Wheel Upgrade - Wheel Fitment learnings that I thought I'd share
Have you heard about offset and backspacing on wheels? I had not until I started researching this wheel/tire upgrade. I thought I'd provide some education for those interested.
If you've already got tire/wheels that are the approximate height but you want to increase the width, or like me, change to a larger diameter wheel w/lower profile tire, then I can share some insight.
In my case I wanted to maintain the basic height and width of the combo but wanted larger diameter wheels. This would require me to go with lower profile tires to maintain the total height. To figure out the new height I needed to understand how to determine tire height from the numbers you typically see. Here's how to understand the numbers on a tire size:
285 - Width of the tire in millimeters (mm). Divide by 25.4 to get to inches.
40 - Aspect ratio of the tire. sidewall height is this percentage of tire width
R - Simply means Radial.
18 - Diameter in inches of wheel opening, or what diameter wheel it fits
So to determine the height of a tire from the above, use the following:
((Aspect Ratio/100)*Width) + Wheel Diameter = Total Tire Height
In my case:
(40/100 * 285)/25.4 + 18 = 26.97"
The total tire height calculated should be compared against what you currently have. If you go taller, you'll need to understand if you can clear the tire in your fender wells and when you turn the steering lock to lock.
To get to a shorter height, you'll want to look for lower aspect ratio (profile) tires.
Once you figure out the tire height, you need to understand the width and positioning of the tire in the wheel well. For me, I'm staying with the same overall width, but positioning was the key. To understand positioning, you have to understand backspacing and offset. I've attached a diagram to help explain these two related measurements and I'll explain why they're important. First take a look at the diagram and you can probably quickly see the difference.
Offset and Backspacing basically tell you the same thing, the relative position of the mounting flange in the wheel. Offset is the distance (typically in mm) of the mounting flange either in front of (considered positive offset) or behind (considered negative offset) of the centerline of the wheel. However, the more useful measurement to me is backspacing. It tells the you the distance (typically in inches) from the mounting flange to the back edge of the wheel (which may or may not be exactly the tire's edge). Using this measurement, you can figure out how far back from the hub's front edge your tire/wheel will stick into your fender well. Obviously you can then determine if there are any interference that will cause rubbing or worse damage to the tire/wheel.
Most companies publish backspacing, but some publish offset. If you know offset but you want to know backspacing then use the following formula:
((Wheel width) + 1)/2 + (Offset)/25.4 = Backspacing
You add the extra inch due to the thickness of the mounting flange. Offsets are either negative or positive numbers. So make sure you use them in the formula above that way to get the correct backspacing. For my 9.5" rear wheels, they have a +6mm (positive) offset so the backspacing is:
(9.5 +1)/2 +6/25.4 = 5.47"
Interestingly, modern wheels tend to have the mounting flange towards the front of the wheel and you can see that on most designs since the lugs sit at the front of the wheel. This results in a deep backspacing of 6-7" depending on wheel width. The more classic hot rod wheels, or "deep dish" wheels, have the flange pushed to the back and a more shallow backspacing. Frankly these deep dish wheels are more rare now, but the look I am going for. These are harder to find now probably due to the larger disc brakes and calipers on newer cars making it hard to fit the wheel around the calipers.
In my case I had to first determine my current backspacing which is 3.7" (both front and back). Then as I shopped for 9.5" wide wheels that had a deep dish yet a backspacing close to 3.7". However, what I found was that most wheels in the 9.5 to 10" width have a backspace of 6" or more. The wheels I selected are Ridler 695's which have a backspacing of 5.5". So, what do I do? If I simply used the 5.5" backspacing, the wheel would sit 2" deeper into the wheel well. This would not look right since I have flares and the tire face would be sitting way back in. Also, the back side would be sticking further in and probably hit the parking brake bracket at a minimum. So my only choice was to use a "spacer"
Some folks are not supportive of spacers, and honestly I don't have any experience. If you need one as thick as I do, 2", then it will come with it's own lugs, and mount with recessed lug nuts to the existing lugs on the hub. Frankly, I would be uncomfortable using a spacer that didn't have it's own lugs which just shortens the bite on the existing lugs. By using this spacer I'm moving the wheel back out by the 2" and putting it where it should be. This of course is adding that 2" on each side of the car into the driveline. Clearly, going cheap on these is not a good idea. Mine are from a company out of Germany and CNC'd to very specific specs, so I hope they work.
We'll see how they drive. I'll also post pics later this week on how the new setup works. Wish me luck and I hope this info helps others.