My son competed in his second Pinewood Derby a couple of weeks ago. Last year he finished dead last. This year he finished 3rd from dead last. I would like to ask for some diagnostic help from more experienced Pinewood Derby racers. Last year and this year I read a good bit of the material available on the Web and I thought we paid attention to all of the important factors. Based on the behavior of his car on the track in both years, I suspect that we had a weight location problem.
Last year his car would begin down the track with the other cars in a tight group or even a bit ahead. As the car hit the straight away it would slow down significantly as the other cars shot away. Each Scout runs 4 races (1 on each track) and every race was the same. The outcome was the same this year, except that 2 other cars happened to be a bit slower.
Does this sound like an issue of weight placement? Is this what the car would do if there was too much weight towards the back of the car?
The material on the Web seems to be contradictory. The average advice seems to be to place as much of the weight as possible towards the back. One web site qualified this by saying that you should not put the additional weight any further back than the rear axle. The Pack information recommended weight centered around 1/2" to 1 " towards the rear from the midpoint between the axles. Another web site recommended the weight be centered 1/2 to 2/3 back towards the rear axle.
Last year I had read some old advice that was apparently applicable to the old S tracks and we had placed a lot of weight in the back. This year we tried to avoid that. We used a 2 oz bar weight countersunk between the axles. We also drilled holes on each side about 1/2 inch forward of the rear axle to add more weight. Since we were still short, we drilled 3 holes in the back and added more weight there. We ended up weighing in at 141.74 grams. I am afraid that the extra weight in the back tipped the center of gravity too far back, but I just do not know for sure.
Buy the cars in bulk, factory made and shipped to the scout hut under armed guard so no parent could figure out how to get an edge; let each cub pick out his racer, blindfolded ...then go straight to the starting line.
Thrill in victory, and participation, no agony in defeat ... and a good time was had by all.
I am puzzled also about weight placement. I have 2 sons who were in the derby race yesterday. This year we put the cg about 1 inch forward of rear axle. Last year 1 inch aft of front axle. No difference in number of races won or speed noticable between years. This year we polished the axles on one and not the other and treated the wheels the same yet on the heat where they raced against each other they tied, twice since the first time they could not tell a winner but on the second race the third car won. One car won 4 out of 9 and the other (with polished axles I might add) won 1 out of 9 races. I tend to think it is pure luck but there are 3 families whose kids always place in the top 5 or 6 out of 35 or so every year. They claim they do not do anything excessive except make sure the car runs straight. We are friends so I believe them.
My 8 year old son bought the kit, and I totally forgot about. He wakes me up at 8am on a saturday morning, saying "Dad, the derby is today, we forgot to make my car"... great, we have to be there at 11am.
Most of my tools are over at a house I am rehabbing, about 30 minutes away. Just going over there would take up at least an hour. So I see what I have on hand. I have a cordless drill, a hammer, some nuts, washers and bolts, a jig saw and a dremel tool, and some old red door paint and some brown spray paint in the shed.
My son says Yugioh uses red and brown colors, and he has some yugioh stickers. Ok, that solves that. I go ahead and draw a rough futuristic car shape on the block of wood and cut it out with the jig saw, sand it down and shape it further with the dremel tool and a sanding wheel.
Of course I have no scale at the house capable of measuring the weight of the car in oz, so I have to devise someway to easily change the weight, so I drill in hole in the "back window" and screw in a bolt with about 10 washers on it.
I paint the car red with the door paint, and then do a faded two tone with the brown spray paint. Boy was it ugly, but my kid thought it was pretty cool. I had my son stand there with a hair dryer on it until it was dry (enough) and he added his stickers, then I attached the wheels.
We get there just in time, I proceed to weight in the car, it's weigh overweight, easy enough, brought a wrench with me and removed some washers. After getting the weight just right, I used some graphite on the wheels.
