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.
Step 1: The 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.
Step 2: Jigging the Basic Shape
Remember that while Styrofoam can be hot wired, you must use epoxy resin as polyester will dissolve it. The floor was made of denser 3/8" urethane that cannot be hot wired.
After the skis were covered with fiberglass the vertical portions of the legs were glued on. The outer legs are set farther out than the front legs, somewhere I think I was picturing the curves of some sports car. The body and head were sculpted of foam and shaped as the skis. Fiberglass was applied once the desired shape was achieved. The leg cores were then added to the skis and the ski /leg sides jigged to the center vertical and horizontal planes of the body.
Additional layers of fiberglass were going onto the lower legs, so rather than trim the excess fiberglass from the first layup foam spacer blocks were glued onto the bottoms to allow the skis to sit parallel to the floor during jigging.
Step 3: Head, Shoulders and Haunches.
The legs, shoulders and haunches were built up with additional layers of foam and fiberglass.
For the head I considered lots of jigging with strips of 3/8" or 1/4" urethane contoured over them but in the end decided there would be enough free handing around the face to stick with layers of precut foam that wouldn't be trying to change shape as material was sanded off.
Step 4: Build the Cargo Lid
The upper portion is sculpted and hollowed out.
Again, I considered 3/8" foam bent over jigging but decided to continue with the hot wired foam arches I used on the more tightly contoured head. Part of this was because I had to deal with the curves of the rear hips rather than have everything come to a taper front and rear.
Two inch increments were marked off front to rear on the horizontal and vertical planes. Measuring along them gave the dimensions to cut the arches. They are not perfect circles, adjustments were built in with multiple radius points on the foam. As with the rest of the cutter, both sides of a section were pinned (or can be screwed or 5 minute epoxied) together, hot wired together, trial fitted, rough sanded and installed as pairs rather than doing one side at a time.
Step 5: "If It Didn't Carry Cargo It Would Just Be Silly" ( Making the Lid Open)
The hinge was put on the left hand (Driver's) side so that when properly parked the lid would open from the sidewalk side of the street. The reality then and now was people park on whatever side of the street doesn't have the "No Parking" signs up for plowing purposes, regardless of the direction they were driving.
Considering the typical building's door hinge pin, I used 3/8" rod to eliminate any worry of it ever bending or breaking. I also used stainless steel rod to eliminate any worry of the hinge rusting. The bearing on the end was capped off with grease in it, to supply grease to the hinge rod over the years. It would have been easier to remember to oil the hinge from time to time. Make sure any surface getting epoxy is properly degreased.
On the side that opens, the tops of the right shoulder and right rear haunch, each have a thick nylon spacer washer embedded in them. A straight edge was run from the axis of the hinge to the shims to align them with the direction the lid would be coming at them. Corresponding pins in the lid drop into them to keep vibration from shifting the lid as it moves down the road. Since it isn't expected to do a lot of bouncing, three pairs of magnets are embedded in the surfaces to hold the lid down rather than trying to fabricate a latch.
Step 6: Ears and Tail
While we wanted swept back ears, we did not want the lid to hang up on them when it opened. Eventually the top and back surface of the ear foam was covered with fiberglass, the foam then completely removed from underneath, ear canal dimples were cut into the sphere of the head and more glass applied to the underside of each ear.
The tail was made of 3" thick foam, made by layering 2" and 1" foam together. Curves were in each section, with new sections at different angles to give a 3-D curve. To get a more or less round tail cross section, rasp file the 3" X 3" cross section into an octagon, then rasp that into a 3" diameter tail.
Care was taken that the tail was far enough out from the rear right haunch that it would not hang up when the lid opened. I then touches the torso between the rear haunch and front shoulder to prevent too much stress traveling along the tail to the back. In reality, three layers of fiberglass later the tail and lid are so stiff that this contact point could have been eliminated and the tail left completely free of the torso.
Step 7: The Hitch
The 1" hitch shaft is welded to a 7/8" solid steel cross shaft, turned down on each end to fit into bronze bearings. The cross shaft came polished and would have fit directly into larger bearings with some sort of thrust surfaces on the ends but instead was turned down on each end to use the smaller bearings side-to-side thrust surfaces.
The hitch was made removable to ease storage or stuffing it into the car if it had to travel. Bearings were encased in flocked epoxy and glass with steel spacers providing the holes for the bolts. As it is important to have a hitch that doesn't come off on the road, it was overbuilt with eight grade eight 5/16" bolts and lock nuts through steel plates holding it on. Keep in mind a single 5/16" pin holds it to the snowmobile. Shorter bolts shown in the bearing block picture were keeping things aligned. The bolts holding them on go up through the bottom steel plate, through the steel spacers embedded in the bearing block and into a coupler nut (a.k.a. tall nut) embedded in the body. Shorter bolts inside the body go through identical steel plates inside the body into the same coupler nuts, assuring the bearing block cannot be pulled or sheared from the body.
The bearing blocks were slid onto the ends of the cross shaft of the hitch, the hitch was put in place with the bearing blocks mounted with connector nuts through holes in the body and the voids then filled with fiberglass and flocked epoxy filler. (Don't forget to use mylar tape or other release agent between the epoxy bearing blocks and epoxy applied to the body.) After setting, the hitch shaft was removed to allow clean up around the bearing blocks and to curve and add the mouse to the hitch shaft.
The center layer of foam in the mouse was cut off along the bottom, when reunited to the sides of the mouse this created a tunnel for the hitch shaft to pass through. Two nuts embedded in the body allow the mouse to be bolted to the shaft but also make it removable. Mouse ears were made out of foam, covered with fiberglass, the foam dug out, the ears were trimmed and glued to the head and shoulders. Additional fiberglass through the front of the ears tied them to the head.
The cheese was made by wrapping fiberglass around foam cores. The front of the wedge was then carved as needed, a hex shaped hole was cut in back and the shaft slid through the hole. The nose of the cheese was then glassed over and the various sized holes molded into the cheese with doming punches.
Step 8: Finishing Touches
The eyes are magnifying glasses with the irises painted on the inside / backside. I made the eye openings before I bought the magnifying glasses and had a bit of trouble finding the right diameter.
I was going to take it down state for painting and airbrushing but wanted to get it on the road, so I settled for spray cans. I suspect it will be repainted into different breeds from time to time.
Whiskers are 1/8" carbon fiber rod put through pre-drilled holes and anchored under the nose with silicon. In the event it comes home with a broken whisker it should be fairly easy to pluck and replace it.
Step 9: Road Test