Backspacing vs Offset
 

So I got a Porterbuilt drop member on a 67 frame / rear axle...


I found some;

20x8.5 w/5.25 back spacing

22x9.5 w/5.5 back spacing

Are these going to fit...do I need to be concerned with offset or is that the metric of Backspacing???
Posted via Mobile Device
Posted via Mobile Device

Re: Backspacing vs Offset
 

offset is the distance from the center of the rim to the mounting surface of the wheel,, this can either be negative or positive, going towards the front of the wheel being posiive and towards the rear of the wheel being negative..

the backspacing is the distance from the mounting flange of the wheel to the backedge of the rim ,,,hope this helps you out

Re: Backspacing vs Offset
 

Yeah I keep readin that and when I think I understand then I get confused again. Haha

So if a 20x8.5 has a 5.25 BS, then that's the measurement from the back of the rim that meets up with your rotors/drums to the back of the wheel?

So why do I care about Offset? If a rim is 8.5 wide and the BS is 5.25 that leaves 3.25 for the center of the wheel and whatever Lip you have on the face of the rim itself.

Is Offset and BS virtually the same thing just a different way to measure it OR do I need to be concerned with BS AND Offset?

I have a Porterbuilt Dropmember and want these wheels and the dealer says yes the front are 5.25 BS and the rears are 5.5 which is what Nate (Porterbuilt) said they use...do I need to be concerned with the different offsets
Posted via Mobile Device

Re: Backspacing vs Offset
 

They are somewhat related. Backspacing is a constant, no matter the wheel's width.
Backspace is an easier way to determine fitment, mainly because the inner part of the rim is more likely to interfere with some suspension parts. Total width figures into things too, but more in a fender clearence during suspension travel way.

Look at it like this,
An 8" wheel with a 4" backspace has 0" offset.
That same 8" wheel with a 3" backspace has 1" negative offset
Again 8" wheel with 5" backspace, it would have 1" positive offset.


The move backspace you have, the more likely you are to have control arm rubbing problems when turning. If you have too much negative offset, the wheel swings in a big arc and you get fender rubbing.

A quick drawing, using the same size wheel, fender and spindle pivot point, the only change is the off set. See how it not only pushes the tire outward, it swings back and forth rather than just pivoting on a center point?

Re: Backspacing vs Offset
 

no they are not the same

Re: Backspacing vs Offset
 

I hope those measurements are correct, they re the same I went with.

Re: Backspacing vs Offset
 

Keep in mind that BS always includes the flange protrusion beyond the bead seat, which generally runs about 1/2" for most wheels. In other words, an 8" wheel will measure approximately 9" over the flanges. This means that an 8" wheel with 0 offset will have about 4.5" BS.

Ray

Re: Backspacing vs Offset
 

The front will work for sure.
The rear could be an issue.
Which rearend do you have?
67-70,... or the wider 71-72 rearend?
I would wait on wheels until you have the truck assembled enough to measure the rear wheel clearance yourself.