It’s easy to make a tire balancer for your slot cars.
You will need some tools including a hand drill with adjustable chuck, a Dremel with a cut off tool, a Sharpie and a small vise with a 2 inch throat. Don’t forget safety glasses.
First, start with a shopping spree at McMaster-Carr (or you favorite full service mechanical supply store). Buy the following:
Neodymium-iron-boron magnets (qty 2), part number 58605K65 or something similar.
If you need 2mm, 3/32 or 1/8 axles to grind you can buy them here as well. Search for drill blanks.
If you don’t already have a small vise, you can also buy one here as well (see 5226A5 for an example or check out Harbor-Freight for cheaper alternatives).
Once your parts arrive you can start building your tool. Remember to put on your safety glasses whenever using rotating power tools.
Using the Dremel with a cut off disc, cut the axle to working length (approximately 1-1/2 inches is good depending on your vise).
The next thing to do is to grind points on the axle. To grind the points, chuck the axle into the drill chuck as far as it will go while leaving about ½ inch protruding out. Hold the drill with one hand and start the drill. With the other hand start the Dremel. Grind a 45 degree point on the end using the side of the cut off disc. Do not press too hard or you will break the cut off disc.
Once you have a suitable point turn off your tools and flip the axle around in the drill chuck to the other side. Repeat until you have a point on both sides of the axle. The hard part is done!
Take the 2 magnets and polarize them South to North. Note that it may be easier to separate the magnets if you place a plastic spacer between them.
Next open the vise jaws (at least 1 inch). Keeping the polarized magnet orientation attained previously, place a magnet on the movable jaw and the other magnet on the stationary jaw. Adjust the jaws so that your axle has about 1/16th inch clearance when allowed to free float between the magnet faces. Give it a try. The axle should hover and spin freely. Some fine adjustment may be required?
To actually balance tires, gears, etc., place a single item onto the axle and let it spin. Note how it orients itself where the heaviest part spins and oscillates to the bottom. Mark the top of the item for reference and check it again. You will likely see a pattern where the item’s orientation is similar at the end of each spin. You will need to either remove weight from the heavy area (where the part orients downward) or add weight to the upper area where you have previously marked. The part is balanced when it does not spin with the same orientation outcome each time (random orientation is achieved). How to best achieve this is left as an exercise for the reader……

Thanks, Randy. I'd try this just to see the things spin around.