There were about 70 cars competing, my son won 1st place. I don't think I have ever seen him so happy. I was the hero for at least one day. :)
Amen. I remember in the early 60s when The Derby was big... really big. And SUPPOSED to be a an event between scouts. Sure, a little help from Dad, but just guidance. We all built our cars and had a great time looking at each other's work... you remember... the left over decals from models stuck on the sides, chunks knocked out of the wood bodies from using your boy scout knife too aggressively, paint where it shouldn't be...
Then the Dads took over. The cars looked great, no more airplane decals on your car - just car decals, there were paint >schemes
We race on a forty foot Piantedosi wood track, with a starting pin height of about 45", and the slope runs out flat at about 12' on a level line from the pins. The pin to finish line distance is about
35' measured on the track.
We use tungsten weights that come in 3/8" d. cylinders and we drill the car body out, starting on the centerline of the car and running parallel to the centerline for one row on either side of it. The drilled holes have about 1/8" between each and they are drilled to depth with a forstner bit on the drill press.
We insert the weights into the center line of holes and attempt to get the car to five ounces, with one ounce riding on the front axles. We set the weights to be flush with the bottom of the car.
We straighten the axles in a press and hammer the nail head into the press so that it is square to the shaft, and then file off the webbing below the nail head. Then we lightly file the underside of the nail head into a coned shape.
Then we insert the axle into the drill press and polish it with wet-or-dry abrasive, starting at 600 grit and moving through the grits to a finish with 1 micron polishing cloth. We wipe them down with alcohol and put them in a baggie with a powdered graphite/moly lubricant. We treat the underside of the nail head the same as we do the shaft.
We use a jig on the drill press to bring the wheels into round and then treat the tread with the same regimen of abrasives. We cone the bore of the wheels and we polish with the same set of abrasives. We use the same abrasives on the area of the wheel that contacts the underside of the axle. We polish the inside edge of the wheels with the same abrasives and then we take a rag and push graphite/moly into the surfaces of the coned hub, flat face, treads, etc. We use a piece of drill rod and some liquid abrasive to polish the interior of the bore. Then we clean out the wheels with compressed air, then swab with denatured. Then we put the wheels in a baggie that has some graphite/moly in it and shake them up, then let them sit overnight - they come out a cool looking graphite gray and - we hope - the graphite gets into some of the crans and nookies.
We drill holes in the body at the bottoms of the axle slots, so that the axles will go in straight. We load the wheel hubs up with graphite/moly and push the axles through, trying to keep as much lubricant in the bore as we can. We push the axles into the body until the wheel bores are 1/32" away from the body. Then we spin the wheels for a bit using the air compressor to direct a stream of air over the wheel to make it spin.
Then we set the car on a piece of plywood that is eight feet long and set to a five degree slope, being level side to side. We put a dot on the twelve o clock position of the axle head and we turn each axle a quarter turn at a time until the car can run a straight line over eight feet.
We then run the air compressor over each wheel until we think the lube is gone and test the car for straight again. A little more tweaking to make it straight. Then we pack it with lube and we're ready to race.
The boy ran third out of fifty his first year and second last year. Then he went to the Districts and ran third out of 150.
To answer your question - I think that you have a friction problem, or an alignment problem, rather than a weight problem.
First my credentials: My son's cars won first place at the pack level
3 years in a row. One of those years he raced against more than 90 other cars in a double elimination tournament such as you describe. The other two years he raced against probably 40 cars. Perhaps coincidentally, I have an engineering degree.
Assuming your weight is close to the maximum 5 ounces, I think only one thing can cause a car to run that slow - friction at the wheels and axles.
Forget about aerodynamics. The car is going less than 12 miles per hour and aerodynamics plays a small part at that speed. It may make a difference between first and second, but not between first and last.
Forget about fine-tuning weight placement. It may get to be an issue among the fastest 2 or 3 cars, but not among the slowest. You don't want the front end to be so light that it leaves the ground at the track joints. You don't want it so heavy that the car can't steer itself easily. Other than those issues, I don't think it's very important.
