## Tuesday, January 20, 2009

### Pinewood Derby part 2, axles and wheels

The nails that make the axles have a few bumps on them out of the box. To smooth them out I mount the pointed side of the nail in the end of a power drill leaving the whole surface that will contact the wheel exposed.
I spin the nail and first use a small file to get the bumps out.

That's just not going to be smooth enough for me though so I will then start using sandpaper, keep doing it over and over with a small strip of increasingly fine sandpaper.

If you look in the paint section of the hardware store you will find some pretty fine sandpaper. What I have is 600 grit wet sandpaper. I've heard of finer sandpaper but Home Depot didn't have any. This will give you a pretty smooth nail. I also like to make sure that the underside of the nail head is smooth, that area will also contact the wheel.

There is a theory that if less area of the axle comes in contact with the wheel it will spin faster so some people make grooves in the nail, reducing the area of the wheel in contact with the nail. I'm not convinced this does anything because friction depends on the mass of the car and the coeffecient of friction of the materials which are rubbing and it does not depend on the ammount of area in contact. If you reduce the area in contact, you also increase the load per unit area so it all evens out. Another "myth" that I've seen searching the internet and heard a few fathers talking about last year is that if you raise one wheel up so it doesn't touch the track it will make your car faster. By the same reasoning I say this is false and while I've witnessed fast cars that only have 3 wheels touching, I think it could make your car a little more shaky. So I'm going to stick with what I've learned from Newton.

After getting super smooth axles you should think about the back part of the wheel that will rub against the car, smooth this out and apply graphite here. Also put graphite on the part of the car that touches the wheel here. One more thing that I just thought of this year is the rubbing of the inside of the wheel on the raised part of the track that keeps the car in the lane. I'm going to make sure that part is smooth as well.

Wow, do I sound obsessed or what?
Hope this is helpful to someone out there.

Brandon or Michelle said...

Brandon again...

I commented on your bonus post before seeing this one. One final thought:

Even if friction isn't a function of contact area, the coefficient of friction is a function of roughness.

Jonathan G.Thwaits said...

yes, for sure

petrecon said...

The early (40s?, 50s and 60s)kits were shaped to make derby cars that mimicked the Indy and 1/4 midget cars of the time - tall, narrow body with wooden axle pieces glued into notches in the bottom of the body. The wheels were narrow plastic disks with a tread pattern molded around the outer edge. Nails were used as today for the wheels to run on. The cars bounced along like Chitty-Chitty-Bang-Bang with wheels wobbling. Some of them bounced off the guide strip, and others actually stopped on the incline, slightly cross-wise on the guide strip.