pinewood derby

[El-Kablooey]

My son and I have a pinewood derby race coming up for cub scouts. A few of us dads are rather competitive at scout events. I have some ideas, but I would really like some input from some of you guys if you have any pinewood derby advice to give. Thanks!


BTW- I don't think jet propulsion is allowed according to the scout handbook! :(

[Bruno Ogorelec]

I don't know your local rules, but the general principle is to minimize drag.

At those speeds aerodynamics are not important, but you have to have rigid, hard, narrow wheels and use the best axle bearings you can afford.

Avoid wheel wobble. Make the 'chassis' rigid, so that the axles do not wobble either. They should track as true as possible even under side loads.

If you have steering, there's an important safety consideration -- Ackerman angles. Having each wheel steer around an arc is vastly safer than having the entire axle pivot.

Mass is also important. Cart weight is usually limited, so a fat son is definitely an asset in this particular pursuit -- unless the rules regulate total weight (driver+cart).

[pezman]

Maybe you could introduce an "eXtreme" pinewood derby that takes place after the main event:
http://www.et.byu.edu/~wheeler/benchtop/xderby.php

Maybe you already know about the following sites:
http://www.pinewoodpro.com/

If the track has a curved slope (i.e. starting relatively flat and then gettnig steeper at the top, then curving from steep to flat at the bottom) , then I think that there is some optimizing that you can do regardnig the center of gravity of the vehicle.

BTW, I was surprised how little free information there was on the topic ...

[larry cottrill]

Bruno, you are thinking of what we call 'Soap Box Racers' - I believe the Pinewood Derby uses cars that are carved wood blocks less than a foot long. So, no operator weight is permissible. I think that there are restrictions on wheels, axles and bearings that can be used, so that the issue of basic design is theoretically what determines the winner.

I believe these cars are allowed to contain a steel plate that is of a sanctioned standard weight - is this correct? If so, the car should be designed so this weight is carried as low as possible. All other things being equal, the lowest CG wins in downhill racing. I would try to get that weight plate right down next to the track - a real ground-hugger. The car should be a streamlined flat blade with most of the wheels sticking up over the top side edges.

I haven't seen many of these, but the ones I've seen all looked more-or-less alike: the common mistake is trying to make them look like a full-size car rather than optimizing them for what they are meant to do. Forget "scale" appearance and make one that is really different, in the right ways, and I'll bet you will have a winning car.

L Cottrill

[Bruno Ogorelec]

Bruno, you are thinking of what we call 'Soap Box Racers'

Indeed I was. Sorry. I was not aware of the existence of the 'pinewood' competition. Thanks for putting me straight.

[Anthony]

Bruno, you are thinking of what we call 'Soap Box Racers'

Indeed I was. Sorry. I was not aware of the existence of the 'pinewood' competition. Thanks for putting me straight.

So was I.

Bruno, up for a good ole' soap box race? Hehe.

[El-Kablooey]

Thanks people. Bruno, you may have been thinking of a different type of race, but you brought up a very good point anyway, wheel wobble and tracking. I think that will probably be the most important variable to the pinewood car, with c.g. right behind. Although speeds are low, aerodynamics could stil be important, as races will be close. I'm thinking a somewhat weight forward and very low cg as larry suggested, that should help make the rear wheels trail the front and be more prone to try to go straight. As for aerodynamics, I'm not sure. something tells me an airfoil shape would be best, nothing seems to slice through the air like a wing. I don't know alot about aerodynamics though, it just seems like the usual wedge tapering down to the front would have the opposite effect of what I want, creating drag and downforce at the rear of the car. I guess we'll see what happens, I'll post pics and results in a couple of days.
Thanks again! This forum is the best, absolutely the most informative and intelligent group of people I've ever known on the net.

[Hank]

Hello- El-K, I competed in the Pinewood Derby for three years back in the 60's. Weight was the deciding factor along with rolling resistance. I built to within five grains of the limit and polished the regulation nails to a mirror finish.
Good Luck.
Hank