Technical Wheel Offset For Dummies Like Me (Up Date)

Let's take the metric out of it for you.
4" backspace @ 1/2" negative offset..............wheel mounting surface 1/2" inside tire center line
4 1/4” back space @ 1/4" negative offset......wheel mounting surface 1/4" inside tire center line
4 3/4” back space @ 1/4" positive offset.......wheel mounting surface 1/4" outside tire center line

Now here is the big secret that most people don't factor in. The starting measured rim width will be 1" wider than the advertised width of the wheel. This is the 1/2" per side of the outside lip. As an example, a 15x8 wheel actually measures at 9" wide, and it is the 9" that you use when converting to offset. This will make the figures listed above work. Hope it makes sense....

Here is a good explanation from a guy on another forum.

Offset can absolutely be measured. It's just the distance of the mounting surface from the centerline of the rim. The problem is that people don't understand that backspace is measure from the outside lip of the wheel, so when they try to convert backspace to offset they forget the extra ~1/2" of rim lip included in the backspace.

So, for example, a 15x8" with a 4.5" backspace. That's a 0mm offset, but people think that's wrong because the backspace is 4.5" not 4". Well, the 8" width listed in the spec is where the tire bead sits, not where the outside lip of the rim is. A 15x8" is actually 9" wide if you measure outside to outside, which is where the 4.5" backspace comes from. So the centerline is at 4.5" from the outside, which means the mounting surface is at the centerline.

And offset can be a very helpful specification. If you're changing the width of the rim, for example, you can just get a rim with the same offset. The additional width will be split between the inside and outside, if the offset is the same the centerline of the wheel is the same. If you keep the backspace the same and use a wider rim, all of the additional width is on the outside.

Another example- an 18x9 with a 6" backspace. If you remember that the 18x9" is actually 10" wide outside to outside, then you know that the centerline of the wheel is at 5". So if it has a 6" backspace, then it has a 1" offset, which makes is a +25mm offset (well, 25.4mm but it will be rounded down). Offset is always listed in mm, which also confuses folks.

 

Classic’s Man!

 

@Johnny Gee , have another look at post #61. I added a bit more info........

 

@Moriarity that catalog pic you posted. Is that from 20 years back of the original’s? Because the 4 1/4” back space of yesteryear sure helps norrow things down.

 

61 is too coarse a factor to be useful. The factor is 61.02374409. That's a longer figure to remember than 16.387064, which is the (exact) number of cc's to a cubic inch.

This came up in a recent discussion on Facebook, and it's something I've been struggling to put my finger on. The way to express engine displacement in metric units is not in litres, not really. We use cc's to express engine displacement, and then we round that to litres to give an idea of what kind of engine it's comparable to. It's more classification than capacity.

It's like, in, say, 1980 there must have been about twenty or more different 2-litre engines made by various European manufacturers. Nobody for a second supposed that any of them actually displaced exactly 2000cc, and when calculating flow requirements or assessing the effects of an overbore, the figure to use was the cc's, e.g. 1962cc in the case of an Alfa-Romeo twin-cam.

That's why I urge people to convert cc's to cu.in., and vice versa. Converting litres to cubic inches makes no sense: it's like saying a 348, a 350, a 351, and a 352 are all "350-cubic-inch engines". Or calling all of them 0.2-cubic-foot engines.

 

So how many liter’s of air does a 275/60/15 tire hold? Maybe this will help with figuring overall tire width.

 

Point taken. But 61 is still a useful factor for determining whether a 4.8L engine is larger than a 327 cu. in. engine. Which it's not.

 

Maybe this will help

 

You should care what the offset is because it gives you information about how the backspace is measured. Since every wheel will be a bit different in the lip dimensions, it gives you information that isn't otherwise published. It's actually really simple. 0 offset means the mounting surface is centered. So an 8" wide wheel listed as a 4.5" backspace (say a Wheel Vintiques) is actually 4" backspace if you measured to the bead. The 0 offset is telling you it's an 8" wheel with 4" on each side of the wheel mounting surface.

As said above, the offset gives you information about how the backspace is measured. Using both together makes sure you know that you're getting exactly what you need. I never used to use offset either, but now you need to use both.

While the bolded statements are technically correct, they don't agree with the table you posted. You did correct yourself in later posts, but what you wrote here was confusing. The relationship is between backspace and total wheel width (edge to edge), not published wheel with (bead to bead). As you stated later, that's why a 4.5" backspace 8" wide wheel in your table is 0 offset, because the backspace is measured as a function of the total 9" wheel width.

