Ideas for a mini Pinewood Derby wheel "lathe"

[vitamink]

Hey folks,

So I'm a Pinewood Derby enthusiast. One of the things I'd like to contribute to the hobby would be an inexpensive device to easily improve the radial runout of the stock wheels sold by the BSA. Currently, you need an actual lathe to accurately turn wheels with any reasonable degree of success. There's a hand tool available, but it's quite difficult to use with any degree of success...definitely not something a kid can do!

So the world of slot cars has inspired me a bit, and I see that there exist several wheel lathes for these cars...some professional, some homebuilt. For example:


The typical way of working a Pinewood Derby wheel is to mount the bore on a pin with a push (interference) fit, which is then turned in a lathe. I imagine for this project I would either employ a machine turned shaft with the end turned down to the appropriate pin size (maybe .098" or .097"), or a shaft with an ER11 collet chuck on the end, holding a pin gage of the appropriate thickness.

My immediate problem that I'm trying to solve is: What can I use to hold the shaft that will let it turn with very little radial runout (I'd like the finished wheel to be under .002" of runout), and will resist deflection when pressure is put on the wheel from the cutting or sanding tool employed to turn down the radius?

I have tried a pillow block bearing, but it had a lot of "play" in it, and didn't hold up to even the smallest amount of deflection. Can somebody suggest a better mounting solution?

If I can give any more details of what I'm trying to do, please let me know!

 

[Tony Wells]

To get around having to sort through different "spindle" sizes, make up a axle assembly with opposing cones that will grip and drive the wheel. You can thread the outboard end and use a nut and washer to capture the wheel. If then you need support on the outboard end, make a correct height block with a bearing in it that is a little smaller than the shaft. The shaft in turn should have a male point, similar to a lathe center, but run only on the edge of the bearing in the block. How you mount the block will determine how much pressure you can get to push on the pointed end of the "spindle" and be assured of little to no runout.

 

[vitamink]

To get around having to sort through different "spindle" sizes, make up a axle assembly with opposing cones that will grip and drive the wheel. You can thread the outboard end and use a nut and washer to capture the wheel. If then you need support on the outboard end, make a correct height block with a bearing in it that is a little smaller than the shaft. The shaft in turn should have a male point, similar to a lathe center, but run only on the edge of the bearing in the block. How you mount the block will determine how much pressure you can get to push on the pointed end of the "spindle" and be assured of little to no runout.

Hi Tony, thanks for the response!

I'm having a little trouble, though, visualizing the setup that you are describing. Is it possible that you could point me to a diagram of something similar?

 

[BGHansen]

The BSA sells an arbor (or did ~14 years ago when my son raced) that you can mount wheels to. It's like the arbors Dremel sells for cut-off wheels. You may be able to mount it in a drill motor and run it at high speed, creep up on a block of sandpaper. Don't know if the rules have changed or not, but back in the day reshaping the wheels was not allowed, only truing them up.

Pinewood derby racing is basically an exercise in the elimination of friction. The maximum possible speed can be calculated by converting potential energy ( mass x gravitational constant x height) into kinetic energy ( 0.5 times mass times (velocity ^ 2)). Mass cancels out (like Galileo dropping 2 cannonballs of different weight which hit the ground at the same time), so the formula boils down to "Velocity = Sqrt ( 2 X 32.2 ft/sec^2 X height)". A track with a 5 foot tall starting block has a maximum theoretical speed of 17.94 ft/sec or 12.23 mph.

Our son won a lot of races following the rules. We also ran an open race that we won multiple times (no rules). Typical "within the rules" tricks are to work the nails near the head and polish them. They had flash near the head which could be needle filed off, then polished. We trued up wheels on the lathe with the BSA arbor. Then ran the nails with wheels mounted in a drill chuck up to speed and added graphite. Once the graphite was worked in, we spun them up to speed in the drill motor, released the drill motor trigger and timed how long that particular wheel/nail spun until it stopped. We'd get wheels that would spin for anywhere from 20 seconds to about 75 seconds. Naturally, the 75 second wheels/nails went on our son's car.

Another "trick" is to adjust the camber on one of the wheels so it isn't touching the ground. Idea is there's less rolling resistance with 3 wheels touching instead of 4. Also camber the tires so just the edge is touching.

Another one is to carefully push the car on a flat surface (we used the school's gymnasium) and see how straight the car runs. Turn the nails and/or put a little pressure forward/backward to true the steering. Hitting the track bumpers equals friction.

