Skis are not straight these days. The middle of the ski, the ‘waist’, is usually narrower than the tip and the tail. This phenomenon is called ‘sidecut’. Sidecut is represented by a series of three widths (in mm) at the widest point of the tip, the narrowest point at the waist and the widest point at the tail of the ski. For example 113-65-98. The greater the difference between the narrowest point at the waist and the widest points at the extremities of the ski, the more extreme the sidecut. This influences the sidecut radius of a ski.
The stated ‘radius’ of a ski is the so-called sidecut radius. It is the actual radius of the circular arc that goes through the widest point of the tip, the narrowest point at the waist and the widest point at the tail of the ski (as described by the sidecut), in meters. The smaller the sidecut radius of the ski, the more self-steering the ski is. The sidecut radius actually describes the largest turn radius that the ski can carve with minimal bending. The more you put the ski on edge and bend it, the smaller the turn radius on snow becomes.
A sidecut radius up to 14 m is considered ‘small’; a sidecut radius over 18 m is considered ‘long’ for most recreational skis. This is most relevant for skis that are made for carving on hard-packed or groomed snow.
Camber is the natural pre-bent shape of the ski. Most skis, when you lay them down flat on an even surface without any pressure or weight on it, have the center a bit lifted from that surface. When you put pressure on it, this pressure is evenly distributed throughout the length of the ski. When that pressure is released, the center part of the ski will come up again. The length of the part of the ski between the two contact points in the front and the back of the ski relative to the total length of the ski is the percentage of the ski that has camber. Some brands list the height of that camber as well (a few mm). Camber helps in loading and unloading forces in the ski while skiing. One could say that the more camber a ski has, the directer it becomes (length f camber), the greater the edge hold (length of camber), and the more potential there is for a powerful rebound (height of rebound).
Rocker is the opposite of camber (sometimes it’s even called ‘negative camber’). Rocker refers to the shape of the rocking chair, the end turned up. A ski is rockered if the part forward of the front contact point of the ski (when on an even surface) is slightly lifted. Rocker is sometimes referred to as ‘early rise’. And that is exactly what it is. It helps the ski with floatation in deep snow and it eases the ski into a turn when tipped on edge. There can be rocker in the tip as well as in the tail of a ski, for easy release in deeper snow.
The length of a ski seems an obvious thing. Mostly, it is stated in centimeters. The length mostly refers to the material length. In reality, most skis are shorter than the stated length, given all the camber and rocker that is involved. The total length is one thing. The ‘effective edge’ is the length between the widest points at the front and at the back of the ski. The ‘contact length’ is the length of the cambered portion of the ski between the front and aft contact points with the surface.
A longer length with a constant sidecut increases the sidecut radius of the ski. Also, it means more material, and therefore more weight and surface (part of floatation). Because of the greater swing weight and the larger turn radius, a longer ski typically requires slightly more power to maneuver. However, taking a shorter ski doesn’t necessarily make it manageable if the longer length of the same ski is too much. You should find another ski altogether if that’s the case.
A sidewall is the vertical part on the side of the ski. Sometimes, it’s not on the entire length of the ski, or not all the way to the top of the ski. In this case, we’re talking about a ‘partial sidewall’. A sidewall helps in transferring forces to the steel edges of the ski, and therefore it helps with grip and edge hold. Also, it helps dampen vibrations and makes for a more consistent flex of the ski. Usually, a sidewall is made of ABS (Acrylonitrile butadiene styrene), a type of plastic that is also used in Lego. It is very strong, but not very light. Many lightweight skis have smaller or no sidewalls.
Most skis (except absolute beginners or children’s skis) have a wood core. The core of a ski determines the main flex and strength of a ski. Mostly, a few different wood types are used in skis. Softer wood for softer skis or for parts in the ski that need to be softer. Harder, more dense wood is used in places (or skis) that need to be stronger. Ash, aspen, poplar, beech, bamboo are wood types most common in skis.
Skis that do not have a wood core, typically have a synthetic composite core. This is lighter and much cheaper to make. Composite or ‘foam’ core skis are typically less stable and stiff as wood-core skis. And, over time, composite cores can lose their properties and the ski can become really unstable and floppy.
Besides a core, skis often have several layers of other materials. To dampen vibration, to add stiffness, to toughen up a ski. Metal is often used. The most used metal in skis is called Titanal (in many cases mis-translated into Titanium, which has nothing to do with it and is about 20 times more expensive). Titanal is the brand name of an aluminium alloy. The thickness of a titanal layer can vary between 0.3 mm in softer skis to 0.8 mm in stiffer skis. Other materials that are used to absorb vibrations are rubber, fiberglass and even basalt.
Carbon is also often used in skis. Carbon is lightweight and helps with ‘pop’, spring energy in a ski. Also it can help stiffen a ski, but only in one dimension at the time. The strength of carbon fiber is in the direction of the strings that are applied. That’s why you often find certain types of weaves of carbon fiber in skis.