Pinewood Derby track question: Engineering advice wanted

[Roger Feeley]

I help the neighborhood kids build their cars every year and every year I wish I had a test track. The track will be easy to build but I don't want to mess with a timer. I think I can build a timer, I just don't want to. I would probably make it by hacking the on/off button on a cheap stopwatch. The alternative would be something more expensive than I want to mess with. Anyway, it's just a test track and a timer is just one more thing to store and keep working.

It occurs to me that if I make the slope on my track so that it runs slow, I can substitute how far the car goes for how fast it goes. I simply make the starting slope low enough so that the car doesn't build up enough speed to reach the end of the 32' track I will build. A kid can run the car, note the distance and make adjustments. Run it again and see if it goes further. I will build a two lane track so kids can run one car against another. I figure, I need at least 12" of width so why not make it two lanes?

Additionally, running the car slower means that we would have a better chance to observe the cars behavior. Is it going left or right? That sort of thing.

I've read that aerodynamics really aren't much of a factor in Pinwood Derby cars so I don't see that making the track slow would be a bad thing.

Comments?

[Tom Bender]

It's a shame that kids can no longer build their own cars and can't get any help from their parents. Glad you are helping out. A few years ago I helped my neighbor build his son's car. Said son not present.

Rant over. Yes I think you are on the right track (sorry)

[Bill Carey]

Helped my son build his and now help my son help my grandson build his. I agree with everything you said Tom. In my experience THE 2 most important things are center of gravity and letting the car run on 1 rail or the other. Which is why your test track is a great idea. When they bounce from one to the other rail they lose a lot of speed. So your test track can help them keep their cars just barely kissing one rail all the way down. It's a small advantage but worth it. Well done Roger - you're one of the good ones.

[Roger Feeley]

It's a shame that kids can no longer build their own cars and can't get any help from their parents. Glad you are helping out. A few years ago I helped my neighbor build his son's car. Said son not present.

Rant over. Yes I think you are on the right track (sorry)

Rant fully justified! And thanks for the validation. I'm not a physicist.

I have a number of rules to which the parents must agree before I will help:
1. I accept no remuneration of any kind. I don't even want cookies.
2. I want one parent there the whole time to generally observe
3. I will not touch the car. Only the child will touch the car.
4. While in my shop, the parent will not touch the car.
5. We will be using power tools and power tools can be dangerous. I keep plenty of car sized scrap around and I do a lot of practice and pre-visualization with the kid during which I will touch the work. When it comes down to actually working on the car, see rule #3.
6. In the interests of safety, I reserve the right to take measures to avoid accidents which may include touching or even striking the kid. At this point, I tell the story about how I clocked my wife once to save her thumb.
7. Parents are allowed and even encouraged to intervene if things are going south. What they aren't allowed to do is 'help'.

I've helped quite a few kids over the years. We generally use a metal lathe to turn the little bits off of the nails. We use a band saw. I really don't draw the line anywhere. When using the band saw, I generally whip up some special push sticks to insure the 3" rule. We talk a lot about body position and balance and "Where do you want your hands to go if the work slips?" I probably overstress the notion of not trying to save the car if something catches it. "Just let it go", I say.

All that preparation and focus on safety is really necessary because mine is probably the only shop for miles. I'm in the DC area inside the Beltway. Finding any kind of unfinished basement around here is a miracle because land is so expensive. These kids have never seen anything like what I have.

I usually spend 3-4 hours per kid. I've never had a kid lose interest but, in spite of all those rules, it's really the kid that drives the work. I just ask them what they want to accomplish and then I teach them how to do it.

My oldest grandson is 2 1/2. I ought to be pretty well tricked out by the time he's ready. He can already tell the difference between a ball-peen hammer and a dead blow. Can't say the names but I know what he means. He's a bright kid but aren't they all?

[Lee Schierer]

What I learned from several years of car building with my son.
Most important, let the child decide the type of car he wants and let them do everything that they possibly can. Don't sweat perfection.
1. Build the car to the maximum weight. Make provisions to add or remove weight because your scale may not match the"official Scale" the night of the weigh in.
2. Trim all flash off the wheels and nails
3. Once you have the nails where you want them secure them with super glue.
4. Streamlining does help with speed
5. If allowed squirt powdered graphite onto the axles just before the race.

[Roger Feeley]

What I learned from several years of car building with my son.
Most important, let the child decide the type of car he wants and let them do everything that they possibly can. Don't sweat perfection.
1. Build the car to the maximum weight. Make provisions to add or remove weight because your scale may not match the"official Scale" the night of the weigh in.
2. Trim all flash off the wheels and nails
3. Once you have the nails where you want them secure them with super glue.
4. Streamlining does help with speed
5. If allowed squirt powdered graphite onto the axles just before the race.

