so with 33x12.5 you think rubbing would be an issue? would i need wheel spacers?

 

I'd like to clarify some confusion:

What we CALL the rim width, is really a NOMINAL rim width...it is NOT the width of the rim.

That sounds confusing...but, just like BS is measured in inches, and offset is measured in mm, it seems all rim related issues are supposed to be confusing.


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Reader's Digest Version of Rim Width Confusion - The rim width you see published/labeled is the width that the TIRE fits between....there are of course two lips or flanges, that stick out on either side of this area, the flanges that keep the tire from falling off of the rim, outside of where the rubber seats.

They do NOT count the flange width when labeling a Rim width. Most of these flanges are about 1/2" each....so a 17" x 7.5" rim is really about 8.5" wide.

By that I mean if you lay the rim on its side, and measure the height from the ground to the top of the flange, it would be 8.5", not 7.5" wide.

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Reader's Digest Version of Back Spacing -

If you look at the above rim on its side, lets say with the part of the rim's hub that mounts to the truck facing up...(its hub face facing up), and measure up from that face, to the top of the flange...that's the back spacing.

I like to measure BS by laying something flat across the center of the rim, so I have a nice straight line across the top of the rim....giving me the top of flange height....

.....then I take a ruler, and just put the ruler's bottom on the hub face, and measure up to the flat thing's line...bingo, the BS.



If there's 5" of backspacing, there's 5" up from the face to the top edge of the flange....no matter what the overall width of the rim is....its always the same BS, if that distance is the same....period.

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Reader's Digest of Offset Confusion -

Offset measurements USE the outer flange widths. IE: Your 7.5" width rim, has its offset measurement based upon the full 8.5" rim width.



So - Lets say you have a 7.5" rim, which is 8.5" wide...

That means the center of the rim is halfway across the width, or 8.5/2 = 4.25"


This is a very important number...that's called ZERO OFFSET, DEAD CENTER of the rim width (The FULL rim width though, NOT the 7.5" LABELED rim width)

The THING that is AT that zero point is the same HUB FACE used to measure the BS.



So - the Offset CHANGES with the rim width, as the zero point is BASED upon the rim width.



There's Positive Offset and Negative Offset - just ways of saying if the HUB FACE moved off center closer to the brakes (Inboard), or closer outboard on the rim.

Positive means its off-centered closer to the outboard side (More BS typically)

Negative offset means it moved off center closer to the brakes/Inboard side (Less BS typically)

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BS and Offset together:

OK, lets say we have a 7.5" x 17", 0 (Zero) offset rim with a backspacing of 4.25".

That means that the backspacing has pushed the hub face right damn smack to the rim's center point.

If I change the offset, AND leave the rim width the same, I will HAVE to change the BS too...because the hub face is moving either inboard, or outboard, when I change the offset off of that zero offset center line of the rim.

If I move the hub face outboard...the BS increases, as the hub face is getting further from the inboard flange...and, by the distance its being offset from the center.


POINT: an inch is 25.4 mm, so, to convert the offset's mm to the BS's inches, divide the mm by 25.4 mm/inch...and you get the number of inches for the mm, etc.


Now, lets say I have a 9" rim (Which is of course 10" wide....), with the same 4.25" of BS.

My hub face is 4.25" from my inboard flange edge.....and, 5.75" from my outer flange edge.

And - the offset is NOW based on 5" being ZERO OFFSET, BECAUSE the rims is 10" wide...so, 5.75" is 0.75" off center....or, as they like to say, its OFFSET FROM THE CENTER LINE BY 0.75".

Notice I could ALSO say its offset 0.75" from my INBOARD flange edge, as its ALSO 0.75" from 4.25" to 5".

As offset uses mm...I need to go through the drudgery of multiplying my 0.75" by 25.4 mm/inch....to get how many mm of offset 0.75" actually is...

0.75" x 25.4 = 19 mm of offset (A manufacturer may round that to 20 mm, etc...)

So, we'd have a rim with 4.25" of BS, but ~ 20 mm offset on a 17 x 9 rim, and zero offset on a 7.5" rim, etc.



The inboard CLEARANCE for the two 4.25" BS rims would be equivalent.



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Reminder....the tire is ALWAYS centered in the rim...all we are moving is the hub face...so only the BACKSPACING affects how far out, or in, the tire sticks or tucks.

(Because it changes where the INBOARD EDGE OF THE TIRE WILL BE, with the location of the inboard flange)

A wider rim doesn't make the tire stick out more....essentially, if its a 12.5" wide section width tire, and you have placed the inboard edge of that tire at some point...you KNOW the outboard edge is 12.5" outboard from that point.

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So - a FATTER TIRE can stick inboard, and outboard, by the extra fattness, regardless of the rim width. (Half outboard/half inboard...)

The BS will control WHERE the INBOARD fattness starts though.



