How lite is too lite?

[gWebber]

While reading through some old forum post , I saw one that mentioned that a PWD car (in pro classes anyway) doesn't need to be 100% pine.

This got me thinking about construction techniques and technologies using pine for a few parts and these thin plywood sheets I've read are often used as stiffeners.

In my idea the plywood might need to be about 3/64 to 1/16 to be strong enough (I think), but maybe as little as 1/32 thick ... Or assorted parts might be diffrent thickness depending on stresses.

But unless I'm crazy, the plan I think would make for an insanely lite frame, likely lighter than a "ladder" frame.

So, how lite is too lite?

 

[Vitamin K]

In the Eliminator/Unlimited classes, some of those bodies get crazy light. I think we're talking 8 grams or less.

The catch is, the only benefit to a superlight body is that you get more flexibility with weight placement. If you're going to set an insanely low CoM, you need to be running the razor's edge of tuning to control that car. Not that this isn't a worthy goal, just be aware of the tradeoffs.

 

[bracketracer]

One of my bodies at Nationals was under 4 grams bare.

You can buy 1/64" plywood. It works fine but I bet you'll find that it's heavier than you think.

How lite is too lite? If it breaks it was too lite......lol

Curious to see what you come up with. My test track is always open if you want a session.

 

[Vitamin K]

I'm interested in the possibility of using bamboo to build/support car skeletons. Stuff is crazy light, and has good rigidity.

 

[Crash Enburn]

I'm in the middle of building a new chassis for my avatar car. The chassis is all pine, and with cut-outs to form the ladder, it weighed in at slightly under 9g. Adding the ply skin will bring the weight up a tad.

Regarding stiffness, Spacewalker has a video or two out there showing how he builds his ladder (or as he calls them, 'wing' — he built and flew model aircraft for years) chassis. He builds with a pine frame, balsa rungs, and then 1/64" Birch ply to end up with a very light, and very strong chassis.

 

[Kinser Racing]

Once you get to much flex in the body, you've gone to light.

To much flex is a judgement call.

 

[gWebber]

I'll cover everyone replies in one reply here best I can.

Vitamin K - yes, I assumed tuning it might get very tricky and certainly beyond my skill set.
Bamboo is an interesting idea too.

BracketRacer - it might not be all plywood, I suggested that as it seems it can be fairly strong and stiff by using the grain.
Actual end product might be a mix of things that would need to be based on knowledge I don't have.
As for test track, this idea is far beyond my means to build, but maybe someone could.

Crash Enburn - yes, I've seen Spacewalker's video, that's where I got the idea of the plywood. My thinking is this might be (if it
would work) be a few steps beyond a "ladder" frame.

Kinser Racing - I think I've addressed the flex issue enough in the design, but without building it one never knows.

Just to clarify, I can draw the basic idea and design notes, but it would take some one with these skills to do it :

Knowledge of the assorted woods in question.
Enough PWD knowledge to see possible issues in the design.
Drafting skills to create an accurate design plot.
Access to a Laser Cutter to make the parts (this is a big one).
Ability to tune what might be completely insane.

If I find some time I'll sketch up some rough ideas and people can tell me if I've lost my mind.

 

[microbrush]

Look at Bulldogs video https://youtu.be/pqJDE7oz2mk for making a 7 gram body.

You can take a little more meat out of it than this and get it down a little more. Plywood would need to be glued on and that adds weight. Even strengthening the light body with CA glue adds weight. Using the dremal stand you can make the wood thin. Making the body very thin and strengthening with bamboo still yielded me a body the same weight I get with the method above. This is the weight of my son's MA car with the fenders added. I guess our prep needs improvement. I messed around with thin and even a hole in it with bamboo reinforcement. I have not raced the bodies as they seem to have way too much flex. Here is how I made the light body. The bottom of the car is flat and so I routed out the top of the car. Now the car only needs a top.

 

[microbrush]

These ones weigh about the same! Which one would you choose?

 

[gWebber]

These ones weigh about the same! Which one would you choose?

Most likely the one without the holes as the one with holes isn't really a rigid structure.
But just suppose for a moment my design was thinner everywhere and was still (hopefully) as ridged.

Yes, I'm aware adding glue adds weight.

 

[BulldogRacing]

I'M WORKING ON THE ULTIMATE PINE WOOD DERBY BODY VIDEO. I will be posting this under the subscription area of this site. As a sponsored racer I want to give back to John. I will show step by step how to remove many variables and guess work in making a body light, fast, and strong. I know some of the very top racers were using this exact design to place at Nationals! I will show how to make a light body frame in a short time that will blow you away. This method has paid for my racing because I have sold many of these to the pros. Now you will have the secrets to a pro body. This video will be worth 5 times what you paid for your membership, how do I know? Because I have made a lot of $ selling these bodies! Now you will see the secrets to a perfect body! I will have the video finished in a short time, until then go fast! P.S, this video will make my last 6.9 gram video look like pre-school!

 

[Kinser Racing]

Don't use a laser cutter, it warps the wood.

 

[microbrush]

Here is the thickness of the thin one with the hole in it. This is why I am saying to make it that thin you need CA glue and other things and by the time you do that the weight increases too much. But, is thinner better? I have to put some wheels on it and test it. There still is more flex than the other body. The weights are about the same and one includes fenders and his one does not have fenders yet and to attach to a thin body is somewhat difficult.

 

[Quicktimederby]

My 2 Laser cut body's ran pretty good at National's. I've had 0 warping with them. The heat is in the cut not the wood, you can grab the block right out of the cutter and it is no hotter than room temp.

Don't use a laser cutter, it warps the wood.

 

[Kinser Racing]

My 2 Laser cut body's ran pretty good at National's. I've had 0 warping with them. The heat is in the cut not the wood, you can grab the block right out of the cutter and it is no hotter than room temp.

Good to know. I just remember the ones that I have gotten from somewhere else were really warped.

 

[gWebber]

Ok, so I don't know if the drawings will make sense, but the idea is based on modern laser cut slot car drag racing frames.
They use aluminum (around 9 grams bare) and such but the idea should work in wood too

The frame is made from several sections that would be glued together with CA and the cutouts and wood thickness likely would have to go through some experimentation to find what is the limits before it's too thin, too weak or too flexible.
Upright cross members set up to make a triangular tube and the top plate closing it off should make it pretty ridged

Still I think it could be as thin as 1/16 or 1/32 (sides, base) and 1/64 (top) for a majority of parts with pine or bass blocks to hold the axles.

Maybe I'm crazy.

 

[B_Regal Racing]

Contact these folks:

http://www.sweenks.com/

They have made PWD laser cut bodies and may be able to help for a small fee. They do good work...

 

[Kinser Racing]

You can make an open (latter) style body covered in vinyl or Monokote and get it to under ten grams. It'll take a lot less time, money and you'll get results.

 

[bracketracer]

gWebber, I would suggest that you use a one piece block for the two rear axles. Single blocks are ok up front but you don't want any misalignment to happen in the rear.

 

[gWebber]

Contact these folks:

http://www.sweenks.com/

They have made PWD laser cut bodies and may be able to help for a small fee. They do good work...

It's not something I could afford to build, just tossing around ideas.