Backspace / Offset Question

[Tony_SS]

When a wheel claims 4" of backspace, does that number include the negative offset?

[alowerlevel]

Offset and backspacing are 2 different ways of describing the same thing. ie a 8" wide wheel with 4" of backspace = a 8" wheel with -12mm offset, or a 8" wide wheel with +12 offset = a 8" wide wheel with 5" of backspace. This might help alittle better https://www.rsracing.com/tech-wheel.html

[Tony_SS]

Right.. but some wheels I'm looking at claim both:

15x10
4" back space
-34mm offset

So does this typically mean that the wheel has a total of 4" backspace? OR does it mean the actual back space is really 2.67"?

It sounds like when the say 4" of backspace, they are factoring in the offset already.

[John Wright]

Tony, check out this link....it has those dimns shown so you can see where they are from.

http://www.rims-n-tires.com/rt_specs.jsp

[gearheads78]

Offset is from the centerline forward (positive) or back (negitive)and backspace is the rear edge to the mounting face. Keep in mind an 8" wheel with a 0 offset will not have a 4" BS it will be closer to 4.5" because of the lip of the wheels. The wheel width is measured tire bead to tire bead. Perfect example is my 18 x 10 wheels measure 11" edge to edge with a tape measure.

[alowerlevel]

Right.. but some wheels I'm looking at claim both:

15x10
4" back space
-34mm offset

So does this typically mean that the wheel has a total of 4" backspace? OR does it mean the actual back space is really 2.67"?

It sounds like when the say 4" of backspace, they are factoring in the offset already.

Yeah -34mm offset on a 10" rim = 4" of backspace

[Tony_SS]

Thanks for all the help and links!

[JRouche]

You gotta thank the tire and wheel folks (tire rack/discount tires) for the confusion. They like to use offset numbers cause alot of their customers are dealing with a pretty stock car and so when a guy with a stock mustang or nissan is looking for wheels they want to know how much its gonna "offset" the stock placement.

But yeah, diff words to get at a similar number. Back spacing is what we use, and its more informative IMO because it tells you exactly where the wheel mounting surface is in relation to the hoop. And wheels are an inch wider when using backspace numbers cause you commonly have a 1/2" of wheel beyond the tire bead seating area.

Tire manufactures use the bead seating surface for tire fitment. But we need to add the 1/2" of wheel on each side for clearance issues. So a 10" wheel for tire fitment purposes is actually 11" wide. JR

[FirstGenZq8]

the BS calculating page that john posted has its good and bad days. yesterday it was up, now it's down. it'll probably be back up soon.

when their page is down i use the BS/offset link in my signature. it'll take you to CCW's wheel calculator. it works fine, i just like the wheel/tire illustration on the rims-n-tires.com page.

[BAD68GTO]

You gotta thank the tire and wheel folks (tire rack/discount tires) for the confusion. They like to use offset numbers cause alot of their customers are dealing with a pretty stock car and so when a guy with a stock mustang or nissan is looking for wheels they want to know how much its gonna "offset" the stock placement.

But yeah, diff words to get at a similar number. Back spacing is what we use, and its more informative IMO because it tells you exactly where the wheel mounting surface is in relation to the hoop. And wheels are an inch wider when using backspace numbers cause you commonly have a 1/2" of wheel beyond the tire bead seating area.

Tire manufactures use the bead seating surface for tire fitment. But we need to add the 1/2" of wheel on each side for clearance issues. So a 10" wheel for tire fitment purposes is actually 11" wide. JR

Amen to that, Discount Tires said "all rim places use offset" when I tried geting a rim price weeks back and was condecending on me even using the term backspacing. I was like WTF, not everyone uses offsets and he was like "they do" and anyhow :bsjerk: I still have not purchased rims as of yet for my 68 GTO because I have not found "the right" rims.