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Wheel specs - Wrapped around axle

3K views 19 replies 7 participants last post by  SLADE 
#1 ·
My math skills are a bit off today. I suppose if I think about this a little more, I'll figure it out. But

Offset - Distance between mounting surface and centerline of wheel.

Backspace - Distance between mounting surface and inside edge of wheel.

I had this wheel on my 2008. 17x8.5, 0 offset, 4.75" backspace. These seem to make sense. Wheel a little over 9" wide, half that would be 4.5", so with zero offset, backspace would be in that range.

I think these are on my new Jeep. I say "I think" because they were on the vehicle when I bought it, and don't have exact part number. Haven't looked in detail to get the exact model, but looking at the picture, they look the same. Both are ProComp.

The specs on the 2nd one have about the same width 17x9, same backspace of 4.75, but an offset of -6.

If this was what was really on my Jeep, I'd expect the wheel to be way outside the fenders, but they are not. Looks about like the stock ones. Plus, I don't see how it could have -6 offset and same backspace, for essentially the same size wheel. If I have same width wheel, if offset goes negative, doesn't backspace get smaller, by definition? How can they both have the same backspace? -6 inch offset from center would push wheel way outside the vehicle and the mounting surface would seem to be outside the wheel, since at -4.75 offset the mounting surface would be right at the inside rim.

What am I missing here?

I suppose my vehicle might have a similar looking wheel, that doesn't actually have -6 offset, but this doesn't look like -6 offset. But my real question isn't about what I have but is it even possible?

Can a 17x9 wheel have a 4.75 backspace, and -6 offset? Is that even possible. Wondering if the description is wrong.

 
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#4 ·
For very close differences you are right. My point was, the half inch difference doesn't seem to be significant in explaining the -6 difference in offset. Is that -0.6, or -6? Is "-6", 6 inches?

I do see that the chart says they what you say, but I must be misunderstanding what the definitions are for backspace and offset.

Offset - Distance from centerline of wheel, to mounting surface, right?
Backspace - Distance from mounting surface, to inside edge of wheel, right?

I'm using this as a reference. I know I'm going to be saying "duh" soon. Just has to sink in. But it seems if the two tire widths are 8.5 and 9, that changing the offset from 0 to -6 would dramatically change the backspace, for the same width wheel. These are not the same width, but are only approximately 0.5 inch different width.

 
#6 ·
Offset is always measured in millimeters.

2.54 centimeters in an inch, 25.4 millimeters in an inch, so 6 millimeters is less than .25 inches.
See, there's the DUH! I thought it was in inches, which is why it didn't make sense. ;) EVERY other dimension is in inches. There were no dimensions saying mm. Now I get it. Thanks. I knew there was a duh in my future.

I was kind of thinking it might be something like that. When I saw the chart with numbers like 102, I figured it might not be in inches. ;)
 
#8 ·
Make it easy on yourself and don't even worry about offset.

Backspacing is more relevant and constant.
Thanks, I was just going over the build list on my 2008. This was my 2008:



And trying to see what I wanted to change and what I could keep. Was looking at specs for the two ProComp wheels and trying to see if I should keep what's on the 2013 or go with what I had on the 2008. Think I'll stay with the 17x9, that are on the 2013.
 
#12 ·
And I'm guessing you already figured out a lot of wheel specs only include offset. Understanding how to convert offset to BS is just another tool in the arsenal.
Indeed. But, "offset" alone does give some useful information. "BS" alone gives different information.

Offset alone gives an indication if the wheels are further apart or closer, when compared to another offset (Wheel spacing).

BS alone gives an indication of how far mount point is inside wheel.

Both, with wheel width, tell everything. So, I really don't see how one is preferable to the other. Either can be calculated, if you know wheel width. BS might be the more critical, since it's dealing with clearances within the wheel itself.

But you are right, I did learn a lot, in this short discussion. ;)
 
#14 ·
Although this argument is dumb and the above is all true, (backspacing or offset) AND wheel width are both important to figuring out fit. On a Jeep (or other offroad), the inner clearance is more likely to be of concern so the backspacing number is generally more helpful. That isn't true on most other vehicles.

