First Time Feedback

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
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ERicJ
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First Time Feedback

Post by ERicJ »

Just built and raced our first car. The car performed really well from a results perspective. But in watching the post-race video I noticed a significant amount of side-to-side wasted motion/energy. Any thoughts on what caused that and how to eliminate it (I'm assuming that's bad)?


https://youtu.be/1e_MLyhpxUw

Things I think we did right:
  • Center of Mass was a little over an inch in front of the rear axle.
    BSA stock axles filed to de-burr, straightened, and wet sanded polished.
    Dry graphite in wheels and on axles.
    Tungsten weight added to 4.95oz.
    Right front wheel unintentionally very slightly elevated to result in sporadic 3-wheel ride.
Things I think we did wrong:
  • Left rear axle installed at angle causing wheel to rub on car.
    Aerodynamics of car not optimal, but son was happy with car-like look.
    Didn't sand to smooth [aerodynamic] finish.
Note that these pictures are after the race and the wheels are all chewed up from my son playing with the car on the cafeteria floor.
Left rear wheel:
Image

Right rear wheel:
Image

Left front wheel:
Image

Right front wheel:
Image

Bottom:
Image

Top:
Image
Speedster
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Location: Toledo, Ohio

Re: First Time Feedback

Post by Speedster »

Howdy ERicj, Welcome to Derbytalk.
WHAT A VIDEO! THAT WAS REALLY COOL!

I was very surprised to see the distance among the cars. That was sad.

Your raised wheel did not negatively affect the cars performance in any way. It actually saved 25% of the Rotational Energy formerly shared by all four wheels. 3 wheelers are faster then 4 wheelers.
The shape of the car followed Physics, Light in the Front and heavy in the Back. A very slim wedge car is not cut that way so it will go through the air faster. A Pinewood Derby car will only travel about 11 MPH and that's not fast enough for air resistance to have much of an affect. It is shaped that way so the Team can put more weight where it is needed. I presently like 2 ounces behind the rear axle slot and end up with a 3/4" balance point in front of the rear axle slot.

Remember, the axle does not turn. Wheel bore polishing is important. Alignment is Super Important.

If the left rear wheel was running parallel to the car the wheel would have migrated to the axle head with the Negative Camber. It looks like there was Toe-in on that wheel that would force it to the body of the car causing friction all the way down the track.

I recommend you get a copy of Troy Thorne's 2013 Edition of "Build a Winning Pinewood Derby Car" $10.00. It can be purchased from Fox Chapel Publishing.
Learn how to set up a Railrider.

Is your Team entering another race? If so, once you get the book you will be able to pull the wheels and align everything.

RAILRIDING.
While narrowing the car 1/16" behind the Dominant Front Wheel is preferred it is not totally necessary. I use the right front wheel as the Dominant wheel. The wheels get a Credit card distance clearance except for the Right Rear wheel gets the thickness of 2 Credit cards. The Right Rear wheel will ride close to the rail but it should not touch the rail.

Again, Thanks for that Wonderful video. If the car races again I would love to see more videos.
ERicJ
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Re: First Time Feedback

Post by ERicJ »

Thanks for the feedback and advice. Here are the other videos I made from the race:

Lions and Tigers

https://youtu.be/0xpnmZh6R2Y

Wolves, Bears, and Webelos

https://youtu.be/dmmwCdYhUD0

Non-Scout / Adults

https://youtu.be/JIx3832tcpA
ERicJ
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Re: First Time Feedback

Post by ERicJ »

That's the only race for this car this year. But already thinking about next year:

How do you get the Center of Mass optimized without drilling holes to add the weights which you need to figure out the Center of Mass? Chicken vs. Egg.

I was thinking that maybe instead of drilling weight holes vertically that I'd drill them horizontally. Two holes, on in front and one behind the rear axle. That way I can add/subtract weights to each hole to optimize the Center of Mass.
Speedster
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Re: First Time Feedback

Post by Speedster »

You're on the right track if speed is your goal.
Let's have some Fun. I'm going to build a car that will follow all The Laws of Physics and we'll do it fast.
5 ounces = 141.75 grams. I will use grams.

Go to Hodges Hobby House and find the wedge body block, Slim SSW, Slotted Stock Wheelbase, $2.45. That's the one I use at our workshops. We will use the 2 rear holes,one behind the rear axle slot and one in front of the rear axle slot. Do not cut any wood away from it at this time. This particular car has the 3 holes already drilled with a 25/64" drill bit. The rear hole is the only one that needs to be drilled all the way through.
My bare block weighs 49.36 grams.
2 ounces of incremental 3/8" tungsten cylinders in Blue package at your Scout Shop weighs 2 ounces. Place them all in the rear hole. Install 4 wheels and axles enough to hold them in place. 120 grams.
Cut 3/8" lead rod so the total weight will be 3 grams less then 141.75 because we need to allow 3 grams for paint. Place in hole in front of rear axle slot.

At this point you should have a COG at 1" in front of the rear axle slot which is generally recommended.

If you take the lead out of the hole, add a 1/2" by 1 3/4" tungsten plate to the top of the car behind the rear axle slot, add enough lead in the hole to bring the weight back up, you will now have a COG of about 7/8" in front of the rear axle slot. If you now start removing as much wood as you possibly can you will probably bring your COG to 3/4" in front of the rear axle slot. I wouldn't go any farther then that. You can now do anything you want regarding where you have your COG.

Please check your email.

If you cut your block you will not need the 3rd hole. The holes should be drilled before the block is cut. That gives a nice large surface to lay on the drill press deck. If you actually use Hodges block simply fill holes with 3/8" wood dowel so you don't add a lot of weight with wood filler or automotive body filler.
Last edited by Speedster on Wed Jan 22, 2020 5:22 am, edited 1 time in total.
ERicJ
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Re: First Time Feedback

Post by ERicJ »

Ah, okay. Those Hodges' car bodies are exactly what I was thinking about.
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