Bead to bead wheel width is what matters for tire mounting and width, but the sea of published numbers are confusing.

Including some that you wrote, but you did clarify it in later posts

 

If you want your tire centered around your mounting face (equal amounts of tire inside and outside of the mounting surface), you want 0 offset. 6mm is roughly 1/4"; 12/13mm is roughly 1/2"; etc. + means the wheel mounting surface is closer to the wheel face, - means the wheel mounting surface is closer to the rear of the wheel.

Backspace is no longer related directly to offset, as backspace uses the full wheel width and not the published width. You have plenty of information to choose what you want here.

 

What I want centered is the tire when all is said and done. As near possible spacing at inside tire edge to leaf spring and outside tire edge to 1/4 panel.

 

If you want the tire centered in the wheelwell (I think that's what you're saying), you'll have to measure your wheelwell relative to the wheel mounting surface (front face of the drum). If your wheel mounting surface is centered in the wheelwell, you just need a 0 Offset wheel. If your wheel mounting surface is not centered in the wheelwell, you'll need to adjust your offset accordingly to get the tire centered width-wise in the wheel well. That is if that's what you're after.

I'm not sure of your technical knowledge so to provide basics I would suggest the following procedure:

1. Jack the rear of the car up and take both rear wheels off.
2. Support the axle on jack stands and let the car down so it is at ride height. Bounce it on the stands slightly to settle it to ride height.
3. Use a straight edge like a metal ruler or carpenters square held squarely against the drum surface where the wheel mounts.
4. Measure from the inside of the straight edge to whatever you want the tire centered between on both the inside and outside of the wheel well.
5. The difference between those measurements in mm will be the wheel offset you're looking for.
6. Repeat measurements on both sides of the car to account for the axle not being centered under the body.
7. If the measurements are different side to side, I would probably take an average of the two.

The only way to properly fit wheels to your car is to do actual detailed measurements on your car.

 

^^^^^^^^^^THIS is all that matters!
And this!

Cripes, I love this thing!

 

Agreed, the stance and wheel/tire combo hit it out of the park on that car.

 

I'm bilingual. I speak both 'backspace' and 'offset' fluently.

 

I like to draw a sketch, showing the spring, inner panel, outer wheel tub, brake drum/flange, and where the existing tire sits relative to these things. Then I can do some maths (British spelling) to figure out what I have, and what I can fit in it's place.

you can figure out the offset on the current wheels by laying the wheel/tire on the floor and measuring the mounting surface to the ground, and lay something across the top to measure the actual width of the tire.

But making a sketch lets me write things down in a way that I can remember what's going on.

 

Here's a measurement guide too: crw-35-580547500.pdf (summitracing.com)

 

i usually buy wheels i don't need, that don't fit, that i end up selling for way less than what i paid for them.

 

Oh No!

 

I’ll look it up when I get back from running medical errands with the father-in.

 

Just saying

 

"Offset" is an overly confusing term and almost no one agrees on what it actually means. It's really not useful outside of steering geometry and it's usually best/simplest to think in terms of width and frontspacing/backspacing when buying or making wheels since they tell you where the actual edge of the wheel is.

 

I got lot’s of cardboard. And that’s how I will approach what back space to get. Either way, spend money on a jig or use scraps. They are useless if you can’t replicate wheel manufacturer specs.

 

Here's more for the pig plie from my secret digital stash.........

backspace


positive offset wheel

negative offset wheel

positive offset

 

Backspacing is measured from the edge of the rim, they've got that straight edge sitting on the bulge of the tire.

 

Not the best photo.......
But that is how it's done when the tire is mounted and the tire bulges. You have to measure the gross depth and subtract the fraction between the bulge top and the rim if you take a photo with a stick that's longer than the rim circumference.
I should have posted a warning that the photo has math in it. Sorry.

 

It's a measurement of the distance (offset) of the mounting surface from the centerline of the wheel. It's not really that complex or confusing.

Backspacing is arguably more confusing, because there's a lot of spots to measure to, but the term itself sounds simple which means it can be more easily misused.

 

@Moriarity section width is 10.70”.

 

The distance of the mounting surface from the centerline of the wheel itself is practically useless and has to be combined with the width and converted to frontspacing/backspacing to see if the wheel is actually going to fit. Offset only tells us how the scrub radius is affected by itself. You can put a wheel on with the exact offset you had but if it's a wider wheel it still could interfere or stick out past the fender.