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Ski Geometry - Shape & Flex

This blog explains why modern skis are the shape they are.
We’ll start from the basics and build from there. We’ll look at the skis from the top, the plan view, and then from the side, and then see what to look for when choosing a ski for the type of skiing you'll be doing. 

LENGTH
The first things we notice about a ski are it’s length and shape. All other things being equal, the longer the ski, the more efficiently it will glide on the snow, so longer skis are faster. This is why nordic touring and cross-country skis are generally longer, to glide better. However gliding efficiency isn’t the only parameter that counts, and all other things being equal, longer skis are more difficult to turn and are heavier than shorter skis. What you’ll be doing with the skis also affects ski length. When skiing on prepared pistes, the ski is in contact with a relatively hard surface and even a short ski will have directional stability. Venture into deep powder with the same ski and it’ll be very “tippy”, prone to diving forward or falling backward since the short length doesn’t offer enough support in soft snow. So powder skis tend to be longer than piste skis. We'll look further into ski length in another blog.

SHAPE / SIDECUT
As well as coming in different lengths, skis also have different widths and shapes.
Some are narrow, some are wide, some have a shape like a “Y” while others more like an “X”. What’s all this about?

Almost all downhill skis, whether alpine or alpine touring, have a wider upswept tip at the front, a slightly narrower tail and are skinniest in the middle.
We measure this shape or SIDECUT taking the width of the tip, the waist and the tail, for example 125-85-112 mm.
Skis are this shape so that when we put weight and lean the ski on the edge, the ski bends and makes a parabola, or curve, and the ski will turn following this shape.
For the same length, the bigger the width difference between the wider tip & tail and the waist, then the faster the ski will turn.

The sidecut of the ski determines the TURN RADIUS of the ski . This is determined by the sidecut of the ski and is measured as follows: if you were to place the skis on the ground and trace the curve of the ski from tip to tail with a pen, then continue drawing that curve until it comes full circle back to the ski. The turning radius is the length in metres of the radius of this circle.

For fast turning slalom skis this may be as low as 10 m, while for wide off piste freeride skis it could be 24 m or more.
Most “all mountain” downhill and touring skis have a turning radius of about 16-20 m.

We’ll see as we progress here that each ski parameter affects the others.
If we consider a ski model with 2 lengths that share the same sidecut, then it’s evident that the shortest ski will have a tighter turn radius than the longer ski.
The manufacturers respond to this in two ways:

1. Giving all lengths of the same model the same sidecut and accepting that the turn radius may vary dramatically from the shortest to the longest ski.
Faction CT 2.0
Length / Sidecut / Radius
166 cm: 131-102-127 mm / 16 m
178 cm: 131-102-127 mm / 19 m
188 cm: 131-102-127 mm / 21 m

OR

2. Changing the shape / sidecut of the different lengths, to maintain a more constant turning radius for all lengths, although the change in width also changes the ski's behaviour.
Fischer Transalp 90 Carbon
Length / Sidecut / Radius
162 cm: 123-87-106 mm / 18 m
169 cm: 124-88-107 mm / 18.5 m
176 cm: 125-89-108 mm / 19 m
183 cm: 126-90-109 mm / 19.5 m

Traditionally shaped skis have a wide blunt tip, a narrow waist then the widest point of the tail at the very end to provide the longest possible EFFECTIVE EDGE and maximum ski-snow contact. Typically seen in carving and race skis, these skis have an “X” shaped silhouette which offers more grip but also requires more effort on the part of the skier to exit the turn and initiate the next turn.

Nowadays many mountain and touring skis are designed to have the widest part of the tip set back several cm from the front of the ski, with the ski narrowing or tapering to the very front of the ski.
The tail of the ski can also be tapered, and TAPER confers several benefits including better flotation in powder and improved agility. We’ll look into this in greater detail below.
Since tapered skis have a more complicated silhouette than traditional skis, their sidecut is sometimes described using 5 numbers instead of 3, to include the tapered tip & tail widths. For example: 124-133-100-118-112 mm.

