tires and wheels 101
Want to install a new set of wheels?? Can't figure out what offset or backspacing you need or want to run?? Well read on and you'll know everything you need to know about positive and negative offset and backspacing...
I'm going to concentrate on backspacing and offset because most of us know what our bolt pattern is... 97-03 F150=5x135 04-08 F150=6x135
So what is backspacing and offset?? Backspacing is the distance in standard inches from the hubs mounting surface to the outer edge of the inboard side of the wheel (the side closest the the frame when mounted) You can measure this by placing the wheel on the ground pretty side down and run a straight edge across the wheel. Now run a tape measure down to the wheels mounting surface (that touches the hub when mounted) and measure up to the straight edge! there ya go!! You've figured out your backspacing
What is offset? Offset is directly related to backspacing, if ya know what one is and you know how wide the wheel is you can figure the other one out!... Offset is the distance in millimeters from the direct centerline of the width of the wheel to the hubs mounting surface! a negative offset would mean the hubs mounting surface is closer to the inboard side of the wheel thus pushing the wheel out or away from the frame and a positive offset will suck the wheel into the wheel well You can figure this measurement out from your backspacing measurement and the width of the wheel. A 10" wide wheel with 4.5" of backspacing yields a negative 12mm offset because half of the wheels width would be 5". The mounting surface is 1/2" off of the centerline toward the inboard side of the wheel beacause of the 4.5" of backspacing... 1/2"= 12.7mm
A 7" wide stock wheel with 4.5" of backspacing yields roughly a Positive 25mm offset (25.4mm to be exact but most companys round it off)
For the next set of examples I'm going to use a 10" wide wheel with 4.5" of backspacing and a negative 12mm offset
So go ahead!!!! run out and throw a straight edge against your hub assembly, rotor, brake drum, whatever mounting surface your wheel touches and take some measurements! You can get a pretty close idea of where you'll stand by doing just that!
If ya got 4.5" of backspacing on a 10" wheel? measure off the straight edge to the steering knuckle 4.5"! You got 7"? you're clear!!
If ya wanna run a a 13.50" tire on that 10" wheel? that has a -12mm offset? Well half the tire is 6.75" and the offset is -12mm so convert the -12mm to roughly 1/2" and subtract it from half the tires width because it's a negative offset and you're pushing the tire away from the frame so you get 6.25"! There ya go! measure from your straight edge to look for tire clearance!
You can even go as far as making a mock-up. Drill a hole in a stick of wood, bolt it onto one of the lugs, run it up 33" for your 33"x13.50" tire and screw a block that's 13.5" wide and hang 6 1/4" into the centerline and look for clearance (just make sure to keep the stick running directly through the centerline of the hub, you start running that at an angle and you'll be way off) This is really just an example of how much you can figure out with some quick math, I wouldn't go as far as the mock up! Haha just measureing off the straight will give you all the info you need... and make sure to look for the height of the tire off the rims bead too, not just at 33" but where the tire balloons off the bead of the rim, like on a 16" wheel you have 8.5" of sidewall...
And this will only be a rough mock up! it'll get you close but sometimes a 33" tire will run 34" on an 8" wide wheel and 32 on a 10" wide wheel because of ballooning and crowning. When forcing the tire on a skinny wheel it will crown and you'll get more height on it. On a wider wheel it may balloon out creating no crown at all, not to mention inflation, differences in actual measurements etc etc...
I know this is long but it's fairly thorough Haha