Most roller skis are made from a metal extrusion that’s fabricated in such a manner that you can mount roller ski wheels. When not equipped with pneumatic tires or shock absorbing wheel forks, aluminum shaft roller skis can be very uncomfortable, especially on poor pavement. With solid rubber wheels the vibration damping of the ski is mainly a function of the shaft construction and the design of the wheel forks. For roller skis that have larger pneumatic tires, the primary vibration damping is the air in the tire and there is no need to use the hybrid composite shafts and composite wheel forks.
HISTORY OF V2 COMPOSITE SHAFTS
In 2006 we began experimenting with wood core hybrid composite shafts. We wanted to reduce the vibration on solid rubber wheel roller skis, and wood cores can dampen vibration quite well. The first cores were made from Baltic birch plywood which is very strong, but heavy. To reduce the weight the core was made in two equal halves. The inside of the core was extensively machined on a CNC router and the two halves were bonded before reinforcing the core with woven carbon fiber and epoxy. We made about one thousand of these shafts before changing the core to Italian poplar plywood. Most wood has a fairly low modulus, or in laymen’s terms, wood is flexible compared to metals or composite structures. Unless stiffened with carbon or other high modulus fibers, wood shafts must be quite thick. Poplar is a very weak wood compared to Baltic birch, but it is 45% lighter and could be fabricated as one unit rather than two halves. We stiffened the flexible core with lightweight unidirectional and biaxial carbon. We made thousands of these shafts and their performance was exceptional, but they were very labor intensive, and since there is a lot of variation in plywood and dimensions vary greatly depending on humidity the manufacturing process was not as precise as we would like. We found a lightweight alloy with superb damping and except for modulus the specifications for physical properties were very similar to the 6061 T6 aluminum shafts we had been using over 25 years. To increase stiffness, we made the walls substantially thicker than on our aluminum shafts. An American company we had never used before extruded the shafts. They produced two prototype lots that were tested both in the lab and on the road. The first lot did not meet the specification, but the second lot was approved. Unfortunately in the production lot there were microscopic defects in a number of shafts that over time propagated and caused premature fatigue failures. We had to reinforce the inside of all the shafts with composite technology. We decided that for the next generation shafts we would manufacture the critical portion of the shafts in house.
BENEFITS OF THE NEW XLK HYBRID SHAFTS OVER PREVIOUS V2 ROLLER SKI SHAFTS
For the XLK shaft we use a very lightweight precise core with good vibration-damping properties and wrap the core with a combination of unidirectional and biaxial fibers. The core itself is not under high stress; it’s the composite fibers that provide the strength. The biaxial aramid fibers (Kevlar™) have very high tensile strength and are extremely tenacious compared to carbon. Kevlar™ also has better vibration damping properties than carbon. Although Kevlar™ has very high tensile strength, the compressive strength is lower than the tensile strength, so under the biaxial aramid fibers we have three layers of unidirectional fibers that have very high tensile and compressive strength. For optimal physical properties we use a biaxial sleeve with a diameter that provides a nominal weave angle of 25 degrees. The epoxy we use to bond the fibers is 3X stronger than any other known epoxy. An outside supplier manufactures the precise core but Jenex manufactures the critical composite structure that provides the strength and fatigue resistance. All composite shafts are compression tested at 2,225 Newton (500 pounds). Every pair of shafts with the same serial number has equal deflection. The wheels are the heaviest part of a roller ski. Rubber is quite heavy and so are the wheel hub and bearings, but the XLK shafts help reduce the overall weight. The materials used in the XLK hybrid construction have better damping properties than any previous V2 roller skis. The biaxial and unidirectional fibers over the metal core make the skis feel light and crisp.
XLK and XLA WHEEL FORKS AND WHEELS
The XLK / XLA wheel forks, with fenders, are produced in Du Pont’s™ toughest thermoplastic and further strengthened with the addition of short carbon fibers. The non-metallic wheel forks also help in reducing vibration. For improved safety all skis accept the patented Speed Reducers and Brakes. The XLK / XLA skate and classic skis are available in four (4) different speeds: Fast, medium, medium-slow and slow. The weight of an XLK98M skate ski is only 1650 grams a pair and the aluminum XLA 98M 1750 grams. The classic XLK900 weigh just 1735 grams and the aluminum XLA900 1870 grams.
ALUMINUM SHAFT SKIS
The aluminum shafts are made in 6061 T6 and are produced by an extremely competent US extruder. When working at Teradyne Connection Systems we designed many aluminum products used in military aerospace applications. The company we use for V2 aluminum shafts was the only extrusion company that met the stringent requirements of the primary military contractor.
ALUMINUM WHEEL FORKS FOR XL AERO AND XLK / XLA 9840
All V2 forks are made in a monolithic U shape, which makes them stronger than individual left and right forks mounted to the shaft. On the 150mm XL Aero aluminum shafts the forks are welded to the shaft, which is one of the strongest methods of securing aluminum to aluminum. U shaped forks are designed to protect the shaft. On V2 skis that have been used by novice skiers, who have not yet learned how to ski properly, we see a lot of damaged wheel forks.
