Ski Boot Performance
What is it that actually drives performance in a ski boot?
Is it all just about the flex ratings? There are several elements that all work together to determine how a boot will react and respond to your efforts to drive your skis.
Lets start with...
With the advertised rating being your best guide, when we discuss forward flexion we are referring to the rigidity of the boot as you bend your ankle and drive your shin forward. This is a big part into finding a boot that matches your anatomy well. The best thing you want to be able to is drive your knee forward to a position vertically above your toe.
Along with flexing forward, in order to be able to turn your skis you need to tip your boot over to the side. These two things work in tandem to really provide the angulation and power loading to drive your skis. When we talk lateral stiffness it's really about how stiff the boot is from the sides. Not only does the hardness or (durometer) of the plastic play a role, but also the thickness of the boot shell walls that are the two main aspects of this. The hinge position and the cuff height also play roles that contribute to efficiency.
Now that you have loaded up your boot all this energy need to travel down to your skis. The main area of this energy transfer is through the rear of the boot where the lower shell and upper cuff interact. This area is referred to as the spine of the boot.
You will see in this area that boots will have different materials and strategies to maintain the rigidity to get maximum power moving down the boot. Things of note will be cuff bolts, denser plastics and even carbon fibre integrated into boot spines.
Rebound is one of the biggest elements in boot performance. When we talk rebound its the reaction and return of the boot and your legs back to the initial and more upright position. Rebound is provided in several ways. The wall thickness of the plastic and how it deforms at its most flexed point (front of cuff and shell instep or through the tongue of a cabrio boot), springs the boot back. Also the tension created by the energy loading at the spine works just like a spring to draw the boot back upright in order to setup for the next turn.
Then as we move down the boot, we can look towards more the actual connection of boot to ski. This is through the binding and contacts on the toe lugs and the base of the sole. For performance through the sole, the key is to prevent any displacement or twisting of the sole (at a micro level). The stiffer this part of the boot is the better again all this energy and power you have created actually is utilized and moved through to the skis. The same method are employed by using stiffer plastics, using a more rigid sole geometry or by adding external stiffeners such as plates.
Here we can discuss how the boot boards (zeppas) come into to play. The closer the shape matched the internal boot last shape the more efficient it will be for foot movements and steering. And a plastic or foam boot board providing faster response than rubberized options.
You will notice on higher performance boots that the liners employ lower profile, denser foams and denser materials all designed for closeness of fit and response. Generally you can see how that plays a part, obviously the softer and more comfortable the liners are the more lag in response.
*The caveat to this is my own opinion of needing to find the right balance with liners, because feeling and proprioception are major factors in actual performance. Stiffest liner isn't always the highest performing in real terms.
The most important part in all of this, is the fit you can achieve with a boot. The closer the boot fits to the shape of your foot and the better you can dial it in matching your anatomy, is the key that unlocks performance. With optimal fit, the more precise your movements can be and the most efficient and responsive your boots become.
All of these things must play a role to determine performance. As you can see it is a multi faceted approach that you can think about in relation to your own ski boots. Whether you are looking to improve your skiing ability and form, or shave a second off your race times, I hope you will look beyond simple flex ratings to optimize your ski boots!