Sled Building Boo-Boos

“Make dogsled runners out of aluminum? Ha! Leonard Seppala tried that with surplus aircraft parts 40 years ago and the sled broke into pieces…” So said Cindy Molburg.

Cindy Molburg was the editor of Team and Trail, the one and only international mushing magazine for many decades. When a story was not covered in T&T the doubters would say it never happened. She had seen everything and knew everybody in the sport. But if someone, no matter what their reputation and record, even a legendary musher like Seppala, wants to build a proven wood sled design with substituted aluminum or fiber reinforced plastic parts, the outcome will usually be disappointing.

Two years ago I took pictures in Thunder Bay, Ontario, showing a team of sled dogs being loaded into a Douglas DC -3 for a charter flight to Greenland.

I remember flying with my family across the Alps from Rome to Zurich in a DC-3, probably around 1958. The flight attendants were  busy after take-off and before landing passing out candy and gum to chew as a way to relieve the pressure in our ears from the altitude changes. These aircraft were not pressurized like more recent airplanes expected to cruise at 10,000 meters or higher..

About the DC-3 shown in the picture, it has modern retro-fitted turboprop engines but otherwise the flaps are still fabric and the airframe is likely 60-70 years old. Old in airplane life  years but there has never been a DC-3 crash caused by structural failure.

My point is, if Seppala had known  a few things about building with aluminum like the engineers at Douglas Aircraft, his sled would still be flying 60 years later like the DC-3.

When I interviewed for a job at Boeing in 1969, the recruiter hired me with a degree in Engineering and Applied Science, a liberal arts type of engineering, not the typical engineering accredited program, and only one class in structural engineering. (We were discouraged from taking courses that were too practical, those were called “cement mixing.”) He said, all you need to know is MC/I and P/A, (the two basic formulas for calculating stress in bending or compression and tension.) That turned out to be true, because at the new hire level as a stress engineer, that was all I did need, and the more advanced concepts like fatigue strength were applied by veterans with the company, and the design principles like geometry, specifying structural elements and their dimensions, rivet head shapes and holes, stress raisers and notch sensitivity, etc. were the responsibility of a different group called project engineers with their accumulated experience going back to the Wright brothers. The overriding principle of least weight, optimum life performance design was called “one hoss shay” from a poem by Oliver Wendell Holmes written more than a hundred years before. About that another time.

Do you ever notice the flex of the wings on large modern aircraft in turbulence? Do we still fly wooden airplanes and use wooden skis? Progress comes from adapting designs to use new and better materials.

The structural and design properties of aluminum extrusions and rolled bars are very different from wood. If you try to use a solid aluminum part with the same dimensions as wood you run up hard against the greater density of aluminum. With hollow and roll-formed profile shapes you can take advantage of aluminum by specifying different cross sections, provided you know what you want.

Using aluminum to match the stiffness, weight and breaking strength of wooden runners, that would be a waste of the material. Less weight and greater breaking strength are usually better, although in a range of similar shapes there is a trade-off between the two; what about stiffness?

What you expect for stiffness in a sled runner may be conditioned by the past limitations of wooden runners. Remember the Noodle Sled? That was a design from the early 90’s with plastic and aluminum parts that made it ultra-flexible… Joe Runyan said it  was great on trials with moguls; maybe he has a photo. Or maybe I do somewhere in the archives.

With stiffness there is a presumption of greater breaking strength within a range of similar shapes: thicker will be stiffer and have greater breaking strength. In general the presumption is false; it does not follow that stiffer is always stronger. Twice as stiff could be half as strong. A stiffer cross section can have lower breaking strength and, for other reasons as well, may not be stronger in practice if you run into a tree or hook a stanchion on a truck bumper.

You can calculate breaking strength for any material and shape combination, and there is a defined standard testing procedure to verify your computation, but it is a proxy, just like the proxies used in scientific/nutrition studies. Flexibility is a virtue with benefits in mechanical properties as much as it is in biological systems and behavior. An elastic part may be able to “shed” and redistribute or absorb a shock or impact better. “Something’s gotta give.” That is what I told a musher who complained her sled “only bumped into a railroad trestle.”

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One thought on “Sled Building Boo-Boos

  1. Pingback: More Sled Building Boo-Boos | Everything I Learned From My Sled Dogs And More!

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