tjspiel

If any of you have, or have had kids in cub scouts you probably know what a pinewood derby car is. It's basically a hunk of pine a couple of inches wide and seven inches long. It has plastic wheels which use nails for axles.

They come in kits a few weeks before race day. You can shape the wood however you like though the rough dimensions need to remain the same in order for the car to fit on the track. The track is a multi-lane hill with a long flat section at the bottom. My son's pack has an 6 lane track with electronic timing.

This is serious business. There are several websites devoted to tweaking the performance of these cars. Our local hobby store has half a wall dedicated to pine car plans, tools, enhanced (though not necessarily legal) parts, and accessories.

Anyway, it's interesting how the performance tips compares to cycling. Since all the propulsion is provided by gravity, you want to add weight to these cars, though the value of getting the weight up to the maximum allowed (5 oz) isn't quite as important as it's made to seem.

One thing similar to bikes is the focus on wheels. Some people sand or shape the wheels so that the contact patch is minimized. The legality of this varies. It is legal to only have 3 wheels touching the track.

The other big thing and I'm surprised I don't see the equivalent discussed here very often is the friction between the wheel and the axle. This is the major focus of most people concerned with making faster derby cars.

We often talk about performance here but very rarely do you see anybody recommending better hubs, bottom brackets, rear derailleurs, or chains. I'm curious as to why that is. Is it expense? Or is it that there is really little difference between a cheap hub and an expensive hub in terms of friction?

kegoguinness

Interesting post! I am just now reading a book called "Bicycle Science". The main point I get from the book w/r/t hub friction et al is that it is such a relatively small figure as compared to wind friction (aerodynamic drag) and rolling resistance that it doesn't make a big difference in overall power requirements of the rider to maintain speed, whatever.

The book notes that the larger the wheel, the less that hub friction tends to be an issue. Thus, with a tiny pinewood wheel, hub friction would play a much larger role than with a bicycle wheel, especially a full-sized wheel that is larger than a car tire. I an't no scientist so take for what it's worth, but you might find the book interesting.

tjspiel

That is interesting and would explain why the pinewood wheels they sell in the hobby store are slightly larger than the stock wheels. Cheaters !

It make sense too since a larger wheel covers more distance in one rotation than a smaller wheel.

I knew 700c wheels were better than 26 inch ;-)

demoncyclist

that is why the wheels have to be the BSA official ones from the kit to be legal.

Onuts

On the underside of the nail head (axle) is a miniture casting ridge that the wheel will run up against. Buff it off with a dremel. You're welcome.

tjspiel

Thanks.

Our family's venture into pinewood derbies was preceded by some friends of ours. Further my son's den has a history of taking the top spots in the pack and having several cars move on to districts so I've gotten plenty of tips from people wanting to maintain that tradition.

This is actually our second year. My son placed 2nd in the pack this past weekend, - same as last year. He placed high enough in the districts last year to compete at the Mall of America but that's as far as he got and probably the same will happen again ;-)

It's one of these difficult parenting choices. I want him to succeed but I want it to be fun too and if he does well, I want it to be because of work HE did. There's only so much axle polishing he's willing to do and that's fine with me.

bdcheung

Grind up some graphite.

Put the axle/nail into an electric drill so that the head sticks out.

Apply some of the graphite to the exposed portion of the nail, where the wheel will eventually be.

Grab a shoelace and drape it over the nail - this will act as a buffer.

Run the drill, buffing the graphite into the nail.

jsharr

I am our pack's pinewood derby chairman. Axles polish and alignment, wheel hub polish, burnished graphite on axles and max weight placed low and to the rear.

Last year, the car my son built was unbeaten. This year, he lost a few races and finished third, most likely because I shared all our build tips with any scout that asked.

demoncyclist

M<y son is a Tiger, so this was his first year. He designed the car (with just a few tweaks from me), we had a workshop night (a bunch of band saws available for cutting, and other help for the less than handy parents) so the cutting was handled by adult hands. He did the sanding and painting ( I showed him how masking tape works). The only thing I did was prep the axles and get rid of the molding burrs on the wheels. He honestly wouldn't have cared if the thing didn't even roll. He just wanted the car.

ItsJustMe

The rules in our pack were very strictly enforced. No modification of stock tires beyond removal of any casting burs. No polishing of the axles; only removal of the stamping burr was allowed. Basically, if you did anything beyond removing burs, working in dry graphite powder, and aligning the wheels, you were DQ'd.

There is hardly any difference (maybe none) between cheap and expensive bicycle hubs, the main difference is weight and durability.

Some of the same performance items apply though; with pinewood, you'd go a lot faster if you shaped the wheels to minimize the contact area. Cyclists do the same, they run very high pressure, very narrow wheels to reduce rolling friction.

Some things don't really apply. For instance, on pinewood, you want to be as heavy as possible within the rules, because your only interest is going downhill as fast as possible. With bikes you have to go up the hill first.

jsharr

Good job dad!!!! This is how it is supposed to work. Each year your son will add a bit of knowledge to his skill set and by the time he is a webelo, you will be an innocent bystander!!!

demoncyclist

I'm the new Asst. Cubmaster (haven't even finished sewing the patches on my uniform), so I will never again be a bystander, and I'm way too old to be innocent. The best I can do is stay out of the fray.

