Shrink Plastic on PWD

[Masco]

Excuse my naivety, but I've been thinking (which is dangerous), and based on my understanding there are elements which have the biggest impact on speed.

1) Weight and COM - maximizing potential energy and stability
2) Aerodynamics - wind resistance

One approach is to maximize the potential energy by removing as much of the chassis as possible and moving that weight to the ideal COM location. I assume this is why I see so many designs where the cars have a front wing (to rest on the starting pin and trip the laser at the finish line), then a narrow section unit you get to the front axle (where you need to extend across the entire body), then another narrow section between the front and rear axle until you get to the rear axle (where again you extend across the entire body). As of today (12/16/14) the cars you see at http://www.derbyworx.com on the home page are an illustration of this design.

However, I've also seen the classic "wedge" shape.

I assume the benefit of the first design is that you are removing as much of the wood weight as possible, resulting in more potential energy. While the benefit of the wedge is that it's more aerodynamic, because you only separate the air one time over the length of the vehicle, while on the other design the air is separated three times (front wing, front axle and rear axle). I am looking for confirmation on these assumptions from someone with more engineering and PWD experience than I have.

If correct, then I'm thinking that using some sort of very light weight covering over the car would prevent the air from having to separate multiple times.

A simple example would be a solid wedge car vs a wedge shape with a bunch of holes drilled in through the body. if all other variables are constant (including total weight, and COM location) the solid wedge would be more aerodynamic and have less potential energy, while the wedge with holes drilled in it would be less aerodynamic and have more potential energy. Correct?

Now, cover the car with the holes drilled in it with light weight film (could be shrink wrap, scotch tape...) and the result would be that the wind resistance between the two cars would be the same... correct? So, you have just maximized aerodynamics and potential energy with this wedge shape.

I'm very curious what others think about this hypothesis, and whether the delta in potential energy would actually result in any measurable difference in speed.

[Vitamin K]

The "Ladder Chassis" is not too uncommon among those with a thirst for speed, and monokote is a common covering used to wrap a holey chassis.

Perhaps you'd find this thread by Lightninboy informative?

[Speedster]

Welcome to Derbytalk, Masco.
I recommend you acquire Troy Thorne's book, "Build a Winning Pinewood Derby Car", 2013 addition. Learn how to build a rail rider/railrunner. Learn how to prep wheels, axles. Run the car on 3 wheels if rules allow. Learn how to use the "Search" function on Derbytalk. There is a wealth of information on DT. Forget about aerodynamics. If you follow all the rules of Physics your car will be aerodynamic.
Have fun working with your scout. After all, it's his car. Good Luck.

[Masco]

Speedster - Thanks, I just ordered the book on Amazon! And I will try to do a better job of using the search function

This is my sons third year and we just watched this video https://www.youtube.com/watch?v=-RjJtO51ykY and we both learned a lot. Outlined the physics in a way not only my son could understand, but me also...

[LightninBoy]

Excuse my naivety, but I've been thinking (which is dangerous), and based on my understanding there are elements which have the biggest impact on speed.

1) Weight and COM - maximizing potential energy and stability
2) Aerodynamics - wind resistance

....

I'm very curious what others think about this hypothesis, and whether the delta in potential energy would actually result in any measurable difference in speed.

I think you are on the right track. You are right to be concerned with weight and COM as the key factor. But I think you are missing a couple key pieces in that list and are over-emphasizing aero by quite a bit. And this is coming form a guy who's been obsessed with aero over the past year.

IMO, here is the list of PWD speed factors ...

1. Weight and weight placement (COM)
2. Alignment
3. Axle / Wheel Prep
...
...
...
...
4. Aerodynamics

Although aero is last and a couple notches down, it absolutely is a factor particularly as the races become more competitive.

[ngyoung]

I recomend you do not buy that book. Everything in it you can find free on this site, other PWD forums, and YouTube.

LBs list of priority is pretty close to mine. He's one of the fastest builders in pro league racing currently so definetly heed any advice he gives you.

First for priority I put axle polishing when using burred BSA axles. Leaving those raw is a huge speed killer. YouTube has plenty of videos on how to polish them.
Next rear wheel alignment.
Weight placement.
Make it a 3 wheel rail runner.
Negative cant the rear wheels positive cant the dominant front wheel
Set steer to 4" when rolled 4' on a slightly inclined flat surface.
COM is important but doesn't need to be super precise.
3/4" or more past the rear axle is pretty conservative.

