Offset vs backspacing?

[Samckitt]

Someone please correct me if I am wrong, if a wheel is not measured by backspacing but by offset; what exactly does that mean? If the wheel is 10" wide & has a 19mm offset, does that mean it has about a 5.75" backspacing? (19mm = .748")

[Rhino]

Offset is the width in mm (positive or negative) that the wheel mounting surface is inboard or outboard from the wheel center line.
To calculate it you need the true rim width (usually 1" wider than advertised) and the backspace. The equation would be as follows

Offset = (backspace - ((wheelwidth + 1) /2)) * 25.4

Essentially you're taking the backspace and subtracting half of the true wheel width. This gives you your offset in inches. Simply multiply that by 25.4 and you have your offset in mm.

[parsonsj]

Everything you wanted to know.

jp

[Samckitt]

Offset is the width in mm (positive or negative) that the wheel mounting surface is inboard or outboard from the wheel center line.
To calculate it you need the true rim width (usually 1" wider than advertised) and the backspace. The equation would be as follows



Offset = (backspace - ((wheelwidth + 1) /2)) * 25.4

Essentially you're taking the backspace and subtracting half of the true wheel width. This gives you your offset in inches. Simply multiply that by 25.4 and you have your offset in mm.

I'm trying to figure the backspacing & the guy gave me the offset.
So backspacing would be (((wheelwidth+1))/2)+(offset*25.4)

On a 10" wide wheel with 19mm offset it would be 6.24"

[Samckitt]

Everything you wanted to know.

jp

That link didn't work.

[Rhino]

The link did work for me.

Give me a sec to work out the math, but that does seem right.

[nullshine]

Strange... the link that John posted worked for me.

https://www.rsracing.com/tech-wheel.html

Try clicking this one? It's a really good site if you can get it to work, has lots of information about proper measurement of wheel dimension, hub patterns and such, all spelled out.

Anyways, if I am thinking correctly a 10" wide rim (advertised) is actually 11" (as Rhino said)... So the inboard half the rim is 5.5", plus the +19mm (0.748") offset will equal your backspacing.

(5.5" + 0.748" = 6.248")

Scot:
I think the only thing you were missing on your calculation in the first post was the extra 1/2" on the backside (due to the actual rim width of 11").

So yes, your calculation in post #4 is correct.

[Rhino]

Solving for backspace would work out as follows.

backspace = (offset / 25.4) + ((wheelwidth + 1) /2)

But you are correct, it should be 6.24" backspace

[Samckitt]

I just tried the link again & it worked this time. Strange.

You are correct I didn't take into account the axtra .5" for the lip on each end.

I have a 9.5" rear wheel now with a 5" backspacing and am thinking about going to this 10" wide wheel with 6.25" backspacing. If I do this, plans are to mini tub it by notching the frame, I would think that would get me about an extra inch, so a 1/4" spacer would allow it to fit without problem (?)

[nullshine]

I just tried the link again & it worked this time. Strange.

I love it when computers go wacky!

You are correct I didn't take into account the axtra .5" for the lip on each end.

I have a 9.5" rear wheel now with a 5" backspacing and am thinking about going to this 10" wide wheel with 6.25" backspacing. If I do this, plans are to mini tub it by notching the frame, I would think that would get me about an extra inch, so a 1/4" spacer would allow it to fit without problem (?)

I think you're on the right track.

If you can gain 1" of clearance on your frame by notching, then add a 0.25" spacer that would (kind of) bring the backspacing of your 10" wide, 6.25" BS rim down to the 5.0" of BS you are running now.

The only other thing you need to worry about is the outboard side of the wheel to the wheel well lip.
Your current 9.5" wide, 5.0" BS (10.5" wide overall) has 5.5" of protrusion from the axle hub.

The 10" wide, 6.25" BS (11" wide overall) you are considering has 5.0" inches of protrusion from the axle hub (with spacer).
(Overall width - backspace + spacer thickness)

So the 10" wide, 6.25" BS (11" wide overall) rim will actually tuck in 0.5" more than your current rim.

Looks like you're in the clear, as long as you can get 1" out of your frame, and no other components are in your way. You would have the same clearance on the backside of the wheel (assuming tire bulge is the same) and 0.5" more clearance to the wheel well lip.

