Wheel Mathematical question

[Nima]

Hi:
I am confused about understanding wheels back spacing and offset, even after all these years!
First thing first: Back spacing is the distance between wheel mounting surface to the lip of the wheel on the back side, Offset is the position of the wheel mounting surface in regards to the center of the wheel based on its width. Correct?
Now, this seems to be a simple calculation unless my understanding of the backspacing and offset above is wrong. For example, If a wheel is 8" wide with 4" back spacing, that means the wheel mounting surface is 4" away from wheel rear lip which means the mounting surface is at the mid center of wheel which is 4" which means the offset has to be zero!
So why and how an 8" wide wheel with 4" back spacing can have positive or negative offset? What am I missing?
Thanks

[special-K]

I don't know, but I've never gotten into the whole modern wheel thing. Wheels used to all have a backspace that would allow any wheel with the same bolt pattern to fit a vehicle with the same bolt pattern. When buying wheels you just got the diameter and width you wanted and unless you measured for your own curiosity, you never knew the backspace. From that standpoint and through all those years, to me "offset" equates to what we referred to as "dish", the opposite of backspace, the depth to center on the front, which would be the balance of the backspace figure when subtracted from the width. You could do the math if you really needed to know what those figures equated to in what you refer to as "offset", but I don't know why that would matter. I've never run across any cases of wheel offset not being the balance of the width after subtracting the backspace. Do you have an example?

[Sheepdip]

This might help
https://mgaguru.com/mgtech/wheels/pi...eloffset_2.gif

[lolife99]

Originally Posted by Nima

Hi:
I am confused about understanding wheels back spacing and offset, even after all these years!
First thing first: Back spacing is the distance between wheel mounting surface to the lip of the wheel on the back side, Offset is the position of the wheel mounting surface in regards to the center of the wheel based on its width. Correct?
Now, this seems to be a simple calculation unless my understanding of the backspacing and offset above is wrong. For example, If a wheel is 8" wide with 4" back spacing, that means the wheel mounting surface is 4" away from wheel rear lip which means the mounting surface is at the mid center of wheel which is 4" which means the offset has to be zero!
So why and how an 8" wide wheel with 4" back spacing can have positive or negative offset? What am I missing?
Thanks

Wheels are measured at the bead surface for the tire,... not overall width.
A 8" wheel actually measure 9" overall.
So a zero offset 8" wheel will have a 4.5" backspace.

[lolife99]

Look at this thread,...
http://67-72chevytrucks.com/vboard/s...d.php?t=530806

[Stormin08]

a high offset (Positive) usually means deep backspacing, a low offset (negative) means less back space and deeper dish.

think in the terms of a FWD car of a high performance car with the track width pushed out wide...you need a deep offset to get under the car. recent heavy duty trucks are receiving the same treatment...look at the newer gm hd trucks, wheels have like 6" BS. an 8" wheel that has 50mm positive offset / or 6" bs.

[Nima]

Thanks for the all replies. This subject has been bothering me since it is one of those things that you never learned and you always have taken somebody's else world for it without understanding it!
First, it doesn't sounds right to me that these two values have been measured by different measuring standards, one by inch and one by millimeter!
This is where this old brain of mine hurts! If you have an 8" wide wheel ( which would be 9" wide from outside lip to lip approximately) with zero offset, which means the mounting surface is at the exact center of the wheel (4" from the either side of the inside of the lips and approximately 4.5" from the outside of the lips) this wheel has to have 4.5" backspacing. In another word an 8" wheel with 4.5" back spacing, mathematically, has to have zero offset since if you change the offset (wheel mounting surface) the backspacing will change automatically but then how come they are different offset ratings for such a wheel?!
Does this make sense?

[lolife99]

Not everyone wants a zero offset wheel.
That’s why they make different offsets.

Personally I only talk in “backspacing”.
Those number match my tape measure.
Sometimes people run different width rearends to gain a deeper lip wheel. As in a narrowed rearend.
I like my wheels tucked inside the fenders more than most.
That requires custom backspacing.
Bagged vehicles for example, need more backspacing than a static dropped or stock truck.

[1976gmc20]

Originally Posted by special-K

I don't know, but I've never gotten into the whole modern wheel thing. Wheels used to all have a backspace that would allow any wheel with the same bolt pattern to fit a vehicle with the same bolt pattern. When buying wheels you just got the diameter and width you wants and unless you measured for your own curiousity, you never knew the backspace. ...

Yeah, that's what I always thought until I tried to adapt 6-hole wheels from an 88-98 pickup to a 4wd squarebody. The bolt pattern works fine but the center hole and backspacing don't. The center hole can be "fixed" but the backspacing causes the wheel to hit the tie rod end.

