Engineering a Hands-Free Toboggan

Engineering Product or Process???: I had a difficult time deciding whether this should be one Instructable or two. This project is both an example of the engineering process and a product of the engineering process. I have tried to create an example (exemplar in educational jargon) that I can show to my high school students to demonstrate the kind of discovery process that I want them to experience. I have also tried to make a cool sled steering and braking system. The cool sled steering and braking system is the focus of all of the steps below. The entire discovery process is crammed into the video "Engineering A Hands-Free Toboggan." This is part of my ongoing search for a compelling way to teach and assess the engineering process.

About The HFT:

The HFT (Hands-Free Toboggan) module is a removable module that attaches to a typical plastic toboggan to give you hands-free steering and stopping capabilities. It attaches with three wingnuts, so you can remove it quickly if you want the classic experience of a basic plastic sled. The entire contraption packs up into a fairly small bundle for storage or transportation.

The inspiration for this design came from my family's experiences sledding down the Burrows Trail (Camel's Hump Mountain), here in Vermont. We usually steer and control our speed with our hands and feet (like jtowle demonstrates in this Instructable), but on this long descent we can build up so much speed that our arms are in danger of getting dislocated by a passing tree or rock. And placing our feet in the snow to slow down throws snow in our faces. My thinking was that if I could control my speed and steer without having to use my arms or stick my feet out of the sled, I might be able to do things like video my kids, eat a snack, drink some cocoa, or at least enjoy myself more thoroughly. I haven't had a chance to test it on a long run in the mountains, but you can see from the video that it works well on our hill behind the house. We're supposed to have a big storm this week, so I'm hoping to do a real test soon.

Materials:

1. Plastic toboggan sled. [It's best if the rim is an even height along the back and sides
2. Paracord (appx. 5 ft)
3. 1/2" Electrical Metal Tubing (EMT) (appx. 5 feet)
4. Plywood (appx 1/2")
5. 1 1/4" Drywall screws
6. 1/4" x 1" bolts (at least 5, maybe 10 or so)
7. 1/4" x 1.5" bolts (2)
8. 1/4" regular washers (get a dozen)
9. 1/4" fender washers (6)
10. 1/4" nuts (at least a dozen)
11. 1/4" lock nuts (4)
12. 1/2" washers (maybe 2 -- if your pulley swivel eyes have small holes, you could use 1/4")
13. Small piece of sheet steel -- appx 16 gauge (there's a workaround if you don't have this)
14. 2 Small pulleys with swivel attachment
15. Short segment of 1/2" pvc (about 1')
16. 1/4" Bungee cord (4') -- sold in bulk at Ace hardware
17. 1" webbing (12') -- I used polypropylene
18. 1" Zinc Ladder Lock Webbing Buckles (2)
19. 1" side release no-sew buckles (1)

Tools:

1. Torch (or other source of fire for sealing frayed ends of fabric)
2. Angle grinder with metal cutoff wheel and flap disk
3. Saws (jigsaw, chop saw, circular or table saw)
4. Drill and bits (1/4" and 3/16")
5. Impact Driver and phillips bit
6. 1/2" Conduit bender (there are workarounds if you don't have one or can't borrow one)
7. Vise grips

1. (fig 1A) Cut a rectangular piece of plywood for the base. In one dimension, the base should be wide as the outermost edges of your sled. In the other dimension, the base should be about 10 inches.
2. (fig 1A) If your sled is longer, you may want to move your platform and seat farther forward. In which case, you might want to include an extra lip (shown in green) to provide an easily accessible spot for the central mounting bolt that holds the module to the sled. Fig 1F shows the mounting bolt positions (yellow) on this sled. This is a short sled, so I have the steering module pushed back as far as possible. The orange line represents the lip of the sled. That's where the bolts attach.
3. (fig1B)bCut out and drill two copies of the cam pattern from plywood. You can print the patterns (see attached SVG files) or sketch them out yourself. If you want your contraption to work like mine, the most important thing at this point is to drill the holes in the same place, relative to one another. The larger holes are 1/4" diameter. The smaller holes are 3/16".
4. (fig 1C) Cut a 5" long segment of 2x4.
5. (fig 1D) Insert 1.5" long 1/4" bolts through the cam bases as shown. Tighten down with nuts and fender washers until the head sinks into the wood far enough so that it won't rotate. Then cover with some hot glue to keep it in place (fig 1E). The purpose of securing the bolt is so that you don't have to grab the head later on, when you screw the lock washer on to the other end. The lock washer will rotate the bolt if it's not held in place.

