My fellow Autopians, how many times have you gone out to buy wheels for that sweet ride of yours and the guy behind the counter starts talking about offsets, wheel width, backspacing, and suddenly you feel your head spinning and about to explode? You just want some wheels and this guy is talking way too much. Well folks, The Autopian is here to help. Think of us as chewable, low dose Asprin.
Let’s first talk about some terminology and then what that all means for your wheels and car. We’ll start with offset and backspacing since these seem to be the most misunderstood and confused terms in the world of wheels.
[Editor’s Note: In the Jeep world, this conversation about offset/backspacing is a big deal, as people often try to install oversize tires, which may not fit without the appropriate wheels to “push” the tires outboard a bit more. But how much does each wheel “push” the tire outboard? To get to the answer, one has to post to an internet forum a question about backspacing, wait for three dozen people to respond with answers that mix up offset and backspacing, wait a bit longer for those folks to start arguing with one another, and then ultimately just buy the same wheels that someone else did. Or one could just read this blog. -DT].
Offset
Offset is the distance from the center of the wheel to the mounting face where it attaches to the suspension. It’s easiest to see if we imagine slicing a wheel in half like you would slice an apple and then look at the new face you just created:
If we measure from the geometric center to the mounting face of the wheel we can measure the offset. In the case of our wheel, it is 50 mm:
Of course, trying to do this on a wheel you have sitting in your garage is pretty much impossible. It’s easy to find the mounting face of a wheel, but try to find the geometric center. You can’t, unless you have some complex measuring tools. Fortunately, you don’t have to. Most every wheel manufacturer puts the wheel offset into their casting in some form, often along with the wheel size. Here’s an example:
On this VW wheel you can see the size (8Jx18) followed by the letters “ET” (which comes from the German word Einpresstiefe, or “press-in depth”) and the number 41. What this is telling you is that the wheel is 8 inches wide, has a “J-shaped” bead profile (which is an industry standard), is 18 inches in diameter, and has an offset of 41 mm. Other manufacturers have different ways of showing this information. I’ve seen some that show the wheel size followed by “+” or “x” and the offset, while others use the letters “IS” instead of “ET”. The letters “H2” next to the wheel size means there are two humps in the rim profile to help the tire seat when it is installed.
On steel wheels you will find the same information stamped into the rim at some location.
Backspacing
The other way wheels are often designated is with backspacing. Backspacing is the distance from the inside flange to the mounting face of the wheel. As you can see above, more backspacing means more positive offset or less negative offset. Here’s another look at backspacing:
In the case of our fictitious wheel, it is 170 mm. It is a dimension that I have never seen stamped or cast into any wheels, but fortunately it is easy to measure. Simply lay the wheel on a flat surface face down, lay a straight edge across the flanges and measure down to the mounting face:
In the case of our VW wheel, it was 151 mm. There is some controversy about whether or not you should lay the straight edge across the inside edge of the flanges or on the outermost bead, but as long as you’re consistent in how you measure it, you’ll be OK.
Why and When Should You Care
Now that we know the difference between offset and backspacing and how to find each dimension for any wheel, we need to understand why they are important and under what circumstances we need to care about them.
Let’s suppose you aren’t happy with the wheels on your car and would like to put wider wheels and tires on it for better performance or a different look. Whichever wheels you choose will need to fit on your suspension. Let’s also assume your car has a MacPherson strut front suspension and came with 8″ wide wheels from the factory. We’ll look at the same section through the wheel but I’ll include the bearing and strut this time. Ignore the gap between the bearing and the wheel — this is where the brake rotor goes. I also haven’t included the knuckle just to keep things simple:
Notice how close the rim gets to the damper. In some cases this distance can be as little as 15 mm.