Lastly but not least, some exclusive powder skis have REVERSE SIDECUT with the widest part of the ski in the centre. These skis have more in common with surfboards than traditional skis and are designed for use in the fluid world of bottomless powder.

Up to now we’ve looked at the shape of the ski from the top, the plan view of the ski. But skis have three dimensions, so what will we find if we look at the ski from the side?

CAMBER, FLEX & ROCKER

Just as the sidecut of a ski forms a curve looking from above, when we look at a traditional ski from the side we find that it’s not flat like a plank, but has an arc across its length, with the tip and tail touching the ground and the centre of the ski raised with a gap underneath. This arc is called the CAMBER of the ski.
Camber was invented by Norwegian telemark skiers in the 1850s. By carving the wood to create an arc they were able to create much lighter skis. The arc spread the weight of the skier over the length of the ski and prevented it from sinking in the middle under the weight of the skier.

When we lean the ski to the side to turn we have to flatten the camber, so that the curved edge contacts the snow. The resistance of the camber to being flattened is called the FLEX of the ski. You can feel the flex of the ski by holding the tip firmly in one hand, placing the tail on the floor and pushing firmly with the other hand at the ski centre. If you try this on several different skis, you can feel the difference in stiffness and also how the stiffness is distributed along the length of the ski with some skis having an even flex throughout the length, while others will be stiffer underfoot with softer tips and tails. There are alternative methods to check your skis' flex....

Skis have flex and camber for several reasons. If the ski was a flat plank it would not maintain good contact with the snow and be difficult to control. Giving a ski camber and flex means that the force applied to flatten the camber is transferred to the ski edge, which then maintains contact with the snow. The ski follows the curve of the sidecut and remains stable at speed.
 

Skis are built with different flex stiffnesses according to user and ski type.
Skis for beginners, kids and lighter adults have a softer flex than skis designed for strong heavy aggressive skiers.
Race piste skis have very stiff flex to ensure maximum control at high speeds.
Powder skis have a softer flex, at least at the tip, to provide tip flex in softer snow conditions.
"All Mountain" skis designed for mixed on and off piste use have medium flex to manage all snow conditions reasonably well.

In the early 2000’s as skis were becoming wider and “parabolic”, designers started incorporating concepts and designs from surf and snowboard technology.

In 2001 Shane McConkey coined the term REVERSE CAMBER while designing the Volant Spatula. The Spatula had a smooth arc from tip to tail, but in the opposite or reverse direction of traditionally cambered skis.


With a rocking-chair like side profile, reverse cambered skis offer superb soft snow performance with the long rockered tip helping turn initiation and agility in powder snow. They are however difficult to use on hard snow and when traversing.

Ski designers wanted to combine the advantages of traditional and reverse cambered skis. The term “ROCKER” was first used with respect to skis in 2002 by DPS Skis founder, Stephan Drake. The term was borrowed from boat, surfboard, and shoe design (e.g. rockered hull, rockered sole). Almost all modern skis have a hybrid design with traditional camber underfoot and rocker at the tip, or at both the tip and tail. The length of the rocker is called EARLY RISE and is measured in cm.
Rockered skis are easier to use and offer higher performance in a variety of snow conditions.

Even a small amount of tip rocker can make a big difference to the ski’s performance.

Tip Rocker Pros:
• Easier turn initiation
• Better flotation in powder and spring snow
• Smoother progression through variable snow and crud
• Shorter Effective Edge gives increased agility, the ski seems shorter and easier to turn
• Easier for the skier to keep a balanced stance in variable snow

Tip Rocker Cons:
• Rockered tips can vibrate and chatter on hard snow
• Rockered skis do not carve or track quite as well on hard snow

Skis with both tip & tail rocker have an even shorter effective edge and the fact that the tail is not in contact on hard snow gives these skis a different character. Rockered tails enable skis to pivot more easily. In powder, rockered tails stay afloat, positioning the skis with a better tips-up attack position, and are helpful for landing switch.

