Ok so I was trying to read someone's web page about Corvair wheel offset and what aftermarket stuff is applicable etc., except it really got me confused by the way it was written and didn't answer my question. I probably could have looked else where but I figured I'd get a straight forward answer here.

Question #1: Late Model Corvair offset is -1 inch (-25 mm). So if I acquired a wheel that had a zero offset, would that require a 1" spacer to keep the 4" of backspace I need?

Question #2: I want to do a 15 or possibly 16 inch wheel. The original tire size was 185/80R13. What height of tire would I need for a 15 or 16 inch wheel to keep the same diameter as the factory tire size?

Answer to question 1- if I'm understanding you correctly, yes. The 0 offset wheel will have the backing plate 25mm closer to the centerline of the wheel- allowing more wheel to be on the "backside" as it were.

2- This requires math. IIRC your 185 80 13's will be approx 25" tall rolling. Here's where it gets tricky. Wheel math is a motherberkeleyin witchcraft. I've mastered it- but it's not easy. It's all going to depend on how WIDE the tires are. The way it works is your "aspect" ratio (the second number) is the percentage height your sidewall is compared to treadwidth. IE 185/80 means the sidewall is 80% as tall as the treadwidth or... 148mm, or approx 5.8in. x2 (cause you have top and bottom) gives you 11.6in. So 24.6 rolling diameter is more accurate, but there's other factors, sidewall ballooning at speed, tread depth, the rays of the sun... you get the idea. So... to answer your question, I need a width number first. Assuming factory width so that 185mm (which is STUPID skinny for a 15 IMO) you'd need something like a 185/65/15... if my math is right.

https://www.1010tires.com/Tools/Tire-Size-Calculator

Or use that.

Since the answer around here is always "Miata"...

http://www.miata.net/garage/tirecalc.html

my brain hurts when i try to keep track of wheels of different widths when things are stated in "offsets".. just use the backspacing measurement and it's easier to follow along..

In reply to novaderrik:

This, I have no idea what genius thought up offset, backspacing is so much simpler to think about.

Backspacing only gives you part of the equation, it can stay the same for different widths so you may end up with 80's poke.

1) no spacer, you're wheel is going to sit 1" further out.

2) use one of the calculators above^

Offset is useful because it's independent of wheel width while backspacing is not. Well the backspace doesn't change, but many other things will and your wheels will stick out more.

Answer to 1: Yes, assuming the wheel widths are the same.

Answer to 2: Use a tire size calculator and see what sizes are close to the diameter you need. Here's the one I usually use:

http://www.miata.net/garage/tirecalc.html

Kenny_McCormic wrote: In reply to novaderrik: This, I have no idea what genius thought up offset, backspacing is so much simpler to think about.

See and I find offset easier to think about, because as mentioned, it gives you the entire picture unlike backspacing.

z31maniac wrote:

Kenny_McCormic wrote: In reply to novaderrik: This, I have no idea what genius thought up offset, backspacing is so much simpler to think about.

See and I find offset easier to think about, because as mentioned, it gives you the entire picture unlike backspacing.

if a 7 inch wide wheel with 4 inches of backspacing just barely clears a part of the suspension or inner wheelwell, then a 9 inch wide wheel with 4 inches of backspacing will also clear it exactly the same- you can add width to the outside until you run into interference from that direction...

there is a lot more thinking involved when you do the same calculations using offset as your methodology- especially if you are doing the rim width in inches and the offset in millimeters as is common practice...

personally, i think the concept of "offset" was invented by screwball German engineers that wanted to prove that they were smarter than everyone else...

^On the other hand if you want to go with a wider wheel and you want to make sure your scrub radius and suspension motion ratio stay the same and you only know backspacing, you need to do the same calculations to find the offset...

novaderrik wrote:

z31maniac wrote:

Kenny_McCormic wrote: In reply to novaderrik: This, I have no idea what genius thought up offset, backspacing is so much simpler to think about.

See and I find offset easier to think about, because as mentioned, it gives you the entire picture unlike backspacing.

if a 7 inch wide wheel with 4 inches of backspacing just barely clears a part of the suspension or inner wheelwell, then a 9 inch wide wheel with 4 inches of backspacing will also clear it exactly the same- you can add width to the outside until you run into interference from that direction... there is a lot more thinking involved when you do the same calculations using offset as your methodology- especially if you are doing the rim width in inches and the offset in millimeters as is common practice... personally, i think the concept of "offset" was invented by screwball German engineers that wanted to prove that they were smarter than everyone else...

Ok, so keep using backspacing, I'll keep using offset........and the rest of the world will continue not to care.

willtheyfit.com

Gives a much better idea of overall fitment than the other two calcs.