What would happen now if we wanted to put 10″ wide wheels on our suspension? If we chose a wheel that has the same offset as our 8″ wheel, it would look something like this:
You can see that the offset is still 50 mm but the wider rim means that the backspacing has increased to 196 mm. Put this wheel on our suspension and we would have this situation:
Notice that there is interference between the wheel and the damper. Clearly, this wheel would not fit on our car.
On the other hand, what if we chose a wheel that has the same backspacing, meaning that the distance from the mounting face to the inside flange stays the same. The wheel would have an offset and backspacing like this:
In this case, while the backspacing stayed the same at 170 mm, the offset changed from 50 to 25 mm. Putting this wheel on our suspension would look like this:
We’ve fixed the interference with the damper and you might be thinking that everything is fine now. Unfortunately, it’s not that simple and it gets back to what I talked about in my wheel spacer post. Take a look at where the centerline (the vertical orange line) of the wheel falls relative to the bearing for both the 8″ and 10″ wheels:
On the 8″ wheel, the wheel centerline falls between the inner and outer ball bearing races while on the 10″ wheel, it falls outside both bearing races. The centerline of the wheel represents where the forces coming up from the road are concentrated, and the location of this line relative to the ball bearing races is critical to the longevity of the bearing. Using a wheel that places this line outside the ball bearing races will have a detrimental impact on bearing life and could even lead to catastrophic results such as in this video (okay, it’s not likely this will ever happen again):
Witnessed and recorded the most INSANE car crash yesterday, you can see Autopilot also swerve and avoid the rouge tire for me $TSLA pic.twitter.com/csMh2nbRNX
— Anoop (@Anoop_Khatra) March 25, 2023
The other issue to be concerned with is the impact moving the wheel centerline has on the suspension kingpin offset and scrub radius. Both of these critical dimensions grow as a result of reducing the wheel offset in the same way as adding wheel spacers does, which could cause some unexpected and undesirable suspension behavior.
Lastly, there is of course the question if such a wider wheel will even fit with the rest of the car. If we compare the two wheels one on top of the other in our section, we can see how much further outboard the outer edge of the 10″ wheel has moved:
I’m not showing the tire in this view, but you can imagine the outer edge of the tire has also moved outboard the same amount. Like we saw with wheel spacers, we need to make sure the outboard edge of the tire will fit with the car’s fenders, especially when experiencing jounce. If not, we will need to flare the fenders or choose a narrower wheel.
Of course, none of this matters if you’re planning to get new wheels that are the same width as your old ones. In that case, just choose wheels that have the same offset as the original ones, and everything should be fine.
Great article. One thing missing that some of the commentors are missing too is that the relationship between backspace and offset isn’t as simple as it seems. The reason is that wheel widths are measured between the flanges and backspace is measured outside the flanges. So to convert from offset to backspace you need to consider the flange width.
I suspect the flange is a standard dimension. I’m not sure what it is, but appears to be about 1/4″ on the wheels I’ve measured. So, for an 8″ wide wheel with zero offset, the backspace would be 4.25″ not 4″.
Hubert, I don’t understand the need to physically measure backspace. If you know the offset and wheel width, then backspace is simply the positive offset plus half the wheel width. Right?
Offset is the dimension that’s important, backspacing is the dimension that’s easy to measure.
It’s not like measuring offset is actually difficult (measure two things and do simple maths), so using backspacing at all is baffling to me.
I’m a mechanical engineer, MIMechE even. I also used to drift competitively and had over 40 different wheels for my car, none of which were the correct OEM size.
“Will it fit?” is very simple maths.
“Should I fit it?” is complex and has radically different answers depending on who is asking.
I regularly get calls from a guy who wants to buy new wheels. He has two degrees in engineering. Every time he uses “poke” as a dimension I want to hang up and block his number.
I came here to say the same thing, measuring the offset doesn’t require any special tools vs measuring back spacing. The only difference is that you have to do two measurments and be able to do some basic math.
this is quality content thanks!, I’ve spent countless hours in many wheel/rim calculators before purchasing my new sets, it’s good to finally get the science behind it all in an understandable form. It would be cool if you could gather all these technical articles into some Autopian Car Knowledge database, so it would be easily searchable.