ItsJustMe

My son's first three years worth of cars (Tiger in the back; note the fins). He did most of the grunt work, I did the rough shaping after he designed it since I used pretty sharp wood tools to gouge out the rough shape.

We mainly went for looks since speed seems almost random. The newest one (in the foreground) won best design and best paint across the pack.

Honestly, if you just want to win, you enter a wedge; a wedge will win every time but it's butt-ugly. Where's the fun in that?

tjspiel

Nice cars !

It looks like the designs of the newer cards did incorporate some performance enhancements (intentional or not). On those cars the edge of the wheels won't rub as much against the car body.

tjspiel

How can you tell if the axles have been polished?

tjspiel

I wonder if there's a correlation between scouting (or interest in the outdoors) and bike commuting.

EGUNWT

Resistance in bearings increases *mostly* linearly with speed. So your drag from your bearings etc is twice as high if you're going twice as fast. Some bearings are a little more, some are a little less.

Resistance with respect to wind goes up with the square of your speed. So if you're going twice as fast, your wind resistance goes up by a factor of 4.

Once you get the bearings up to a "good enough" state, the cost of a significant improvement is very expensive, compared to, say, shaving your legs, getting a slicker set of kit, getting rid of the spare tire, etc.

The fun thing about pinewood derby is the scale. You can make a lot of really inexpensive changes to trivial things that will give you a little bit of an edge and they add up. The same changes on a bicycle are more expensive just because of the scale, so things tend to add up faster. A really great aerodynamic skin (paint) for a pinewood derby car would probably cost about 30 cents. A new set of kit, aero bars, and one of those sperm helmets would probably run you a lot more than that .

demoncyclist

One neat trick I found was to put a drop of superglue right where the hub of the wheel is going to touch the body, then while the glue is setting up, dust some graphite into it, then polish the graphite after the glue dries. That way the hub is rubbing against the slickest possible surface.

RogerB

Better yet, install the axles with negative camber so the wheels never touch the body.

This year, a "formula" car won pack overall. (Yes, it was my son's car.) He's my second scout. My older boy did the derby 5 times and placed overall the last three years--I think with two firsts.

There's nothing random about speed. The random part is how much speed everyone else puts into their cars.

As for who does the work...I think it depends on the kid. Regardless I try to use it as a physics teaching tool, as well as an object lesson in prioritization. (You want speed or looks? We don't have time for both.)

A wedge is not necessarily always the winner, but it is just as competitive as any other properly designed car and very easy to build with minimal cuts and easy finishing. I'd like to build a "wooden block" car someday just to prove my point. (Although it would have to be hollowed out considerably.)

As for bike hubs vs pinewood "hubs:" The bike hubs are all pretty good and a minor part of the equation. The pinewood "hub" is a piece of crap unless you prep it, and a major factor esp when all you have to do is beat the next car to the traps.

wahoonc

Friction is the enemy in any pure gravity race, with wind resistance being the other one, above a certain speed. FWIW the ABCE in MSP has a gravity race every year. The guy that one it the year I was there had removed all the grease from his bearing and bathed them in oil. He won by a healthy margin over the other similar bikes, by almost a full block if memory serves me. A dragging brake, or sticky hub will make its presence known time and time again on a long ride.

Aaron

somedood

The car gets energy for it's forward momentum from gravity, and to get the most speed you want to have the largest amount of potential energy built up when the track flattens out. Since the track slopes, the further back the weight is the higher the weight will be relative to the ground. This means that the weight will have fallen farther than the same car with the same weight up front when the car becomes level, and will have more potential energy because of it - that is what will propel the car further with all else being equal during the point where the track becomes level. Weight further back is the biggest and easiest boost for speed you'll get on a pinewood derby car.

I know you mentioned that getting the most weight isn't as important as it seems, but I'll put in my 2 cents and say it is the very most important thing for the cars propulsion. Gravity will pull on the cars with the same acceleration regardless of weight, but once the track becomes level all the stored up energy is what will keep the car moving. More mass = more energy.

icedmocha 

Pinewood derby is a perversion of boy scouts. WHen I was a scout the kids parents basically did everything, added teflon, designed/balanced cars. The kids set the cars on a track, that was it.

bdcheung

My mom (single parent) didn't do **** for my pinewood derby car and I still kicked the other dads' asses.

AlmostTrick

I've seen this strategy backfire when over done on a less than perfect track. When the front wheels hit a joint, they bounced up and derailed the car, stopping it quickly.


I agree with maxing out the weight. All scales are a little different. Build the car slightly over 5 ounces and bring your battery powered hand drill along to the official scale. Drill shallow holes in the bottom until legal. Don't forget to cover the holes with electrical tape to remain aero.

Artkansas 

Depends on whether they rode their bike to scout meetings I imagine.