Introducing the Ski Flex Index (SFI)
The last 10 years freeride and freestyle skis have changed a lot. An important reason for these changes was the the appearance of the snowboard. Skis got shorter and twice as fat in 10 years, most freeride skis got twin tips. These changes have not come easily. The public awareness of ski shapes has forced ski companies to change their presses, core planers, base extruders and so on. Skis are now being shaped the way they were supposed to. New, better shapes appear every year. This is how ski builders experimented before engineers standardized all shapes the last century. So what has all this got to do with flex?

For years we have seen countless discussions about ski stiffness and flex end up in quarrels about what ski is stiffest or softest. It seems as if all real discuss of flex patterns is obstructed by a general lack of language. We can not discuss ski flex when we don’t know what ski flex is. So why don’t we have a "ski flex index" when there is one for boots? One possible reason is the complexity: A ski can have variations in relative stiffness in different areas of the ski, different pattern for different length, non linear flex properties, materials react differently to temperature, skis get slightly softer with age and so on.

At the bottom left of this page there is a PDF which contains flex curves of selected freestyle and freeride skis for the 2008 season. The skis have been measured in the same machine, at the same temperature and with bindings removed. To avoid the softening effect of long time use, all the tested skis are as new as possible. The reference pont for all the curves is the true centre of the ski; this is the centre line of the diagram (0). The curves are drawn through 16 points with 10cm intervals.

To understand the flex you should look at the whole curve, not just the highest point of the arc. The middle (where the binding is mounted) is less important than the front and back area.

Ski Flex Index
A longer ski needs more stiffness to distribute the same amount of pressure to the tips of the ski. Flex curves from two skis of different length is therefore not directly comparable. To compensate for this, the stiffness has been recalculated. Through a formula based on average flex curves from skis of different sizes, I have made an index for flex/length.

SFI is a row of five numbers, each number corresponds to a part of the ski: Back tip, back ski, mid ski, front ski and front tip. A completely average ski would have the numbers 5 5 5 5 5, while softer skis will have lower numbers, towards 1 1 1 1 1. A very stiff ski can be 9 9 9 9 9, but in theory the numbers can go higher, since absolute stiffness is a practical impossibility. 2 5 5 5 2 is an average ski with very soft tips, 8 5 4 5 8 is a ski with very stiff tips and a pretty soft mid. 7 6 5 4 3 is a ski with a stiff tail, average mid and a soft front.

There are many theories about ski flex and what is the “best” flex pattern. I have made this diagram as a contribution to that discussion, it is a tool to give your arguments a better foundation. Read it, make an opinion and fight for it.

Endre Hals