Spend your time and energy on wheels and axles. Look closely at the nail that comes in your kit. It has a spur under the head that wants to dig into the plastic wheel and stop it from turning. The part of the axle that the wheel rides on needs to be soooo smooth it shines like a mirror. There are lots of ways to get there. I chucked the axle up in a cordless drill and had my boy run the drill while I applied:
A small file to the offending spurs under the head.
Narrow strips of sandpaper starting with 300 grit and progressing to 800 grit.
Jeweler's rouge applied on a felt wheel spun by a Dremel Tool while the axle was spinning in the drill. We probably spent 30 minutes on each axle. If the axle is a little smaller in diameter where the wheel runs on it, that's a good thing.
Don''t ignore the wheels. The running surface of the wheel needs to be as smooth as a baby's bottom. Chuck the wheel hub in a drill and sand, then polish the plastic. It will take a high gloss if you polish it. Find a way to polish the inside of the hub where it runs on the axle. I found that the shank of a Dremel bit is just the right size, but DON'T spin the bit in the hub, you'll get it out of round. Instead, put a little grinding compound or jeweler's rouge on the shank and spin the wheel on the shank. I used an air blower to spin the wheel at high speed. Wash the wheel with lots of hot water to be sure you get all the abrasives out.
Adjust the tracking by bending axles as needed so that the car rolls straight when pushed across the floor. You don't want the car to scrub of speed by hugging the center rail all the way down the track.
Lube the axles with graphite lubricant.
If you do all those things, I can't promise a win, but you shouldn't be embarassed.
DonkeyHody "Even an old blind hog finds an acorn every now and then."
Wheel balancing! And Wheel bearings . . . and machined axles. And, synthetic bearing lubricants. Tried a taller gear ratio? ~:o))))) NuWave Dave in Houston
I think that "luck" is a big part too. 25 years or so ago, my son and I built ours going by the Dad watches, kid does plan. Got to the race and it was too heavy. Drilled many holes (some clear through), Crazy looking thing took first place in a probably 25 car race. The kids nicknamed it the "Swiss Whiz".
Sure - my son's first year. 1st place (64 kids) in each of 4 heats of all 64 cars, so first over all for the troop. Way, way, fastest car there. At superderby (top 5 from each pack - 125 cars all) 1st place after 3 heats, 4th heat rubbed the rail, came in 13th overall. Still just over .01 seconds for 4 heats between him & #1 overall. The cars are all fast at regional.
The physics are simple. ALL the energy of the system at the start is potential energy dependent solely on height & mass. You maximize these by building the car as long as allowed (7"), car at the maximum height, with they weight as far back as will maintain stability (cg in front of rear axle, but close to). However, the difference in stored energy is relatively small given the parameters, as long as you're at max length & weight (the change in height of cg is pretty small range).
You said your sons car kept up down the slope. Heading down the slope is the conversion of potential to kinetic energy (height to velocity). If you're losing on the flat run it's all friction. Air friction is negligible. My son's second year we did a football helmut (of wood) on top of main body. Hilarious looking, wobbled, but amazingly came in
8th overall. Last year, much to my chagrin, he made superderby again (trust me, once is enough) - but barely and got womped (took 8 hours to get through, and was obvious this was one of the slower cars). This year his car is an "arrow" - we haven't even assembled yet.
So what causes the difference - reducing friction. Where he (and other top cars) all pull away is in the flat run. Ways to reduce friction, in sort of importance (most to least):
1) trial roll the car, adjusting the axles, until it runs straight. If it rubs against the center guide rail the entire way ain't gonna be fast (of course, if the car isn't set up straight, out of your control).
2) File the burr formed when the nail head is stamped. This will definitely cause friction, and is easy to remove. A further refinement is smoothing/truing the axles to a true round, but minor. The wheels spin on a single area (bottom) of the nail, so as long as that is smooth should be fine.
