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Winning Pinewood Derbies - Intro

Having two sons in scouts I have been through a lot of pinewood derby races.  I remember the very first race with my oldest son.  He didn't win and there were a couple of cars that smoked everyone else.  Obviously their father new more about pincecar racing than I did.  I felt like I let my son down, at the very least his car should have done better.

So I thought about the car, the race, friction and gravity.  Learned what I could from what was on the internet, not much in the 90's.  Even borrowed a track and did some testing.  By the time my youngest son was racing his cars were almost unbeatable.  Now that they are grown up I decided it was time to pass along what I have learned.

I won't get too much into the actual building of the car, you can find that on plenty of sites.  What I will tell you are thinks that will make the car run faster! Some of these you will have seen on other sites or books. One thing that you will find, on this site, where I can, I back up my conclusions with actual test data not just conjecture!  You can be confident these tips will make a difference.  Where else will someone show you actual data to back up their recommendations for free?

I feel however that I must say you are doing yourself and your child a disservice if you don't work on the car together.  Particularity as they get older they can do more of the work, (sanding, painting, etc.).  There are some things that you will still need to do because of safety or they just don't have the skill yet.  however the main thing is you are working together.  I know how hard this can be, it was for me, but as time went on my sons would do more, but it was hard for me to let go sometimes.

 

My son, (second from left) with the first of many derby trophies.

 

Winning Pinewood Derbies - Basics

So what does it take to make a winning car?  Below I out line some of the basic and key things you need to do.  You may read tips in books or at other sites but often they are just conjecture.  Most are good but some you can't really be sure if they work.  Now it may not be possible to test everything, however I have made an attempt to test some of the key ideas to see if they work.  In addition looking at the data you can get an idea of how much of an impact a modification might have.

So here are some of the basic things you need to do

  • Make the car streamlined, the race is short enough that areodynamics doesn't play a huge part but you still want to avoid blocky designs , streamers or flags.
  • De burr and polish axles, (article coming soon).
  • Make the weight of the car close to maximum, (but don't go crazy trying to get it exact).  Article here
  • Lubricate your axles with a graphite / molybdenum lubricate.  Article here

If you want to take your car to the next level then I recommend the following:

  • Lengthen the wheel base, (coming soon)
  • Make the center of gravity around 4 1/2 -5 inches from front of the car.  Article here
  • Round and polish wheels (article coming soon)
  • Cut speed axles (article coming soon)
  • Select the four fastest wheel/axle combinations. Described in this article
  • Put the slowest wheel/axle combination on the front and raise it off the track. Article here.

Remember to have fun.

 

Winning Pinewood Derbies - One Wheel Up

One thing you often see recommended is putting one of the wheels up so that it doesn't run on the track.  The idea is your reducing the amount of friction by going from four wheels to three.  The question is does this work?  So I decided to put it to the test.  I did one wheel up for each trial with one in the front and then one in the back to see if it makes a difference.

As you can see in the tables leaving one wheel up does make a big impact on the performance of the car.  It is interesting that leaving a front wheel up has a larger impact than leaving a back wheel up.  I rationalize it by noting that the center of gravity is towards the rear of the car.  Thus since most of the weight is near the back then having both wheels down helps to distribute the load and keep the car running true.

The final test ran was to alternate which wheel in front was up.  The reason I do this is after I prepare my axles and wheels I give them a spin test.  This is done by holding the axle in one hand and spinning the wheel and then timing, (I just count or you could use a timer), how long the wheel spins.  Longer is better, if is spins more than 30 seconds you are well on your way.  It should spin smoothly and without wobble. I then take the best three pairs, (axle & wheel), and these are the wheels I put down and the slowest one I leave up.  So for this test the slowest wheel is on the left front. As you can see from the last table leaving the slowest front wheel up does produce the best results.

 

My recommendation it to put the three fastest wheel/axle combinations down and put the slowest one in front and raise it off the track.  However do not raise it to high as it still need to be able to ride the center strip of the track and help keep the car on course.

 

      Trial (sec)        
Test 1 1 2 3 4 5 Avg STD
All Wheels Down 2.313 2.328 2.310 2.320 2.330 2.320 0.009
Front left Up 2.242 2.258 2.254 2.254 2.255 2.253 0.006
          difference 0.068  
               
      Trial (sec)        
Test 2 1 2 3 4 5 Avg STD
All Wheels Down 2.482 2.452 2.481 2.476 2.452 2.469 0.015
Back Left Up 2.468 2.46 2.447 x x 2.458 0.011
          difference 0.010  

 

Test with the slowest wheel/axle combination on the left front of the car.

      Trial (sec)        
Test 1 1 2 3 4 5 Avg STD
All Wheels Down 2.350 2.345 2.351 2.330 2.344 2.344 0.008
Front left Up 2.243 2.245 2.249 2.254 2.286 2.255 0.018
Front Right Up 2.294 2.3 2.303 2.297 2.297 2.298 0.003
          Front Left Difference 0.089  
          Front Right Difference 0.046  

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