Monday, November 17, 2008

Make Your Own DIY Ski Pulk From Your Burley Trailer

I've wanted to make a DIY pulk for pulling kids around on XC skis for a while now. I have been toying around with trying to convert one of our other child carriers into a pulk capable of hauling kids. We have a baby-jogger stroller and a Burley D-Lite bicycle trailer (not compatible with Burley's new ski kit), and it seemed that one of these would work just great. After a few seasons of trying stuff and failing to get something that worked all the way around, I put the project on the shelf for a while and moved on to other things. But with the cooler weather has come some new inspiration, and a few ideas that seem to have solved some of the big issues I couldn't overcome in years past.

Why the Burley? Well, the Burley already provides a weather-proof shelter with seating and a way to strap kids in, so a lot of the difficult requirements for a kid-pulk are already taken care of. While I love the plastic sled pulks and have made one of my own that I like alot, their design breaks down when it comes to providing shelter and a secure spot for kids to ride along. Plus, if you've ever used a Chariot trailer, you know that the Chariot rocks and is way better than the plastic sled pulks - kids like them more, and they are alot smoother to pull because of their skis.

So, now that we have the trailer part provided by the Burley, here are the areas that needed a different solution:
  • Pulling system: must attach firmly to the trailer, must be easy to dismantle, must fit in a car, must not be too heavy, must minimize single points of failure, and must allow for rigid separation between the trailer and the skier so that the ski trailer doesn't crash into the skier on downhill sections.
  • Harness system: must attach firmly to the skier at the waste, must not ride up or down on the waste, must have a comfortable way for the pulling system to attach, must be easily removed when the skier wants to take breaks.
  • Ski attachment system: best if it provides some front-to-back swing motion so that the trailer can swing up and down independently of the skis (allows skis to move over mounds and dips without forcing the trailer to follow the same plane as the skis), must provide some rigid vertical support so that the vertical segments that attach the skis to the trailer won't collapse inwards or outwards, must be removable without too much effort, and must make use of cheap second hand or broken skis.
And of course, all of the parts can't add up to much, and the project shouldn't require any special tools or skills (welding, for example). If I'm going to be pulling around a home-made pulk, I don't want to spend as much as a Chariot would cost me.

Harness System
The first part I tackled was the harness system. I ended up using a waist belt commonly found at a hardware store in with the tool belts (although this one was specifically a back support for lifting). The one I found is similar to this one, but mine didn't have a buckle on it (in fact, the one that is linked here looks like it could be even better). This was the most expensive part of the whole kit at $20. I also decided to add a locking belt made of nylon webbing and a plastic fastener to add some increased support for the attachment points in the pulling system.

Pulling System
Next up was the pulling system, for which I came up with two designs:

Design 1: Fashion two pulling poles out of metal poles (1/2 inch EMT electric metal conduit, easily found in your hardware store's electrical section). Use a post or other similar surface to bend the poles slightly at one end so that it creates a bit of a rise from the trailer up to the skier's belt. Drill holes in the ends with the bends in them, and use the Burley's included cotter-pin fasteners (the ones that fasten the bicycle attachment arm to the trailer) to fasten the poles to the trailer. Flatten the other ends of the poles with a hammer and drill holes into the flattened part. Pass caribiners through the holes and attach the carabiners to the waist buckle.

This design seems ok, but I didn't like the way it fastened to the waist harness - I used this same attachment scheme on another prototype and while everything stayed attached, I found that my hands and arms would sometimes rub against the carabiners and sharp metal edges. The loops on the harness also seem like points of failure. But the two-pole design is a pretty tried-and-true design with respect to easily pulling the trailer, and is also nice because it allows you to back the trailer up pretty easily if you need to turn around. NOTE: If I had more time, I might follow some of the suggestions in Ed Bouffard's site for creating a better waist harness and fastener system.

Design 2: Use the Burley's bike-trailer attachment and some 1/2 inch EMT conduit. Bend the trailer-side portion of the conduit into a triangle to mimic the frame on the bike where the Burley's bike attachment arm attaches (use the same method of pipe-bending shown in Design 1 above). Use some other conduit attachment parts to to attach the pole to the waist buckle. See the photos for details, but it basically consists of a bunch of parts you can find in the electrical section of your hardware store and a cotter pin that I was able to find in the "fasteners" section of Lowes / Home Depot. You'll need an old drill bit to drill some holes in the metal pole and the threaded 1/2 inch conduit fastener. Also, make sure that the triangle part of the pole that you create is perpendicular to the 3-holed metal part that attaches to the waist harness (see the 3rd photo below).

This seems like the most elegant and easy to use system. The use of the stock Burley bike trailer attachment allows for a bunch of flexibility that could be useful, and the fact that it attaches right onto the back of the waist belt means you don't ever get the poles rubbing your hands or arms. The possible drawbacks that I can foresee here are that the joint between the bike attachment and the pole either causes the trailer to dip up and down frequently (think of your kids riding the waves in a small boat), or the joint dips down and drags in the snow if you don't have enough tension on the line (perhaps this would happen going downhill).

The jury is still out on both of these, but I'm certain Design #1 will work, it just might need some refinement, and I'm hopeful that Design #2 will work because I think it's more lightweight and flexible.

Ski Attachment System
Once I had the pulling system down, I needed to figure out how to attach skis. I started out trying to find a way to attach skis using the same method of attachment as is used by the wheels (buy some wheel axles, put some kind of legs on the axles, attach skis to the legs), but frankly, this is a rather complicated and delicate proposition, as there are a lot of parts that carry a lot of the structural weight of the trailer. I tried a few designs, but they were all brittle and ended up breaking in some form when I added any significant weight to the seats in the trailer.

I also tried just attaching skis directly to the Burley trailer, but couldn't figure out a way to do this that lifted the trailer high enough off of the ground.

So recently, I decided to try something much simpler, which consists of using the wheels, which already provide some front-to-back swing for the skis in relation to the trailer, as well as providing a nice rigid vertical structure on which to attach the skis. The design simply consists of a few cruddy skis that I found at a local thrift shop cut to roughly 3 foot lengths, along with a 2x4 laid on its side and cut with a jigsaw to match the profile of the wheel. The skis are attached to the 2x4 bracket pieces with screws driven through the bottom of the skis into the 2x4. These ski brackets are then attached to the wheel using simple hose clamps. The hose clamps don't allow for super-easy ski package removal if you want to use the trailer for biking again, but since I figure I'll be skiing way more in the winter than I'll be biking, it's a tradeoff I'm willing to make. The brittle part of this design is the 2x4s - they could easily split, so we'll have to see how they hold up. I'll probably fashion a few extras and bring them along with a drill for my first test runs on the snow : )

I haven't test driven this design yet, but I'm hopeful it'll work. And I'd love to hear any constructive ideas of any sort on this design, or any other thoughts from folks who have similar ideas. Any thoughts can be sent to leifontheroad at hotmail dot com.

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