1. (Fig 2b) Arrange the 2x4 on the rectangle as shown.
2. Screw the 2x4 to the plywood by inserting 1 1/4" drywall screws from the plywood side. I forgot to get a picture of this.
3. (Fig 2a) Use 1 1/4" drywall screws to attach cam bases to the module base and the 2x4. I like to pre-drill with a small bit (maybe 1/16"), to prevent splitting. But if you have lots of 2x4 pieces, maybe you would prefer to risk splitting.
4. (Fig 2D) Don't forget to screw down both sides of the cam base. I forgot to do this once, and my cams ripped off when I hit the brakes. The blue circled screws are the ones I tend to forget. The screws will be too long for the plywood, but that's okay. You'll be cutting them off.
5. (Fig 2C) Use a cutoff wheel to cut off the ends of the screws that went through the plywood. Then grind them down with a flap disk or other grinding implement.

1. (Fig 3A) Position the base on the sled. So that you can drill through the rim of the sled and use those drill holes to bolt the sled to the module base. Make sure that you're going to have room to access wing nuts on the bolts. Figure 3A shows where I placed my bolts. I drilled from the sled side, one hole at a time. Each time I drilled a hole I place a bolt in it, so that all of the bolts would be aligned in the end. The sled is plastic, so you can stretch it a little one way or another if your holes seem to be off. Figure 3B shows the bolts from the other side.
2. Trim the module base so that it covers the sled but also shares an edge with the cam base (fig 3C). You can see me sketching part of my cutting path in figure 3B.

You can make the paddles out of other materials, but EMT conduit works great (though bending it to the shape you want can be tricky). Whatever you use for the backbone of your paddles needs to be strong and as light as possible.

1. Figure 4A shows how the metal conduit needs to be bent for the paddles to contact the snow at the right angle and at the right place.
2. I made a template using a piece of wire (figure 4B) that I bent into shape until it looked like it would be a good backbone for a paddle.
3. (Fig 4C) I then used a conduit bender to bend 1/2" EMT into the shape of the template. I bent it cut it, and ground away the sharp bits. You can see that at one point it was helpful to place the pipe on a stool (because it would not lay flat on the ground, which seems to be the easiest position in which to bend tubing).
4. (Fig 4D) I somewhat flattened both ends of the pipe. This makes it easier to insert screws later on. Figure 4E shows these flattened surfaces with screws (from a later step)

1. (Fig 5A) These directions relate to the left paddle. The right paddle is the same but reversed.
2. I first orient the cam (which my son calls the "lemon") so that the center 1/4" hole is at the middle of the "clock." I place the 1/4" hole (red in the diagram) at 3:00, and I place the 3/16" hole (green) at about 10:00. The whole lemon-shaped cam is asymmetric, so if you can't manage this arrangement, you probably need to flip the cam over.
3. I position the metal tubing so that the main arm is aligned vertically in the picture (along the blue dotted line). This blue line is offset from the center of the cam by 2-3 inches.
4. You should probably clamp the tubing in place, but I just hold it and screw the drywall screws right through the tubing and into the cam, using an impact driver.
5. (Fig 5B) Add several more screws
6. (Fig 5C) Cut off and grind down the screw ends.
7. (Fig 5D) How things should look now.

