When someone asks what is the "back off set" about a rim are they asking the depth from the back of the rim to the inner well?
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Re: back off set?
Back of center of wheel to the outside edge of the rim on back of the wheel. -
Re: back off set?
the highest part of the back center?Comment
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Re: back off set?
I should ask for a 66 standard rim this measurement should be?Comment
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Re: back off set?
The aftermarket uses the term "backspacing", which is the dimension from the inner edge of the rim flange to the wheel mounting surface. OEM's didn't use that terminology - they used "offset", which is the dimension between a plane defining the center point of the rim (between the bead seats) and the wheel mounting surface; if the center of the rim is inboard of the wheel mounting surface, that's "positive" offset - if the center of the rim is outboard of the wheel mounting surface, that's "negative" offset.
1966 Corvette wheels had 0.44" positive offset.Comment
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Re: back off set?
One note is that on steel wheels, you can take wheel width, divide by 2, and then add the offset to get the backspacing, or at least close enough for government work.
On aluminum wheels, the flange can be thick enough that this value is not accurate enough to keep you out of rubbing.
For example, a 6 inch rim with a +0.50 offset would be 3-1/2 inch backspacing.
However, what is important with Corvettes is to ensure the wheels are compatible with Corvette brake calipers. They overhang the plane of the rotor hub, so the backside of the wheel mounting flange cannot be flat, or the calipers will hit. Some manufacturers make their wheels special to fit, others require 1/4 inch spacers.
If you use spacers, you have to add that to the backspacing specified to get the wheel to sit in the correct location, or it will sit another 1/4 inch out. You also have to ensure the lug nuts have enough threads of grip (at least as much as the stud diameter) or you can lose a wheel.
Wheel Offset and Backspacing diagramsComment
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Re: back off set?
Ok, Thanks guys nice link Mike...so it looks like you really need the tire off the rim to get an actual measurement....unless you have handy those dial verniers shaped like a C clamp to get around the tire. Suppose I can find a way to go from the inboard edge to the outboard edge without removing the tire...assuming the distance between the inboard flange edge surface and outboard flange edge surface will give you a clue.Comment
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Re: back off set?
"unless you have handy those dial verniers shaped like a C clamp to get around the tire."
Almost any place that spin balances tires has a set of those in order to measure the rim width -- a necessary input to the balancing machine. Check at tire shops in your neighborhood. If you can't borrow it for a while, maybe you can bring the wheel/tire to the store to make the measurement.TerryComment
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Re: back off set?
There are these wonderful tools available at any Home Depot, Loew's, and most Mom & Pop lumberyards. They are called 2x4s.
Cut one to roughly 15 inches, set on ground.
Lay wheel on it sideways.
Cut another, lay on top of wheel.
Slide a tape measure down the center hole of the wheel to the top of the lower 2x4. Measure to the bottom of the upper 2x4.
You have the rim width.
Then measure from the back of the mounting hub to the upper 2x4, if you have the wheel face down, and you have the backspacing.Comment
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Re: back off set?
Hear ya Mike, those tools are also available in the 1X3 version at reduced prices.Comment
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