Sled Building Materials Spreadsheet

“An’ it ain’t no use to sit and wonder why, babe
If you don’t know by now…”

MATERIALS: WHITE ASH 7075 ALU TITANIUM 3-2.5 TITANIUM 6-4 MARAGING 350 POLYESTER GLASS EPOXY HT CARBON
DENSITY p 0.60 2.7 4.5 4.5 8 2 1.6
YIELD STRESS S 8.00 80 105 160 350 120 230
MODULUS E 1.80 10 15.5 16.5 30 6 19
STRAIN at YIELD e % 0.44 0.80 0.68 0.97 1.17 2.00 1.21
WORK to YIELD W 17.78 320.00 355.65 775.76 2041.67 1200.00 1392.11
WORK to YIELD/p 29.63 118.52 79.03 172.39 255.21 600.00 870.07

The data in the spreadsheet fields apply directly to rods or bars in compression or tension from loading in the length; axial loads. With some simplifications it is possible to interpret for comparisons of bending properties of skis and runners.

If all other factors were equal this spreadsheet suggests that the material properties of the two fiber reinforced plastics FRPs in the last two columns (with far higher work to yield per unit weight) would easily make the strongest lightest sled runners or skis. For I beams or laminated/stressed skin with light cores,  the most strength and weight is in the skin or flanges with proportionally little weight wasted in the core or web.

For similar profile beams in bending, stiffness will increase with increasing Modulus and depth/thickness and width of the beam.

Strength of beams usually corresponds to the load on a beam spanning a given distance and the bending “moment” of the beam at failure. These are values calculated under static conditions. A ski or runners are not subject to the same kind of static loads. The load increases and decreases over time as the runner comes up and over a bump. A more flexible runner will “shed” part of the load by bending.

For these reasons the Work to Yield field and the corresponding Work to Yield divided by density give more useful comparisons of different materials.

Unfortunately attaching other components to FRPs can be a problem, forming a curve in the front of the runner also a problem, and making runners or skis with light weight cores from these materials by hand lay-up methods can increase the cost to be prohibitive. Another factor is the cost/benefit for weight saving at different baseline weights. If a pair of aluminum runners can be made weighing 3 kg/7 lbs at reasonable cost is it worth the extra cost in $ to make them lighter to save a given amount of weight considering also other downstream costs and difficulties to use FRP? Cost and fabricating could also disqualify the Titanium materials.

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