That was my expensive education about backspacing. I suppose one could use spacers but I've heard various opinions about the safety of those and besides they cost as much as new wheels.

For some reason the trend of nearly all "newer" vehicles is wider axles and high backspace wheels. The only reason I can see for this is maybe the extra width is needed to allow for the CV axles of IFS FWD and 4WD vehicles ???

[Stormin08]

you really should measure from inside the lip, verify with an overall width for clearance.

i prefer BS to offset, but if understand 25.4 mm is 1", you can figure it out...just remember Positive vs Negative

he bad is NOT ALL wheel manufactures make all BS/Offsets, you can get close...but not always.

i run a COYS WHEEL, currently a 7" wide front with 4.25 BS, i would love it to have 4.5...

[lolife99]

Originally Posted by Stormin08

you really should measure from inside the lip, verify with an overall width for clearance.

i prefer BS to offset, but if understand 25.4 mm is 1", you can figure it out...just remember Positive vs Negative

he bad is NOT ALL wheel manufactures make all BS/Offsets, you can get close...but not always.

i run a COYS WHEEL, currently a 7" wide front with 4.25 BS, i would love it to have 4.5...

This is why the new "Speedway" wheel is so popular.
Billet Specialties decided to come up with an affordable wheel (aka Chinese made like all the other cheap wheels),.. that actually FIT most old cars and trucks.
THEY GOT THE BACKSPACE NUMBERS CORRECT,... when all the other companies were just guessing.
They come in gray centered wheels and full polished as shown below.
Contact member Way2lo2 here on the board or check them out at http://www.billetspecialties.com/whe...ies/speedway-/

I think Summit Racing even sells them.

[The Rocknrod]

Unless of course you have a step-side.

[lolife99]

Originally Posted by The Rocknrod

Unless of course you have a step-side.

Why do you say that?

[The Rocknrod]

Originally Posted by lolife99

Wheels are measured at the bead surface for the tire,... not overall width.
A 8" wheel actually measure 9" overall.
So a zero offset 8" wheel will have a 4.5" backspace.

So that concerns me as I thought about an 8" wide 17" Dia. wheel in the back of my stepside. With the bulge of the sidewalls I'm not sure, what do you think?:

[lolife99]

Originally Posted by The Rocknrod

So that concerns me as I thought about an 8" wide 17" Dia. wheel in the back of my stepside. With the bulge of the sidewalls I'm not sure, what do you think?:

Looks like you have 12 inches of total room..
If you have a 17x8 Wheel you need a 4.5 inch bs to center it in the fender.
And that leaves 1.5” of wheel gap on either side of the wheel.
If I’m looking at your spec sheet right.

What size tire do you plan to run?

[The Rocknrod]

Not sure something 27" Tall that will fit.
Why do I keep thinking a 4" bs will center it in there? Where is my thought process flawed?

[lolife99]

Originally Posted by The Rocknrod

Not sure something 27" Tall that will fit.
Why do I keep thinking a 4" bs will center it in there? Where is my thought process flawed?

Like I said above,... wheels are measured at the tires bead surface.
The overal width of most all wheels are 1” wider overall.
A 17x8 Wheel will actually measure 9” wide over all.

[The Rocknrod]

Oh then that's a good thing. Finding an 8" wheel with a four inch backspace was getting tiring. They were all 4.5" backspace. I was thinking custom wheel cash was in the future.

[Stormin08]

Originally Posted by lolife99

This is why the new "Speedway" wheel is so popular.
Billet Specialties decided to come up with an affordable wheel (aka Chinese made like all the other cheap wheels),.. that actually FIT most old cars and trucks.
THEY GOT THE BACKSPACE NUMBERS CORRECT,... when all the other companies were just guessing.
They come in gray centered wheels and full polished as shown below.
Contact member Way2lo2 here on the board or check them out at http://www.billetspecialties.com/whe...ies/speedway-/

I think Summit Racing even sells them.

if your CHINESE made is correct, which it probably is ( as most cast are)...then the issue i had was this, once the stock is sold out...a 1yr plus wait or very high prices due to the tariffs.

i am all for the tariffs, but some one should start casting again within the US..

[Custom 68]

Great thread, I am gonna bookmark it!

[lolife99]

Originally Posted by Stormin08

if your CHINESE made is correct, which it probably is ( as most cast are)...then the issue i had was this, once the stock is sold out...a 1yr plus wait or very high prices due to the tariffs.

i am all for the tariffs, but some one should start casting again within the US..

Boris Maryanovsky at StreetMachinery in Euclid, Ohio has them in stock and ready to ship.
He was the one that worked with Billet Specialties to develop these wheel.
(they are Chinese, like Coys and Ridler, etc)
He's one of the biggest dealers of Billet Specialties wheels in the country.
I follow him on Instagram and Facebook.