Tire Sizing Jig

Discussion in 'Wheels & Tires' started by Crawdaddy, Dec 2, 2010.

  1. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    Many moons ago, I remember seeing a car show where they showed this nifty jig to determine what is the largest tire you can fit in a wheel well. If I remember correctly, it attached to the wheel studs like a rim would, and it had an adjustable arm to determine the diameter, and another adjustable arm at the tip of the first arm to determine the maximum width. Anyone know where I can get that tool, or how to make one? I need to figure out my maximum size for the Suburban, but there's not a lot of data out there for it, and no shop is willing to just mount a bunch of random tires on the rim and test fit them.
  2. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    That would be a cool tool if you could find or make...I have never even heard of one. It might be expensive for a one-time use though. I may have gotten lucky, but when I purchased my tires, the tire shop was more than willing to try on a couple different sizes to ensure I would be happy with my purchase. However, if that is not possible for you, then maybe if you gave them an extra $20 for their time, they wouldn't mind. I think that may be even a cheaper way to go.
  3. randomsandwhich

    randomsandwhich Rockstar 3 Years 100 Posts

    i used a tape measure, balsa wood and my old tire to determine what type of clearances I would have, it was a lot of number crunching and scratching my head and numerous re-measurements but I was able to get a good idea. I don't even know how accurate I was but I don't have any rubbing issues with my selection.

    this idea may work as well

    Last edited: Dec 2, 2010
  4. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Wow! That looks pretty cool but I can see how you would almost need to be an engineer to build one. Were you able to figure in the offset and/or backspacing with what you made?
  5. randomsandwhich

    randomsandwhich Rockstar 3 Years 100 Posts

    i didn't really make anything lol

    I used the balsa wood to find out what clearance I would have with problem areas. In my situation this was the UCA and the fender well near the doors as there is metal there and I did not want to hack metal to make wheels/tires fit. With that I jacked the truck up (for reason that this would put the suspension in max droop and would put the uca closest to the tire/wheel) I then took tape measure and got rough estimate how much clearance I had with uca and tire, then I cut lengths of balsa wood to get an exact measurement of clearance. I did the same thing for the fender well as well as with the truck under normal stance (tires on ground. I wasn't worried about the tire being compressed into wheel well has the uca would move further way and create more clearance).

    So with my measurements I knew how much inches I had of clearance, now if your using the wheels on the truck already then you could convert those numbers to mm and calculate how much wider of a tire you could run. Not worried about height or sidewall as I didn't think this would cause much difference.

    As far as backspace/offset I used same measurements and looked at various wheels I liked. I knew that I could only go in so far before I would rub with the uca and so far out before I would be getting into the fender well. With the wheels I liked I figured how much clearance I would have from the wheels b/s. for example ..

    my stock wheels were 7.5 in wide with 31mm offset and xx backspace my new wheels were 9in wide with 4.5in backspace 0 offset .. so the new wheel is 1.5 in wider, which told me it would be 0.75 in wider each side, so from my measurements I subtracted the .75, i then took the difference in backspace so for example stock wheel backspace 6 in new wheel 4.5 minus would equal 1.5 in so I added .75 in of clearance to the uca (more clearance because new offset is pushing wheel out) and subtracted .75 from the fender well. With those calculations told me how much wide of a tire I could go with.

    I'm recalling this from memory so It may be very confusing, possibly doesn't make sense and I can't tell you if this actually works. But its what I did .. but it took about a week of number crunching, re-measurements and researching to see what would work.

    You could probably use the that tire fitment calculator somewhere on here and use what measurements you had and see how much wider you could go with your current rim.

    Anyways that's what I did and I don't suggest anyone attempting it, your kids/wife/girlfriend will get very frustrated when you tell them to be quiet while you sit in your garage pulling your hair out, attempting to figure this stuff out on your own. :grrrrrr: Please don't use this as a guide, I'm just telling what i did. i dont even know if this makes sense
  6. mfleetwood

    mfleetwood Epic Member 5+ Years ROTM Winner 5000 Posts

    Too funny! Yes, it makes sense from a logical standpoint but now I am even more convinced you need to not only be an engineer but now probably along the lines of an aerospace one to get it done so that is measures correctly. I can definitely see the hair being pulled out as I read through this. I think rims alone are confusing when you start talking about how to compensate for offset and backspacing, but then trying to come up with a gadget that also takes into consideration max tire size?? If there is a tool out there like that (reasonably priced and easy to use), someone could make a ton of money off of it selling it to people like us.

    Thanks for sharing your story and the warning to stay away from.

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