Post Job Assessment

After I finished the skis I decided that some stiffness testing was in order.  The stiffness of a ski is critical to its performance.  A backcountry or powder ski needs to be lighter and can be softer.  The soft snow is more forgiving and high speeds are not typically achieved.  An east coast carving ski needs to be relatively stiff to hold an edge at high speeds and avoid vibration.  To test the ski stiffness I placed weights on the ski and measured the deflection.  After searching my house for weights I only found two 5 lb weights.  Since a gallon of water weighs approximately 8 lbs, I filled a bucket up with two gallons to get an additional 16 lbs.

I tested all the skis that I have to get a good comparison.  The goal with the skis this year was to design them so that they would be stiffer that the first generation Goliaths and softer than the second generation Goliaths and Tomahawks. This goal was achieved.  In order off stiffness, here is how my skis rank up from stiffest to softest.

1. Tomahawk
2. Arlia (M)
3. Arlia (R)
4. Panjshir
5. Elan Silver Magic
6. Blizzard Titan
7. K2 Kung Fuja
8. Goliath V1

The graph below shows the test results when 5, 10, and 26 lbs was applied to each ski.  Note that I tested the Arlia and Panjshir prior to attaching the second layer of fiberglass and topsheet; these results are noted by a (PC) after the name.


The calculations that I performed prior to building the skis are summarized in the table below.  These calculations did not take into account the stiffening properties of the epoxy.  The test results show that the epoxy has an average 4.18 stiffening factor for non-metal skis and around 2.4 for skis with metal built into the core.  The skis I built with metal rods in the core were very stiff and therefore the epoxy had less of an effect on the stiffness.  Since I don't have a pair of the second generation Goliaths I could not test the stiffness (these skis belong to my brother).

Overall the skis turned out as I expected and have a stiffness within the range that I had hoped for.  The following is a summary of what I have learned this season:

1. Rather than packing tape, use contact paper to protect the base and top sheet as the application and removal would be easier.
2. The cost of printing out full size templates is worth while.  In the past I printed out the templates on 8.5 x 11 paper and taped the sheets together.  This worked, but the skis were never quite straight.
3. Assuming a mold springback of 15 mm is safe.  The Arlia mold was designed with a 30 mm camber and came out at 15 mm, as planned.
4. Paper is a good medium for appling graphics (acrylic paint) and when soaked in epoxy becomes 70-90% transparent.  Cotton cloth still works well for opaque solid colors.  Solid colored construction paper may work well also.
5. If a cap construction is used rather than a plastic sidewall, then the sides of the skis must be sealed to make sure the core is sealed after edge grinding.
6. 120 grit sand paper / grinding wheel is appropriate for edge grinding.
7. A course jig saw blade works best when trimming flashing off of the skis after pressing.
8. Isospeed 4400 p-tex base material grinds more easily than Durasurf 4001 p-tex base material.  This is likely due to the Isospeed material being slightly harder than the Durasurf material.
9. West Systems Epoxy, 150B Resin / 206B Slow hardener wets up easily and provides enough time for lay-up without hardening.  The pumps are worth buying to easily measure out the epoxy.
10. Mounting edges on the ski mold effectively holds the base in place during pressing and simplifies the process.