Stock wheels. I stick with them and rarely find any that (to me) improve the -look- of cars, and I also know that there is a valid rational engineering reasons to stick with the stock rims. They were designed to work on that car.
I do have a mix of stock wheels on my ’64 F100 crewcab. While all are 16″ and three are orginal to the truck, some are 5″ wide and some are 5.5″ wide (or is it 6″ and 6.5″?, I forget). Dad had two split 16.5″ rims on the rear that I have removed, thus only three original remain. I found a set of 4 that were 0.5″ different in width (I didn’t know that at the time, it was hard enough to find 16″ instead of 15″ wheels), so I have a mix on there now. I suspect that in addition to being an old style suspension, and such a minor difference such a difference is essentially meaningless.
I, an engineer, had this discussion with a friend, also an engineer, several months ago when I was replacing the oxidized (and ugly) wheels on my 20 year old Lexus. My friend could not quite rationalize that backspacing and offset change with wheel width, and was certain the wheels I had chosen (after extensive research) would not fit. Of course, when I showed my friend the wheels mounted on my vehicle, the immediate response was “I told you they would be a perfect fit!”
Great lunchtime read – thank you!
Question I’ve always wondered – when manufacturers offer different sized wheels for a given model, do they commonly all have the same offset so nothing else in the suspension has to be changed, or do they actually modify things so they’ll work?
Yes, that is commonly what they do. If for some reason a wider wheel with the same offset wouldn’t fit, they will make additional changes so that the change in scrub radius and kingpin offset don’t cause a problem.
It all depends but often the offset is different for different width wheels. For example the S-197 Mustang. When fitted with 7″ wide wheels the offset of the OE wheels is 39 or 40mm while the 8″ wide wheels have an offset of 44, 45 or 50mm. 8.5″ and 9.5″ use 50mm.
This actually clears up a lot, thanks. My 1966 Thunderbird’s steel wheels have a somewhat odd rim width that isn’t made anymore (though new tires still fit), and I’ve wanted to get new wheels for it so I don’t have to deal with hubcaps or ugly plain steelies, but there’s so much nuance to wheels that I’m never quite sure what will fit on it. Stuff like this helps demystify wheel fitment for me. Now the big question is whether the new wheels I’m looking at will rub on my fender skirts…
For your 1966 T-Bird, I wouldn’t sweat it. Just find something that fits inside the fenders. Don’t worry about things like scrub radius or kingpin offset. For those old suspensions, it really doesn’t matter. They were designed with massive scrub radii and kingpin offsets anyway so a change in wheel offset really won’t upset things to the point where you can feel it.
Huibert, thanks for this. I’m facing this issue with a ’63 International Travelall: it came with 3 16″ rims and one 15″, and finding a fourth 16″ rim the same size and backspacing has been…challenging. I did find a fourth, had it shipped from Idaho, and came to find out the offset was different than the other three—I’ve been scratching my head this whole week trying to understand why, and you just explained it to me. But knowing older cars are more forgiving is a huge relief.
Have you considered putting wire wheels on it? They might even make reproductions of the original factory wires in common tire sizes.
The T-bird would have came with wheels that have no offset, very common at the time. So stick with wheels with zero offset and with the same width tires they won’t rub. If you want to go with a wider tire measure the existing clearances and see if there is any room.
I love when I find out that something I thought of as being relatively simple is infinitely more complicated that I originally thought.
Now to finally read the February Offset article I missed. Thanks for all the homework Huirbert (/s).
But seriously, these posts from you are amazingly informative. It’s like a free engineering course.
For real, as someone going into mechanical engineering I feel like I’m getting a head start on a bunch of things with articles like this.
Congratulaions on choosing Mechanical Engineering. You’ve chosen the correct engineering field 🙂