Tail Rocker Pros
• Easy turn release in powder and spring snow
• The ski pivots easily
• The ski has a surfy character, good soft snow performance

Tail Rocker Cons
• The ski has an overall softer flex, with less backbone and energy
• Turning stability is reduced in long turns, the ski wiggles more
• Reduced edge grip on hard snow

Some skis do away with camber altogether and combine tip & tail rocker with a flat cambered (ie no camber) mid section:

These skis give good powder performance but offer much less edge grip on hard snow. These skis pivot easily and are very manoeuvrable but are reserved for deep powder conditions. A niche design.

ROCKER & SKI LENGTH

Rocker confers advantages that combine to make skis more agile and easier to ski in variable snow conditions, so that many people find rockered skis “ski shorter” than traditional cambered skis of the same length.
Many skiers will choose a slightly longer ski with rocker and we definitely do not recommend downsizing to a shorter length when choosing skis with tip & tail rocker.
 

MULTI or TRIPLE RADIUS SIDECUT

Several ski manufacturers claim that their skis have multi radius or triple radius sidecuts and that the radius the ski will carve can be varied according to how much pressure is applied to the ski. In fact all skis have always been multi-radius, but the developments in ski design and construction in recent years now make it much easier to build a ski with easy-to-use variable sidecut.
So how does this work?
Let’s consider a ski that has shape with a shorter turning radius in the centre of the ski, and longer turning radius at the tip and tail:

When only a shallow angle is applied to the ski, the camber of the ski is not fully flattened and the ski will “run” on the snow with mainly the tip and tail of the ski touching the snow. If these sections of the edge have a long turn radius then the ski will make a relatively long turn.
If the centre of the ski has a shorter turning radius, then as the ski is progressively given a greater angle by applying more weight and lateral pressure, then the camber will be progressively flattened and the turning radius is reduced.
Skis with this design offer a greater variety of turn lengths depending upon the pressure and angle applied to the ski and the edges.

SUMMARY

Almost all modern skis combine traditional camber with some rocker.
Camber provides edge control when traversing and high speed turning stability in hard snow conditions.
Tip rocker provides versatility and balance in variable snow conditions, so many touring skis and “all mountain” alpine skis have camber + tip rocker.
Tail rocker provides additional performance in powder and skis with tip & tail rocker work best in powder and soft snow conditions.

ALPINE RACE SKIS
Alpine racing skis require a long effective edge for maximum grip and high speed stability. Some are now built with a small amount of rocker for faster turn initiation but the emphasis rests on that long effective edge.

ALL MOUNTAIN SKIS
Most latest generation "All Mountain" skis have moderate tip rocker, camber underfoot and a flat tail or a small amount of tip rocker.
These skis tend to be more forgiving, so beginners can progress more quickly.
These skis typically have 10-20% tip rocker, camber underfoot and 0-10% tail rocker.

TOURING SKIS
Many touring skis have a similar design with a moderate turning radius, avoiding extreme sidecuts since skinning uphill is more efficient and easier with a straighter edge and the ski has to have the versatility to cope with the variable snow conditions encountered when touring. Many touring skis are built with an emphasis on ease of use in all conditions rather than all-out performance.

FREERIDE SKIS
These generally have longer rocker profiles than "all mountain" skis but retain camber underfoot. The wider (and longer) the ski, the more rocker, as soft snow performance predominates.
Freeride skis typically have 20-30% tip rocker, camber underfoot and 10-20% tail rocker.

POWDER SKIS
Powder specific skis have long tip and tail rocker and moderate or soft flex and relatively long turning radius. The long rockers and flat or limited camber underfoot give an agile playful character.
Some models have a stiffer mid section with a shorter turning radius underfoot to offer improved hard snow performance.

FREESTYLE SKIS
Freestyle all mountain skis have a twintip design with rockered tips and tails and a moderate camber underfoot. Freestyle backcountry skis are wider, with longer rockers and can have either a small amount of rocker or more often a flat or even reverse camber. Freestyle park skis are twin tipped but do not always have rocker, the longer effective edge giving added grip on the steep and hard pipe walls. Park skis have a stiffer flex to provide more "pop" for tricks in the park and provide superior stability and grip when landing.

Neil

Neil started skiing in the Scotland when he hired lightweight tele gear and skied Cairngorm to Ben Macdhui. He was bitten and not by the midges! Th ...

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