I agree with you on the streamlining. But aerodynamics can be judged visually. I don't see any need to tweak aerodynamics using a test track. The big difference in performance is in wheel friction and alignment. That gets me back to my original question of whether or not I can use distance on a slow track vs time on a fast one.

This may be digressing a bit. I read about a pack that had a rule where the kid that built the winning car had to tell how they did it. Also, they had to host a build the next year and offer the same advantages to all the rest of the kids. It helped keep things on the up-and-up. I don't know how they could enforce me helping though. I'm certainly not willing to help 50 kids. I guess if they instituted a rule like that here, I would just have to withdraw.

[Doug Garson]

First I'd like to say well done, I think what you are doing is great and will likely make a big difference in the kid's lives. To answer your question I don't know anything about Pinewood derby but I assume these are miniature cars running down an inclined track. So the driving force is gravity and the resisting forces are friction in the wheels and bearings and air friction. I think your "slow track" will enable the kids to optimize the cars to eliminate friction in the bearings and wheels run straight (shortest distance) and to a lesser extent air friction just fine. A full speed track with a timer might be marginally better but as you said, more complicated. I'd love to see some photos of the cars if you have any. I'm a retired mechanical engineer but not an expert in this field.

[Bill McNiel]

Roger,
I could not agree more with your approach. Your shop safety rules and no touch rules are dead on as well. Kudos Sir!

I believe that the distance vs speed evaluation is a valid concept.

Your participant rules are exactly what all should embrace. Thank you for bringing some faith into a trying time.

[Eric C Stoltzfus]

This guy has some great tips on making pinewood derby cars as well: https://www.youtube.com/watch?v=-RjJtO51ykY

[Stan Calow]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

[Pat Barry]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

Its called Pinewood derby. LOL

[Mitchell Ristine]

I helped a friend with his son's. I wondered at the time if using a dense wood like ipe or lignum vitae would allow smaller profile with maximum weight.

Hitting the max weight is important. But equally important is the placement of the weight. Having the weight of the vehicle equally spaced over the whole vehicle loses potential energy. Get all that weight as low as possible over the rear axle.

[Ole Anderson]

Boy this thread brings back old memories. My dad helped me build my car, I helped both my son and grandson build theirs. Even helped build a track once.

[Roger Feeley]

Its called Pinewood derby. LOL

Pine is probably ok. If you look at the design of most cars, there isn't much wood left. If the kid wants to keep the car, a better wood might be a good idea. Most kids paint the cars so there wouldn't be any way to tell the wood species.

Man, you ought to see all the little gizmos for tweaking these cars. There's a whole industry around making them fast. And a lot of physics. The local Boy Scout supply is next door to my local Woodcraft so I dropped in there to pick up a couple of kits to have spare parts for my little trainees. I was amazed to see all those tuning gizmos for sale at the BS store. To me, those resources are a bit shady (as am I). Somehow it rubbed me wrong that it was being sold in that store. I asked if they were directly affiliated with the local counsel and they said yet. I guess what I think of as sort of cheating is sanctioned by the Scouts. Frankly, I see no way that your average Cub Scout could get their heads around most of that stuff.

Check this out: https://www.derbymonkeygarage.com/pi...ols-s/1827.htm

[Roger Feeley]

Technically, you don't exactly have to have a track to adjust some things. You can go a long way with just a 4' flat rolling surface like an old countertop. Draw a line down the middle to serve as a center line and then parallel lines about an inch on both sides. Incline the surface a bit, say 10 degrees. You want the car to roll but not very fast so you can see what's going on.

Now, find the biggest ball bearing you can. I have a 2" one sitting on my desk. Place the bearing on the center line and let it roll to the bottom. Adjust the surface so that the bearing follows the center line exactly. Now you have your surface ready to test cars.

Set up a car at the top so that it aligns exactly with the center line and release it. What you are looking for is for the car to veer off about 1 to 1.5" either way. This is the most basic setup. If you are doing a 'rail rider', you need the car to veer in a certain direction.

My track idea is not so much for adjusting alignment but for tuning rolling resistance. I realized that If I do the slow speed track, any little defect could affect the speed of the car by a lot. I will be careful to sand everything very smooth.

Interestingly, the local Cub Scout pack has a track with a hump in the middle. You go down the initial incline, flat for a bit and then up and over a hill. I noticed last year that some cars went a bit airborne on that hill. In thinking about that, I realized that they are essentially imposing a speed limit. Imagine an extremely fast car. It hits that hill and just takes off and never finishes. There would be a maximum speed that you can't exceed. Or maybe you would need some sort of front spoiler. Maybe make the ground clearance of the car really low in front. I don't think I like that hump....