found set of wheels, confused about backspace/offset again. Need help

[Mattchu60]

I found a set of wheels on CL. The post says that they are 16x8 wheels and have a 3mm+ offset, but says that they have 4.625" backspacing. Here is the quote below:

"plus 3 mm offset, 4.625" backspace."


From what I thought I knew, the offset was from the middle of the wheel. So 3mm shouldnt be much at all, and the backspacing would be around 4" (half of 8").

What am I missing here? I like these wheels, but I want to make sure I get something with around 0-6mm offset, since the stock ralley wheels are 15x8, with 0mm offset (I believe).

Thanks for any help. Maybe someone could clarify the relationship between backspace/offset. I thought they were tied together, but this makes me 2nd guess myself.

 

[J Knight]

A rim width is measured from bead to bead so an 8in rim is actually 9in wide so the center is actually 4.5in. With that said 4.625bs sounds correct for +3mm offset.

 

[Mattchu60]

Makes sense. With these specs, I assume this would line up around the same as my k10 rally wheels. Is that bascially right?

 

[J Knight]

Im not sure what the 6 lug 8in rally bs is but it is 4.25in on the 5 lug rallys.

 

[cooter55]

Back in the day, GM used wheels that were typically close to a zero offset wheel. Stock rim width of 7 inches made for an 8 inch wide wheel and backspacing was about 3 3/4 to 4 inches. What you have to watch out for with a wheel that has increased back spacing is interference with steering and suspension components, like tie rod ends and ball joints. If you're unsure, take some measurements of your current wheel and then check how much room you have for the additional wheel width.

 

[Mattchu60]

http://minneapolis.craigslist.org/ank/pts/3950099303.html

Here are the wheels I am looking at. They are weld wheels, pretty expensive when new. These are used, but they look pretty clean.

I couldnt find any pictures of these on a square, but I think they would look good.

 

[Mattchu60]

I measured one of the rally wheels tonight. To much surprise, this is not a 0mm offset wheel like I though. The backspacing is around 4", while the "frontspacing" is around 5". I found some information on the net, and found that this equals a -12mm offset.

That said, I'm not sure if I should be messing with a wheel that has 4.625" of backspacing, I don't want to run into issues with rubbing.

 

[77 K20]

I measured one of the rally wheels tonight. To much surprise, this is not a 0mm offset wheel like I though. The backspacing is around 4", while the "frontspacing" is around 5". I found some information on the net, and found that this equals a -12mm offset.

That said, I'm not sure if I should be messing with a wheel that has 4.625" of backspacing, I don't want to run into issues with rubbing.

The black steel wheels I have now are 16x8 and they have 4.5" of backspacing. The ones you are looking at only have 0.125" more.

I am not 1/10" of an inch away from rubbing on my leaf springs. They should be fine.


I have also seen a 1979 GMC K25 use the newer 2500HD wheels. They have 4.875" of backspace and work.

Also tire rack shows wheels with 4.88" of backspace being able to fit.

 

[Mattchu60]

Well I finally figured it out last night as I was laying in bed before falling asleep, I had forgotten a critical peice of information in the whole axle swap process - the WMS on a K10 front axle is 67", and on a K20 front axle it is 69". This is because the mounting surface on the hubs is farther out on a K20, even though its the same axle.

So, If I did my math right - if a K10 wheel is -12mm with 4" backspace, then by adding an inch of length on each side for the K20 axle, I would need a +12mm offset wheel to get the same stance and could allow for up to 5" of backspacing, since all the "stuff" behind the wheel is in the same place on a K10/K20 (axle components like tie rods). Bascially, I gained an inch on each side by swapping axles.