[Ron L]

http://www.1010tires.com/WheelOffsetCalculator.asp

It's been a while since I bought my wheels but I seem to remember that app having a bug. I believe it did not like negative offset numbers so make sure to double-check your math.

[MonzaRacer]

http://www.rims-n-tires.com/info_spe...20/%20Comparer


Try this site I love the way it tells you back space and shows the tire infor too.

[MonzaRacer]

According to the rims n tires 19mm is 6.2 backspace

[DriverzInc]

Did the math myself to check too, and yes, a +19mm on a 10" wide wheel is a 6.248 (or 6.25") backspace.

The way I do it is I take the overall width of the wheel, which in this case is 11" and divide it by 2, which will give you "0 offset", which in this case, it is a 5.5" BS.
Then, I take 19, and divide it by 25.4001 which is how many mm's are in an inch. This gives you .748. Now, since its a positive offset, you just add your .748 to the 5.5" and you have your backspace.

Hope that breaks it down for everyone. If not, guys like me are always here to help.

[JRouche]

Ok, dummy me, I always wondered why there was, in my vision, two measuring systems for wheel offset or backspacing. I always thought it was a metric VS us type thing. Seems the import car world is bent on using offset numbers and the muscle car, hot rod, pro-touring folks wanted to use back spacing. What the heck. I was lost.

And I read and read, learned and listened, still the same. Till one day on the crapper reading one of my favorite novels. Ok, no, not my wifes People magazine but my "Chassis Engineering" book. I kinda take it all around the house

Anyway, up to this point I was still unaware of the differences. Backspacing? Offset? All just words to describe the same thing right, how far a wheel sets out from the mount. Well not really...

So I sat and learned, and my eyes were opened.. All in one paragraph.

For fear of copywrite issues Im gonna paraphrase slightly. What I read.......

In order to reduce your cars front scrub radius you should use wheels with zero or negative offset. Zero offset means that the mounting surface of the wheel is on the same centerline as the tire. Because a 10 inch wheel is actually 11 inches flange to flange, the backspacing for a zero offset on the 10" wheel is 5.50 inches.

That single paragraph summed it up and made sense. Offset is a term used for suspension purposes (not always) and back spacing in used for fitment issues.

I never knew..... Offset is really important, meaning the offset from centerline, if you are concerned about handling. Backspace is a term for fitment issues. Which is just as important. But on different levels.

Oh, I was kinda lucky to have my front wheels on a zero offset, but my main concern was the backspacing for fitment. Cause if it doesn't fit what the hell, its all out the door... So I learned the diff, its not all about fitment, some handling issues come into play.. Nice to know info.... JR

[68Formula]

And I read and read, learned and listened, still the same. Till one day on the crapper reading one of my favorite novels. Ok, no, not my wifes People magazine but my "Chassis Engineering" book. I kinda take it all around the house


In order to reduce your cars front scrub radius you should use wheels with zero or negative offset. Zero offset means that the mounting surface of the wheel is on the same centerline as the tire. Because a 10 inch wheel is actually 11 inches flange to flange, the backspacing for a zero offset on the 10" wheel is 5.50 inches.

Be careful. For some strange reason, Herb's definition of positive vs. negative offset is different than typically used today. He calls negative "mounting surface outboard of the wheel centerline," and then uses an example of 1" negative offset creating a 11" wheel with a 6.5" backspace. Usually a positive offset by most definitions today, creates more backspacing. (The above links are prime examples).

[JRouche]

Be careful. For some strange reason, Herb's defition of positive vs. negative offset is different than typically used today. He calls negative "mounting surface outboard of the wheel centerline," and then uses an example of 1" negative offset creating a 11" wheel with a 6.5" backspace. Usually a positive offset by most definitions today, creates more backspacing. (The above links are prime examples).

Oh yeah, not just that one, there are other typos. Someone has a thread asking about which books are good. I wrote this below.. Thanks for the heads up.. JR

That one and "how to make your car handle" by Fred Phun.. I use both and they cover everything I need. Even still, you gotta watch the reading, I "think" I have seen some typos in both books. But if yer reading it carefully you can pick it up and realize it was just a typo. JR