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 would be training at Sleeping Bear Dunes in western Michigan. While they're certainly not 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 training during the day. It'd be considerably more stable to pull our camping and rescue gear than to carry it on our backs.

While they said we could just pull any plastic toboggan with a line tied to our waist, they suggested that many add tracers to keep the pulk from bowling them over on downhill runs.

... And so the glove was thrown!

I found lots of simple designs online. But the most detailed discussion, and the design I liked best, was Ed Bouffard's at SkiPulk.com. He sells complete ski pulks, and offers a downloadable book for do-it-yerselfers. I downloaded the book.

His design starts with an inexpensive, expedition-quality sled. Then modifies it to be pulled by fiberglass tracers attached to one's hips.

The tracers have ball joint rod-ends, which are connected to the pulk by pinning them into a piece of aluminum channel bolted to the pulk's front corners. The tracers cross each other, forming an X, and eye bolts on the other end of the poles are clipped to a padded waist belt.

Crossing the tracers allows the pulk to track within your ski tracks instead of slamming into stuff you're trying to ski around. Stabilizer fins help prevent side slipping whilst traversing a hill.

His design is based around the The Paris Expedition sled. 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. I got mine from REI, but they do not seem to be carrying them any more, and they are getting hard to source. They are manufactured by ERAPro in Canada. Aubuchon carried them on the east coast, and True Value hardware stores carried them on the west coast. Tho you should call before you drive there.  They are popular with ice fishermen, so you might try places that cater to them ...

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 to the hips with carabiners.

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 basically followed the design in the book, tho I did add u-bolts on the front and back for hoist and belay points. 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 the 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 into the aluminum channels, 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 and 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.

This design has reportedly been used in expeditions from Mount Kahtahdan in Maine, to Denali in Alaska. During my first test run, I found that as I skied around obstacles, the sled tracked around, rather than into them. It worked flawlessly for me at my training 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. It is not designed to transport people safely. Nor has it been tested to lift heavy loads up steep hills. Build and use such a contraption at your own risk. If you somehow maim or kill yourself, or another while using any element of this design, don't blame me!

The downloadable book has been updated, and may not have the same info I used.

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, 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."


Whaddya think? Tell me how your build went.