This Help article describes basic ski design and how that affects ski performance.
In this post, we are mainly focusing on choosing a ski for ski touring. A good ski for ski touring in Scotland (where Arc Guiding is based) is unlikely to work well for backcountry skiing in, say, British Columbia. But the general principles hold true.
Basic ski design
Below is an overview of ski design features. Many of the principles and features mentioned below also apply to splitboards and telemark skis.
Shovel
The front of the ski, also known as the tip. This is the widest part in almost all ski designs. It’s shape determines how well the ski rises up onto the surface of the snow and how easily the ski starts a turn. To further improve these qualities, ski tips can be ‘early rise’ or ‘rockered’ (see below).
Waist
The middle of the ski (below the foot) is always the narrowest section of modern ‘parabolic’ (or ‘carving’) skis. It is also one of the most influential parts of a ski’s design. The waist width determines how well a ski ‘floats’ in soft snow and also how quickly it changes edge when turning. Thus, skis with wider waists tend to be less nimble than those with narrow waists.
Tail
The rear end of the ski. The tail width affects edge hold towards the end of a turn – making it grippy or skiddy, for example. Rockered tails make it easier to rotate the ski when flat. ‘Twintip’ skis have very upcurved tails to help with skiing backwards.
Base
The base is made from polyethylene (‘Ptex’). It is usually hot-waxed to improve glide. Good quality bases tend to be ‘sintered’. Another type is an ‘extruded’ base, in which the plastic has been liquified (melted) to form the sheet. This type of base tends to occur in cheaper ski designs. It is less porous, so holds wax less well. But is in some ways easier to maintain.
Sintered : Production process where granules are forced together with heat and high pressure without becoming liquid. Importantly, this type of base is tough and also has many tiny pores, which ‘soak up’ ski wax.
Ptex ‘candles’ and a metal scraper can be used to easily repair gouges. Deep gouges which expose the core are called ‘core shots’. Unfortunately, these usually need patching in a workshop by a trained ski technician. Beware of deep gouges near the edges. If these catch a rock, the entire edge could be ripped out.
Topsheet
The protective layer covering the top of the ski. A thicker topsheet provides better protection from scratches, but it does add weight.
Pro tip: Many piste-oriented skis have ‘rails’ integrated into their top surface to accept a specific binding. However, if you want to mount a touring binding you will need a ‘naked ski’ (ie: no rails) .
Core
The core determines the ski’s overall shape, weight, vibration absorption and flex pattern. Most touring skis have a core of glued and laminated hardwood (eg: Aspen, Maple, Ash, Poplar, Paulownia). Additional materials may include fibreglass, carbon fibre and Kevlar. Camber, rocker and flex are all determined by the skis core (see below).
Edges
Clearly, sharp edges improve grip on icy and hard snow, particularly when carving. Ski edges are made of hardened steel, so will rust if left wet for a long time.
Edges can be deburred and sharpened at home using an edge sharpener. But it’s important to remember to maintain the manufacturer’s base and edge angles (usually 0 or 1 degree). Lastly, edges are often detuned slightly at tip and tail to make them less ‘catchy’.
The curve of a ski’s edges determines its sidecut and turn radius (see below).
Ski geometry
Turn radius
This is the size of arc a ski naturally wants to turn when on its edge. It gives a quick indicator of how manoeuvrable the ski is. Most resort skis have a radius of around 16m or less, whereas off-piste skis are 18m plus. However, many factors influence turn radius, including :
- Waist width
- Sidecut
- Ski length
- Camber
- Rocker
- Ski flex/stiffness
- Edge angle
- Skier skill.
Sidecut
- Size of the sidecut = Widest measurement (shovel) minus the narrowest measurement (waist).
- Shape of the sidecut helps determine the ‘feel’ of a ski through a turn and its turn radius.
The turning radius, plus shovel, waist and tail widths are usually printed somewhere on the ski.
Ski length
If you go up a size in a certain model of ski, you will often get a proportionally wider turn radius. A longer ski will also provide more float, due to it’s higher surface area. Furthermore, longer skis tend to be more stable at speed. Conversely, a shorter length will be lighter and more easy to rotate and steer. It may also be a little less cumbersome when carrying.
Camber
A ski base design where the unweighted ski has two contact points: one near the tip and one near the tail. The waist gradually curves upwards. ‘Classic camber’ increases the skis ‘pop’. Note that all skis bend into reverse camber when turning.
Contact point: Where the unweighted ski base touches the snow
Rocker
A ski base design where the unweighted ski has one contact point at the waist. Here, the tip and tail gradually curve upwards from the middle. This shape can be thought of as the opposite to camber. It makes a wide ski easier to rotate and hence is often found in off-piste skis. Powder skis might be fully rockered, in contrast to piste skis being fully cambered. Increasingly, off-piste skis display both camber and rocker, which enables them to also perform well on piste.
Ski flex or stiffness
There are two types of flex in a ski: lengthways stiffness and torsional stiffness (twisting). Lengthways stiffness affects balance and control fore and aft. Torsional stiffness, on the other hand, affects edge hold.
Different ‘types’ of skiers want different flex patterns. Hence, light or nervous skiers and park riders do better with a more bendy ski. Heavy or aggressive skiers and racers need a stiffer ski. In fact ‘skier type’ is an important factor in working out ski binding DIN settings.
Skis for touring in Scotland
For ski touring in Scotland, skis wider than about 98mm underfoot can become quite unwieldy. Although they may have lots of float in difficult conditions, their edge grip and turning radius is likely to be compromised for harder, icy and narrow conditions that are all-too-common.
Summary
Although skis look quite simple at first, it can be seen that they involve a great deal of engineering. Understanding the basics of ski design will help you choose a ski for ski touring and perform your own ski servicing.
Ski design FAQ
Modern skis are generally sized to come up to between nose and top of head height. Because they are significantly wider and more easy to turn, they do not need to be sized as long as the straighter style of skis of the pre-1990’s. A slightly shorter length may be better for beginners, lighter skiers, tighter turns or mountaineering. A slightly longer length may suit a heavier skier, deeper snow or higher speeds.
There are an increasing range of skis that are designed to perform equally well on and off piste. Skis marketed specifically for touring will tend to be lighter, those for powder will be wider and those for freeride may be stiffer, but these are generalisations. A key requirement, however, is that they are ‘naked’ skis, ie: they do not have binding mounting rails. This means they can be fitted with a touring binding.