1. (Fig 6A) Drill 1/4" holes about 1" rearward (relative to the sled) from the mounting holes (circled in red). Install 1.5" 1/4" bolts into these holes (orange) from below the base, and tighten them into place with a nut and fender washer. Tighten until the bolt head on the back is flush with the plywood. This bolt head needs to sit on the rim of the sled without pushing into it, so it needs to be recessed. The purpose of the fender washer is to give you cinching power to pull the bolt head into the wood without doing the same thing on the top of the wood.
2. Fig 6B shows how the bolt head should look.
3. Fig 6C shows the pulley already added. I forgot to take a picture showing what's underneath, so I created 6D to show the details.
4. (Fig 6D) To install the pulley, remove the fender washer and replace it with a regular washer (yellow). Then add a regular nut (gray). Tighten the nut and then slip the swivel eye of the pulley (pink) over the bolt. Then add another washer (yellow), followed by a lock nut. Don't over-tighten the lock nut. The pulley needs to be able to swivel around.

1. (Fig 7E -- oops!) Cut the seat out of plywood. The dimensions are shown. While you're at it, cut notches for the cam return bungees.
2. Fig 7A shows the positions of the seat stops and the hinge.
3. Fig 7B shows how I cut the seat stops (from a 2x4) with my chop saw. The seat angle is 76 degrees, so I made two cuts at 38 degrees, as shown.
4. Fig 7C shows the 2x4 piece sliced into two halves.
5. Fig 7D shows how I attached the seat stops and the hinge.
6. You can't actually see how I attached the stops. I attached them to the seat back using 1 1/4" drywall screws, inserted from the seat side.
7. After much second guessing of myself, I decided to bolt the hinge to both the module base and the seat back. The 1/4"bolts are indicated in blue, and you can see that I also used a few screws (shown with red).

I have a history of trashing nice things in an attempt to make them better. So when my wife saw that I was "modding" the sleds, I remembered that it was important to leave as small a footprint as possible. That's why the only modification to the sled itself is three 1/4" holes. The problem there is that this steering module is held on by only three bolts, and I was worried that a traditional bolt head or 1/4" washer would rip through the plastic. Hence the need to make "big mounting washers" (see Fig 8A).

If you're not worried about aesthetics, you can simply place a strip of wood under the sled rim and then screw drywall screws down through the module base, through the rim of the sled, and into the wooden strip. This works great! But it leaves ugly little screw holes that people who attend to details will inevitably find. So here's how to make the washers...

1. (Fig 8B) Find some stout metal sheet. Drill some 1/4" holes in it. Then trace outlines of "washers" that will fit inside the rim of the sled.
2. (Figs 8C and 8D)Cut the washers with an angle grinder (or whatever you have). Then grind them to remove any sharp edges.
3. Fig 8E shows the bolts from the top, tightened into place with wing nuts.

1. (Fig 9D) Before you really get started with this step, use a fender washer and a lock nut (circled in orange) to mount the cams to the cam bases. Adjust the tightness of the lock nut so that there's the best combination of low wiggle and low friction.
2. (Fig 9A) Now rotate one of the paddle arms into the position where you would like it to begin contacting the snow. In Figure 9A, the red line on the table indicates the imagined height of the snow. Hold some plywood next to the paddle arm so that the bottom of the plywood sits on the snow. Trace a blade shape (as shown) on the plywood.
3. (Fig 9B) Cut out the shape, then, again, hold it next to the paddle arm. Repeat the process from step 1, and once you get the blade where you want it, either trace the arm's outline on the blade, or clamp the two together. I used the tracing method.
4. (Fig 9C)
5. Screw the arm and blade together, using the tracing to make sure that you keep them properly aligned.
6. Pound the ends of the conduit so they're flush with the plywood (for maximum safety).
7. Grind away anything sharp.

Tow ropes are great, but they can be a nuisance when they get pulled under the sled. The tow rope keeper prevents this. If you're worried that sitting on this metal wire hook will be uncomfortable, I attest that it is not.

