I've been asked a few times about what tire backspacing means so I figured I would write down a few things about it. This is as things have been explained to me over the years and some comes from mechanic books that I have read.
Backspacing:
This is the measurement the back of the wheel to the mounting surface.
Offset:
This is the measurement from the wheel centerline to the mounting surface. You can have Positive, Zero or Negative offset. Positive offset is when the mounting surface is closer to the front of the wheel than the center line. Zero offset is when the mounting surface lines up with the center line of the wheel. Negative offset is when the mounting surface is closer to the back of the wheel than the center line. For some reason they like to measure this in Millimeters.
How does this apply to the wheel:
A large backspacing or positive offset brings the wheel further into the vehicle, tucks it under the fender. A small backspacing or negative offset brings the wheel further away from the vehicle, sticks it out from under the fender.
Stress on the hub bearings:
It has been explained to me that the wheel generally come from the factory with the center of mass lined up over the hub bearings. This way there is the least amount of stress on the bearing assembly and ball joints. If you bring the wheels in or out with a different backspacing you increase the stress placed on these components. For us offroaders moving the wheels out will allow you to run larger tires and keep your turn radius.
How to Measure your existing wheel:
Figuring out backspacing and offset is fairly easy, just requires a little bit of math (fractions, multiplication, division, addition, subtraction and even a little geometry). I can hear the complaining already. Wheels are generally given by their inner flange measurement. i.e. a 15x9 wheel will be 15" in diameter with a distance of 9" between flanges.
Step 1: Find the width of the wheel. If you already know the wheel size just at 1" to account for the flanges on either side. If you don't know the size and the wheel does not have a tire mounted to it, just measure across the flanges (outside to outside). Now like most of us we have tires mounted on our wheels. You will need a helper this time. To do this take two straight edges wether it be rulers, blocks of wood, or pieces of metal. Lay them across the outer edge of each wheel flange, make sure they are parallel (see your using geometry). Measure the distance between them. So our first math problem is finding the wheel width outer flange to outer flange. So in our example of a 9" rim we would add 1" to that to make 10"s, or if we measured it would be 10".
Step 2: Divide that number by 2. So now 10" / 2 = 5". This is our wheel center line.
Step 3: Lay a board, straight edge, or piece of metal across the outer wheel flange on the backside of the wheel. Measure to the mounting surface and see what this is. Let use 4.5" as an example since this is what seems to be pretty popular for a backspacing.
Step 4: Minus the center line from the backspacing number. So 4 1/2" - 5" = -1/2".
Step 5: Convert the -1/2" offset to Milimeters. 1" = 25.4mm. Covert -1/2" to decimal -.5 and times that 25.4. This equals 12.7mm.
So from this measurement we can determine that we are running a wheel that measures 15" x 9" with a backspacing of 4 1/2" and a negative offset of 12.7mm.
How to figure out what backspacing a wheel has with knowing only the offset:
Sometimes you will see wheels stated as being 15x9 with a +12mm offset. So what is the backspacking? That's pretty easy.
Step 1: Add 1 to the wheel width and divide by 2. so (9" + 1" = 10"), (10"/2 = 5") This is your center line.
Step 2: Convert the +12mm to inches. Divide the 12 by 25.4. (12 / 25.4 = .47). We will round up to .5 for math ease.
Step 3. Since this is positive offset we add this to the centerline. ( 5" + .5" = 5.5") so our backspacing is 5 1/2". If it was a negative offset we would just minus the number off of centerline.
