From: djm@kbs.citri.edu.au (David Mills)
Subject: Re: Learn on short or long skis?
Organization: Collaborative Information Technology Research Institute
Date: 20 Dec 92 23:51:19 GMT

mmendel@eniac.seas.upenn.edu (Mark Mendel) writes:

>In article <djm.723860511@phobos> djm@kbs.citri.edu.au (David Mills) writes:

>>
>>There are three main things affected by ski length:
>>	- inertial stability -- easy of turning,
>>	- camber -- weight distribution, and
>>	- friction -- edge grip
>>
>>The first category.  The shorter the skis, the easier they are to turn. 

>	He then goes on to imply that the reason short skis are easier to turn
>is because of thier lower longtitudinal inertia.  Poppycock.  But before we can
>discuss the real difference between short and long skis you must
>understand the physical mechanisms by which we make skis turn.   

>	 Ski turns come in two flavors, carved and windshield-wipered
>(most turns are some combination of the two).  In a windshield-wipered turn,
>the ski skides laterally as it turns, following trajectory not unlike the
>automobile part named after this turn.  

Wrong there are many non-carved turns that are not windscreen wiper turns.
The word christie (german for skidded turn) covers one such family of turns
(i.e. stem-christie, parallel-christie etc).  Turns where a bump is used to
initiate (pivoting on the top) is another.  Turns in powder where neither
carving nor windscreen wiping is possible another, etc., etc., etc.

>	Carved turns are harder to understand.  Following
>the description in  How the Racers Ski (by Whitterell, if I am not mistaken) 
>if you put a ski on a flat surface and tilt it onto one of its edges,  because
>of its sidecut (i.e. the ski is wider in the tip and tail) the edge will only
>touch down only at the tip and the tail.  Now bend the ski backwards into 
>reverse camber (by wieghting the ski) and keep bending until the whole 
>edge makes contact with the flat surface.   Guess what?   The line of contact
>of the edge with the surface is actually an arc.  Turn the flat surface into
>snow, and start moving the ski forward and will follow that arc, making a 
>wonderful turn with no lateral motion (i.e. no sliding sideways).  Turn the 
>snow into ice and it still works just fine.  You cannot do that with a 
>windshield wiper.

Yes thats one of (and possibly the main) technique used by ski reacers.  But
not all gates allow this, and more often so in a slalom race where the turns
are usually too short to allow this completely.  The principle still applies,
but a controlled element of skidding is introduced into the turns so as to
reduce their radius.  The turns in a giant slalom are closer to a pure carved
turn, but still not all turns are pure carves.  And so on into Super-G, Down
hill.

>	The difference between long skis and short skis is that:	
>Long skis carve, and short skis windshield-wiper.      

Sorry Mark, but that is a load of poppy cock (to coin a phrase).  Your
description of a carved turn is reasonably accurate.  But very few skiers,
even those with long skis are able to carve a turn.  It requires balance,
steering, edging, leveridge and weighting skills that take years to learn
(certainly not before a skier has done more than 100 days skiing, and mostly
not till long after that.

A pure carved turn also requires speed.  Otherwise it is not possible to exert
enough constant pressure on the ski throughout the turn to maintain it in the
reverse camber.  Also, the radius of the resulting turn will be quite large,
varying with the ski type (> 6m diameter for a slalom ski and > 8m diameter
for a giant-slalom ski -- as estimates -- different ski make will vary too).
Thus even skiers who are able to carve a turn mostly don't.
All other turns (and carved turns to some extent) are executed by the
application of a rotary steering force (applied by the feet and legs when done
correctly, but by inertial rotation as a reaction to a shoulder swing when
done incorrectly).  Because a longer ski has greater inertia (and also partly
because of friction cause by the edge -- greater edge angle greater friction
also longer ski greater friction) then the force required to overcome the
inertia (and edge friction) becomes larger.  i.e. the ski is harder to turn.

Further, it is possible to carve a short ski (but the ski will travel more
slowly than a longer ski doing the same carved turn), and probably more people
on long skis windsceen wipe than carve.  Carving is a skill (a very difficult
skill), and having a long ski does not suddenly bless you with that skill.

>Mark Mendel

David.