A slippery slope - ski technology
With Christmas behind us, for many people the next holiday to look forward to is a skiing trip. Skis have been around for about 5,000 years and were first used as a means of transport rather than as a form of recreation, as today.
Skis have very demanding material requirements. They must be light, strong, waterproof and smooth, and able to resist bumps, scratches, cold temperatures and strong sunlight. The ski pioneers of Scandinavian nations used long femur bones from animals to make the first skis, which were held in place using leather thongs. In Mediaeval times, skis were made of wood and what we today call a ski boot was a simple leather shoe, attached to the ski with a binding made from leather or willow branch. In the 18th Century, Norwegian soldiers used heel straps on their skis to enable them to go downhill faster without losing control. They were also the first skiers to use poles.
Steam was applied to the wooden skis to provide the upward bend at the tip, which assists with smooth travel of the ski over the snow. The heat in the steam causes the cell walls to plasticise so they can be compressed. Ski styles differed between regions, but a popular style in the 19th Century was the Osterdal, which consisted of a short ski (about a metre-and-a-half long) and a longer ski (about three metres long). The short ski was used to push off against the snow, and the bottom was sometimes covered in fur. The longer ski was used for gliding and was grooved on the bottom to guide it smoothly across the snow.
In the 19th Century, skiing developed into a sport and later, in 1924, was one of the sports at the first Winter Olympics in Chamonix, France. It was also in the 19th Century that the development of train travel meant city dwellers could reach mountain resorts, and skiing became a recreational pasttime. In the early 20th Century, two poles began to be used to help skiers’ balance. Despite these advancements, the skis themselves continued to be made from wood, with the addition of steel edges to improve glide.
The perfect wood for a ski is light, flexible in length and stiff in cross-section – which doesn’t exist. The result was the development in the 1940s of a sandwich structure of wood laminates. Using different woods in different areas improved the flexibility control. Also, the glue between layers resisted movement of one layer relative to the other and thus reduced twisting.
With inexpensive wood becoming more difficult to source, all-metal skis became common in the 1950s. These were not very successful, in part because they were easily dented and also because wax could not be used on the metal underside, so they often got stuck in the snow. American skier Howard Head developed a sandwich structure for skis, consisting of a wooden core covered in aluminium. Glue and heat were used to fuse the two materials together. These skis were not popular with racers because of chatter on uneven surfaces and at high speeds. Head then developed skis made with lightweight and flexible plastic laminates along with steel edges, and the popularity of his skis increased. By this point, bindings had evolved from simple leather straps to leather devices that attached the toe to ski but left the heel free.
Today, skis are excellent examples of composite structures and are typically one of three types. At the lowest end, laminated skis consist of materials sandwiched on top of each other, with the centre layer being the core. Torsion-box skis consist of a core with fibres wrapped around it, while finally single-shell skis are at the highest price bracket and consist of a core surrounded by a shell.
The inner core of modern skis, which determines strength and longitudinal flexibility, is typically made from wood or foam. Wood is cheap, but controlling its properties and ensuring that both skis in a pair will behave identically is a challenge. Foam, typically polyurethane, was introduced in the 1970s and is lighter and more easily controlled during manufacturing than wood. It also offers better vibration absorption. A third possible material for the core is aluminium honeycomb, which makes skis light and strong, but also flexible and prone to vibrations.
The outer layers of 21st Century skis are typically glass- or carbon-reinforced plastic, or the para-aramid, Kevlar. Kevlar was developed by DuPont in 1965 and has excellent strength-to-weight and impact protection properties, thanks to the many interchain bonds in its structure. Its specific strength is 10 times that of high-strength steel wires. Interestingly, competitors in slalom races wear Kevlar gloves.
In some layers, the reinforcement fibres, laid parallel to the ski axis, are loaded with tension if below the core and compression if above the core, and support the bending moment. Fibres are chosen accordingly, for example Kevlar is stronger in tension so is used in the base. In other layers, fibres are laid at 45° to the ski axis to provide torsional stiffness. Vibration damping devices, such as rubber strips, are included in the composite structure. The edges of skis are made from steel, which may be regular strength or hard tempered, and which allow the ski to grip the snow while turning.
The underside, or base, of skis nowadays is typically coated in high-density polyethylene. This polymer ensures sliding of the skis over the surface, whatever the type of snow. However, polyethylene has poor abrasion resistance so a polyethylene candle is used to patch up scratches. Another problem is that polyethylene is easily broken down by ultraviolet rays through photodissociation, so a frequent application of wax is necessary. Hydrocarbon wax or fluorocarbon wax can be used – the latter consists of negatively charged fluorine atoms and repels water and dirt better, but is more expensive and difficult to apply, so is generally only used by racers. A graphite additive is used in the wax to conduct static charges away from the bottom of the ski during use and hence reduce friction due to static electricity.
The mention of the polyethylene underside brings us to a very important area of physics that is crucial to skiing. For skiing to be possible, there must be low friction between the base of the ski and the snow. When the temperature is higher than -10°C, the friction between the ski and the snow causes the snow to melt and a water film is created. The water film acts as a lubricant with a coefficient of friction, μ, of around 0.02, whatever the material of the ski base (μ=0.16 for two lubricated steel parts). However, at temperatures lower than -10°C, the lubricating film is not created and the coefficient of friction increases in the case of wood and metals, but not in the case of polyethylene and polytetrafluoroethylene.
Even 50 years ago, the notion of smart skis would have been unthinkable, yet in 1995 the first smart ski, K2 Four, was born. These skis have a small card made of a piezoelectric lead-based ceramic and containing an electrical control circuit. It is placed in front of the binder, where vibrations typically initiate. Mechanical vibrations are changed into electrical signals that are sent to the control circuit, which in turn sends out electrical signals to the ceramic that make it change shape and hence dampen the vibrations. Head Intelligence Technology consists of fibres in the ski that respond to the mechanical forces on the ski and as these increase, the torsional stiffness is increased. The fibres are angled at 45° so that when the skier is manoeuvring difficult turns, the increased stiffness means the edge of the ski is pushed into the snow. If skis were so stiff all the time, cruising over soft snow would not be pleasant. Some high end Head skis are even smarter. The electrical energy generated from mechanical effort is stored and released in a pre-programmed way, in time with the oscillations of the skis.