Talk to me about wheel offset and backspacing for a 4x4

[Balzer]

I have learned these trucks are a bit sensitive to what wheels are put on them.
My fist GMT400 had 15x12 wheels and a crappy lift and it ate up everything in the front end. (dont even know what offset and backspace they had)
My 97 had 16x10 with zero offset and 4.75" backspace and didnt really wear out parts much.
The wheels on my truck now used to be on my dads 94 ecsb and are a 15x8 with zero offset and Im not sure what backspace. And my dad never had any problems with front end wear with these wheels.

The wheels I have been looking at are 17x9 and come with either zero, 12mm or 25mm offset.

I feel these truck are better off with the proper wheel. I have seen friends with wide wheels and near stock tires that also eat up unit bearings and ball joints. And I have seen friends run 35's on stock wheels and never have a problem.

So what is stock offset and backspace/ and what does a guy choose to keep close to those specs while running aftermarket wheels and bigger tires?

If it matters my truck currently has a skyjacker 2" lift, I may stay with that and run a 33" tall tire or I might later go to a 6" lift and run 35" tires. I would like to run a 17x9 wheel either way. I also want to add stock fender flares and would like the wheel/tire to be close to the edge of the flares. They can poke out a bit but not more than 1" or so.

I should know more about offset and backspace but I really dont know much at all other than what I have stated above.
Thanks in advance.

 

[JollyGreen]

Well, I believe the stock offset is like a 31mm offset. If you want to run larger tires you'll want to run a wheel with a negative offset, something like -12mm or less offset. Offset is the measured distance between the hub mounting surface and the wheels center line. Typically speaking, the higher the offset/backspace, the more the wheel will tuck inwards towards the suspension or away from the fender (positive number). The lower the offset/backspace, the more the wheel will push out away from the suspension or towards the fender (negative number).

For Example:

If the offset on a 9" wide wheel is +12mm, the hub mounting surface will be 12mm forward from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 5.5"

If the offset on a 9" wide wheel is -12mm, the hub mounting surface would be 12mm towards the back of the wheel from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 4.5"

Calculating the backspacing using the rim width and offset:

First - add 1" to the rim width and then divide by 2 to find the wheels center line (you have to account for the wheel flange which is why you add 1" to the rim width).
Second - convert the offset which is in millimeters into inches. There is 25.4mm in 1 inch so divide the offset by 25.4. Lastly - add the converted offset to the wheels center line if the offset is positive for the correct backspacing. Subtract the converted offset to the wheels center line if the offset is negative for the correct backspacing.

For Example:

17x9 +12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*+12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" + 5" = 5.50" Backspace


17x9 -12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*-12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" - 5" = 4.50" Backspace

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[Balzer]

Thank you that was a better explaination than I could find on the net.

 

[JJZ71]

That is really good Jolly Green. Even I understood it and I have issues with some of the basics!!!! No kidding, good right up.