Backspacing & offset on a stock wheel for a 99XJ

[MAP]

I've read several threads on backspacing of wheels and hear that 4 inches of backspacing on a 15x7 wheel could cause more stress on steering components and on wheels bearings.

This is what I know from reading other posts.
The stock wheel for my 99XJ has a 15x7 wheels, 5 lugs with 4.5" lug pattern and 5.25" of backspacing. I have a 2" budget boost.

Now if this wheel is 7 inches wide, the centerline is 3.5" from either side. This means that the 5.25" of backspace gives a positive offset of 1.75".(5.25" backspace-3.5" centerline) So the mounting plane is actually closer to the street side of the wheel. Actually 1.75" from the street side edge.

So on a 15X7 wheel, in order to get a 0" offset you would have to get a wheel with 3.5" of backspacing. On a 15X8 wheel a wheel with 4" of backspacing will give you 0" of offset. In this case the mounting surface would be right at the centerline of the wheel.

On a 15X7 wheel, 4 inches of backspace would still give you a positive offset of 0.5".

As long as one uses a wheel that gives you 0" or positive offset, would this cause any extra load on bearings or steering components?

I would think that a negative offset(where mounting surface is closer to the back side of the rim/wheel) is where one would expect to have problems.

I do understand that any changes from the stock backspacing will shift the tire from it's original stock location and could cause rubbing on the fenders or control arms.

Can I get some feedback on this. Is my understanding correct?
Does anyone disagree with this?

And of course the reason for this post is that I would like to get new wheels that stick the tires out a bit for a wider stance yet not negatively affect the wear of bearings & steering components or negatively affect handling.

Thanks,
MAP

 

[RCP Phx]

Two things,in your first calc it would be -1.75" of offset.Second,your not taking into consideration the location of the wheel bearings.

 

[Kittrell]

And of course the reason for this post is that I would like to get new wheels that stick the tires out a bit for a wider stance yet not negatively affect the wear of bearings & steering components or negatively affect handling.

Thanks,
MAP

Most people run a 3.75" BS'd wheel with no ill effects. Look for a set with 3.75" or 4". Either wheel should keep a 12.50" tire off of the control arms, and don't worry about bearing wear and such. Just don't run a 2" BS'd wheel, ever wonder why the tires on the ghetto low riders look like this / \

 

[norcal420]

Yeah, don't sweat it.

4" BS, 15*8 w/ 33*12.50s is all good...

DR

 

[MAP]

Two things,in your first calc it would be -1.75" of offset.Second,your not taking into consideration the location of the wheel bearings.

Could you elaborate on your second statement?

Thanks for the other feedback guys.

MAP

 

[RCP Phx]

The actual relationship between the centerline of the tire to the centerline of the bearing hub.Jeeps deep backspacing accounts for the addition of the axle-stub/wheel flange dimension bringing the tire back over the centerline of the bearings.Little hard to describe I hope that makes sense.

 

[Eagle]

The actual relationship between the centerline of the tire to the centerline of the bearing hub.Jeeps deep backspacing accounts for the addition of the axle-stub/wheel flange dimension bringing the tire back over the centerline of the bearings.Little hard to describe I hope that makes sense.

Quite some time ago MaXJohnson put up a CAD drawing that illustrates this. Perhaps he still has it and can either put it up again, or provide a new link to wherever he has it.

 

[MAP]

About 2 years ago I replaced my factory spare with a full size spare.
The tire is a 225/75R15. The steel wheel has white spokes and is supposed to have factory dimensions. Anyway, I got the tire and wheel at one of those big chain Discount Tire stores.

I had never installed the spare to check the fit until today. Good thing I did, it only had 15PSI of air in it.

It appears that they gave me a 15X7 wheel that has 3.75" to 4" of backspacing instead of the factory 5.25".

I installed this in front and back for fit and to check for any rubbing. I jacked up the axle under the wheel to try to raise it as much as possible to see if I could get something to rub and it fit fine.

I liked how the tire sticks out a bit compared to the factory tire.
Here's a pic.

I also saw something strange but it appears this is how it's supposed to fit.
I saw the back of the steel wheel and realized that the wheel doesn't seat right at the bolt holes. It seats only along a ring that runs around the bolt holes. It appears as a dark ring on the white paint. I'm assuming this is how it's supposed to seat.

The Ecco wheel seats right at the bolt holes.

Any comments?

Thanks,
MAP

 

[MAP]

Any more input on this topic?

MAP

 

[MAP]

I just realized something.

Backspace is measured from the farthest edge of the wheel flange.

The backspace measurement takes into consideration the edge of the flange on the wheel, but the wheel width does not since it only measures the distance between the inner mounting surface of the flange.

So in order to calculate a 0" inch offset you need to take into consideration the height of the flange. If the height is .25". Then you need a backspacing of 3.75"(not 3.5" of backspace) on a 7" wide wheel to achieve 0" of offset.

Hope this makes sense. So on our 15x7 wheels, 4 inches of backspace still gives .25" of positive offset.

MAP