3) Remove the "nub" off each wheel where it was injection molded. Helps to true the wheels as well, although I've always found them pretty much round.
4) Lubricate w/graphite (all we're allowed) a lot, and spin the wheels a lot, before the meet.
5) Camber the axles up a few degrees. This causes the wheel to want to move away from the body. Wheels rubbing against body is bad. Camber too much though and they push too hard against nail head, again increasing friction.
6) You can shave (carefully) the profile of the wheels so they run on an edge, not the whole surface. Note that cambering the axles should have this effect, and over-profiling the wheels can result in disqualification.
7) Raise one axle so car runs on 3 wheels. Not so sure. In theory, less rolling friction (3/4 to be exact...) and no energy stored in spinning up the 4th wheel. However, if the car wobbles at all, varying which 3 wheels are touching, I could see this being a net negative as different wheels would be spinning up/slowing down. Hard enough to get the damn thing to roll straight on 4 wheels (stay on 4 wheels!!).
8) "Aerodynamics" The first car was pretty much aerodynamic - but had "shark fins" on the back that didn't help. The 2nd was the aforementioned football helmut. The 3rd was a "missile laucher" with a big ass rocket out the front. I don't even remember last year's car (thank God this is the last). Some guy's cars are professionally finished - I mean race car lines and a paint job better than my real car. I prime the car (spray) and my son hand paints whatever he wants. He also spoke shaves (if needed), rasps, and sands the body before priming (thus this year we needed a round of Bondo to fix some "design misfeatures"). Needless to say, 6 - 10 year olds aren't too great with hand tools and sanding is boring as hell. He used the spokeshave at 6
- it can be done - just need to supervise. They're not the greatest looking cars, but they run okay because we pay attention to the fundamentals - max length, weight distribution, reduce the real friction factors (non-aerodynamic). Any drilling, etc he does. I do the bandsawing and any chisel work (because the risk of a slip). Obviously, a 10 yr old whose helped make 4 cars can do a lot more than a 6 yr old on his first.
It's a funny thing, but there was a time in the US when kids were allowed to lose, all on their own. Those losers won two world wars, put a man on the Moon, invented the atomic bomb, and did numerous other things including creating an economy so powerful that the rest of the world looked on in awe and terror as its products dominated their cultures.
Since then, what have the "winners" done? Near as I can tell they mostly quit the first time things didn't go their way.
My grandson just did it this past weekend, he finished about the same. Somewhere near last. He had fun though. He called me all upset, because after the race a wheel broke off. Placing the weights just forward of the rear axle seems to be the right place.
To me, it is the transmission of the concept of craftsmanship.
The same mentality that makes you create a piston fit drawer, a hermetic dovetail joint, a perfect deep gloss lacquer finish - these are all created by the repetition of simple processes, and the truth of success in any of them is - Character.
Going the extra mile. Being unreasoning in your pursuit of excellence. Doing what needs to be done to make it as good as you possibly can.
I don't have my boy stand in front of the drill press for an hour to win this race - I want him to understand what it takes to win his race, whatever that may turn out to be.
I have been somewhat facile about winning the Derby - I am much less so about the lessons that the process teaches.
The goal with my son is never actually to beat anyone else but himself.
Kipling had a great poem about it. I've had my boy memorize it.
Pat - If you don't mind me turning this discussion over to my betters:
If you can keep your head when all about you Are losing theirs and blaming it on you, If you can trust yourself when all men doubt you But make allowance for their doubting too, If you can wait and not be tired by waiting, Or being lied about, don't deal in lies, Or being hated, don't give way to hating, And yet don't look too good, nor talk too wise: If you can dream--and not make dreams your master, If you can think--and not make thoughts your aim; If you can meet with Triumph and Disaster And treat those two impostors just the same; If you can bear to hear the truth you've spoken Twisted by knaves to make a trap for fools, Or watch the things you gave your life to, broken, And stoop and build 'em up with worn-out tools:
If you can make one heap of all your winnings And risk it all on one turn of pitch-and-toss, And lose, and start again at your beginnings And never breath a word about your loss; If you can force your heart and nerve and sinew To serve your turn long after they are gone, And so hold on when there is nothing in you Except the Will which says to them: "Hold on!"
