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Here is a snowmobile cutter I built over the last winter. It is a cat chasing a mouse chasing a chunk of cheese. The cheese is the hitch and the cat is the actual trailer. It is sized to carry a banana box and a bit more. The cat's tail is a handle to open the storage area and is held closed with magnetic latches.
As I haven't made anything with foam and fiberglass in some time I wanted a project with many compound curves.
People ask if it will carry passengers. I briefly considered a rumble seat but despite our well maintained roads it seems passengers deserve a suspension. Someone without a snowmobile might consider a smaller dog or person pulled version or toboggan.
As I haven't made anything with foam and fiberglass in some time I wanted a project with many compound curves.
People ask if it will carry passengers. I briefly considered a rumble seat but despite our well maintained roads it seems passengers deserve a suspension. Someone without a snowmobile might consider a smaller dog or person pulled version or toboggan.
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Signing UpStep 1The planning and layout
I started with a general sketch of the cat. The mouse and cheese were ideas that came later and were not originally planned.
The sketches were then turned into various scaled drawings with the layers of foam inserted within. The approximate shape of each layer was freehanded over the various surfaces but a lot of time was not spent trying to match coordinates between views. It was easier to tweak the layers as they were cut and shaped.
The measurements from the drawings were scaled up and drawn onto pink extruded polystyrene foam insulation. I started with a simpler sub assembly, the skis, rather than the main torso. The parts were cut out and glued together to create the appropriate thickness. Each section was then sculpted and shaped. This step was accomplished using a mix of hot wire cutting, saws and sanding.
As the legs were fairly simple, I started with them.
The sketches were then turned into various scaled drawings with the layers of foam inserted within. The approximate shape of each layer was freehanded over the various surfaces but a lot of time was not spent trying to match coordinates between views. It was easier to tweak the layers as they were cut and shaped.
The measurements from the drawings were scaled up and drawn onto pink extruded polystyrene foam insulation. I started with a simpler sub assembly, the skis, rather than the main torso. The parts were cut out and glued together to create the appropriate thickness. Each section was then sculpted and shaped. This step was accomplished using a mix of hot wire cutting, saws and sanding.
As the legs were fairly simple, I started with them.
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Styrofoam and polystyrene foams are available locally in sheets up to 4' X 8' in thicknesses up to 2". They can be cut with a hot wire that can be made as long as you want (although four foot is my practical limit) so you can easily cut curves or textures out of the long axis of a sheet (or airfoils for an airplane out of blocks, as we home builders have have). On the other hand, if it is going to be around gasoline or other solvents, order urethane or other fuel resistant foam. These foams CANNOT be hot wired but the urethane sands and sculpts VERY well. Automotives and hardware stores have polyester resins and glass, keep in mind wax is put in the polyester. It absorbs humidity and cures tacky for the next layer to stick all the better, the wax shields it for a glossy finish but can cause adhesion problems on the next layer. Polyester resin will dissolve styrofoam, polystyrene and other readily available foams, on the plus side it is much more UV resistant and costs less than epoxy. Epoxies won't dissolve foams, stick to just about anything but cost more and you can develop allergies similar to poison ivy, especially if you are casual handling it. Epoxy takes longer to cure, generally a good quality.
If you want fine detail, use urethane and coat it with epoxy or polyester resin without the glass, it will be fragile without the glass. I'm not a big fan of Bondo but did use it for filling some parts of the cat cutter. It is polyester based and will absorb water, if you wet sand the finish you may see your paint fall off later. Composite airplanes are filled with a mixture of epoxy and micro balloons, the balloons are tiny glass spheres that are so light they behave like dust. They are inert but play havoc on your lungs if you inhale them. Mixing more balloons with less epoxy will give a softer, lighter material that can be sculpted into very fine detail. The epoxy base needs a day or two to set but you could probably make a 30 minute polyester concoction. Keep your mix ratios consistent so you don't have hard and soft streaks in your carving. While the catalogues say it has no structural strength, it will take a fair amount of compression, it is my understanding it provides the buoyancy to the remotes that go into places like the Titanic.
I have also made molds out of combinations of wood, cardboard, foam and modelling clay. The oils in the clay keep it from dispersing into the clay. Interestingly, epoxy doesn't stick to duct tape or packing tape, so you can wrap an object or make a parting line with tape as a release agent, the strapping tape leaves a glossy finish. Every wiggle, scratch, thumb print in clay as well as dust caught under the tape will appear in the fiberglass. I haven't tried it yet but you could sculpt something in plaster, coat it with a parting material and make a mold.
Again, go on line to Wick's Aircraft http://www.wicksaircraft.com/ Aircraft Spruce and supply http://www.aircraftspruce.com/ West Marine http://ecatalog.westmarine.com/ or other such places and search for Composite Materials. Keep an open mind for projects, the cat's whiskers are made of 1/8' diameter graphite rod similar to what's used in fishing rods and golf clubs, they also have it in rectangular bars that would make killer kite sticks.
(B) I mail ordered most of the materials, including the metal hitch shaft and bearings, from Wicks Aircraft Supply in Illinois. The whiskers are their 1/8" diameter graphite rod. Type "Composite Materials" in the search box. http://www.wicksaircraft.com/ In years past I have also purchased materials from Aircraft Spruce & Specialty Co. http://www.aircraftspruce.com/ You may also check on line or at boat dealers for West Marine or other products.
(C) I helped build a home built airplane (Quickie Q2) in the 1980's, maybe you can find someone in your area building or modifying a plane, boat, car, etc. Wicks has a "Composite Practice Kit & Book" for $95 that looks promising. If you can split the cost with others you will also have people to bounce ideas off as you go. I wanted to build something with a lot of compound curves. However, JoAnn's fabrics has smaller diameter solid foam balls that would save time sculpting a head. Also consider the urethane foams, they sand very easily, but cannot be hot wired as the heat gives off toxic gasses. Start simple with just a figure head for a boat, truck grill or something more boxy like a mail box.
(D) I used a couple of different weaves, both about seven ounce per square yard. Weight and the cost of extra fiberglass were not a consideration so I put three layers on the outside and something like eight on the bottoms of the skis. The tail got three layers while the lid has two layers outside and one layer inside, and ended up so stiff the contact point where the tail touches the right side of the body was unnecessary. You can make simple sample lay ups to test and break, but as a bike trailer should be light weight I think one layer inside and out will work, with a second layer where you want durability. You can always repair collision damage