Wheel A has 4.5 BS
Wheel B has 4.75 BS
Wheel C has 5 BS

Which is better for my Jeep? The 5" BS, because the other wheels have a width of 40 feet and one with 5" BS is 9" wide.

At the end of the day, backspacing and offset are both measurements that have to be used with the wheel width to gain any meaningful understanding of the wheel. They can be used to calculate each other and are both valid.


Now who wants to talk about which measurement is more useful for scrub radius? :laugh2:
 
#15 ·
First, I didn't say "guaranteed to always provide wheel spacing differences". I said, "gives an indication", which is it's point. It gives the wheel to wheel centerline spacing.

Which has the wider track? A wheel with 13 offset, or one with 0, for a given wheel width?

Which is likely to have more clearance within the wheel? A wheel with 4.25" BS or one with 5.5?

You prove my point. You can't answer the 2nd question, UNLESS you provide additional information. I said ALONE. If all you know is BS, you can't make the claim that A is furthest apart. The only way you can say A is furthest, is to specify an offset and wheel width. I said ALONE.

In general, all other things equal, an offset gives "an indication" of wheel spacing (centerline to centerline).
In general, BS gives "an indication" of how much the mounting point is inside the wheel.

And the way that is referenced, is for two wheels of the same width, an offset gives an "indication" of wider track.

And for two wheels of the same width, a BS gives only an indication, of how much the mounting hardware protrudes within the wheel well.

Obviously, you need to consider everything.

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.

If you move the wheel centerlines on any given vehicle further or closer, the track is widened or lessened. If you change no other dimension, only offset affects that.

Backspace: Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel.

If you know nothing else about the wheel, the only information you get from BS is the amount of distance the mounting point protrudes within the wheel.

That is all I was trying to say. Where am I wrong in anything I have said? I may not be explaining my point quite right but I think I do now understand how BS is measured and how offset is measured, and what they mean.

Best I can tell, if you know wheel width, and either BS or Offset, you can easily calculate the other. But there is a real reason there are two different terms. They have different purposes. BOTH are necessary for the full picture. But each does have a specific purpose, in engineering terms.

If I ask myself, "why would I need to know ONLY the BS?" The question answers itself. Inside wheel clearance.

Thanks, the discussion helped me, if not anyone else. ;)

Now who wants to talk about which measurement is more useful for scrub radius? :laugh2:
I do have another measurement question but it's probably best left to a different thread. Has to do with lifts as they relate to whether the vehicle should typically be perfectly level or slightly higher in back. But that's for another day. I have to research this first, but I have noticed something in the 3 Jeeps that I have owned (an observation/concern).
 
#18 ·
You need to know BS to determine inside clearances. For most aftermarket wheel installations that change from OEM width - this is the most important thing to spec FIRST - as this is the first place you will see mechanical interferences - ackerman arms, control arms, frame rubs etc.. THEN:

You need to know OffSet to determine best outer fender clearances (redundant on the JK but quite critical in full fendered applications) and to pick the best final diameter of tire to control resultant change to scrub and camber curves. You can see rub issues - but scrub and bump changes are left to the "just live with it" approach in order to fit the desired wheel to the clearance issue - because we can only FEEL those changes and butt chassis dynos have no "standard" calibration from one dyno unit to the next).

The reason OF is the most given spec in parts lists - OF need got determined at the OEM drawing board - THE OEM spec is expressed MM (because global parts and machining sources are easier to harness in MM than Imperial). OEMs use billions of wheels a year - after markets maybe low millions... so since OEM uses MORE, their need in MM supersedes the need to express in Inches. (and thats the same reason we went to Metrics in sizes tires)
BS can be calculated and therefore only given in the spec chart as a "courtesy" on some wheel makers part.

