ENERGY RETURN, VIBRATION DAMPENING & FATIGUE FAILURES OF ROLLER SKIS
What is fatigue failure? The rupture of a material as a result of repeated loadings at a stress much lower than the yield strength of the material under static conditions. If the stress is high enough, but only a fraction of the yield strength in a static condition, all materials will fail under repeated cycling. However, some materials are much more fatigue resistant than others. If the stress is low enough steel and titanium can be theoretically cycled forever. But even at very low stress aluminum, copper, brass and several other materials will eventually fail from cyclical loading. Because the density of aluminum is very low most airplanes are constructed of aluminum. With aluminums relatively poor fatigue resistance, aeronautical engineers have to determine the lifespan of the aircraft based on how many times the plane will take off and land and how many hours it will fly. Roller skis are under extreme stress as the shaft is supported only at each end. In order to reduce road vibration there must be some deflection of the shaft, but the deflection must be limited or the shafts will fail prematurely.
The shape of a beam can significantly affect the fatigue life. Two hollow rectangular tubes made of the same material and of the same outside dimensions and geometry and both having the same moment of inertia (I) can have vastly different fatigue life. That’s because the relationship of the inside geometry (I negative) to the outside geometry (I positive) can dramatically increase or decrease fatigue life. (In a hollow rectangular tube, the moment of inertia I is determined by subtracting I negative from I positive. The formula for deflection in this type of a beam is PL3/48EI where P is force, L3 is the distance between the end support structures cubed and E is the modulus of the material)
Numerous fatigue studies show that a very smooth surface can dramatically increase the number of cycles before fatigue failure. Fatigue fractures start from areas of localized concentration of stress, usually areas with a nick or a scratch mark. That’s because most materials are notch sensitive. Almost all aluminum roller ski shafts that fail are due to bruise or scratch marks on the bottom of the shaft or on the wheel forks where these indentations create high stress areas that cause fatigue crack propagation. When wheel forks are welded to roller ski shafts the strength of the aluminum of both parts in the welded area is reduced. Failure at the welded joints after repeated cyclic loading is referred to as fatigue weld propagation.
A weld that does not create fusion between the two parts is visually very apparent and if a load is applied to the weld joint it will fail immediately. The pneumatic wheel roller skis that we produced in the late 90’s used rivets to fasten the wheel forks to the shafts. However, after a period of time of cyclical stress the rivet joints would loosen. We have been using welded joints on the pneumatic wheel skis for 14 years and they have been very reliable with only a few fatigue crack propagation failures due to cyclical stress.
MATERIALS USED IN ROLLER SKIS
Wood: Many roller skis are made with wood frames. Wood has good fatigue resistance and absorbs vibration quite well. However, even the strongest wood materials, like oak and hickory, have a modulus of only 20% that of aluminum so the shafts have to be thicker to withstand the load. This can make them heavier unless they are reinforced with carbon. To keep the weight down wooden shaft roller skis usually have more deflection under load than aluminum shafts. Because of the larger deflection and lower modulus the energy return of wooden shafts is also lower. Without a lot of carbon reinforcement skis based on wood construction are less responsive than pure carbon composite shafts or the V2 XLQ shafts that have exceptional energy return.
Steel: Very good fatigue resistance and inexpensive. Also very good impact resistance. But steel is 2.8 times heavier than aluminum and about 3.5 times heavier than shafts made with carbon composites. Today I don’t think any roller skis use steel shafts, but it is a very good material and with such high tensile yield strength and good vibration dampening, steel should be re-evaluated for roller ski use. For bicycles steel is still the most common and longest lasting frame material.
Carbon Composite: Very good fatigue resistance and extremely light. Pure carbon composites structures are very expensive to make and have very poor impact resistance unless the carbon frame has a strong core like the V2 XLC. Because ski poles have no core to absorb energy carbon ski poles break easily under impact. (The number of ski poles broken in Sprint Races is astronomical)
Aluminum: Light weight and easy to extrude, but aluminum has a relatively poor fatigue resistance. The frame has to be stiffer than carbon, wood or steel frames or it will fatigue rapidly. One of the smoothest roller skis made using an aluminum frame was produced in Switzerland. However, due to the low moment of inertia the shafts flexed so much they fatigued rapidly and had to be replaced on a regular basis. Many skiers who trained on the Swiss skis had to replace the ski shafts several times a year. We know skiers who replaced the shafts every 60 hours of useage.
When manufacturers of mountain bikes switched from steel to aluminum, frame failures increased rapidly. Steel frames have a very high fatigue resistance and when the lighter aluminum frames first became popular many engineers did not realize that you couldn’t design an aluminum frame just using the modulus, yield and ultimate tensile strength of the material. If an aluminum frame is allowed to flex like a steel frame, it will fail very quickly. At one mountain bike race in the late 90’s we counted over a dozen broken aluminum bike frames. When after very short usage thousands of aluminum bicycle frames fractured, the designers realized that the aluminum tubes had to be much bigger in diameter so they would not flex. The re-designed aluminum frames, with large diameter tubes, were much stiffer and as a result needed shock absorbing wheel forks. Today’s aluminum mountain bikes use stiffer frames and are generally equipped with very sophisticated wheel forks.
V2 Hybrid Shafts: The V2 XLQ shafts are the most advanced shafts we have ever produced and are composed of five different vibration dampening materials. The backbone of the ski is a very light vibration absorbing Japanese propriatory alloy. The deflection of the XLQ shaft under the load of a skiers classic "kick" or a skiers skate "push off" is similar to the low deflection of aluminum shafts, but the vibration absorption is almost 10X greater. This makes the XLQ skis very lively and crisp with exceptional energy return. To further reduce vibration and noise the hollow section of the shaft is filled with highly compressed closed cell foam. The carbon reinforced composite wheel forks have outstanding vibration dampening and are fastened to the shafts with a 3M vibration absorbing adhesive. The same vibration dampening adhesive tape used to mount windows in skyscrapers so that in earthquakes the windows do not shatter. The outside of the shaft has a textured Swedish made coating. This finish also reduces both vibration and noise. The result is exceptional energy return and the lightest and most vibration absorbing shaft we have ever made. We are positive that for paved surfaces the XLQ are the most snow like roller ski on this planet.
FATIGUE TEST PROTOCOL
In our standard test for aluminum shaft roller skis we apply a load (P) of 2000 Newton (445pounds or 202Kg.) for 500,000 cycles to a 50mm X 38 mm (2” X 1.5”) block mounted in the center of the ski. For the more fatigue resistant carbon composite skis and for the XLQ shafts we apply the same load for 1000,000 cycles. We have a custom made machine that applies a load at a rate of 60 cycles per minute. A typical aluminum skate shaft supported so the span between the supports is 410mm will deflect about 4.2 mm under a load of 2000 Newton. After 500,000 cycles the center of the shaft has moved up and down 4,200,000 millimeters. The V2 carbon composite and the XLQ shafts are more fatigue resistant than aluminum and the shafts can be designed with a lower moment of inertia. In the 90’s we tested aluminum shafts from other roller ski manufacturers and most failed long before our design criteria. For a printable version in pdf format click on the link below.