The Measure of Skating Skis
Every time we test skating skis you hear the same dialog. Crouched intently over their clip boards, the testers mutter “glide, no glide, squirrelly, stable . . .” as they concentrated hard to dredge up from their neural nets the impressions of each ski. One was heard to exclaim, “Well I know for certain - fit is important!” But how important is it? Skating skis are the test of ski fit. They have to glide well, run straight and provide a stable platform for balance. And if we can put some numbers on the factors effected by fit perhaps it will give us perspective. If we know something about the cost of poor fit it should help us make rational choices.
Length: Long enough to support your weight and a little longer to provide tracking stability. Increased length gives no glide advantage, decreased length -- same thing. Stability increases with length -- too short a ski reduces balance and increases tempo by 2-3%.
Flex: Stiffness related to body weight and track conditions. Too soft a ski (20% or more) will reduce glide by 4 to 5% and stability even more. The net effect can be to increase tempo by 15% and further shorten glide (6 to 8%). Too stiff a ski in hard track effects edge stability and downhill control. In soft track even a good ski slows down because it plows the soft track. If the ski is 20% or more too stiff it will reduce glide by 5 to 10%
Pressure Distribution: The way the ski distributes your weight over the length of the ski determines how far the ski sinks into the track, determines how the ski steers, edges and flows over the snow. The practical effect of this shape is complicated but we can put some boundaries on it. Glide -- wrong ski right flex (e.g., hard track ski with heavy pressure on tip and tail used in soft track) up to 10% plowing effect reducing glide. Stability -- wrong pressure distribution (everything else OK). Tempo increase of 3 to 5% due to poor balance.
Torque: This is the measure of skis resistance to twisting. It effects the glide and the stability. Too stiff yields very stable but slow. Too soft the reverse. This is a quality that is part of ski design and des not vary much. Over all effect with today’s skis 1 to 2% unless flex is wrong.
Base Material: High graphite, low graphite, split base, all racing bases are sintered high molecular density polyethylene. Using the wrong base for the day (with the right wax) is likely to be of no importance, or at worst reduce glide by ½ of one percent.
In prior issues, we introduced new measurements to allow calculation of forces acting on the shovel or tip of the ski. Measurement of compaction forces -- shovel splay or tip lift and frontal geometry and compaction angle to allow analysis of plowing. We have measured and identified the significant factors which slow the skis down and used those factors for comparative evaluation of performance. Let’s summarize what we know. There are five major factors which slow the gliding ski: 1) friction, 2) vibration, 3) compaction force, 4) aerodynamic drag and 5) plowing force.
Friction is a complex phenomenon not completely understood but related to the ski base material, the structure (roughness) of the surface and the composition and application of the wax. Friction is measured by force necessary to maintain glide relative to the total load on the ski -- the coefficient of friction. Thus, a coefficient of 0.02 means that 3.2 lbs. of force will keep a 160 lb. skier sliding. A coefficient of 0.05 would require 2½ times that force or 8 lbs. The details of wax, structure and base material will continue to fill the pages of the Ski Research News. (Stay tuned!)
Vibration is a loss of energy which like other energy losses reduces the speed of the gliding ski. In the next issue, we will cover this factor in detail. It effects both glide and stability and should not be neglected.
FC -- Compaction Force as measured by the Ski Research Group is pressure on the front end of the ski (Newtons/cm2). It makes measurement of the slope of the shovel of the ski. The slope is a function of the compaction force, the velocity, the compressibility of the track. For different snow density, we can estimate the depth of compaction and calculate the energy loss. All other factors equal, it allows comparative analysis of compaction losses which are probably as much as 5%.
Aerodynamic Drag Cross country skiing has only lately recognized Aerodynamic drag as an energy loss worthy of analysis. The advent of head to head racing; the pursuit races, the mass start relays and the sprint races have shown the advantage in drafting. Quantitatively, aerodynamic drag is a function of frontal area, and velocity squared .
Plowing Force is like aerodynamic drag only the flow is of snow around the shovel rather than air around the skiers. The factors of importance are snow density, frontal area (and shape) and velocity. The plowing or pushing of the snow out of the track along with the compaction of that track represent the most significant factor in soft track glide -- accounting for as much as 10% loss in glide.
In summary, the energy losses:
Friction 2%
Vibration 1% plus stability loss
Compaction Force 2 - 5%
Aerodynamic Drag 1 - 2%
(drafting -- up to 4%)
Plowing Force 5 - 10%
So there it is -- bad news and good news. The bad news is obvious. If you pick your skis at random by length you have dropped $500 in a very bad lottery. The good news is skis can be measured and tested to reduce these risks and the numbers we have given should help put the choices in perspective.