Alternative Boot/Binding Telemark Interfaces - Design Choices   
posted 22 Sep 97     revised 25 Oct 97
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The Dynafit interface has offered the AT market a lightweight
alternative binding with reasonable performance (and a few annoyances
not inherent to the interface).  There is no reason that the telemark
market doesn't deserve the same innovation.  Some have made the case
for NNN-BC being an improvement over 75 mm, but this format has no
takers in the all-plastic boot market (the only boots I'm interested
in) and has several drawbacks.  The bar/sole standard does not seem
strong enough to handle the torque generated by the stiffest boots and
a powerful skier.  Another serious problem is that the format is not
acceptable when hiking or climbing in rocky terrain.  It might be
possible to come up with some kind of cover for the bar to address
this issue, though nobody has done it that I'm aware of.

The two big questions I see for alternative interfaces that will lead
designs down very different paths are:

1] How will the release be designed?  

There are two classes of release in ski bindings that I'm aware of: 
1] direct release - the boot is the only thing that comes off the ski
(all downhill bindings and most AT bindings) and 2] plate release -
the boot is attached to a plate that releases from the ski (all
telemark release bindings and the Ramer AT bindings).

This choice will affect the shape of whatever metal fitting is
installed in the boot - a direct release will require the interface to
allow the binding part to let go of the boot part with the right
release dynamics (for example, sufficient elasticity).  An example of
this design would be to have the boot/binding interface be similar to
the Ramer dislocating ball/socket release.

A direct release has a few advantages such as being able to design a
brake so that whenever the boot is not on the ski, the brake is deployed.
This makes it easier to carry the skis and to enter or exit the 
binding without the ski taking off.  This seems difficult to do with a
plate release (imagine an "OR" brake where it is deployed if the plate is
off the ski OR if the boot is off the plate).  Another advantage is
that the lack of the metal release plate might lead to a lighter design.

Plate release has a few things going for it though.  It is possible to
design for minimum quiver costs by having a cheap part mounted to the
ski with the release part having the most cost (e.g. Ramer).  Brakes
are certainly still possible (e.g. Voile), but the boot must be very
securely attached to the plate, or a strap attaching the boot to the
plate will be necessary at lift areas.  Since the release problem is
decoupled from the interface problem, both can be optimized
separately, and a 75 mm interface can use the same plate design and
thus the same ski can run multiple boots.

2]  What will hold the ball of the foot close to the ski for an effective
telemark turn?

All the possibilities I can think fit in two classes: 1] direct hold -
the ball of the foot is held on the ski by fittings on the boot near
the ball of the foot (no current implementation that I know of) and 
2] hold via heel - grabbing the heel of the boot with material that is
connected near the ball of the foot (all cable bindings as well as the
Voile (plastic) plate).

The direct hold is possible to implement with either type of release
and might be easier to design into a step-in or pole-actuated binding.
The binding is cleaner (nothing needs to be secured when throwing the
skis in a car rack), and there is no cable failure problem.

The hold via heel has a few advantages to compensate for its possible
problems.  Telemark performance can be fine tuned by adjusting where
the material (cable, plate, etc.) is pivoted and how much tension the
material exerts on the heel as it comes off the ski.  If the sole of
the boot wears out or is not as stiff as you like in the first place,
compensation might be made by this adjustment.  Also, when a cable is       *
used with fittings it is possible to fully unload the cable tension         *
and still have lateral stability.  Thus one can set a touring tension       *
so that the heel comes off the ski easily and doesn't start to get          *
restricted until a point deep into the touring stride (some skiers          *
feel the complete freedom of an AT pivot is undesirible - kick turns        *
are harder and an occasional "knee on the ski" touring fall can             *
happen).  Another tension could be optimized for telemarking and            *
ideally a pole could flip between the two choices.  (Pole-actuated          *
pivot point locations would accomplish the same swithching of modes,        *
but is much harder to see how this could be realized.)  If hold via         *
heel is implemented with a direct release, the loading on the cable
could cause a release or possibly interfering with the release
depending on the design.  A plate release causes no problems.

In addition to these two big questions, there are a few other issues
that come to mind.  How useful would it be to have backward
compatibility with the Dynafit binding?  You could then use your tele
boots in a Dynafit set up (but only if the size were set correctly).
I think of this as more of an issue to a manufacturer who can then
cover more markets with one boot.  I'm guessing compatibility in the
other direction will be more difficult and probably rarely utilized
since Dynafit compatible AT boots don't have soles that bend.  Another
issue is the fitting on the heel.  For Dynafit fitting compatibility,
the heel fitting design can't be changed much, but still might be
useful in implementing a heel-lock unit (optional of course - those
who don't want the weight, complexity, or possible loss in safety can
either not attach the unit for that day or not buy it at all).  For
non-compatible fittings, all sorts of shapes for heel fittings are
possible.

If anyone has any thoughts on this matter that they are willing to
share, follow-up posts or email will be read with interest.  I'll
repost a summary later in the season. - dara

Notes on weight: The lightest 75mm release binding is currently the
Voile CRB 3 pin/cable (the cable-only model is not recommended for
plastic boots) at 1360g (Couloir VIII-1) whereas the Dynafit AT
binding is listed at 570g (Backcountry Oct 95) and 740g (C VIII-2).
(Somebody's scale isn't working?)  The Silvretta SL, another binding
using fittings, weighs in at 910g (B Oct 95) and 1050g (C VIII-2).

Dara Parsavand | 735 Pine Street | Boulder CO 80302 USA 
parsavan@prony.colorado.edu  (use this address if different from header)