offset vs. backspacing

[ICEMAN6166]

http://www.crawlpedia.com/wheel_dimensions.htm

[slixtyfive]

Everything i've ever seen or read about 'backspacing' indicated that it is actually the measurement from the wheel mount surface to the 'inside' edge of the rim. In other words, lay the wheel down on a flat surface with the outside of the wheel up and measure from that same flat surface up to the back of the wheel mount surface for your backspacing measurement. So kinda the 'inverse' of what is shown in the diagram above......?

[Toyz]

Your first sentence is largely correct. I guess one could measure in the way you describe, but it might be easier to lay the wheel with outside down and measure from mounting surface to a straight edge laid across the inside edge. That will put you fairly close to accurate backspacing. Correct backspacing will be larger with wider wheels, offset SHOULD remain nearly the same.
There's a lot of technology at work there. On dual bearing hubs, ideal situation is to have centerline of wheel centered between the bearings. Such things as "kingpin angle of inclination" also come into play, especially with modern anti-lock braking systems and diagonally separated systems. This is a major reason for the recent proliferation of high positive offset wheels.
Paul

[Toyz]

To further expound on this; F-series trucks prior to the late nineties, except dual wheel applications, came from the factory with a near-zero offset regardless of original wheel width. This holds true whether equipped with a 15x 4.5 wheel or a 16.5 x 9.75 wheel. CORRECT replacement wheels will stay close to this minimal offset to avoid poorer handling and additional bearing loading. This does not indicate that wheels with improper offset can't be run successfully; they often are, with little consequences.
The popular Jaguar, and especially Crown Vic, conversions are by no means exempt from this. Going to a neutral or negative offset on these applications not only widens the track, but can affect scrub radius due to the line formed by the KAI no longer being located outside of wheel centerline.
Too much information? Probably for most of our situations; but certainly worth consideration when replacing a high positive offset application with one with much less offset. Think of a dually wheel mounted on an F250 hub for instance. Railroad company vehicles do this for obvious reasons; they also tend to enforce low speed limitations when these vehicles are operated on roadways!
Paul

[slixtyfive]

Toyz - Aren't you describing the exact same measurement?

My comments are just based on the descriptions I have typically seen for ordering wheels. From my experiences, the 'backspacing' as I noted above is what I have seen. Sometimes 'offset' and 'backspacing' are used interchangably. I'm not saying that is correct but just what i have experienced. I'm not trying to start a debate on this - again just pointing out that you need to be careful and be sure that communication lines are open when discussing these measurements.

[Toyz]

Yes, I am not sure what you are referring to as far as the diagram goes. It appears generally correct to me. As stated in a previous post, you are correct; back spacing and offset is often confused. Again, offset can, and should, remain very close to original where possible, while backspacing is dependent on, and should change with, wheel width.
To use Jeff's example; his new wheels with -12 mm offset stick out considerably further due to the increased width more so than the total of less than 1/2 inch change from stock offset.
Paul

[slixtyfive]

I guess I made the assumption that the left side in the diagram is the 'outside' of the wheel since that is the general profile of wheels i typically deal with. But that assumption may be incorrect since it doesn't identify the which side is which.

[Toyz]

Sounds as if your monitor is about as poor as mine! The axle side is lightly lined in.
Paul

[slixtyfive]

Really??? I didn't see that.

Now that I look closer I do see a couple light lines but I still can't make out exactly what it is showing...

My apologies....

[bird55]

Thx for postings, and good to see the discussion. It does make a difference and never ASSUME. I had very close tolerances on the last setup I did and all this was very important when ordering expensive aftermarket wheels! Always something new to learn.