Great build Chris and excellent writeup and articles in OJ!! Thanks for all the effort to put this out.
I'm finally starting a real build-out using my 02 F-350 as my foundation.
I've had it since new and 178K of work/travel/adventure miles. The wife is tired of ground camping in the Oz Tent RV-5 so I'm upgrading to a Hawk FourWheelCamper on a custom flatbed. I've been looking to run 37's as I did the swap to 4:10 gears years ago when I began running 35's. The suspension is a Donahoe 6" (now Icon) but will need some upgrades once I get the camper/bed finished and the load figured out. What I'm seeking is advice on wheels. I want to run 37x125R17 and need to know the right backspace/offset I should seek for proper fitment. The forum searches for this info have been a bust.
Anyway, you've definitely given me some mod ideas and I love what you've done so far. Subbed.
I don't want to hijack this thread, but I've got some insight here.
I have had the same issue as you since I have owned my Excursion. Whenever trying to research fitment, there is a ton of discussion among various message boards about "lift required" to fit X size wheels, without real discussion on the backspace/offset.
I actually just took some pictures and measurements this past weekend of my Excursion that is sitting on 285/75R17's (~34) on 17x8 4.5" backspaced wheels at full lock on both sides. I was planning on sketching up some lines to see what backspace/offset I needed to keep me off the leafs with a 37" setup.
I had someone holding the wheel at full lock to the right.
Here are some labeled pictures:
PS overall:
PS rear liner clearance:
PS leaf clearance (just hardly kisses spring at full lock):
DS overall:
DS front bumper clearances:
DS leaf spring clearance:
As you can see, the limiting factor is the rear of the tire touching the leaf spring at full lock. As you move up past 35's, you will need to trim the front bumper and pull the liner back. Neither of these things worried me. I also would like to get some measurements while the tire is full stuffed, I just didn't have the time or resources to get that done.
You can do some calculations for the leaf spring clearance as you increase the tire diameter.
In the sketch screenshots below, I've constrained the smaller tire to just touch the leaf spring, and then constrained the larger tire to be at the same center point and angle as the smaller tire. I used my 33.8x11.2 tire and a 37x12.5 tire for comparison's sake here. I used a guess of 30 degrees worth of steering as an assumption.
As shown, the larger tire goes ~1.4" past the leaf spring. I'm sure someone could have easily figured this out, but I just whipped this up on my lunch break because I had my software open.
Obviously, I have made some assumptions and your mileage may vary. Hopefully some of these images and thoughts of mine can be useful to you. Thanks for giving me an excuse to actually write some of this down instead of think about it in my head.
-Russell