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The Physics of Gravity Powered Race Cars


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#1 Magoo

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Posted 19 April 2014 - 16:13

Being so terribly sharp, you lot will find this interesting, I think... an examination of the mechanics of Pinewood Derby cars -- the little gravity powered racers used in the Cub Scouts. Very popular in the USA, almost  a rite of passage for little boys, but honestly I don't know if people elsewhere have ever heard of this form of motorsport.  

 

 

Anyway, do you agree with the video host's physics? Disagree? Have some ideas of your own?

 

 

 

 

Video: Hacks and Mods for Pinewood Derby Builders | Mac's Motor City Garage.com 

 

 

 

 

k22qc.jpg



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#2 Canuck

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Posted 19 April 2014 - 20:19

Comment before reading / watching:

I just finished my son's Cub-based version which is in essence the same thing, but with marginally different rules.  After abiding by what was perhaps too strict an interpretation and helping him only where it was in the interest of his safety last year, he was absolutely creamed by some creations that had all the hallmarks of not being prepared by 7 and 8 year old boys but their fathers.  This year I took a different route all together and leaned on all the resources at hand - a rocket scientist friend of ours (mine), a budding aero researcher, more than a few YouTube videos and lots of plans for R&D (that never matured beyond plans.

 

The rules for his car have specific dimensions to ensure it fits the track and doesn't interfere with the other cars, a max length, max weight and a brief but fairly inclusive "no bearings, no bushings, no washers" rule.  Given the nature of the competition, rolling friction seems logically to play the biggest role - wheels that run parallel to each other came up a number of times. 

 

Despite the prep, the execution suffered.  I had concerns all the way along about how I was going to successfully build the side pods as drawing them with a .100" thickness over the top of the wheel was easy while keeping them from breaking was another matter all together.  It doesn't help that as a woodworker, my incompetence is exceeded only by Homer J. Simpson.  Maybe.  In the end, the car raced without wheel covers and several ounces of lead weights covered by epoxy and what can only be described as sub-optimal wheel prep. 

 

I carefully deburred the "axles" (nails) and wheels, removing any flashing lines and reducing the surface area that might rub between the wheel and the body.  And then I lubed them.  I eschewed the established advice of using sewing machine oil or dry graphite and instead used a molydisulfide / graphite spray lube.  Something in the product softened the plastic in the hubs and rendered them useless.  This of course was not apparent until they'd been installed.  It goes without saying this all took place perilously close to the race start time.  We purchased a new car kit (cheaper than buying 2 sets of wheels that are sold in packs of 3), hastily installed them as best we could and ran it.

 

He didn't win  - that honour went to a brick with properly prepped rolling stock - but he was very competitive and landed in the top 5 and left with a huge smile, the ultimate goal given the tears of the year prior.  Next year is his last round before moving up and racing trucks so we've got one more shot.  This time we'll focus on the wheels and add it to what we learned.

Plan.jpg

Number7.jpg

His car is #7 - shown with other cars for scale.  The #4 pink LEGO brick took all the winning.  Speedy thing.

 

Final.jpg

Painfully close, but his final race as he got spit out the back by an alligator.

 

 



#3 gruntguru

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Posted 19 April 2014 - 22:34

What is that quote about the "aerodynamics of a brick"?



#4 John Brundage

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Posted 20 April 2014 - 15:29

My oldest son was in the cub scouts. We did the project together. We discussed body design. He decided on a shape and made a picture of what he wanted the car to look like. The completed car was in the shape of a teardrop with the larger radius forward and tapering off to the rear. I did the rough cut on the band saw and he did the finish shaping, sanding and painting. I taught him how to use verniers and to measure and locate the centerline. We milled the bottom to accept lead tire tape weight, and covered it with body filler to keep it from falling out. We also deburred the axles and radiused the edge of the heads of the brads. The wheels were polished to remove any imperfections.  The rules limit what can be used for lubrication. If I recall, graphite powder was on of the few allowed and was what we used. Ideally the car should be the maximum allowed weight for it to be quick. We made the car using our scales and made it a tick under weight as we did not want to be over on the official weigh in. My son's car won every heat.



#5 GreenMachine

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Posted 20 April 2014 - 23:22

Can't get this to paste as a link, but you/your son could try this ... http:/aigrf.BlogSpot.com.au/  :eek:



#6 MattPete

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Posted 23 April 2014 - 02:01

My only claim to racing fame is that I was the Pinewood Derby Champion of my pack when I was in Webelos.

 

There was no engineering involved, except for the imagination of a young boy.  In retrospect, the car resembled a Lotus 56 wedge, with a Lego astronaut (complete with helmet) glued to the top (the driver).  The grumbling from the other boys was that I won because only 3 of my 4 wheels actually touched the track.  Honestly, it wasn't engineered that way -- who expects a 10-year-old to nail all wheels in level?  Anyway, I've always wondered if that didn't inadvertently reduce the friction....



#7 gruntguru

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Posted 23 April 2014 - 02:22

. . . . .  Anyway, I've always wondered if that didn't inadvertently reduce the friction....

and the rotational inertia.



#8 Magoo

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Posted 23 April 2014 - 12:59

In the film, one of the seven critical design aspects was running on three wheels. 



#9 Canuck

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Posted 23 April 2014 - 21:56

I admit to being surprised by the significance of weight position. Single biggest contributor.

I am left wondering if, as in the case of the friction induced from rail-riding being over-ridden by the benefits of a smooth trajectory, if placing the weight such that the COB is on the rear axle and a front wing to pin the nose down will prove faster than a COB in front of the axle without the necessary aero device. Perhaps the angle of the start line is steep enough that one could get away with placing it behind the axle with a wing element that comes into play before the track levels out.

#10 Magoo

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Posted 24 April 2014 - 00:27

If the weight bias is toward the rear, I wonder if we don't want the bicycling end at the rear also. 



#11 gruntguru

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Posted 24 April 2014 - 02:47

Depends what produces the lowest friction - 3 evenly loaded wheels or one heavily loaded and two lightly loaded. Probably the former.

 

Of course the ideal car doesn't have that option - the closest would be two evenly loaded (rears) and one lightly loaded front (say 45%/45%/10%).



#12 Kelpiecross

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Posted 24 April 2014 - 13:45


My experience with small "Matchbox" - sized cars is somewhat limited but it seems to me that the best/longest/smoothest running cars are those with some very basic form of suspension.
Cars with no suspension seem to lose speed by vertical bouncing - presumably the energy for the upward motion of the bounce comes from the car's forward speed and this speed is not recovered when the car falls back down. This seems to happen even on the smoothest of surfaces.
I think the "Hot Wheels" series of cars has this type of "suspension"

#13 MatsNorway

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Posted 24 April 2014 - 15:26

Make one chassie with all the wheels on and a suspended mass. Its tricky to make true four wheel suspension hence our "stiff" rear axels used on various vehicles even today.

 

O-rings on a rod works as a damper. And is used on simpler rc competition models.

Big rods alined to the width of the car.

http://www.redrc.net...bmidb12rr-5.jpg

http://www.redrc.net...dy12thPod-1.jpg

http://www.teamcrc.c...i/slider-up.jpg

 

It might work to just glue it on the arm and lube the rod. Different lubes different damper stiffneses.
 


Edited by MatsNorway, 24 April 2014 - 15:43.