, so you'd lose even more space.

If you go 15x7 with a 4.5" offset, it will be close. You only get an extra 1/4" to the quarter, so, by your measurements you end up with 3/8" to the quarter. That's not really enough. It might work most of the time depending on your ride height, how many passengers you carry, how much stuff is in the trunk, or how enthusiastically you corner. But it will probably rub under some of those conditions. What you really need is a 15x7 with 4.75" of backspace. That would give you a 1/2" to the springs and 5/8" to the quarter, which would be great. But that would be a custom rim.

Nice job of explaining it Blu, and a good illustration.

The only thing I would add for clarification to what Blu said is that when he is describing the offset (+6, +3, +16), that number is in millimeters. For some reason backspacing is typically called out in inches and offset is called out in millimeters. Don't know why, but wanted to clarify because it confused me for a minute too.

72bluNblu said:

Ok, lets get the definitions straight first-

Backspace- On the inside of the rim (car side), the distance measured from the outside lip of the rim to the wheel mounting surface. You can measure this by laying a straight edge across the outer lip and measuring to the wheel mounting surface

Offset- The distance between the centerline of the rim and the wheel mounting surface. Offset can be positive or negative, positive offsets move the wheel mounting surface "out", so, more backspace. Negative offsets move the wheel mounting surface "in", so, less backspace.

Here's an illustration. Nevermind that I modified this for an 18" wheel, it applies to any diameter 9" wheel. This one is a +25mm offset, which is a 6" backspace on a 9" wheel. A zero offset 9" wheel has a 5" backspace.

View attachment 1715111111

The different measurements are nice to have, because of how things change relative to the measurement. If you go from a 7" wide wheel to an 8" wide wheel and you want the centerline of the wheel to remain in the same spot, you keep the same offset because the offset measurement is independent of wheel width. But if you use backspace and change the wheel width the backspacing must also change to keep the centerline in the same place.

So, lets say you have a 15x7" with a 4.25" backspace, and replace it with a 15x8" with a 4.5" backspace. You have more backspace, but you actually lost clearance on the outside (fender side), not the inside. You can see that with the offset. A 15x7" with 4.25" of backspace is a +6 offset. A 15x8" with 4.5" of backspace is a 0 offset. So, the center of the rim actually moved a 1/4" toward the fender, even though the backspace is more, because the width of the rim changed too.

Your measurements are pretty helpful. So, with a 15x7" and 4.25" of backspace you have 1" to the springs and 1/8" to the quarter. You need a 1/2" to the quarter, minimum, if you don't want any rubbing. And you only need a 1/2" to the springs, so, you have plenty of room. So you need at least a 15x7" with 4.625" of backspace (4 5/8"). That's a +16 offset. And that's the bad news. The 15x8 with 4 5/8" backspace is only a +3 offset, so, it would actually be WORSE than the 15x7 +6 you have now. And the section width of the tire would probably be wider on the 8" rim, so you'd lose even more space.

If you go 15x7 with a 4.5" offset, it will be close. You only get an extra 1/4" to the quarter, so, by your measurements you end up with 3/8" to the quarter. That's not really enough. It might work most of the time depending on your ride height, how many passengers you carry, how much stuff is in the trunk, or how enthusiastically you corner. But it will probably rub under some of those conditions. What you really need is a 15x7 with 4.75" of backspace. That would give you a 1/2" to the springs and 5/8" to the quarter, which would be great. But that would be a custom rim.

Click to expand...

Hey Blu,
Very informative on the explanation. A little drawing/engineering background perhaps? That was exactly what I couldn't find on the net the night I was looking for it. Maybe it should be a sticky in the wheel and tire section for others to see?

Does the tires sit equally in the same spot on each side of the car?
Maybe the axle is shifted to one side or the other.

sireland67 said:

Does the tires sit equally in the same spot on each side of the car?
Maybe the axle is shifted to one side or the other.

Click to expand...

They usually are off up to 1/2" side-to-side.

Frnknsteen said:

Nice job of explaining it Blu, and a good illustration.

The only thing I would add for clarification to what Blu said is that when he is describing the offset (+6, +3, +16), that number is in millimeters. For some reason backspacing is typically called out in inches and offset is called out in millimeters. Don't know why, but wanted to clarify because it confused me for a minute too.

Click to expand...

Because us Americans use backspacing, usually. The Europeans or Asians use offset. It's really the same thing, although offset is easier to understand, but backspacing is easier to measure.
To convert inches to mm multiply by 25.4, to convert mm to inches divide by 25.4.

sorry, duplicate post

Not to be a dick to the original poster but to help anyone else who might read this thread. Every time one of these threads pop up and someone says that "AB&C used this size wheel on their car, but it won't fit mine?!?" I cringe. I post a link that tells how to measure YOUR car using only a set of jack stands and a washer tied to a string. But people insist on being lazy and using what complete strangers told them should fit their car? It's your money, keep buying wheels and crossing your fingers hoping that they will fit.

How To Measure For Proper Wheel Sizes

BrianT said:

Not to be a dick to the original poster but to help anyone else who might read this thread. Every time one of these threads pop up and someone says that "AB&C used this size wheel on their car, but it won't fit mine?!?" I cringe. I post a link that tells how to measure YOUR car using only a set of jack stands and a washer tied to a string. But people insist on being lazy and using what complete strangers told them should fit their car? It's your money, keep buying wheels and crossing your fingers hoping that they will fit.

How To Measure For Proper Wheel Sizes

Click to expand...

The OP did measure his car, and provided those measurements. Which I used to figure out the backspace he needed.

Yes, every car is a little different and if you’re completely maxing out the wheel/tire size that’s a big deal. But most folks aren’t using every last 1/8” or even every last 1/4”, so general recommendations can be very helpful to a lot of people. And some people just aren’t good at doing the calculations or comparing different wheel and tire specs. Nothing wrong with that, I answer PM’s all the time about wheel and tire combinations. The more measurements that are taken then better the level of fit you can get, that’s true.

And no matter what, when you max out tire size there’s some finger crossing because the published tire specs don’t always match the real world tire measurements. Different brands and models of tires can vary quite a bit on actual measurements, even for tires that are listed as the same size on the sidewall.

BrianT said:

Not to be a dick to the original poster but to help anyone else who might read this thread. Every time one of these threads pop up and someone says that "AB&C used this size wheel on their car, but it won't fit mine?!?" I cringe. I post a link that tells how to measure YOUR car using only a set of jack stands and a washer tied to a string. But people insist on being lazy and using what complete strangers told them should fit their car? It's your money, keep buying wheels and crossing your fingers hoping that they will fit.

How To Measure For Proper Wheel Sizes

Click to expand...

Hey Brian,

Thanks for that informative contribution to this thread. Perhaps I didn’t find your write up or link when I was searching for wheel threads. Maybe that’s why I did things the way I did them. I guess I didn’t realize there was only your way to do things.
As a side note, just so you are aware, a lot of the world's problems could be solved if we all agreed to stop being dicks. From stealing someone's parking spot, to starting unnecessary wars, someone deciding to act like a dick is always what caused it. Hope you have a pleasant day. :-D