Measuring accurate ET of wheel?

[pantera]

Does anyone know of an accurate way of measuring the ET of a wheel?

Many thanks,

Dan

[Bill Verburg]

Originally Posted by pantera

Does anyone know of an accurate way of measuring the ET of a wheel?

Many thanks,

Dan

Accuracy will depend on the tools and skills of the measureer



the formula is
ET = backspace - .5*overall width

I use stiff lath that lays across the flanges then measure to the mounting face w/ a micrometer

error comes from parallax and deflection but you will be very close to actual if you are careful

[pantera]

Thanks Bill, this what I have for measurements:

17 x 8.5 with a backspace of 6 inches

17 x 10 with a backspace of 7.5 inches

Trying to figure what the ET's are for each wheel?

Dan

[Arne2]

Originally Posted by pantera

Thanks Bill, this what I have for measurements:

17 x 8.5 with a backspace of 6 inches

17 x 10 with a backspace of 7.5 inches

Trying to figure what the ET's are for each wheel?

Dan

Rim width is to the bead seats. To measure offset accurately, you need to measure the overall width of the entire wheel, which can't be done while tires are mounted.

The offset on many modern wheels will be stamped or cast into the wheel somewhere. Look for something like "ETxx" or "etxx".

For an approximation (generally good to ± 3mm or so), we can assume that the overall width of the wheel is approximately 1" more than the bead seat width. So the 17x8.5 is probably 9.5" overall, and the 10" is probably 11". This gives:

Front
6 - (9.5/2) = 1.25" or approx. 32mm

Rear
7.5 - (11/2) = 2" or approx. 51mm

Does this help?

[Bill Verburg]

Originally Posted by Arne2

Rim width is to the bead seats. To measure offset accurately, you need to measure the overall width of the entire wheel, which can't be done while tires are mounted.

it can w/ lathe of the proper length, here I have one on each side so that overall width can be measured

Originally Posted by Arne2

The offset on many modern wheels will be stamped or cast into the wheel somewhere. Look for something like "ETxx" or "etxx".

often, but not always

Originally Posted by Arne2

For an approximation (generally good to ± 3mm or so), we can assume that the overall width of the wheel is approximately 1" more than the bead seat width.

again, often, but not always. Notable exceptions are Fuchs and BBS

[pantera]

I very much appreciate you guys taking the time to go into detail on this. The only thing that has me confused is what you come out with a front wheel ET. This is for a set of Fikse 993 fitment that I'm selling. I would have thought the ET's would have been closer to the high 40's for a 8.5 front rim?

Dan

[Bill Verburg]

Originally Posted by pantera

Thanks Bill, this what I have for measurements:

17 x 8.5 with a backspace of 6 inches

17 x 10 with a backspace of 7.5 inches

Trying to figure what the ET's are for each wheel?

Dan

Glad to help but you still have not specified what the 8.5 or 10 measure, is that bead width or overall width?

people usually talk about bead width but overall is what is needed

[KTL]

If you're trying to push the limits of what your body or suspension will allow, you certainly want to be accurate. Bill's method accomplishes that with stiff pieces of wood that span the rim edges. It's critical to use the rim edges IMO because often times a "stretched" tire installation can have the rim edges protruding past the tire sidewall. Therefore the rim edge is a critical clearance.

Measure the rim width from edge-to-edge. Divide that in half.

Then measure down to the wheel hub/mounting surface from your stiff straightedge laid across the wheel. This is the backspacing. Subtract the backspacing from the 1/2 rim width and there's your offset.

[pantera]

OK, what I need to do is remeasure the overall width since the tires are mounted as indicated by Bill's picture. I'll do that this evening.

Can't thank you enough for your help guys,

Dan