Yet another "legal" trick is to bias the weight to the rear of the car. We biased our son's cars to about 4.5 oz. on the back wheels and 0.5 oz. on the front. The idea was to move the center of mass as far back as possible on the car. When the car is in the starting blocks, the center of mass will be higher than if in the middle, so it's effectively at a higher height than a car with the weight at the front.

For the "open race", we machined the wheels to a knife edge for less rolling resistance. We also used thrust bearings on either side of the wheels. Also put a rare earth magnet in the front of the car. Our starting block was a pivoting dowel with 16D nails sticking up as stops. Our "open" cars were literally thrown down the track at the start; the cars stuck to the nail and when the dowel was spun it gave the cars a good pull for a push start. Totally "legal" in an open race; sure perplexed a lot of dads when they'd see our cars win by large margins.

We never lost a heat on the open races, think my son's cars lost one or two heats but ended up the overall winner every time. Great fun to work with "the boy"!

Thanks for posting and bringing back old memories!

Bruce

 

[vitamink]

Hi Bruce, thanks for your response!

The BSA sells an arbor (or did ~14 years ago when my son raced) that you can mount wheels to. It's like the arbors Dremel sells for cut-off wheels. You may be able to mount it in a drill motor and run it at high speed, creep up on a block of sandpaper. Don't know if the rules have changed or not, but back in the day reshaping the wheels was not allowed, only truing them up.

I've actually got one of these mandrels, it looks like this:

The problem I run into is that the runout of my drill press (or hand drill) is already worse than the runout of a stock wheel. I'm shooting for getting the wheel down to .001" - .002" of radial runout. I think my drill press measured .006" last time I checked it. My hope is that I can build a contraption that will let me rotate a wheel with a lot less runout than that. My current problem is finding a mount for a rotating shaft.

Pinewood derby racing is basically an exercise in the elimination of friction. The maximum possible speed can be calculated by converting potential energy ( mass x gravitational constant x height) into kinetic energy ( 0.5 times mass times (velocity ^ 2)). Mass cancels out (like Galileo dropping 2 cannonballs of different weight which hit the ground at the same time), so the formula boils down to "Velocity = Sqrt ( 2 X 32.2 ft/sec^2 X height)". A track with a 5 foot tall starting block has a maximum theoretical speed of 17.94 ft/sec or 12.23 mph.

Our son won a lot of races following the rules. We also ran an open race that we won multiple times (no rules). Typical "within the rules" tricks are to work the nails near the head and polish them. They had flash near the head which could be needle filed off, then polished. We trued up wheels on the lathe with the BSA arbor. Then ran the nails with wheels mounted in a drill chuck up to speed and added graphite. Once the graphite was worked in, we spun them up to speed in the drill motor, released the drill motor trigger and timed how long that particular wheel/nail spun until it stopped. We'd get wheels that would spin for anywhere from 20 seconds to about 75 seconds. Naturally, the 75 second wheels/nails went on our son's car.

Another "trick" is to adjust the camber on one of the wheels so it isn't touching the ground. Idea is there's less rolling resistance with 3 wheels touching instead of 4. Also camber the tires so just the edge is touching.

Another one is to carefully push the car on a flat surface (we used the school's gymnasium) and see how straight the car runs. Turn the nails and/or put a little pressure forward/backward to true the steering. Hitting the track bumpers equals friction.

Yet another "legal" trick is to bias the weight to the rear of the car. We biased our son's cars to about 4.5 oz. on the back wheels and 0.5 oz. on the front. The idea was to move the center of mass as far back as possible on the car. When the car is in the starting blocks, the center of mass will be higher than if in the middle, so it's effectively at a higher height than a car with the weight at the front.

For the "open race", we machined the wheels to a knife edge for less rolling resistance. We also used thrust bearings on either side of the wheels. Also put a rare earth magnet in the front of the car. Our starting block was a pivoting dowel with 16D nails sticking up as stops. Our "open" cars were literally thrown down the track at the start; the cars stuck to the nail and when the dowel was spun it gave the cars a good pull for a push start. Totally "legal" in an open race; sure perplexed a lot of dads when they'd see our cars win by large margins.

We never lost a heat on the open races, think my son's cars lost one or two heats but ended up the overall winner every time. Great fun to work with "the boy"!

Thanks for posting and bringing back old memories!