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Why do we bother with offset at all? -

Offset tells us where the contact patch is located relative to the steering geometry. (BS does too, but offset is based upon the CENTRAL POINT, which is primarily used in steering geometry/handling calculations.

Things like Ackerman Angles, and other engineering factors, are changed when you change where the TIRE'S center is.

When you move the contact patch pivot point a bit, you change the handling/steering of the rig.

Its not always crucial, but, it does change things. On some rigs, it changes it enough to be a an issue, on other's, not enough to worry about.

On high speed performance applications, at the limits of adhesion on a emergency/racing situation...it might mean the difference between winning/loosing/crashing/not crashing....for most of us in trucks puttering along...its less likley, albeit not impossible, to make a difference in our lives.

Ricers/Racers etc use mm, and worry about offset a lot. Truckers worry about BS more.

They are more worried about steering geometry, we are more worried we can't stuff the tallest fattest big meats under the truck that can fit. Both of us are F-ing around with our steering and handling though, just using different ways of measuring it though.

(Different priorities)



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Summary -

1) The widths rims are labeled with are really an inch less than the REAL widths, because God Hates you, and wants you confused. (Add an inch to get the real widths)

2) The hub face of the rim is ONE main point of reference for BS and offset.

3) BS only measures hub face to the inboard flange edge distance - so, the more distance, the deeper into the inside of the rim you have to go to reach the hub face/the tighter the rim is tucked into the wheel well...no matter what the rim width is. Its in inches.

4) Offset is only measuring how far from the center line of the rim the hub face is...so zero offset is halfway between the two flange edges (5" from either side on a 10" total width rim, or a rim LABELED as a 9" rim) Its in mm.

5) For tire clearance, all you need to know is the BS...more BS = Tighter Tuck, less BS = more like an Outrigger.

6) If all you know is the offset, and the labeled rim width, you can COMPUTE the BS. Add an inch to the labeled rim width, divide by 2 to get the true center of the rim width...and the BS is the distance from the hub face to the inboard flange that results.

7) An inch is 25.4 mm

8) Positive offset means its hub face is off-centered closer to the outboard side (More BS typically)

9) Negative offset means its hub face is moved off center closer to the brakes/Inboard side (Less BS typically)

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Well - I hope that helps a bit.

 

TEEJ,

So are wheel spacers in effect changing the offset and backspacing of stock wheels? Is there any real difference between a 1.5 wheel spacer on a stock Rubi 7.5" rim and an aftermarket rim that has 1.5" less backspacing?

glwood6

 

A 1.5" spacer reduces the BS by 1.5", because it essentially moves the hub face inboard, towards the brakes, so that the rim sticks out from the well further by 1.5" than it did before.

Technically, its not really changing the rim's offset, because its not changing the rim...but, from a reality standpoint, and a steering geometry stand point, it IS changing the effective offset...because its essentially shifting the hub face off center, towards the brakes, by 1.5", 0r changing the offset by about 38 mm.

As far as the difference between using a rim with 1.5" less BS, vs a 1.5" spacer, no, its the same really as far as steering and clearance go...

So if $50/rim spacers make more sense than $ for new rims, sure, its the same end result...they both merely move the hub face inboard to make the rim stick out more.




BTW - If you live in NJ, or other state that doesn't ALLOW the tire to protrude out past the fenders....you won't pass inspection, or, you can be pulled over by the cops, etc....esp if its rainy, and you're tire spraying the bejeebers out of said cop behind you, etc.

If you have spacers, you have the option of removing them for inspection at least.

 

Same here. 17" Pintlers with 12.5" wide tires. No rubbing...

 

I noticed the 17" Pintlers come with various BS...so, you'd need to know WHICH 17" Pintlers you had as far as BS goes to know how to compare. If the OEM BS is 6.25", and a 1.5" Spacer seems to cure the rub problem for 12.5"/315 mm tires, a Pintler (Or any other rim) with 1.5" less than 6.25" should work as well (6.25" - 1.5" = 4.75" BS)

Most people seem to be indicating that 4.75" of BS avoids rub, so, all are in agreement.

If the tire is another inch wider, say a 13.5", logically, you'd need another 0.5" of BS reduction to clear the extra 0.5" inboard it would extend.

Pintler makes some 17" rims with 3.75" BS too though for example, but, warns that these may require extended fender flares where local laws forbid the tire to protrude out from the fenders....but that extra inch of BS past the 1.5" would allow clearance theoretically for a 14.5" tire.

 

Here is another link that might help explain offsets for you.

http://www.discounttiredirect.com/di...eelOffsets.jsp

 

I live in NM, no knowledge of law forbidding tire past fender flare. I, however, do not like tires outside of flare: I am putting on Spidertrax spacers to bring the 11.5" tires on stock Rubi rims out flush with the outside edge of the flare...hopefully. Anyway, thanks for the insight!

Gary