Lead part - cast or turn?

The bounding box isn't the issue. A long-wheelbase car, done properly, will fit in the bounding box.

In packs and districts where they really get into this, they have a separate class for long-wheelbase cars. They're MUCH faster. The wheels go right out to the corners of the bounding box, and to the corners of the car.

Too bad. I've thought about trying an airfoil on top, one that lifts rather than presses down, but I've restrained myself.

-- Ed Huntress

What principle makes that work? Is it not having to use up potential energy by turning it into "flywheel energy" in the non-touching wheel?

Jeff

removal of a friction source - actually 2 friction sources - friction of that wheel against the track - uses some of the energy as heat. friction of that wheel against the axle - more loss due to heat.

Tillman

But you still have the same load, distributed now across the area of three bearings instead of four. Specific friction (friction per square mm of bearing area) goes up...but how much? How linear is the load/friction curve for mild steel against acrylic, or whatever it is?

Hmmm...maybe it's time to bow out and leave it to the kids...

Beeman.

Consider the limiting endpoints where you have some *huge* number of wheels, as many as would fit inside the bounding box. You just know that car's gonna be a dog.

I think the simple answer is easy - you just run a car and time it. Then take one wheel off and run it again. Just have the weight rigged so it will run ok with three. If it really makes a difference then it should be easily visible in the times.

Jim

Well, that's pretty much what we did, on our 16-foot test track. There was no difference that we could measure.

But one car doesn't control for a lot of variables. So I don't know. I *did* test weight placement, wheel camber, toe, etc. No toe; center the weight; camber didn't have a conclusive result; straight-running on a flat floor was the best indicator of speed on the track, all else being equal...which stands to reason, because friction of the wheels against the center strip seems to be the limiting factor, after wheels and axles are fully prepped.

-- Ed Huntress

D'oh... and to think we always shunned the nail slots which provide this capability and sought out a drillpress to drill aligned holes instead!

I recall a friend roughing something out with a pocket knife while his dad was out of town, and doing suprisingly well given the crude results

- perhaps this is why. Having the event have an ability to ignore common parental input can be an advantage...

At the time, I was wondering if leaving the mold flash on the wheels was the reason - started to wonder about evening it up rather than removing it.

(Despite storing the official track at our house, my dad wouldn't use it for test runs - made our own 10 foot test track)

At a bearing store I saw a spraycan of dry film lubricant based on molebdynum disulfide, and I think it has some teflon in it as well.

I bought a can. When the stuff dries, it stays put on the metal, doesn't come off on my fingers. It is seriously slippery stuff!

I cleaned up a 40-year-old Palmgren drillpress vise and lubed the "ways" and screw with this stuff. WOW!!!! I put a knurled knob on the screw in place of the little crossbar, and now I'm thinking about putting a little spinner on the knob! I can run the vise open and closed from end to end just by dragging my sleeve on the knurled knob, though the vise is snug enough that there is essentially zero wobble and play in the vise.

(This little vise is used on the bench for small things, rather than on a drillpress)

Lead melts at about 640F. Some silicones will take that .... but you might have better luck with 30/70 body solder. It's almost as heavy as lead and melts at considerably cooler temperature.

You could cast your weights directly into the wheels using CerroBend. It's not as heavy as lead, but it's pretty heavy -- and it melts at

157F. It's specific gravity is 9.4 vs lead's 11.35. CerroSafe goes at between 168 and 190F, probably about the same density. McMaster Carr carries these products.

CerroSafe was developed specifically because it is safe for kids to handle, as in casting toy soldiers.

No, it's not. It's 37.7 % lead and 8.5% cadmium, worse actually than CerroBend.

The "safe" refers presumably to slight initial shrinkage allowing a duplication casting to be removed without cracking the part you are using as a mold, allegedly it then expands back to the original dimension.

The plain tin/bismuth alloys (example CerroTrue) would be less toxic, but the lowest melting point there is 281 F, so you can't use the water bath and as a result have to be carefull not to overheat.

It might have meant safe temperatures.