Material for False Bottom with Hollow Bodied Cars

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.
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SlowSteve
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Material for False Bottom with Hollow Bodied Cars

Post by SlowSteve »

I recently made a car that has a fairly rectangular body which is still fairly thick (3/8" or so) and to maximize the amount of additional rearward mass I could use I hollowed out the body using a dremel. So as to not degrade the aerodynamics of the car (assuming that leaving the underside of the hollowed out car uncovered would do so), I stretched masking tape along the underside of the car to give it a flush surface.

The only problem with the masking tape is that is doesn't really stick very well and it is fairly fragile. Have any of you tried using some other type of tape, film, veneer or something else to cover a hollow space in the underside of your cars?

Grubartez, I read some of your posts and I see you have built some incredibly light hollow bodies that you then cover to make a flush underside. What have you found that works best for doing something like this?

Thanks!

Steve B.
Last edited by SlowSteve on Sat Jul 12, 2008 12:16 pm, edited 1 time in total.
Grubartez
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Re: Material for False Bottom with Holllow Bodied Cars

Post by Grubartez »

Steve

I like to use 3 ply 1/64" birch plywood. I get it at the local hobby shop. Some of my friends who build R/C planes said to give it a try. I epoxy into place and then use a belt sander to sand up to the edge of the car. It's very strong when epoxied into place.
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Go Bubba Go
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Re: Material for False Bottom with Holllow Bodied Cars

Post by Go Bubba Go »

Grubartez wrote:Steve

I like to use 3 ply 1/64" birch plywood. I get it at the local hobby shop. Some of my friends who build R/C planes said to give it a try. I epoxy into place and then use a belt sander to sand up to the edge of the car. It's very strong when epoxied into place.
What he said, except that I use cheapo Wal-Mart super glue (4 pack for $1.29 IIRC) around the edges to glue it in place.

I buy a lot of stuff at a local Hobby Town where the guys are big on RC airplanes and got the same recommendation from them.

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SlowSteve
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Re: Material for False Bottom with Hollow Bodied Cars

Post by SlowSteve »

Thanks guys, that's exactly what I was looking for. :D I'm call some hobby shops to see who might carry that.

Steve
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Re: Material for False Bottom with Hollow Bodied Cars

Post by ScoutAndDadTeam »

Michaels caries it too.
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SlowSteve
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Re: Material for False Bottom with Hollow Bodied Cars

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ScoutAndDadTeam wrote:Michaels caries it too.
Great tip, thanks!! I stopped by Michael's on my way home and bought some.

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Re: Material for False Bottom with Hollow Bodied Cars

Post by Teeeman »

We've used styrofoam inserts to fill voids and overlay with nothing more than printer paper (glued on with Elmer's).

It will get a slight wrinkle though when you paint.

The birch is more work but WAY better looking.


Balso wood also works.


Some foams might fill voids and sand smooth...


but that spray on expanding foam like you use on windows to seal them around the edges... expands too much and has huge cells when expanded and won't trim up smooth.
Don't recommend it.

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Re: Material for False Bottom with Hollow Bodied Cars

Post by Nitro Dan »

Teeeman wrote:...Some foams might fill voids and sand smooth...
I find blue or pink insullation foam sands very smooth (and is extremely light), but you'll need to coat it with Elmers glue or a laquer primer before spay painting it with anything else; otherwise, it will pit the surface.

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Re: Material for False Bottom with Hollow Bodied Cars

Post by millettbbc »

This year and past years I have done the same thing you are doing but this may be my last year. In order to keep the weight as low as possible in the car next year I am going to hollow the top of the car and make a carved plate to fit over the opening then fill the cut line, sand, and paint. I hope it works for me and it may for you too.
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Re: Material for False Bottom with Hollow Bodied Cars

Post by Kenny »

You might take a page out of the rc airplane builder's book and use monokote or similar film. Application is pretty easy once you see it done using a small heated iron. The heat triggers the glue on the film to stick to the wood. It is extremely light-weight and durable, very shiny and comes in a wide variety of colors. Downside is that it can be a little expensive. Places that stock it would be hobby shops that cater to RC Airplanes. For help you might contact a local RC airplane hobbyist to give you some pointers on application (and they might have some small scraps in the garage they'll give you!)

I scratch build 1/4 scale rc aerobatic stunt planes. Most are nitro powered and some are electrics. Since my son Andrew was a Tiger we have applied a combination of techniques gleaned from traditional PWD and from high performance rc airplane models. This strategy has served us very well. In RC Models it is all about finding a balance between power to weight ratios, strength, and durability. Me thinks PWD is similar in many respects.

RC Plane builders rarely paint their planes because paint is simply too heavy compared with film. And the films look sooo good properly applied.

This year, we'll probably use iron-on film exclusively on our cars.

Be forwarned, that every single little blemish shows through the heat shrinked film, so you want to play extra close attention to a superb sanding and filling job prior to applying the film to your car body. If you do it right you will be rewarded with a very light finish that looks wet when you are done.

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Re: Material for False Bottom with Hollow Bodied Cars

Post by Grubartez »

I've seen a couple of cars before that were covered in the Monokote and they turned out very well. The bodies were super light yet very tough and sturdy. I wish I new how to use it, I would sure build one in a heartbeat. Maybe someday down the road.
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Re: Material for False Bottom with Hollow Bodied Cars

Post by Randy and Son »

You might try using those "Body Skins" custom transfers made by PineCar. They are intended as a sort of whole body decal for a derby car but might just as easily work to cover the opening on the bottom of a hollowed out car.

I had never heard of Monokote before but the description reminded me of this and I wondered if it might work as a poor man's Monokote?

Randy
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Re: Material for False Bottom with Hollow Bodied Cars

Post by GravityRacer »

I don't know about the body skins, but Monocote is really tough. It is still susceptible to sharp force trauma, though. This should be a slight occurence, since it's on the bottom of the car. My current avatar has a 3/32" skin of plywood on the bottom.
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Re: Material for False Bottom with Hollow Bodied Cars

Post by quadad »

I have to ask the question - has any ever proven that having these "voids" on the bottom of the car slows it down ?

I could be wrong, but just my gut feel is that if most cars have pretty solid bottoms up to the front wheels. Its hard for me to imagine that having holes further back than that makes any real drag.

Alternately I consider using the birch or whatever skin on the top to allow my kids to drill lots of holes straight through the body and not have to 'clean them up' much. It would seem to be a quick way to get weight out and have it still look good. Kids seem to get bored doing the "clean up" filing and sanding anyway.
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Re: Material for False Bottom with Hollow Bodied Cars

Post by GravityRacer »

I do the cover up for aesthetics. My fastest racer (a wedge) had an offset hole in the bottom that I did not bother to cover. I have a CFD program that I might try to put the hollow shape into, but I am not intuitively convinced that there would be much effect, so I haven't bothered.
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