Sunday, September 2, 2012

Winning "Jet" Powered Pinewood Derby Car

I finally had an opportunity to participate in a 'no holds barred' pinewood derby for grownups. The annual competition does have some standing by-laws, however.

  1. No open flame or explosions
  2. Any car leaving the track will be disqualified
  3. No sabotaging the other vehicles during the race

Being new to the area, and new to the competition, I set out to at least compete for the fastest car. After spending many hours finishing unpacking my garage to clear out a space to build a car, I had about 4 hours before the competition. I grabbed my tools and set to work.

The Design

Originally, I wanted to use the bell of an outrunner electric motor as the drive wheel, but I knew I did not have enough time to design and build such a pinewood derby car. So, I decided to salvage the parts from an old RC airboat (that didn't work so well) I had collecting dust in my garage.

The Chassis

The Materials

  1. 1/8" Scrap Plywood from the Homely Despot (they usually have scrap wood in the back of the lumber section that you can pick up for pennies on the dollar
  2. 1/2" Square Hardwood Dowel - Your local hardware store probably has 2ft lengths for about a dollar
  3. 3/8" Square Hardwood Dowel - same as the 1/2" dowel
  4. Small Self Tapping Wood Screws
  5. Pinewood derby Wheels and 'Axles' - You can substitute other wheels if you please
  6. 9x4x4mm ball bearing
  7. M4 bolt
  8. M4 nuts

First order of business is figuring out what size track you'll be racing on. From the interwebs, I acquired these rough dimensions

For stability, I wanted to keep the CG (center of gravity) as low as possible. Most Pinewood Derby tracks have a raised guide that comes up 1/4" above the track, and is 1 5/8" wide. A normal pinewood derby car straddles the guide and has a small gap between the guide and the bottom of the car. Additionally, at the end of the track, often the guide will be raised another 1/4" to slow the car at the end of the track. What I wanted was a reasonable sized deck that hugs the rail for stability and to hold the electronics

In all honesty, I just eyeballed the length. It was almost too long and did only barely fit on the starting blocks. I gave the car a little extra width to ensure it would clear the rails, which it did. The width worked out fine, with one exception, which I will note in the Power and Control section below

Power and Control

Materials

I used what I readily had in my garage. The power system that I used hits the sweet spot in price, and is about as cheap as you can get.

  1. Motor - C20 brushless Outrunner 2050kv - First off, you need a motor. I have a couple of these C20 brushless outrunners. Great little motors for the price.
  2. Propeller - GWS EP Propeller (DD-5030 127x76mm) - A good starting propeller match for the C20 brushless outrunner are the GWS DD-5030. The DD-5030 on the C20 brushless Outrunner will draw between 6-7A and should produce enough thrust for our small light car. A pack of 6 should be plenty for now. I had some GWS EP Propellers (DD-5043 125x110mm) available in case the competition got tough, but the car was fast enough without propping up. The width becomes an issue here. A 5in prop will interfere with a large car in a lane next to it. In the video, I could not race next to the waterbottle car. Perhaps a smaller prop might allow you some more flexibility.
  3. Prop Adapter - Prop Saver w/ Band 3mm (10pcs) - In order to mount the props onto the motor, grab some prop adapters. These prop savers fit the motor perfectly and the GWS props previously listed.
  4. Battery - A small powerful battery - 600mAh, 3S 20C rated or higher should do.
  5. Speed Controller (ESC) - Hobbyking SS Series 15-18A ESC - Cheap and simple
  6. Connectors -
  7. Misc
    • Controller - If you don't want to invest in an expensive R/C controller, and don't have one, you can pick up extremely cheap ones from HobbyKing or HobbyPartz.com. The Hobby King 2.4Ghz 4Ch Tx & Rx V2 (Mode 2) is a good choice for getting started. When there are kids around, it is always good to have a human in the loop controlling the car.
    • Battery Charger - If you don't already have a LiPo charger and just want one for this project, you can grab something like the Turnigy 2S 3S Balance Charger. Direct 110/240v Input to get yourself started.

Assembly

Tools Used

For my build I used the following tools

  1. Cheap miter saw and miter box - a coping saw would also work
  2. Utility knife
  3. Electric drill and assorted drill bits
  4. Hot glue gun
  5. Screw driver
  6. Dremel Tool - You could get away without one of these

I only had about 3 hours to throw the car together, so I chose the simplest assembly methods.

  1. Using the utility knife (or if you happen to have better more appropriate tools) cut out the rectangle of the 1/8" sheet of wood to form your main deck.
  2. Hot glue the 1/2" rails to the bottom of the 1/8" thick deck and then reinforce each rail with 3 or so of the wood screws (don't forget to drill pilot holes).
  3. You can choose to round the rails with a Dremel and a sanding head. This helps in case the track it will race on isn't perfectly smooth, especially at the joints between track pieces.
  4. Drill pilot holes about 3/4" back from the leading edge where the deck meets the rail for the front axles. I eyeballed the locations for the front wheels, and it worked out fine. This isn't your typical pinewood derby car, you'll should have plenty of thrust to power through misalignments.
  5. Cut two 1" lengths of the the 3/8" diameter square dowel. Clamp the pieces together and drill a hole through both of the pieces for your rear axle. The hole should be positioned slightly offset from center so that your rear wheel clears the square dowel supports. I used a 4mm bolt for the rear axle with small 4mm nuts on either side of a ball bearing previously mentioned for the rear wheel.
  6. Put the axles onto the supports with the wheel (ball bearing) installed. Then, take the rear wheel assembly and hot glue it to the bottom rear of the car.
  7. Now, you can install your motor mount. This depends on what type of motor you have and what prop you want to use. I'll leave it up to you to figure this part out.
  8. As a final step, I installed strips of velcro on the deck of the car for installing the electronics. The velcro also helps absorb impact if your car makes an abrupt stop at the end of the track.
  9. If you need help with wiring your electronics, there are plenty of guides and forums for RC enthusiasts that can help. The focus for this post is mostly about the mechanical side.

Performance

Video

This car performed better than I could have hoped. I never had the chance to even open it all the way up. I had a more powerful prop combo available if the first set up proved to be too wimpy. However, after the first race it became apparent that I would easy sweep the derby.

Why did it perform so well?

Several factors helped this car outperform my expectations.

  1. The propeller located in rear, 3 inches above the deck, provided down force on the front of the car. This allowed all the power to be applied to accelerating the car without worrying about it popping a wheelie and flying off the track. Additionally, the down force on the front of the car kept the car on the guide rails, preventing it from jumping the rails as some of the other cars did.
  2. High thrust/weight ratio. I still don't know how much the car weighed, but I estimate less than .5 lbs. I estimate that with around 1 lb of thrust, plus sqrt(2)*gravity due to the 45deg starting angle, the car accelerated at least at 2.7G.
  3. Ideal starts. I was able to apply thrust before the gates dropped, allowing my car to accelerate the instant the gates dropped.

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