The video you linked is pretty good to get you started. Like LB said and was shown in that video, aero is a factor but for most scout racing a simple wedge will perform just fine. That is there one thing that I feel you can really let your kid be as creative as he wants. If all the other factors are addressed it will still be fast. If winning is his goal I am sure at his age he will pick a shape that is aerodynamic. The only thing I really disagree with in that video that he got wrong about league racers is bent rear axles to canter the rear wheels and what he said about grooved axles. Pretty much all the book will tell you was in that video.

I highly recommend you comb through the rest of the 5kids racing site that vitamin k linked for the ladder body.

[Speedster]

Masco, the nice thing about the book is, it was written for kids and it has lots of pictures. Its been said, "A picture is worth a thousand words". It can be referred to in seconds because it's directly in front of you as you and your scout build the car. Also, you, and your son, can show it to others, especially if you do workshops. You ALREADY know how to build a fast car. The people on Derbytalk are a unique group. They are usually trying to get one more .001 of a second of speed. The desire for more speed is fine as long as the true reason for the Pinewood Derby is not lost. I wish you well. Have Lots of Fun with your scout. He will remember the time spent with you forever.
Cheers
Bill

[ngyoung]

Most of the active folks here on Derby Talk are prominently focused on the scout's experience. Yes some of the discussion on seeking out every last thousandth of a second but what was brought up so far in this thread is mainly the basics.

[Vitamin K]

I recomend you do not buy that book. Everything in it you can find free on this site, other PWD forums, and YouTube.

I think, for your average Pack parent, the book would be a good investment. It seems well put together, and has lots of nice illustrations and step-by-steps. I don't let my kids on the Internet unsupervised, but I'd be happy to let them read the book featuring Dash and Dottie.

Now, for obsessed folks like you and I...there are other sources of info that might give more of an edge, but Thorne's book covers most of the basics for speed well.

[sporty]

Super speed comes with building skills and is a learning process.
this site covers a wide range of information. Just ask questions. Some of the fastest and best car builders come here and help others and share info. As time and new posts come forth. Info can get shuffled down.

Knowing your rules and what you are allowed to do is fairly important aswell.
sporty

[ngyoung]

Sorry. Looks like I was thinking of a different book by David Meade "Ultimate Speed Secrets". When I first got to researching for my Tiger Cub's derby a friend let me look through it and I didn't see anything in there that wasn't already shared on here. The Troy Throne book seems like it is more geared towards kids. I am still a cheapskate on top of being a derby nut. I still feel everything worth doing at the scout level is all put together at http://5kidsracing.webs.com/cubscoutbuildtips.htm

I can understand having something else to visually show them can help them understand why each step is done and why it helps. I did pick out some of the shorter demonstration videos on youtube to show my son when we got started. The "EASY Pinewood Derby Car WINS using Science!!!" which came out after his first derby season was pretty popular with him though and he is excited for this season.

Beyond that any secrets mainly involve different coatings and waxes for the wheel and axles racing with oil and even those aren't very secretive.

[Masco]

Thanks to all. Being our third season, we have the basics down. This year we are planning on doing a 3 wheel car and putting positive cant on the rear and negative on the FDW. I will probably start another post as we being the build to share and ask questions during the process. I'm glad I found this resource. Thx

[ngyoung]

Got your canter backwards.

Rears like this... /---\

Front wheel with DFW shown on the right... !---/

[FatSebastian]

I was thinking of a different book by David Meade "Ultimate Speed Secrets".

I believe Ultimate Speed Secrets was a pamphlet on which the later "Pinewood Derby Speed Secrets" was later based. Troy's book is essentially the follow-on to Meade's "Pinewood Derby Speed Secrets", for which Troy was the illustrator.

I like books. "Time is money" as they say; a lot of time can be spent rediscovering "free" information on-line already contained in a convenient, illustrated reference book that a kid can hold in his hand. Books are inexpensive relatively speaking, and a good starting point for scouts.

[davidwilkie]

...The only thing I really disagree with in that video that he got wrong about league racers is bent rear axles to canter the rear wheels and what he said about grooved axles...

Curious if by ^^^^ you mean you don't think that rear axles should be canted?...or the method of the canting (bent axles rather than drilling at angle)?