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Measuring Tire Diameter

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  • Measuring Tire Diameter

    I've been truing the tires of my 1/32nd scale cars on a Professor Motor tire truing machine. A feature of that machine, and machines like it, is that you can mount the whole axle of a car on the machine and true both tires at once.

    Problem is, those tires don't end up the same diameter. It is a common issue and the only fix I know of is to reverse the axle in the machine and run it again. With luck both tires will end up the same diameter.

    But I don't know if that really works. What I need is a good way to precisely measure the diameters of those tires. A caliper is a great tool for doing such measurements, except that it does apply some compressive force to the tire, and tires are compressible. They are designed to be! So it is not at all easy to get precise and consistent diameter measurements that way.

    An alternate method to measure tire diameters is with a tire gauge. That's a sheet of plastic with a number of holes in it of various sizes. You try to slip a tire through those holes, and find the smallest hole it will go through. In industry this is known as a Go/NoGo gauge. Works okay, but if you want to measure tire diameters to a thousandth of an inch you are going to need a lot of holes.

    Now me, I'm not one to do anything in a conventional manner if I think I can come up with a better one. So I designed a tool to address that whole comprehensibility issue. The idea is to spread around the compression force enough that a calipers can be used to precisely and consistently measure tire diameters. And you know what? That tool works! I can get precise measurements consistent to +/-0.001 of an inch.

    'Course it would be nice to get more precise measurements, but plus or minus a thou' is pretty good. Not too easy to beat that.

    So here are some photos of my tool. How it works should be self-explanatory.

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    And just for giggles, here are the Mark 1, Mark 2 and Mark 3 that I 3D printed while evolving my design.

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    They all worked, but each iteration worked better.

    So does reversing the axle in my tire truing machine produce tires with the same diameter? I haven't done that study just yet.

    Stay tuned...
    Last edited by HO RacePro; January 30, 2022, 04:25 PM.
    Ed Bianchi
    York Pennsylvania USA

  • #2
    Grand idea...I may have to print one myself πŸ‘
    Kevan - Isle of Man
    Life is like a box of Slot cars...πŸš“πŸš—πŸššπŸšœ


    • #3
      My Heath Robinson method is more crude but works OK for me. My objective is to get the two tyres on the same axle within 0.1mm of each other. That is approximately 4 thou FWIW. On a 20mm dia tyre that is like 0.5% difference. Where did I come up with that target measurement? I pulled it out my ear! But to me it "felt" good enough. How do I measure? Crude method using digital calipers:

      Click image for larger version

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      I hold the calipers as near as perfectly horisontally as I can, open the jaws so it is open less than the diameter of the tyre, lay the tyre on the jaws and VERY slowly open the jaws until the tyre starts slipping though. Very quick and easy and I can get repeatability within a few (say 5) 100ths of a mm. That IMPLIES an accuracy of in the order of 0.05mm, which is 2 thou. As long as the two tyres to be used on the same axle is within 0.1mm I am happy.

      And I really have no clue how that compares with tyre prep by competent proxy builders. But if it has to be more accurate than 0.1mm I give up!

      Attached Files


      • HO RacePro
        HO RacePro commented
        Editing a comment
        Well now that is ingenious! I'll have to try it!
        Ed Bianchi

    • #4
      These are toy cars guys
      I totally get if you’re going to do the job do it right!
      But if you take into account the scale accuracy of the car your trying to finely tune you can throw your calipars on the bench and go by eye
      Peterborough Ont


      • #5
        Dave, you've missed the point. We're not interested in scale, what we're trying to do is prevent the car crabbing down the straights. It's for performance, not looks.
        Ed Bianchi
        Ed Bianchi
        York Pennsylvania USA


        • #6
          I use the tire slipping out of the caliper method as well. Works well enough for me.
          Loan Shark aka Matt
          I am Alive because Organ Donation Worked... TWICE
          Lake Country, BC, Canada
          Shark Pit: 38' Routed MDF


          • #7
            Years ago I tweaked the jaws of my RSM 3 (similar to Tire Razor) so both tyres true to near enough the same and my Hudy has an adjustable stop so both machines true accurately enough that I don't have to measure afterwards.
            I use a digital caliper to find the smaller of two tyres to true if I'm using the Hudy and do the same as Alwyn.
            Kevan - Isle of Man
            Life is like a box of Slot cars...πŸš“πŸš—πŸššπŸšœ


            • #8
              Originally posted by Kevan View Post
              Years ago I tweaked the jaws of my RSM 3 (similar to Tire Razor) so both tyres true to near enough the same
              Yep, I did the same so now my measurement is essentially just a verification/quality acceptance test.


              • #9
                Well I tried out my tire gauge on a set of three custom cars I built -- identical except for their gear ratios. I was able to gauge the tires right on the car. No need to dismount them.

                On the first car the rear tires were almost exactly the same diameter. On the other two the left rear tires were about 0.005 inches larger than the right. I sanded those down using their own motors to drive the rear wheels. So now all three cars have matched rear tires.

                Seems like I'll be using that tire gauge a lot. Having matched tires on all my cars makes all the sense in the world.
                Last edited by HO RacePro; January 31, 2022, 10:27 PM.
                Ed Bianchi
                York Pennsylvania USA


                • SuperSlab
                  SuperSlab commented
                  Editing a comment
                  If you use them on urethane tyres would that then be the Ed Gauge, not Paul Gage?

              • #10
                I just got in a new stock of wheels and tires. Yesterday I glued and trued a set of rear tires for a Ford GT Mark 4 model I've just set up with a printed chassis of my latest design. Truing was done on my Professor Motor truing machine equipped with my 'Slideways Shifter'.

                I mounted both wheels on an axle and trued them. Then I measured their diameters using my new tire gauge tool and calipers. I found there was about a 0.010" difference in their diameters. So I put the axle back in my machine, reversed and sanded off a few more thousandths. Checking the diameters again it looked like they were the same or so close I couldn't reliably measure the difference.

                So, at the moment it looks like reversing the axle can help get the rear wheel diameters matched. But I am thinking of ways to modify my machine so that I can get matched tires on the first try. I'm thinking of a way to use shims to better control the vertical stroke of the machine.

                If I make any progress I'll report back here.

                I don't expect matched rear tires to help with cornering. My new chassis have a central pivot so all four wheels can stay in contact with the road regardless of any slight twist in the track, so a mismatch in tire diameter should not cause any wheel to lift. In a corner the rear tires will slip no matter how closely matched they are because the car has no differential. A mismatch in diameter shouldn't have much or any effect.

                The benefit of having matched rear tires should be in the straights, where there should be no power lost to crabbing and slippage. 'Leastwise that's the theory. I hope to race the car at an IHSR event on Saturday. We'll see how it does.
                Last edited by HO RacePro; February 3, 2022, 09:06 AM.
                Ed Bianchi
                York Pennsylvania USA