Proper term - Offset or Backspacing?

[Larry Callahan]

I am working on our new PT Garage and I'm not sure what the most appropriate term is for one of the questions.

Is it offset or backspacing when describing a wheel?

[clancj]

All depends on the information given. If the desired back-spacing is known, wheel width can be ordered with the specific back-spacing built into it to place the wheel in the desired location.

offset is the difference between the centerline of the wheel (over-all width) and the location of the inner edge of the mounting flange. if the desired wheel width is known, offset can be order into the wheel to place the wheel in the desired location.

"0" offset is absoulte center of the wheel (lip to lip).
"+" offset is the difference between the center-line and mounting flange (toward the inner edge of the wheel).
"-" offset is the difference between the center-line and mounting flange (toward the outer edge of the wheel).
1 inch = 25.4mm

offset: backspacing will vary in relation to wheel width
backspacing: backspacing is fixed regardless of wheel width.

In our world back spacing is king.

[Van B]

Just my .02, but I think backspacing is easier for most to understand.

[j-rho]

That's not exactly right clancj - a zero offset location isn't necessarily the absolute center of the wheel - it is the midpoint between the inner and outer mount planes for the tires. The inside or outside edge of the wheel may have differently sized or shaped lips. You could conceivably have two wheels of the same width and same backspacing, but with slightly different offsets due to lip size/shape. Generally the distance between the wheel mount plane and the wheel lip is about 1/2" (so a 9" wide wheel from a tire-mount perspective is usually about 10" lip-to-lip) but this isn't an absolute.

When it comes to determining inboard clearance, backspacing is the more useful measurement. In some racing circles, you have to run wheels with an offset the same as stock, so in that case, offset is the measurement you need to ensure.

Both values are useful to know.

[clancj]

That's not exactly right clancj - a zero offset location isn't necessarily the absolute center of the wheel - it is the midpoint between the inner and outer mount planes for the tires. The inside or outside edge of the wheel may have differently sized or shaped lips. You could conceivably have two wheels of the same width and same backspacing, but with slightly different offsets due to lip size/shape. Generally the distance between the wheel mount plane and the wheel lip is about 1/2" (so a 9" wide wheel from a tire-mount perspective is usually about 10" lip-to-lip) but this isn't an absolute.



i did state "over-all width", but i assumed wheel lips were symetrical. i did not know a given wheel could have asymetrical mounting lips. (i learned somthing new today). however, wheel measured "lip to lip" / 2 should be good enough for govt work in oreder to find center of wheel.

[Northeast Rod Run]

it seems that when I see stuff listed for my newer cars, with metric bolt patterns, the wheel manufacturers like to list offsets (usually in MM), but with our older cars most of the rims are listed as backspacing (in inches)

I may be wrong here, but that's just my observation

[Larry Callahan]

What I need is a drop down box with numbers inside it to pick from and I don't know what to call it.

[73X]

I would use backspace as a drop down box title since that's mostly what the custom wheel companies most on here order from use. Probably use 1/8" increments ranging from say 3.5" to 9"(just possible numbers) unless you're going to have it as a fill in box.

[clancj]

http://www.rsracing.com/tech-wheel.html#patterns

the above website has a matrix for wheel width, back-space and offset.

[Larry Callahan]

I would use backspace as a drop down box title since that's mostly what the custom wheel companies most on here order from use. Probably use 1/8" increments ranging from say 3.5" to 9"(just possible numbers) unless you're going to have it as a fill in box.

I'm not sure how I will work this one. I want things to be as accurate as possible so I want to us a lot of drop down boxes. One type-o and and a car may not show up in a search.

[Larry Callahan]

http://www.rsracing.com/tech-wheel.html#patterns

the above website has a matrix for wheel width, back-space and offset.

Thanks!

[robertjra]

In the wheel world you need to know bouth backspace and off set if you have any ?'s e-mail me at [email protected]

[Rhino]

I would agree that you would need to know either backspace or offset, but I'd be hard pressed to find a situation when you'd need to know both.

I would assume that most people know, or can find a way to translate between one and another. Similar to if we were discussing using Liters vs Cubic Inches. They're both measures of the same engine displacement, but simply a different way of representing that value.

If I had to vote I'd say that more people are going to be familiar with backspace than they would be offset.

[MrQuick]

Larry have them list both that way most of us can figure out either way. Backspace/Offset

[jjordan]

Offset came to us from Europe, hence the offset listed in mm. Backspace is good old fashioned American know-how: just flip the wheel over face down and measure, there it is on tape in inches. Pretty hard to measure the offset without doing some calculations. Not hard, just not easy to put your hands on. When I am taking an order if the customer has the backspace, offset, or both, we can make it happen.