I purchased some wheels the other day for the Roadster, which unfortunately turned out to have an offset that was different to what was advertised. In order to confirm this I needed to work out how to measure offset, and thought I would share.
The wheels were advertised as “P42”, which according to the interwebs would be about right to fit the NC when they are 17×8 with 205/45R17 on them.
Got them home, chucked one on the rear and WOAH, no way was it going to work. The wheel was sticking out beyond the guard and there was no way it would tuck in and clear the guard lip without rolling the guard, more low and some serious camber.
I’m not prepared to alter the car to make the wheels fit, so I was a tad confused. A quick check of the back of the wheel and it was clear the offset was wrong; it was stamped with “35” indicating the offset was 35, not 42.
Contacting the seller and he was sure it was 42 as he measured it. I disagreed with this as a 42 should work, but I needed to confirm it for myself.
A quick check of the internet and I worked out how to measure it.
Step one, you need to measure the width of the wheel, from barrel lip to barrel lip. This isn’t the width of the wheel as marked (IE: 8″), this will usually be wider than that. In this case I know the wheel barrel extends past the edge of the tire, so when you lay it face down the barrel either sits on the ground or is millimeters from it, so I could measure from the ground. Easy.
You will need a straight edge, and to lay it across the wheel. Measure from the barrel (or in my case, the ground) to the lower edge of the straight edge. Make sure the straight edge is actually on the barrel, not the tire.
My result for an 8″ wheel was 235mm (or 9.25″).
Then you need to measure from the back of the hub mounting face to the lower edge of the straight edge (or the inner barrel lip).
And this one was 154mm.
Now you divide the width of the wheel by 2. 235/2=117.5
Then you subtract the width from the mounting face measurement. 152-117.5=34.5
Now, my measuring isn’t that accurate, so there is a margin of error, but I know the wheel is stamped with 35 so a result of 34.5 is pretty damn close and confirms that it’s 35, not 42. I know mine is a positive offset, but there are also ways to work out if it’s positive or negative. I didn’t need to work that out, so I won’t go into details on it.
This should get you close enough to know what the offset of a wheel is. To be more accurate you would really need to measure the wheel without a tire on it.
Thankfully in this case the seller is a good guy and has agreed to return the wheel as it was his mistake in measuring and advertising the wheels with the wrong offset.
Also many thank to my lovely assistant who held the tape measure while I took photos, I didn’t have enough hands to do it myself.