Most of us are not anatomically balanced, so even though the wheels and bindings are perfectly aligned, the ski might pull in one direction. Few roller ski manufacturers provide a method for wheel alignment. Some suggest that you to bend the wheel fork if the ski pulls to the right or left, but bending can weaken the forks. On the XL150S, 150SC and XLK/XLA 9848 we use a unique wheel alignment device. Draw a pencil line along the side of the alignment lever, loosen the nut slightly and if the ski pulls to the right move the lever forward and if the ski pulls to the left move the lever backward. It’s simple and it works.
On the XLA and XLK that use the composite wheel forks, the front right bolt-hole is oblong while the left hole is round with a very snug fit. If the ski tracks to the right loosen the nut on the bolt just enough so you can move the bolt forward. If the ski tracks to the left move the bolt backward.
DUAL DENSITY RUBBER WHEELS
In June of 2018 we received the first dual density wheels for the XLK/XLA 9848. For skis with solid rubber wheels a soft wheel is much smoother, especially on rough pavement or dirt roads. One of my favorite wheels many years ago was very soft. However, being so soft the wheel wore down in less than 20 hours. The new 9848DD has about 3mm of 80-shore hard rubber over much softer rubber that makes the wheel absorb vibration better. We used a press to apply a load of 75 pounds (330Newton) to a standard 9848 wheel and to a 9848DD wheel and measured deflection. There was a lot more give in the DD wheel, which should make for a smoother ride on the DD wheels.
ROLLING RESISTANCE OF WHEELS AND KINEMATIC DAMPING
The rolling resistance of tires is a function of the amount of energy the tire absorbs. Bearings have minimal influence on the rolling speed of a wheel. A fast tire absorbs very little energy and uses a “high rebound” material. A slow tire absorbs a lot of energy and the tire can become very hot. On a warm sunny day, when the pavement is much hotter than the air temperature, a very slow rubber compound will wear rapidly and can actually disintegrate like truck tires do on the highway on a hot day. V2 roller skis do not use low rebound rubber. To slow the wheels we use a patented system based on kinematic damping. On the W98RS, the slow wheel on the XLK/XLA98S skate ski, we use the same rubber composition as on the W98RM, but we slow the wheel with a patent approved lightweight (10 gram) internal kinematic speed reducer called the ISR. The ISR is exactly the size of a 22 mm bearing and on the slow wheels we simply increase the depth of the bearing bore to accept both the ISR and the bearing. This system increases the rolling resistance by about 15 / 20 %. The heat generated by the ISR is dissipated through the aluminum bearing tube and the bolt.
If you spin a wheel with your hand on an in line skate or roller ski with bearings that are lubricated with lightweight oil, and the bearings do not have contact seals, the wheel can spin for several minutes. Contact sealed bearings with grease lubrication will stop in less than five seconds. The conclusion that most people reach is that the free spinning bearings will make the roller skis a lot faster. Wrong! In order to accurately test the rolling resistance of the wheel, the wheel must be under similar load as when the roller ski is being used. We have developed a sophisticated test machine that accurately measures the rolling resistance of wheels under load. The wheels are driven at a constant speed of 30 kilometers per hour. The machine works just like the cruise control on a car. As you encounter a hill more gas is supplied to the engine to maintain the speed. Our engine is a precise electric motor and we measure the current required to keep the wheel speed at 30 kilometers per hour. The energy required to maintain the speed is measured to 1/1000 of an ampere so the test machine is extremely accurate. We have found that, under load, the non / sealed oil lubricated bearings are less than 2.0% faster than permanently sealed greased bearings.
SPEED REDUCERS AND BRAKES
V2 roller skis have become world famous for the patented Speed Reducers. Over the years the concept has been modified and improved numerous times. The latest generation Speed reducers for the AeroXL150 skis work so well that you can ski in almost any terrain. In the past, top ski racers felt that using Speed Reducers and Brakes was only for “sissies”. Not anymore. Many of the world’s best skiers are now using Speed Reducers and several we know use both Brakes and Speed Reducers.
Our first brake, introduced in the mid 90’s, was not an elegant solution. However, in 2005 we developed a simple brake that really works. In 2007 we introduced an improved version for all XL model skis. By mounting the brake to the wheel fork we were able to eliminate several parts and make the system both stronger and lighter than the Universal brake unit. (The US Patent Office granted the Patent for our brake system in 2010.)
AERO ROLLER SKIS
In the summer of 1999 we introduced the first reliable pneumatic tire roller skis. We dubbed them the Aero. The Aero roller skis have been sold to over thirty countries and over the years we have made many improvements. With the Aero you can roller ski on both paved and non-paved roads. Since the Aero will roll over objects and cracks in the pavement that stop other roller skis, most skiers consider the V2 Aero safer than any other roller ski. Many users have told us they spend a lot more time roller skiing on the Aero than they do skiing on snow and many have also told us that without the security of the Aero equipped with Brakes and Speed Reducers they would not roller ski.
V2 HAS THE BEST ROLLER SKIS FOR ALL SURFACES
The XLK hybrid skis are the best roller skis available for paved surfaces. For dirt roads and rough pavement nothing can beat the V2-Aero pneumatic tire skis or the XLK9848. With the patented brakes and speed reducers you can now roller ski just about anywhere.