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100 RPM Dry Sanding Tires

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  • 100 RPM Dry Sanding Tires

    I have a Professor Motor Tire Truing Station -- one of the many brands that copy the basic features of the Tire Razor brand stations. Well, at least it used to. I've made a number of modifications to my station. It is more a Bianchi/Professor Motor hybrid these days.

    My latest mod has been to replace the comes-with motor and pulley with a 100RPM gearmotor and a custom-printed urethane pulley.

    So why? 'Cause I have good reason to think the stock motor/pulley drives the axle/wheels far too fast. I've ruined several sets of tires by overheating them despite being -- I thought -- very conservative in the rate at which I lowered them into the abrasives. Part of that reason may be as the tires get abraded they heat up and expand, increasing their contact with the abrasives and heating up still further -- a positive feedback loop that never ends well. The result is the rubber becomes hot and gummy and starts to grain and shred. Ugly.

    I have experimented with wet sanding at 100RPM and not have the success I expected. I've done it with a water/dishsoap solution as coolant, and Mobil 1 motor oil too. For reasons I haven't fully doped out the results have not been encouraging. But that's not what I am here to talk about.

    The gearmotor I installed was purchased off of Amazon for US$15. I picked on 100RPM based on a Scientific-Wild-Ass-Guess. The size and shape of the gearmotor was a pretty good fit, and it was designed to operate on 12 volts DC. The motor shaft was larger than the original Professor Motor motor, so I ended up printing a new TPU pulley designed to fit. I had to drill new holes in the mounting plate, but otherwise it was a simple bolt-on fit.

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    As far as actually sanding a set of tires with the new drive, really, no drama. Most importantly the tires stayed cool to the touch while still sanding up true.

    The multimeter you see in the video is measuring the amp-draw of the motor at 12 volts DC. I was lowering the axle/wheel assembly just enough to increase the amp-draw by 10 to 20 milliamps at a time. That seemed to work well.

    So how true were the tires? To be honest true like I have never seen before. Normally I'd expect to see something under a thousandth of an inch total indicated runout as measured by a dial indicator skimming the OD of the tire. But no visible runout at all? I figured something was wrong. Uh-huh. The indicator was working fine. The tire was just that round and concentric. Here, see for yourself.

    Kinda spooky, yes?

    One advantage of re-powering the station is the power supply does not need to be nearly as beefy. The original spec for the Professor Motor power supply was a minimum of 3 amps. I can get by easily with a half an amp.

    I guess I should also mention that the revs at the axle is less than 100RPM. The driven pulley is larger than the drive pulley. I haven't bothered to work out the actual speed. Anyway, my model railroad power supply has a variable voltage output, so I can dial in a very wide range of speeds, if that makes a difference.

    By the way, the shifter moving the abrasive plate side-to-side is my own design. A lot simpler and compact than some you'll see. I've sold a few of those already as kits for US$30.

    My next job is to create one or more additional abrasive plates with finer-grit sandpaper. The objective is to polish the surface of the tires to get even better grip. Stay tuned.

    Last edited by HO RacePro; June 5, 2022, 09:55 AM.
    Ed Bianchi
    York Pennsylvania USA

  • #2
    It's hard to believe turning that slow would cut anything but the proof is in the video. It has to take forever to cut the tires at that slow speed though.
    Saginaw Valley Raceway
    Only Rule: Just enjoy who you are racing with.


    • #3
      Originally posted by dw5555 View Post
      It's hard to believe turning that slow would cut anything but the proof is in the video. It has to take forever to cut the tires at that slow speed though.
      What makes you think it has to be spinning fast to cut? If anything too high a speed destroys the grinding medium, notice how sandpaper lasts much longer at lower speeds.
      Kevan - Isle of Man
      Life is like a box of Slot cars...🚓🚗🚚🚜


      • #4
        Ed, in the first part of your post you mentioned not getting results you expected. However, all the results you are showing look really good. Is there something else going on?

        It doesn't surprise me at all that slowing things down helps a lot. I have a hudy, not a tire razor, but in general making it easier for the material to get clear of the work is going to be a benefit. With that razor style machine, you've reduced the amount of material that must be cleared on each stroke dramatically.

        Did you measure the speed of the stock setup? Just curious if you slowed things down by a factor of 10 or 100?

        Either way, all interesting stuff.


        • HO RacePro
          HO RacePro commented
          Editing a comment
          Wet sanding urethane tires with water/detergent at low speed I was still getting graining and strings of rubber. Wet sanding those tires with motor oil and 2000 grit sandpaper did not seem to be improving the concentricity of the tires. I need to play/experiment/futz a lot more to figure out if wet sanding is useful, on urethane or rubber tires. It should be, but I want better results to prove it.

          I'm going to take some speed measurements on my dry setup. I know the gearmotor is rated at 100RPM. I don't know what the speed of the stock Professor Motor motor is, but I can find out. If it is similar to the gearmotor's motor it'll be something like 3500RPM. So a reduction of about 35 to 1. TBD.

      • #5
        Something else I should mention...

        Slowing things down that much means me and my workshop are no longer being dusted with little bits of black rubber. That's no small thing. I actually bought a shop apron specifically to keep the crud off my clothes while truing tires.

        Instead the shavings accumulate in relatively tidy little piles behind the tires. So much more civilized.
        Ed Bianchi
        York Pennsylvania USA


        • #6
          IMHO, adding lubricant to sanding only does two things. 1) help control temperature and 2) help the sanding medium not clog.

          You've slowed things down enough, I'd be surprised if heat was still an issue. If the dust is releasing from the sandpaper and getting out of the way, then again if say this isn't an issue. I don't know that I would expect additional gains.


          • #7
            Ed, I was intrigued by your comment to cdub where you said "Wet sanding urethane tires with water/detergent at low speed I was still getting graining and strings of rubber". This is probably in line with your post of a picture of polished tyres in your Wet sanding thread where I commented on the fact that it did not look as smooth as I would have expected. I have really not been able to get to slot cars over the past couple of months but for comparison purposes: the following is typical of what I get with urethanes on a Tire Razor going from 180 grit (dry) -> 320 grit (wet) -> 600 grit (wet). And as explained in your "Wet sanding" thread, all I am doing for wet sanding is placing a drop of water on the wet/dry paper and replenishing every minute or so as it gets moved off: the tyres stay wet throughout the process but with zero spray or splashing

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            This photograph is is obviously taken very close up at very good resolution/sharpness with a decent camera and a flash so it greatly emphasises/magnifies any marks in the tyre. Even though I did this pretty quickly (maybe 10 minutes each of 320 and 600 polishing, including time taken to smooth the rounded edges to a degree) as I wasn't too fussed about a perfect job on a Scaley Lotus 49, I can assure you with the naked eye it is VERY smooth. I run my Tire Razor at around 5V when doing this so I am assuming the wheels are turning at a few thousand RPM and I really have none of the issues you mention.

            Just for comparison purposes: this is what an out of the packet, glossy, new PGT urethane looks like photographed under the same conditions. And we all know to the naked eye those look like glass out the mold....

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            And I took the liberty of enlarging the image that you posted in the aforementioned thread:

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            Do you perhaps have better photographs of tyres done with your 100RPM process as, with the above images, I am struggling to see any real benefit to the effort involved compared to just doing some polishing on a Tire Razor? And that cannot be right: surely it should be noticeably better with proper slow polishing?


            • HO RacePro
              HO RacePro commented
              Editing a comment

              I am impressed by the results you've achieved with wet polishing on your Tire Razor! I obviously have some work to do. You've proven it is possible to truly polish urethane tires after truing. So now I need to figure out how to match your results. I need to work on that.