Back-spacing measurement

[clivehorridge]

Is there anyone who can kindly explain how to measure back-spacing.

I have a set of 5 and a set of 4, 16" steel rims all 10" wide. The only difference between the two sets is the shape of the triangular (decorative) holes in the centre plate.

All of the rims measure the same distance from the face of the centre (where it faces onto the hub) to the inner rim and to the outer rim. For this reason I'm calling the "offset" "central".

This weekend I did a bit of flexing and I was pleased that the rears tuck-in nicely under the arch without touching. I haven't rotated the back axle that far before and didn't know how the wheels would behave relative to the arches.






I'm running 305/70/16's on both the BFG Mud KM2's and the Grabber AT2's, so I know the rims are good for my 80.

Question is, I'm trying to order a 5th rim for the second set and the dealer has asked me for the "back-space number". I've recently had all the rims refurbished and painted in tough thick paint, and the numbers that are stamped onto the rims is no longer visible.

Can anyone therefore tell me what the back-space number would be on a rim that measures "central". The dealer doesn't speak English and my Romanian is almost zero. When I try to explain that they are central, he simply keeps asking "back-space number please...".

Any idea's please 'cos I don't want him ordering the wrong sized wheel.

Thanks

 

[Jon Wildsmith]

Back spacing or offset is specified relative to the centre of the wheel so from what you've said the back spacing or offset on your wheels is 0, same as the OEM 80 series 8" wide wheels.

 

[clivehorridge]

Thanks Jon,

A logical answer thankfully!

If they weren't central for example and the distance to the inner rim was greater than the distance to the outer rim, (i.e. the centreline of the rims closer to the centre of the car) how would the offset be measured, positive or negative and in which units, inches or cm/mm?

Sorry for questions on questions

 

[Jon Wildsmith]

If the mounting face is closer to the inside of the rim, that's negative offset. So e.g. fitting 30mm spacers is like having wheels with -30mm offset. A lot of cars have wheels with positive offset, so the mounting face is closer to the outside of the rim. The letters ET are often used to indicate offset, e.g. ET45 just means 45mm positive offset, which is what a 100 series has I think. I think inches are only used in the US these days but I could be wrong.

 

[clivehorridge]

Thanks Jon for your further explanation.

I will now try to communicate this to the supplier that's going to try to get my 5th rim.

Actually, after I posted I realised that have one more rim that needs refurbishment. I'll scratch around on it to find the stampings (hopefully) to give the guy something to write on his order form, just in case my measurements are not as accurate as they should be .

Thanks again.