Compare this (to understand why parts suppliers dont NEED to supply all our answers I'll use a business model I managed successfully) to the bore specs on an engine. There is no "bore spec" on an overbore. OEM makes ONE SPEC - standard. STD is 4.000" on a 350 SBC If I need to go .015 over to clean the cyls - I bore to 4.014 and hone to 4.0148, and order a plus .015 piston. When I clearance the piston - I mike it to 4.0081 along the pin. I DO ALL THE MATH - NOT GM, not the parts supplier. There are aftermarket suppliers who DO spec final bore (and hone stages in the case of Hypers) for me, but most rebuilder parts are just ordered by oversize and machinist does all the math (or looks up at his chart on the wall) - why SHOULD they do my math or keep and publish the knowledgebase and interchanges FOR ME. That's not part of profit margin when hitting price sensitivity targets! Data Maint/publication costs money (unless accuracy doesn't matter lol) - and even 1 item of data cross can matter when trying to maintain profitability YoY while not pricing your customers right over to the low bidder! Same as a rebuilder valve - I don't order a .3274/1.940 intake to replace a 5/16 stem 1.94 valve for that 350 - I order a 1.94 valve in oversize - so its +.015 guide +.030 face. Then ream the guide to .3274, hone to .3275 (.015 larger than stock) and cut the seat to 1.970 (or 1.990 in a high flow port and run the valve to its margin)! I punched guides and cut seats in 20-30 at a time - tossed em on the shelf and only ordered the valves when I had em move into the sale line - that way I didn't have to stock 30 variations - and could still turn around an order same day!

How does this equate to wheels? Easy. Offset is expressed in 6.35MM (1/4") steps ROUNDED to next whole MM (to match global metrics). So -19, -13, -6, 0, 6, 13, 19, 25, 32, 38, etc
My 2016 production vehicle has a 17/8.5/51 wheel. I am going to increase the tire diameter only in my 2017 project design. For every 1" of tire diameter increase - I must take one step of OF off (assuming pin inclination and included camber are in the 3-6degree range most vehicles use). So to go from a 28 to 32 I would want a 17/8.5/32 to maintain the same scrub radius and caster curve. My BS WAS 6.75 - and everything fit. Now its 6.0 so everything will still fit - well except unless I am ALSO going all the way to 35 which are wider than the tires under 33 so need a wider rim - so I then look to how much backspace I must give up (and thus sacrifice scrub) to clear the wider tire... adding width is 1/2 of increase divided by .25 = number of steps in OF toward 0.

So yes in the field, to pop wheels on, its the other way - BS first and OF as an afterthought BECAUSE we are lifting, widening tires widening wheels etc... and the tire's diameter effect is NOT the wheel makers job!

Bottom line - a 33x12.5 tire wants a 17/9/-6 (4.75BS), a 34-35 wants a 17/9/-13(4.5) a 35-37 wants a 17/9/-19(4.25). The problem is - if you get a -6 and later want to go to a 34 or 35 - you have RUBS (which damage tires).
But if you get a -19 - when your running 33s all you give up is some scrub (only damages FEEL and WEAR rate)! This makes the "rule of thumb" of 4.5 to 4.25 BS for a lifted JK - to help you avoid problems later - no thinking.
 
#19 ·
Bottom line - a 33x12.5 tire wants a 17/9/-6 (4.75BS), a 34-35 wants a 17/9/-13(4.5) a 35-37 wants a 17/9/-19(4.25). The problem is - if you get a -6 and later want to go to a 34 or 35 - you have RUBS (which damage tires).

But if you get a -19 - when your running 33s all you give up is some scrub (only damages FEEL and WEAR rate)! This makes the "rule of thumb" of 4.5 to 4.25 BS for a lifted JK - to help you avoid problems later - no thinking.
The bottomline for me is I have 17x9, 4.75 BS and -6 offset, on the vehicle now. Was just school myself to see if I needed to change wheels, for 35 or 37. I have 35 on them now. I will not be using stock fenders. When these tires are used up, I may go with 37", like I had before.

Thanks,
 
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