Thanks for the replies guys, but the reason I ask about offset is because some of the wheel companies don't give backspacing, only offset.

My understanding is that the Corvair has 1 inch inboard offset (-1 or -25mm) and has 4 inches of backspacing.

So, if I wanted to go from a 5 inch wide wheel with -1 inch offset to 5 inch wide wheel with zero offset, I would need a 1 inch spacer. This should keep the 4 inches of backspacing I need to stay clear of the suspension with.

However, I want to go with a 7 inch wide wheel that looks to only be produced with a zero offset. I would believe with zero offset the backspacing would be about 3 inches. So with no way to physically measure backspacing, what would I need to look at?

7" wheel with 0 offset will have 3.5in of backspacing. That actually makes it easy- because your backspacing is half of the wheel, because the offset is 0, or the middle.

Personally, I think better in offset myself, because it gives me less variables, and I'm able to jam the calcs easier. And because I want to figure out where it's going to lay on BOTH SIDES, it's easiest for me to go "Ok, I have 9" of room, and I need X amount of space between the backing plate and the shock tower" or whatever the impending doom is on the inside. Easier to roll fenders than it is to bend suspension out of the way.

I think the short answer is, they'll fit on the inside, I'd have to physically measure the car in order to tell you if they'd fit on the outside.

mndsm wrote: 7" wheel with 0 offset will have 3.5in of backspacing. That actually makes it easy- because your backspacing is half of the wheel, because the offset is 0, or the middle.

Not quite. It will have 3.75 to 4" backspacing because it will include the lip width. different types of wheels have different overall widths. A 7" wheel might be 7.5-8 wide on the outside.

Backspace also changes with tire sizes/bulge. Offset will not.

This is what I got from the site http://autoxer.skiblack.com/tires.html, maybe it will give you a better idea:

Backspacing - The distance from the inside of the rim to the point where the wheel contacts the brake drum. All car 5 1/2" wide wheels have a backspacing of 4 1/4". For reference, more backspace moves the wheel further in the wheelwell, less backspace brings it out. Maximum backspace for an early with 15" wheels is 5", late is 5 1/4".

Offset - If the wheel is centered in and out relative to the face that contacts the drum, then it's zero offset. If the wheel is in or out relative to the mounting face, then it has offset. The stock Corvair car wheel (early or late) is offset 1" inboard (source: Chevrolet Engineering Features, courtesy Larry Claypool). Note: I prefer using backspace to offset, since there is some difference of opinion as to which way is "negative" or "positive" offset - the current catalogs show "positive" as the mounting surface of the wheel being closer to the outside of the rim than the inside. I will generally use backspace.

Lates can easily handle up to 7" wide wheels in the front, with up to 8" (!) wide in the rear with the correct backspacing. Even wider will fit in certain sizes, but you will need to alter the backspacing to move the wheel out somewhat (less backspace), and perhaps roll the inside edge of the fenders. On the inside you should also check the steering arm in the front and the protruding threads on the rear suspension's toe control links for clearance.

That makes my head hurt.

A 5.5" wheel would be closer to 6.5" total width and at zero offset would have a backspace of 3.5". For it to have a backspace of 4.25" it would have a positive 1" (25mm) offset. The same backspace on a 7.5" wide rim would require zero offset (7.5 rim->8.5" total, zero offset at 4.25"). To retain the same centerline that same 7.5" rim would keep it's 25mm of offset and would instead have 5.25" of backspace. (which is the max).

So, for a 4-lug car I would find a nice FWD 4-lug (probably 4x100mm) 7.5" wheel with offset in the 35-40mm range. I would couple that with a 1.25" wheel adapter and be done. Test fit a wheel before ordering the adapters and adjust thickness as required. If the rim has 40mm of offset, and is 7.5" wide, backspace would be 4.25"+40mm~5.8". Add a standard eBay 1.25" adapter brings that down to @4.55", which is probably a good compromise.

For a 5-lug car I would be looking for 4wd S10 rims. Should just about bolt on.

In reply to oldopelguy:

That's why I was saying I was confused.

So would 1" positive offset be inboard or outboard the centerline?

SyntheticBlinkerFluid wrote: In reply to oldopelguy: That's why I was saying I was confused. So would 1" positive offset be inboard or outboard the centerline?

Outboard. Away from car.

That's what I thought. The Corvair website is saying that the wheel offset is 1" inboard but still has a 4.25" backspace.

This is freaking confusing.

do you have an actual wheel you can stick a ruler on? that would clear things up right quick. ignore all the confusing nomenclature. If you know what you have, you can do further measurements and decide what you can fit.

I agree the language from the website is not very clear. maybe they mean the wheel itself sits 1" further inboard towards the car from the wheel centerline. which would give you the +25 offset that also agrees with the 4.25 backspace.