If you can talk with crowds and keep your virtue, Or walk with kings--nor lose the common touch, If neither foes nor loving friends can hurt you; If all men count with you, but none too much, If you can fill the unforgiving minute With sixty seconds' worth of distance run, Yours is the Earth and everything that's in it, And--which is more--you'll be a Man, my son!
Luck has very little to do with it when there are several well-tuned cars in the race. Several times I've seen tounaments that lasted all day, with one car that crossed the finish line first every single time it ran. You can't attribute that sort of success to luck.
Having said that . . .every year Fate chooses to smile on some kid who had little or no help. He takes the nails out of the box and pounds them in with a hammer. The heavenly bodies all line up in the proper orientation, and his car does very well, but it still doesn't beat the ones with polished axles and turned wheels. The rest of those kids usually suffer a different fate. I've seen several cars that stopped before they got to the finish line.
I've held work days in my shop with my tools and assistance to try to help some of these boys be more competitive. But they aren't the ones who show up. None of the single mothers or non-technical dads bring their kids. It's the young dads who are sure they know how to build a fast car, they just need the tools to do it.
In the end, somebody has to be last in every competition. I didn't contribute the genes or the tutoring to allow my boy to be good at football, baseball, basketball or soccer. He certainly knows the agony of being last in those areas. But I can show him how to build a fast pinewood car.
DonkeyHody "We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore." - Mark Twain
Thank you for your replies, especially those with very detailed advice and instruction. The consensus seems to be that we had a friction problem of one sort or another. I tend to think that that may indeed have been the case as we spent no time this year on checking to see if the car was going straight or not. The primary reason for that was that we had nothing but frustration last year in trying to get the car to roll straight across the kitchen floor. The tip about putting it on a slightly angled board is a good one. Both years we did chuck the axles in the Dremel and polished the shaft and the underside of the head with steel wool. We do not have a small file, but I did use a utility knife last year to shave off the burr underneath the head. This year I could not see any burrs underneath the head, so we only applied the steel wool to the shaft and underside of the head. We also sanded the wheels by hand, but no turning by Dremel or otherwise. Apparently we need to do a much more thorough job in this area.
For lubrication both year we used the Dry Lube sold by the Council Shop. It sounds like graphite is a much better solution, if used sparingly.
To those who replied with messages emphasizing the life concepts at issue here, I appreciate your words as well. We certainly are not out to win, but simply to do our best. We do not feel that we have done our best yet. We have neither the time nor money to go overboard on this, but it is much more fun to have a car that at least hangs with the others. To do this day my son will tell you that he had fun in both years, but building the car and participating in and watching the race. He would like to add to that fun by finishing even better. He is fascinated by what aspects we might be able to change or tweak to improve the car for next year. He is ready to start again now! To that end...
I have two practical questions to help us with improving our performance:
How can we test our car at home without spending a fortune on our own track? I have access to a scale, but the only affordable test track that I can find is a single lane 30 foot track on eBay. Since we have two very slow cars, it would great to be able to test a new against them. The 8 foot board is an excellent idea. Are there other affordable alternatives?
On the subject of bending the axles, this is where our frustration came in last year. We would push the car across the kitchen floor and try to bend the axles by hand to adjust them. However, that was simply pressing them against the wood of the axle groove, so of course they probably did not stay bent. We have the alignment/camber/measuring tool that is sold in the Council Shop and we did make use of it this year. We did have a hard time finding an effective way to straighten the axle slots. A drill press is obviously needed. Nevertheless, how do you bend the axles themselves?
Thanks for all of your help. This is still a fun process for us both and we are looking forward to our next car project.
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