Drag Line

New Member
Jan 22, 2013
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Hi all,

I would like to share what I have learned about wheels and possibly take
some of the mystery out of them for you.

Back Space: is a quick way to determine if one wheel is similar to another/different wheel.
It it measured from the inside center surface that will mount to/touch the drum or rotor to the inside rim edge. On the KJ/KK it is approximately 5".

Off Set: is a number that is measured from the exact centerline of the wheel.
1.) if it is a positive value, it probably will not be designated as plus (+20mm) as most manufacturers will state that positive values are a given.
2.) if it is a negative value, it will generally be designated as minus/- (-20mm). But not always preceded by a - sign. Read descriptions carefully.
3.) positive Off Sets move the rim position towards the middle line, closer to the A-arms, negative Off Sets move the rim position towards the outside or the truck fender.
Off Set is useful IF you know what the width of the wheel is from outer edge to outer edge of the rim or from the inner to inner lip of the rim where the tire will seat.
Off Set can indicate ~ what the Back Space is if the following equation is used;
1/2 the rim width (in inches or mm) plus or minus the given Off Set will pretty much equal the Back Space.
Off Set Calculation Example: A Liberty KK OEM rim has a width of 7" divided by 2 = 3.5 inches (88.9mm) plus the positive Off Set 43mm (43/25.4 = 1.7") = 5.2". (5.2" = ~131.9mm total)
This isn't exactly a 5" Back Space but can be useful if you can't put a wheel on your vehicle but need to know how closely it will fit for clearance to steering gear, etc.
On a wider wheel, let's use 8" for a Liberty, the Off Set should be a smaller number because 8"/2 = 4" (101.4mm) + the Off Set need to = 131.9mm.
So in this case, (5"Back Space) 131.9mm -101.4mm (1/2 rim width) = 30.5mm (1.2") so 4"+1.2" = 5.2" Back Space, the same as the 7" wide rim.
If the same Off Set # was used from the 7" width wheel on the 8" wheel, the Back Space would be ~ 5.75" which might interfere (rub) against upper control arms or other Steering parts.
A negative offset of 43mm either 7" or 8" will position the wheel ~1.7" further out from the steering gear components & therefore about 1 & 3/4" past the edge of the fender flair which is why lifting the KJ/KK to an appropriate height is important when adding tire height. Also note that putting a wheel further out from where it engineered to be will put an additional stress on the wheel bearings that might not be designed to handle that stress & MAY result in premature failure.

Hub Center Bore or Hub Diameter: The Diameter of the center bore of the wheel that will fit around/over the axle hub. KKs have a 71.5mm centercap/hub bore. Remember for safety, get a hub centric ring reducer to align a larger hub bore opening to the reduced bore of the smaller hub.
If the center bore is the same as your current wheel, you can use your existing caps, if the hub bore is larger, either find larger caps or run without.

Rim Diameter: Fairly self explanatory. It is important if switching to a "big brake" kit and you need to know if the caliper assembly will fit within the wheel. Side note on big brake upgrade kits, be sure your SPARE tire will fit the new set-up.

Knowing some of this will let you change out wheels from the limited offerings of Stellantis/FCA to a larger variety of other OEM or aftermarket wheels.
Ford, GM, Toyota & others offer a large selection of OEM 5x4.5 bolt patterns in 16", 17" & 18" rims with approximately 5" back spaces and 71.5mm hub bores, you just have to do some research on the style you like.

Any information in this article are for general knowledge only & I DO NOT TAKE ANY RESPONSIBILITY for it's use or misuse.