My pleasure. My son took first place in his Pack and the Districts this past year. We used a lot of the techniques you mentioned, with a few variations:

- We polished the axles to a mirror finish with increasingly finer grades of sandpaper. We also polished the bores of the wheels with pipe cleaners and plastic polish.
- In addition to raising a wheel, we also configured the car to run in a "rail rider" alignment. This essentially amounts to putting negative camber on the rear wheels and positive camber on the touching front wheel, as well as as a small amount of toe-in. The upshot is that this allows the heavily loaded rear wheels to never touch the guide rail, while the front wheel rolls along it, giving you a straight path down the track.
- Instead of graphite, we used Krytox oil. Combined with a slick coating applied to the wheel bores and axles, this provides a truly excellent lubrication that lasts a lot longer and outperforms graphite.

Good times!

 

[Mondshine]

Hi Bruce, thanks for your response!



I've actually got one of these mandrels, it looks like this:

The problem I run into is that the runout of my drill press (or hand drill) is already worse than the runout of a stock wheel. I'm shooting for getting the wheel down to .001" - .002" of radial runout. I think my drill press measured .006" last time I checked it. My hope is that I can build a contraption that will let me rotate a wheel with a lot less runout than that. My current problem is finding a mount for a rotating shaft.



My pleasure. My son took first place in his Pack and the Districts this past year. We used a lot of the techniques you mentioned, with a few variations:

- We polished the axles to a mirror finish with increasingly finer grades of sandpaper. We also polished the bores of the wheels with pipe cleaners and plastic polish.
- In addition to raising a wheel, we also configured the car to run in a "rail rider" alignment. This essentially amounts to putting negative camber on the rear wheels and positive camber on the touching front wheel, as well as as a small amount of toe-in. The upshot is that this allows the heavily loaded rear wheels to never touch the guide rail, while the front wheel rolls along it, giving you a straight path down the track.
- Instead of graphite, we used Krytox oil. Combined with a slick coating applied to the wheel bores and axles, this provides a truly excellent lubrication that lasts a lot longer and outperforms graphite.

Good times!

 

[Mondshine]

I was a Cub Scout leader years ago in the Metro Detroit area. Our pack had CNC machined Pinewood cars designed in the wind tunnels of GM, Ford,Chrysler. Dads wouldn't even let the kids touch the cars until it was time to register.
Sometimes, the fastest cars were so sloppily (kid) built that only 3 wheels touched the ground, making for some very disappointed dads.
We set up an "Unlimited Class" so dads could race (in their own name).
As I remember, friction between the axle nails and the wheels was the killer. That's why the 3 wheel cars were so fast.
Anyway, have fun with it, Mondshine

 

[vitamink]

I was a Cub Scout leader years ago in the Metro Detroit area. Our pack had CNC machined Pinewood cars designed in the wind tunnels of GM, Ford,Chrysler. Dads wouldn't even let the kids touch the cars until it was time to register.
Sometimes, the fastest cars were so sloppily (kid) built that only 3 wheels touched the ground, making for some very disappointed dads.
We set up an "Unlimited Class" so dads could race (in their own name).
As I remember, friction between the axle nails and the wheels was the killer. That's why the 3 wheel cars were so fast.
Anyway, have fun with it, Mondshine

Friction between wheels and axles is a huge piece of the puzzle. Interestingly enough, running on 3 wheels as opposed to 4 isn't a friction saver, since surface area is not a factor in friction calculation. The real benefit you get is that you've reduced the total amount of rotational mass, so the car has 1/4 less energy to invest in getting the wheels spinning, and can instead use that energy to create forward motion.

 

[Franko]

The cars I "helped" make won both years I was involved (20 years ago). I used the 3-wheel trick. The center of mass was adjusted to 1/3 forward from the back and it was the maximum weight. I polished and smoothed the axle nails with fine sandpaper and jeweler's rouge and lubricated with graphite. I turned the nails and wheels on a drill press (wet sanding the wheels) and put just a small amount taper so they rolled on a center edge. Tuning it to run straight was also part of the plan. I think I slicked up the outside of the wheels to reduce friction on the guide rails.

A lathe would have been great, but I didn't have one then.

He was my best friend's son, so I had him come over a couple hours before work started and we had a little science lesson with a white board, as I explained and illustrated all the principals we would employ to make it faster.

The third year, my best friend's boy wanted to win the design prize without much regard for speed. We won that one by carving the body to look exactly like a flint arrowhead using a router and core box bit to dig out chip shapes, and painted it with two shades of light gray wiped to make it two toned and glossy varnish to look just like flint. I attached a rectangular block on the underside for mounting the wheels.

I used the supplied block of wood by sawing the front half of it to a pointed shape and gluing the two wedges I sawed off to make the whole shape into an arrowhead shape. It looked so much like a big flint arrow head that people couldn't believe it was made of wood until they picked it up.

 

[Superburban]

And all these years, I thought it was supposed to be a competition for the kids.