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AWANA and Pinewood Racing Cars

April 2nd, 2017 by Math Tricks | Filed under Physics.






It has been a while since my last post.  That is because we have become engineers of sorts here – building race cars!

Specifically, these are AWANA race cars, which are very similar to Cub Scout Pinewood race cars.  The same methods shown here can be applied to making pinewood derby cars, so stick around if this is the case for you!  This article will be updated as we progress through the process, so be sure to check back often.

The fist thing we did was to expand the wheel base.  If you look at the kit images above, you can see the wheel slots that are cut by the manufacturer.  The biggest undertaking that can be made is to place the center of gravity as far back as possible – which is about 3/4″ in front of the rear wheels.  So, if you expand your wheel base, you can place your weights much further back than you could otherwise.  I am sorry I don’t have images of this because I decided to document this well into the engineering process.  Simply, we used a 3/32″ drill bit chucked up into a drill press.  The holes were drilled such that the wheel edges were almost flush with the rear and fronts of the cars.  Also note that one of the front wheel holes of each car was raised slightly higher than the other; we want the car to ride on three wheels instead of four – more on this later!

Next, we cut down the blocks down, sanded them, and then painted them with spray enamal.  The maximum weight of the cars is 5 ounces (141.748 grams), so we had to cut off a lot of wood from the blocks because they were well over the max weight.  Note also that we had to cut out a lot of material in the front of the cars in order to push the center of gravity towards the back of the car:

The wells you see in the back of the car are for adding lead pellets later in the process.

Side view of the cars – do you see how the front wheel hole on the blue car is higher than the hole on the orange car?  The hole on the right side of both cars is 1/16″ higher than the left for both cars.

 

Don’t let the Force be with you!

Friction is the enemy.  The axles that come with AWANA cars look smooth, but they can be improved.  By using a variety of sandpaper, we were able to take away the roughness of the axles; we wet sanded with 800, 1000, 1500, 2000, 3000, 4000, and 5000 grit sandpaper.

 

The axles were chucked into a drill press, and the axles were sanded at 2200 rpm.

 

After sanding, they were polished with Brasso metal polish, followed by waxing with Johnson paste wax to protect the axles from rusting.

The result is demonstrated in this photo of unprocessed (top) vs sanded and polished (bottom):

 

By wet sanding and polishing, you will greatly reduce the friction on your wheels, and your chances of winning races will be improved.

 

Weigh Down

Adding weight such that the center of gravity was shifted 3/4 inch in front of the rear axial.

The maximum weight for these cars is 5 ounces – a.k.a. 141.748 grams.  So where do you put it?  When the cars begin the race, they go down a sloped race track, which later becomes horizontal.  So, you want gravity to act on your car for al long as possible.  Therefor, you want to add the weight as far back as possible.  Where is the ideal center of gravity?  It is approximately 3/4 inch in front of the back axial.  This will provide adequate stability and weight in the back – a nice balance!  Be careful when placing the weight – you don’t want it back too far!  The weight at 3/4″ in front of the axial is risky, but if you are lucky, your car will be stable enough to win.  Consider placing the weight 1″ in front if your luck is not too good!

 

Race Day Results

 

The good new is that we had the fastest race cars!  The bad news – they were too fast!  Too fast?  Yes – so fast that they could not slow down at the end of the track.  The speed and the lack of material to impart friction during the breaking period resulted in the cars hitting hard into the end of the track.  The hard hits ended up disrupting the alignment, and the cars ended up flying off the track in later races.  One car was the fastest in all 11 races it completed, but one race ended up with the car flying off the track – resulting in an average time less than that needed for the top three.

 

Lessons Learned

Lesson 1 – the fastest car does not always win.  There is an element of luck involved.  The one crash may have been the result of other factors besides bad alignment.  Debris on the track could have triggered the crash – I don’t know – but I would like to think so – LOL!

Lesson 2 – proper breaking.  The cars were very difficult to slow down at the end of the race.  The holes in the front of the car were made to shift the center of gravity to the back, but they also decreased the surface area, and the breaking mechanism was not able to stop them from crashing into the wood block at the end of the track.  In hindsight, it may be good to add a silicone skin on the bottom of the cars to aid in slowing the cars.  Any other ideas?

Good luck with your cars!

 

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