Machined Wheel Preparation

Before you get any farther, read your wheel rules! The following section describes techniques which are guaranteed to be illegal in many Packs and Districts and will get your son's car disqualified if you use them.

Ok, you've been warned. Now, what is the big deal with machining wheels? The primary purpose is to minimize the rotating mass. The rules say you can't reduce the diameter of the wheels. Ok, that's fine. We will remove material from the inside of the tread and sidewall areas, instead. Look at the two photos below. The "before" shows the mass of a set of stock wheels as a bit over 12.5 grams. The "after" wheels weigh in at just under half that.

Stock Pinewood Derby Wheels Modified Pinewood Derby Wheels

So why does the mass of the wheels matter? After all, I told you we will bring the total mass of the car right back to 5.000 ounces before we're done, right? A physicist would mumble something about "rotational moment of inertia". In simple terms, it takes energy to "spin up" the wheels as the car starts. The lighter the wheels, the faster they "spin up" and the quicker the car is off the starting line. The difference is dramatic. Although I haven't quantified it, an otherwise identical car with lightened wheels seems to "jump" off the line and gets a visible head start in the first couple of feet of the sloping part of the track.

What is the general plan? The tread of the stock wheels starts out at about 0.090" thick. We will reduce that to 0.035". The sidewalls are molded at about 0.109" thick and we'll take that down to 0.035" as well. That pulls half the mass out of the wheel, and mostly from around the perimeter where it matters most from the physicist's viewpoint. (The farther out from the center the mass is, the more energy it takes to spin it up.)

Tools and supplies for machining and preparing wheels:

  • Safety goggles
  • Precision metalworking lathe
  • Set of gauge pins
  • Collet sized to grip the O.D. of the wheel
  • Sharp pointed lathe cutter
  • Custom inside-cutting tool
  • Plunger-type dial indicator mounted on vertical holder, with narrow plunger tip
  • Numeric-controlled milling machine programmed to cut a "bolt pattern" of eight holes
  • Chiller (at work, we have what is called a "vortex tube" that takes shop air and by a vortex action, spits out chilled air at the other end) I suppose you could also use a stream of cutting fluid, but the lathes at work aren't set up with fluid, so we use air instead.
  • All of the supplies from the "simple wheel preparation section", except the 220-grit sandpaper

Process for machining wheels

  • Select a gauge pin which is a light press fit into the axle hole of the wheels (You may find that you have to go up or down 0.001" or 0.002" from wheel to wheel because of cavity-to-cavity variability in the wheels.) It is a very good idea to practice this process on a couple of scrap wheels before starting on your "good" wheels selected for smooth running!
  • Chuck the gauge pin in the lathe and press the wheel onto the pin, with the hub (inside) end out away from the chuck.
  • Spin the wheel and bring the pointed tool down to kiss the O.D. of the tread.
  • Taking very light cuts (no more than 0.0015" per cut), turn the O.D. just enough to smooth the tread. This will typically be about 0.007" to 0.010" total removed. You must not appreciably reduce the diameter. The basic rule is not to do more than the "simple wheel preparation" does with sandpaper. Set the diameter zero on the lathe and cut all of your wheels exactly the same amount.
  • Now pick up the "simple wheel preparation" at the point of using the 400-grit wet/dry sandpaper, through polishing the O.D. of the tread.
  • Complete these steps on all of your wheels. Be sure to make a few extras in case you wreck any in mid-process!
  • Replace the small chuck with the collet sized to hold the O.D. of the wheels.
  • Set the custom tool up to plunge into the wheel end-on
  • Chuck a wheel in the collet with the open (inside) side facing out.
  • Set up the chiller to blast cold air on the tool/wheel interface. If you don't, the friction will melt the wheel to a glob of useless goo.
  • Position the tool to just start cutting and then start taking small amounts off the inside of the tread until you reduce its thickness to the desired 0.035". Set the diameter indicator on the lathe's numeric indicator to zero at this point.
  • Now start plunging down into the sidewall until you reach that same 0.035" remaining thickness. This requires pulling the tool out, taking the wheel out and measuring it with the plunger-type dial indicator, and once you get to the right thickness, setting the zero on the numeric indicator on the lathe.
  • You now have the lathe "zeroed" at your desired thickness for both tread and sidewall. Chuck up another scrap wheel and verify that you still get the right thicknesses before cutting "real" wheels.
  • Now cut your good wheels, being careful to take light cuts, back the tool off to break the gummy chip which develops, and cut slowly and smoothly down to the zero point you set.