Rim Offset vs Backspace

[RT NOMAD]

Correct my understanding if I’m way off base.

Backspace is expressed in inches
Offset is expressed in millimeters
Positive offset is when the mounting hub pad is outside the centerline of the rim width.
Negative offset is when the mounting hub pad is inside the centerline of the rim width.
Example: 17x10” with an offset = +18mm, What is the Backspace?

Backspace = Offset (converted to inches) + (Rim Width/2)

Backspace = (+18mm/25.4mm/in) + (10in/2)
Backspace = 0.709in + 5in
Backspace = 5.709 inches

[Scooby]

http://i60.photobucket.com/albums/h37/MFP57/dopeycat.jpg

[jackiert]

Correct my understanding if I’m way off base.

Backspace is expressed in inches
Offset is expressed in millimeters
Positive offset is when the mounting hub pad is outside the centerline of the rim width.
Negative offset is when the mounting hub pad is inside the centerline of the rim width.
Example: 17x10” with an offset = +18mm, What is the Backspace?

Backspace = Offset (converted to inches) + (Rim Width/2)

Backspace = (+18mm/25.4mm/in) + (10in/2)
Backspace = 0.709in + 5in
Backspace = 5.709 inches

You're right. However, I've never done it quite like that. I usually find the backspace first, then use the formula to find the offset.

Edit: I guess I should have read your example. All you need to do is find the centerline of the wheel (measure from inside to outside flange and divide by two). Subtract the centerline from the backspace (inside flange to mounting pad) to get the offset. You're also right about the conversion from inches to mm by multiplying 25.4. I think the example given, you'd have a huge backspace of like 6.25 or so. What is it you're trying to do? Did you find a wheel you like?

[RT NOMAD]

I'm wanting to get a wider rim than the 9" ones I now have to run some drag radials at the track.
I have Enkei RPO3 18x9 rims with a +18 offset.
They make a RPO3 17x10 rim with a +18 offset.
So, I'm figuring that the wider rim will have 0.5 inches more backspace, I think.

[jackiert]

I think you'd be okay. Especially since you have the adjustable coilovers. But even if you didn't, I don't think you'd have any issues with rubbing. I'd go somewhere local and see if you could to a "test fit" of the wheel, or at least one with similar specs.

[PowerWagon896]

Correct my understanding if I’m way off base.

Backspace is expressed in inches
Offset is expressed in millimeters
Positive offset is when the mounting hub pad is outside the centerline of the rim width.
Negative offset is when the mounting hub pad is inside the centerline of the rim width.
Example: 17x10” with an offset = +18mm, What is the Backspace?

Backspace = Offset (converted to inches) + (Rim Width/2)

Backspace = (+18mm/25.4mm/in) + (10in/2)
Backspace = 0.709in + 5in
Backspace = 5.709 inches

You are correct, on most of your assumptions, but offset can be expressed in inches too as backspace cam be expressed in in mm.

One thing to remember is that backspce being the distance from the mounting hub pad to the innermost portion of the rim is usualy 1/2" greater than 1/2 the nominal wheel width. Wheel width is measured @ the tire bead, not the outer edge.

A 10" wheel width will measure 11" OA so a 10" wheel width that has 0" (or mm) offset will have a 5 1/2" backset.

This being the case, your example would have a 6.209" backspace.

Just a bit too much backspace to clear the sway bar on the rear when the wheel has no weight on it & the suspension is @ max bottom travel.

My hands on experience has shown 6" backspace to just kiss the rear swaybar on my Daytona when the car is jacked up by the frame.