wheel offset explained

[mood_indigo]

To start things off, I’d like to get a few definitions out of the way.

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel and for some ungodly reason is usually measured in mm. the offset can be one of three types.



Zero offset - the hub mounting surface is even with the centerline of the wheel.

Positive offset - the hub mounting surface is toward the front or wheel side of the wheel.

Negative offset - the hub mounting surface is toward the back or brake side of the wheels centerline.

Backspacing of a wheel is the distance from the hub mounting surface to the rear outer most edge of the wheel.

Frontspacing is a term not used too often, but is a term I like when discussing a wheels ability to tuck. It is the distance from the hub mounting surface to the front outer most edge of the wheel.

As an example I’m going to use a wheel that is 20x8.5 with a +19mm offset as this seems to be a pretty common size.

The first thing you have to understand is that actual wheel width is about 1 inch wider than stated, so a wheel that is 20x8.5 in actuality measures 9.50 inches wide. That being said the centerline of the wheel, +0.00mm offset, is the same as 4.75in backspacing/frontspacing.

9.50in/ 2 = 4.75

Now to get the offset from metric to a more understandable unit of measure (inches), we are going to divide it by 25.4. This number is a constant as there are 25.4 mm in an inch.

19mm/25.4 = 0.7480314in

For practical reasons, we’ll round up to 0.75in. Since we already established that our centerline is 4.75, and we are dealing with a positive offset, we are going to add 0.75 in to 4.75 to give us a wheel backspacing of 5.50 inches. This in turn will allow us to figure up the wheel frontspacing by simply subtracting the wheel backspacing from the overall wheel width giving us 4.00 inches.

9.50in – 5.50in = 4.00in


more info on wheel/tire/tucking combos to follow

[mood_indigo]

A common question I run across is “will (insert wheel) with (insert backspacing) tuck on my truck??” The answer to that question is more that likely yes (provided you are not talking about a zero or negative offset wheel) it’s those gd tires that get in the way. I hope this clears up some of the confusion.

Because I own a 2001, I’ll use the 99+ trucks as an example, but you can apply this to any vehicle you see fit.

On the 99+ trucks, the magic number is (about) 4 inches. (this number varies from truck to truck) This is the distance from the hub mounting surface to the edge of an unrolled rear fender. I chose to use the rear as an example for simplicity as the front wheels camber in as the wheel travels into the fender, and are typically not a problem. As long as the distance from the hub mounting surface of the wheel to the front outer most edge of the tire (not wheel) is less than 4 inches, you will not have tucking issues. We’ll call this Total Frontspacing.

To figure out your total front spacing, you need to know some basic information. What is your wheel size, offset, and tire size. I’ll start with the stock wheels and tires.

Stock wheel size is 16x7 with a backspacing of about 5.25 inches. We must first get the centerline by adding 1 to the width and dividing by 2.

(7 + 1)/2 = 4 inches

Next we subtract the centerline from the backspacing to get the offset.

5.25 – 4 = 1.25 in offset

Now we need to find our tire size. Again, I’ll use my stock tire size…Michelin LTX M/S 255/70/16. According to the Michelin web site, this tire is 30.2 inches tall and 10.3 inches wide on a 7.5 inch wheel. For every half inch you need to get to your wheel size, you add or subtract 0.2 inches from the width of the tire. In other words, my tires are 30.2 in tall and 10.1 in wide on my stock 16x7.

To find to total backspacing/ frontspacing, we are going to apply the same principle we did to find wheel backspacing/ frontspacing.

The new centerline of the tire/ wheel combo is 5.05. we found this by dividing 10.1 by 2. Again we are dealing with a positive offset, so you add the 1.25 offset to the centerline to get a total backspacing of 6.3 in.

(10.1/ 2) + 1.25 = 6.3 in

This will let us find our total frontspacing by subtracting the backspacing from the tire width.

10.1 – 6.3 = 3.8 in

Since the 3.8 inch total frontspacing is less than 4, you should have no problems tucking this wheel/ tire combo. You can of course get around the 4 inch limit by rolling or trimming your fenders.

*note – you should take measurements when possible. these numbers are close, but not necessarily exact