Sunday, January 13, 2008

Building a Backcountry Pulk

They told us to bring a sled to pull our gear for National Ski Patrol's Mountain Travel and Rescue training. We were preparing to train at Sleeping Bear Dunes in western Michigan. While they're certainly no mountains, the immense sand dunes are perched atop towering glacial moraines, and offer ample opportunities to climb and descend steep terrain.

We'll be on nordic skis, camping two nights, and it'll be considerably easier to pull our camping and rescue gear than to carry it on our backs.

While they said we could use any plastic toboggan and just use line tied to our waist, they did comment that many in the past have added tracers to keep the pulk from bowling them over on downhill runs.

... And so the glove was thrown!

I found lots of stuff online. But the most detailed discussion, and the design I liked best, was Ed Bouffard's at SkiPulk.com. He sells complete ski pulks, but offers a downloadable book for do-it-yerselfers. His design starts with an inexpensive, but expedition-quality sled, which one modifies to allow it to be pulled by fiberglass tracers. The tracers are connected to the sled with ball joint rod-ends pinned into aluminum channel that's bolted to the front corners. Eye bolts on the other end of the poles are clipped to a padded waist belt. Stabilizer fins help prevent side slipping whilst traversing a hill. This design has reportedly been used in expeditions from Mount Kahtahdan in Maine, to Denali in Alaska.

I basically followed the guidance in the book. I did add u-bolts on the front and back for hoist and belay points. I found the The Paris Expedition sled at REI. It's 5'L x 20"W x 6"D, made of .125 mm linear polyethylene with 5 tracks for directional stability, and it costs just under $30. REI does not seem to be carrying these any more, and they are getting hard to source. They are now manufactured by ERAPro in Canada. Aubuchon carries them on the east coast, and True Value hardware stores carries them on the west coast. Tho you should call before you drive there.

Tracers were made from 6' x 1/2" fiberglass poles designed for electric fences. They're strong, and they spring back to shape after being flexed. I got mine for $2.50 each from Kencove Farm Fence. I found a hipbelt on eBay that came off a 1980's Coleman external frame backpack with loops sewn in just the right places for clipping to the tracers.

I got most of the specialized hardware from McMaster-Carr. Their web site is excellent in how it leads you to selecting the hardware which best meets your needs, and it is well illustrated.
  • Two thrust-rated ball joint rod ends with 14° swivel, a static load capacity of 3972 lbs, 5/16-24 threads - p/n 8405K121 for $8.59 ea.
  • Two zinc-plated steel locking pins with round retainers, 5/16" Dia, 1-3/4" useful length. (p/n 98416A125) $1.55 ea
  • Two zinc-plated, load-rated eyebolts w/ shoulders, 500 lbs working load, threaded to 1/4-20, with a 3/4" ID (p/n 3014T251) for $4.30 ea
  • Four zinc-plated, grade 2 coupling nuts threaded 1/4-20 & 1/2-13 (p/n 97088A205). $3.06 ea
  • Two Stainless Steel U-Bolt W/Plate with a working load of 1090 lbs. (p/n 8896T77) $7.04 ea
  • Misc. nuts, bolts, washers, epoxy, etc. were bought at my local Home Depot.

Metals Depot had good prices and reasonably small minimum orders. I ordered two foot lengths of the following:
  • 6063-T52 Aluminum Arch. Channel (Sharp Corner) 1-1/4 X 1-1/4 X 1/8 (p/n C311418) $10.06
  • 6061 Aluminum Flat 1/8 X 1-1/2 (p/n F418112) $ 2.96
  • 6061 Aluminum Flat 1/8 X 2-1/2 (p/n F418212) $ 4.84
  • 6061 Aluminum Angle 1/8 X 1-1/2 X 2 (p/n A3211218) $ 6.70

I cut the aluminum channel into two 1 1/2" lengths with a hacksaw, then used a Dremel Roto-tool with metal cut-off discs to round off the tops. I drilled them to 5/16" to accept locking pins. A little time with file and sandpaper, and they looked like jewels.

I had a local machine shop thread both ends of the rods to 1/2-13 to accept the coupling nuts. I also had them drill and tap both ends so that the rod ends could screw through the coupling nuts and into the rod ends. I'm concerned that this created thin walls in the rod ends and therefore weak spots, But the coupling nut will provide plenty of support, I filled all voids with epoxy, and epoxied everything in place.

I clipped the tracers into the harness and onto the aluminum channel, and clamped the channels on the front corners of the sled to set the proper toe angle. I mounted them with 1/4" x 1/2" bolts, nyloc nuts, with 1/8 x 1 1/2 aluminum backing plates.

I mounted the U-bolts to the bow and stern utilizing 1/8 x 2 1/2" aluminum as backing plates, and 5/16" bolts. Probably over engineered, but then I'm no engineer.

I cut the aluminum angle into 5" lengths, then cut the short side diagonally to create the fins. I bolted them to the bottom of the sled towards the rear utilizing countersunk 1/4 x 1/2" flat head machine screws and nyloc nuts backed up with washers.

To load it, I'll lay a tarp in the bottom, place my gear in, and then do a burrito wrap to protect it from the elements. I'll strap in the load, with 2" webbing and nylon web clips.

Pulling it around a local park, I found the sled could be steered by simply rotating my hips, and that the sled tracked around, rather than into obstacles. It worked flawlessly for our training evolution at Sleeping Bear Dunes.

OBLIGATORY DISCLAIMERS: I am not an engineer, and this is not an engineered design - The backcountry is inherently risky. Pulling a sled behind you on skis adds complexity to an already risky environment. Build and use such a contraption at your own risk. If you somehow maim or kill yourself, or someone else while using any element of this design, don't blame me, I am not responsible!

I have no financial interest in any of the sites to which I link except The Home Depot with whom I'm a former employee and a very minor shareholder.

Other designs:
Joe's Find Your Adventure article "Building my first ski pulk for winter adventure!!"
PMag's article "Build a gear sled the dirt bagger way."

I'd love to read your comments!