Setting the record straight on backspacing!

[ToogyTJ]

I am really starting to get frustrated trying to figure out what backspacing to get. For any experts out there.
What is the difference between offset and backspacing?

I came across this site on the net:

www.discounttiredirect.com/helpWheelOffsets.html (http://www.discounttiredirect.com/helpWheelOffsets.html)

So the way I read it is if you had a wheel with 4"BS with negative 11.43mm offset. That would equal approxiametly a wheel with a backspacing of 3.25" BS with zero offset? and the 4.75 with positive 7.62 offset would be approxiametly a wheel with a backspacing of 5.05 BS with zero offset?
If so it kinda looks like both choices (AR-767) are bad in different ways.
Anybody care to comment on this?

[Lauti]

ToogyTJ,

Don't forget the wheel width in your calculations!

BS: Distance measured from mounting surface of the wheel to the inside lip of the wheel.

OS: (+) to the inside, or (-) to the outside offset of the wheel mounting surface in relation to a center line drawn through the wheel.

[brewmenn]

ToogyTJ,

Don't forget the wheel width in your calculations!


Also don't forget that wheel width is measured from the surface where the tire and the wheel meet. Backspacing is measured from the widest edge of the rim. I believe the difference is about .5 inch per side or 1 inch overall.

therefore:
4"BS with negative 11.43mm offset = 7.9 inch wheels [(4+.45) x 2]-1=7.9
3.25" BS with zero offset = 5.5 inch wheels (3.25 x 2)-1=5.5
4.75 with positive 7.62 offset =7.9 inch wheels [(4.75-.3)x2]-1=7.9
5.05 BS with zero offset=9.1 inch wheels (5.05 x 2)-1=9.1

backspacing = [(wheel width + 1)/2] + offset
offset = backspacing - [(wheel width + 1)/2]
Wheel width = [(backspacing - offset) x 2] - 1

I think this is right
Hope this wasn't to much math. And remember to use negative numbers if the offset is negative

stock 5.5" BS on 7" rims = negitive 1.5 offset or 38mm

[BlackJeepOfTheFamily]

I've posted this several times now ....it just doesn't get any more simple than this :

http://albums.photopoint.com/j/View?u=118677&a=10489024&p=45548347&Sequence=0&res =high


You Can see the picture can't you?

[brewmenn]

Thanks BJOTF I was trying to find that pic.
Note that in the pic. that the offset shown is negative.

Now if we can just save this info somewhere to pull out every time this question gets asked we'll be all set.

[Lauti]

This is a very good picture, Mike!
It shows it all you need to know!
Sorry, I couldn't finish my above post, I would have had more to type, but that's not necessary anymore.
I think it should all be cleared up now for ToogyTJ!

[Wrangled97TJ]

Thanks for posting that BJOTF. The whole back spacing issue makes sense to me now too. :D :beer:

Marty

[Lauti]

I had to think about this again.
The Grizzly wheels are 7JJx15 wheels and have 31.75 stamped inside the wheel. 31.75mm are 1.25", so 7" wheel width plus 1" for the lips, makes 8", half is 4", plus those 1.25" --> this results in 5.25" BS for the factory Grizzly wheels! Right? Or did I make a mistake?

[Wharfrat]

For posterity purposes, I found Lauti's original response to be confusing :

"OS: (+) to the inside, or (-) to the outside ..."

Offset is actually (-) to the inside and (+) to the outside, inside being towards the axle and outside being out towards the street. Here's a website that helps clarify :

http://www.z31.com/wheelsandtires.shtml

This picture also helps:

http://www.gmhightechperformance.com//archives/images/0602/0206sc_mount18.jpg
Plus someone has taken the time to write a nice online calculator :

http://wwgarage.com/website/rim_calculator.html

Although the 1" that gets automatically added to wheel width during the calculation is just an estimation and may result in a +/- 1mm error depending on manufacturer.

[JT's TJ]

Am I the only one who can't see BJOTF's picture?

[Griz]

Good picture Mike, I haven't seen that one in quite a while.
For those who can't see it, it is because you do not have a PhotoPoint account with your cookies enabled.

[Wharfrat]

Although the following diagram is fairly crude and specific to 15"x8" AR767's, it may also help to illustrate these concepts :

"http://www.geocities.com/mule_rocks/AR767.jpg"

Note: You have to manually copy-and-paste this URL into a web browser ... accessing it directly from this forum violates geocities rules.

[BlackJeepOfTheFamily]

Originally posted by JT's TJ
Am I the only one who can't see BJOTF's picture?

LOL !

I cant' even see them :D

...this post is from over a year and a half ago (before photopoint screwed everyone over, it's just been re-vived recently. I remember the pic though, it was a DAMN good one..If I can find it on my PC at home I'll repost it from my own server tonite.

[mad jeep chick]

If you are looking to replace your stock rims with something wider then I would like to offer a suggestion....

All things, such as fender well clearance, springs, etc. being equal and not causing interference in your choice, try to split any added width equally on each side of the centre line. (Example time!!! 7" rims with 5.5" backspacing = 8" rims with 6" backspacing) The load on stock wheels is placed evenly over the bearings. I have heard that if you mess this balance up too much (apparently a little is okay.... maybe) it will cause your bearings to have a short/er life span, could affect your turning radius, stress the ball-joints, and other assorted maladies.

I will let the experts elaborate, because I really don't know a lot about it.

MJC :bigsmile:

[caseyPB]

im soon to get some 15x8 rockcrawlers...first what brand should i go with.....just the AR's or what....

....and second.....what backspacing should i get....its a 31x10.50 tire and i wanna be able to....remove the washers on my steering stops, so i can get the original turning radius as it was b4 the tires...and....i kinda like the look of the tires past the flares....any help...thanx

[Gilkman]

www.eaglewheels.com used to have a picture showing all this, but it is not there anymore....too bad.

[computertechgirl]

:mad:
My reply = no help, but I've been trying to figure all this out too!

I'm about to buy a set of wheels and 31 inch tires and I'm trying to figur eout what backspacing I need if I hope to use the (same) wheels with 33's after my suspension lift next year.

All the calculations and pics are great, but why isn't there a chart somewhere that lists a general (recommended) backspacing for different size wheels and tires on jeeps?

[mappelsauce]

i dont think there is such a thing a a chart for backspacing with tire size, but it woulbe be nice. I guess you could use some of the info here to figure it out. However, the best way to get answers to that is to start a new post with that specific question. It is a very common issue that a lot of jeep owners have dealt with, so may as well see what everybodys actual experience is. You could also try a search. If i remember right, almost everybody i read about used 3.75" to 4.25" backspacing depending on the brand for moderate sized tires. i cant rememeber for the bigger ones (35+).

one last comment. those calculators are nice to try to figure out how to get nearly equal wheels in different sizes. But most jeep owners are getting different wheels because they need to shift their tires outward (due to wider tires rubbing things inside). In that case, the only thing that should matter is the backspacing. In that case smaller is better, and it is not the rim size but the tire width that matters (since it is what is rubbing).

Nevertheless, a good post with info i was not aware of and i hope my post helped.

thanks,
mike