Wheel offset calculator

[Cerberos]

Wheel offset vs. backspace

Last evening I had a close friend, and avid auto enthusiast, ask a rather insightful question with respect to a post on another forum regarding wheel specifications, i.e. backspacing vs. offset. He couldn't figure out why recommended offset would change if wheel width changed.

After a 30 minute conversation and a bunch of scribbled pictures on dinner napkins, it finally made sense to him. In short, recommended backspacing doesn't really change but offset does.

Let's take a look at how this relates to our Raptors. As you go up in wheel width, say from our factory 8.5" to 9", the required offset drops. In this example, you would only need +28mm of offset in a 9" rim to maintain the same backspacing as the stock rim.



Many of us would like just a bit more clearance at the strut, so reducing the offset in the 9" example to +25mm give you 3mm more clearance and still only pushes the outside face of the wheel out 15mm.



Here is a link the wheel offset calculator

 

[dataustin]

Probably the best post ever!

 

[BIRDMAN]

what? offset and backspacing are directly related. how would 1 change without the other?

 

[Cerberos]

what? offset and backspacing are directly related. how would 1 change without the other?

This is pretty much how the conversation got started last night! And yes, you are right, backspacing and offset have a direct relationship for a given rim width. My original intent was to explain how an increase in rim width impacts offset with a given backspace. Here is an more detailed look at the relationship.

It will help to have another diagram that defines the various wheel or rim dimensions and descriptions.



As you can see above, backspacing is the dimension from the outside of the rear edge or flange of the rim to the inside face of the hub. For our Raptors this dimension is 6.08".

Offset is the distance from the centerline of the rim to the inside face of the hub. If the centerline is to the inside of the hub, offset is defined as positive. It it lies to the outside of the hub it is defined at negative. Our Raptors have a +34mm offset.

Backspacing is crucial because it tells us what the furthest inward point is on a rim in relation to a fixed suspension point, i.e. the mounting surface of the hub. The ideal backspacing dimension does not change regardless of overall rim width. What does change is the portion of a rim that lies to the outside of the hub mounting surface.

Let's assume that the ideal backspacing for a Raptor is 6.08" based upon a nominal 8.5" rim width and that we want to maintain the stock suspension geometry but go to a 9" rim. What happens? We still want that furthest inward point of the nominal 9" rim to be 6.08" from the hub mounting surface. That maintains all the stock or ideal clearances at the strut. What does change is the relationship of the centerline of 9" rim to hub mounting surface, i.e. offset. Since the 9" rim is wider, and we fixed the backspacing at 6.08", the centerline of the rim has moved outward in relation to hub mounting surface and the offset has been reduced from +34mm to +28mm.

Now keep in mind that the relationship between a rims nominal width and overall width can and does vary between wheel manufactures. So, this approach should be only one part of the fitment process.

 

[LastLight]

I should have stayed in a math class. This stuff is like a foreign language to me.

For my new wheel selection, I have a 17" x 9" wheel with an option of +12 or -12 offset.

w/ +12
Inner Clearance: 16 mm more

Outer Position: EXTEND an extra 28 mm


Does that mean it will work or not?

Can I run a Nitto Trail Grappler at 35/17/70 and avoid rubbing problems?

thx

 

[LastLight]

I should have stayed in a math class. This stuff is like a foreign language to me.

For my new wheel selection, I have a 17" x 9" wheel with an option of +12 or -12 offset.

w/ +12
Inner Clearance: 16 mm more

Outer Position: EXTEND an extra 28 mm


Does that mean it will work or not?

Can I run a Nitto Trail Grappler at 35/17/70 and avoid rubbing problems?

thx

SO, let me try this again. Stock wheels have a range of 20-34 offset.

If I take the +12 of this new wheel and I add 12.7mm for the additional 1/2 inch of width it equals 24.7mm of offset. Thus, this wheel should work fine on the Raptor.

Just unsure of the 315/70/17 Nitto Trail Grappler fit now and if it will rub?