Before you do that, see this: http://www.1010tires.com/WheelOffsetCalculator.asp

It may help.

Good point, BTW.

And this one: http://marksink.com/tire_wheel_offset/offset.html

 

Please don't shoot yourself! Adding the spacer makes the offset less POSITIVE or more NEGATIVE.

 

So, when determining which spacer to use on a wheel whose offset is different than original equipment am I trying to maintain the original centerline of the wheel, which I believe is a static measurement and always the same on any given wheel or am I trying to maintain the original wheels offset to the mounting hub. The info in the attachment you sent would suggest the wheel centerline is always the distance between the in broad and outboard flanges divided by 2. This dimension minus the wheel backspace, measured from the inboard flange to the wheel hub, gives you the wheels negative or positive offset. It would seem, based on the fact that all the measurements are made from the wheel hub, offset is in fact changing with the addition of a space on wheels of identical width.

 

Correct. Forgedwheeler said, you change the "effective" offset. Similar concept of adding turbochargers to an engine; you dont change the compression ratio, but you change the "effective" ratio.

What makes it more confusing for some is using terms like "more negative", especially if you are working with positive numbers, because technically it doesnt make sense. You cant say +50 is more or less negative that +60. Its just and offset of 10 (or positive 10) less.

I think this discussion is fun, it helps everyone learn a little bit more.

 

if you are going to try and fit the same size wheel/tire but with a different offset by using a spacer then the new wheel will have to have more positive offset than your original wheel because adding the spacer is the same as adding negative offset...to make this point clearer...if you added negative offset to a wheel you would be moving the wheel mounting flange inward and the wheel center would move outward...you get the same effect by adding the spacer...so if your original wheel was a 55mm offset your new wheel would have to have a 62mm offset with the 7mm spacer...that would keep the wheel/tire center in the same position...excuse me while I go and shoot myself

 

Please arrange to have your gun sent to me when you've finished with it.

 

I will assume, therefore, that with the correct wisdom you have imparted to all of us that I can consider you now a "big shot"!!!

 

it's in my will...I hope they clean it up first

 

He can't wait that long.

Make an intervivos gift now. It's safer and no tax consequences!

 

Don't mean to belabor this but I found this that I thought was interesting. I think it answers a few unknowns.

This is the origin: https://www.rsracing.com/tech-wheel.html

I believe it shows what some of us have been saying.


Subtract:
Wheel center line from Wheel backspace to get offset. If backspace is less than the wheel centerline the offset is negativeIf backspace is greater than the wheel centerline the offset is positive

 

Yep, if the mounting pad is closer to the car side of the wheel in relation to the centerline, its negative. If its closer to the street side in relation to the centerline, its positive.

I just had an idea. I can make an instructional video on do-it-yourself wheel offset measuring.

 

This is exactly what I was saying and the consensus said I was reversed!!!

Thus, spacers produce a more positive offset, because they move the wheel's centerline closer to the street side.

Get that instructional video done. Some of the guys here need it!

 

you are wrong Gcalo...the spacer effectively moves the wheel flange inward, which reduces the positive offset even though the wheel centerline may move outward it does not in relation to the inside wheel flange...you are almost there...

 

Haha you still have it reversed. Pablo Picasso Jr to the rescue!



Red is the wheel centerline, green line is the car side of the wheel. X and Y are backspacing distances. Say X is 60mm. By adding a 10mm spacer, which is the pale blue line, you decrease backspace. Y = 50 mm (60mm-10mm) and thats the new backspace you have. The spacer "effectively" changes the mounting pad by 10mm. This 10mm is away from the wheel centerline, which means you have a lower (more negative) offset.

Where the wheel centerline in relation to the street doesnt matter, what matters is the distance between the mounting pad and the wheel centerline.

 

For what it's worth (if this helps anybody)....this has always been the way I thought of it:
ET is the offset measurment in mm.
I means 'pushed in'..... i.e. how much the wheel is 'pushed in'.
If a wheel has a 10mm spacer it is 'pushed in' 10mm less.....so, effective offset is the wheel's ET, minis the spacer.
Final caveat....if your reference point is the body & fender line....(generally where I start).... for every increase in J width (on a different wheel) add 12.7mm in offset in order to maintain the same 'fender line' on the larger wheel. If you do not adjust the offset, the wider wheel will appear to be 12.7mm 'out' further than the previous you considered.