1. (Fig 10A) Get some strong wire (coat hanger thickness). Cut a few inches and double one end back about half-way to the other end. Hold the doubled over end close to the other wire and clamp them together tightly with vise grips.
2. (Fig 10B) Insert a stout piece of metal bar or rod (at least 1/4" in diameter) as shown. Rotate the rod to twist the wire. After it looks secure, bend the free end into a hook (Fig C).
3. (Fig 10D) Drill a hole in the module base, just behind the hinge (green arrow). Slip some 1/4" bungee cord through the hole. Tie a knot to keep it from pulling through the hole. Tie the other end to your hook. Adjust the length of the bungee until you can hook the tow rope to it as shown in Figure 10E.

1. Seal the end of some paracord by melting it. I usually melt with a torch and then follow that up quickly by wetting my thumb and forefinger and using them to shape the hot melted end. Use whatever method works for you, but try not to settle for fraying paracord.
2. (Fig 11A) Pass the paracord down (from the top surface) through the 3/16" hole (shown in orange) of one of the cams.
3. (Fig 11B)
4. Pass continue to pass the paracord through the pulley on the opposite side of the sled. Leave an extra foot or so of paracord beyond the pulley. You can cut it off later, if you want.
5. Tie a knot in the other end of the paracord (indicated in blue). This will keep it from pulling through the cam.

The cord stops are used to adjust the neutral position of the paddles. They attach to the pulley cords, but since they're too big to fit through the pulleys, they dictate what length of cord can pass through the pulleys.

1. Fig 12A shows how to cut a stop from a 1/4" fender washer.
2. Fig 12B shows how to install a stop in the paracord.

Before I started using these bungees, the paddles just dragged in the snow and slowed down my sled. The bungees return the paddles to their highest position, keeping them out of the snow when they're not in use.

1. (Fig 13A) Slip some 1/4" bungee cord through the remaining 1/4" hole of a cam, entering from the top of the cam (green). Then tie a knot in the end the you passed through.
2. (Fig 13B) Slip the knotted end of the bungee through the slit in the seat back on the opposite side of the sled. The green arrow points to the opposite side, where the knot is now held in place by tension.
3. (Fig 13C) Tie a knot in the other end of the bungee (green circle). Before you cut off the excess bungee, make sure that you have enough length. The goal is to have enough bungee tension to return the paddle to its high position, but not so much tension that it's hard to extend the paddle to its low position.

You can use ropes for this. I used ropes on my first few iterations of the steering module, and they worked okay. But they were hard to adjust, and they tended to tangle, so I switched to webbing.

1. (Figs 14A and B)
2. Cut a 12" strip of 1" webbing. This is going to be the "small loop" (shown in orange in both figures).
3. Pass the 12" strip through the indicated end of the ladder lock buckle (orange), and sew it to itself. This is a permanent loop.
4. (Fig 14A)
5. Cut a 6" length of 1/2" PVC. This is the "stirrup." Also cut a 50" length of 1" webbing.
6. Seal the ends of the webbing and pass one end through the pipe. Sew the webbing to itself so that it makes a loop which passes through the stirrup. The entire circumference of the loop should be about 28".
7. Pass the free end of the webbing through the ladder lock buckle.
8. (Fig 14A) Feed some 1/4" bungee cord through the pvc pipe. Tie it to itself to make a loop as shown. The bungee loop should be fairly close to the pvc. It should be big enough to stretch around a boot and small enough to not slip off a small foot.
9. (Fig 14A) Tie the paracord (pulley cord) to the small webbing loop.

1. (Fig 15A)Drill two 1/4" holes in through the module base, near the bottom corners of the seat.
2. (Fig 15B) Cut two lengths (appx. 18") of 1" webbing. Using a hot nail (I heated mine with a torch), melt a 1/4" hole near the end of each length of webbing.
3. Fig 15C shows a bottom view of the module base. You can see that each length of webbing has been attached to the module base with a 1/4" bolt (1" long) and a fender washer.
4. (Fig 15D) Attach the no-sew snap buckle.