Tire Razor Indicator Knob

The Tire Razor slot car tire truing machine does not have a depth gauge or stop of any kind. As such, the only way to ensure you are truing tires to the same diameter, which is vital, is to use calipers to measure them repeatedly. The problem with calipers is that the tires are soft and pliable, and so can compress when being measured, making precise measurement difficult.

This indicator knob is NOT a measuring device, but more of a gauge to enable you to repeat the same diameter, whatever that may be. This fits OVER your existing Tire Razor knob. 

If it's not intuitive how this helps, follow this detailed guide for your next set of tires.

1. After you've printed the parts, you'll need to remove the adjustment knob by completely unscrewing it, so that you can install the Indicator Needle, which is a slip/friction fit on the top bar of the Tire Razor.
2. Then, press on the Indicator Knob, and re-install the knob by screwing it down until the bolt meets the top of the axle bracket.
3. Set up your wheels and tires to be trued as you normally do, and install the axle onto the Tire Razor.
4. Screw the knob and down until the tires touch the sanding plate. It's not necessary to count how many rotations of the knob it takes to get the tires down to the sanding plate.
5. Once either tire is touching either side of the sanding plate, either note the current position based on number and ticks, or remove, rotate, and replace the Indicator Knob (which is a friction fit) such that the #1 is as close to the tip of the Indicator Needle, or just to the left of it, as possible. Note this position so that you know if you go past it while truing the tires. Taking a photo with your phone is a quick and easy way to note the position.
6. Begin truing the tires, and take note of which tire starts to sand first if they do not both do so at precisely the same height. It's not uncommon on machines like this for one side to be very slightly lower than the other. That's the side you want to track the diameter of, if final diameter is important to you.
7. True the tires until you reach your intended diameter, or until one of the tires is completely true. It should be clear that it has been sanded smooth both all the way around the tire, and all the way across.
8. While truing, pay attention to the position of the Indicator Knob and if you pass the original starting position, remember or note that down.
9. Once you are happy with the true-ness/diameter of the first tire to be completely true, note the position of the Indicator Knob. Take a picture if you need to. :)
10. Reverse/raise the axle bracket slightly. No more than one full rotation of the knob should be plenty.
11. Now, remove the axle from the machine, flip it over and put it back on. Now we're going to make the other wheel match!
12. Retuning the Indicator Knob to ALMOST the same place as the first tire ended, continue truing the second tire. The first tire, now on the “high” side of the machine, should not touch the sanding plate much, if at all, but keep an eye on it if your truing method requires a coolant or something to prevent overheating the rubber.
13. Continue truing the second tire until the Indicator Knob matches the position you stopped at for the first tire. If the second tire is true all the way across and all the way around, then you're done!
14. If the second tire still needs some sanding to be perfectly true, you'll need repeat the process from the start of the first tire. Note the position of the Indicator Knob when it's true, flip the axle around again, and true the first tire down to the same diameter, knob position, as the second one needed to go.
15. In the end, both tires should be true all the way across, all the way around, and be exactly the same diameter.

To Print the Indicator Knob

• If you have a printer capable of multiple colors, download the black and white parts and set them up however you see fit. For my tests, I found the best quality was when I had the numbers facing up (with a little support for the center of the knob pocket). I then finished it off with some light sanding to remove any strings or boogers and get it to a smooth appearance.
• If you have a single color printer, you can print the one piece knob, and change filament once you reach the height of the numbers and tick marks. For best results, your printer and material needs to be on point with great layer adhesion and low/no stringing.
• The Indicator Needle should print as provided with no supports or brim/raft.

If you'd like me to print a set for your Tire Razor, find me on the slot car forums as Mr.Flippant or email me at [email protected].

Enjoy!

Update - 5/11/2023 - It was revealed to me that the Tire Razor also came with a similar knob but which was slightly smaller, and had six points rather than seven. I've added a version to fit that knob, indicated by the “6” in the file name.