Rebound

PMTS Forum

Rebound

Postby Arkady » Tue Feb 27, 2007 10:48 am

I am working on "power releasing" as explained on pp.109-117 of "Essentials". The drills seem to work, and I am getting some rebound - sometimes really throwing me hard from turn to turn. I seem to recognize the feeling. The problem is inconsistency.

Occasionally, I can link a few turns with good rebound (both in free skiing and in the gates), and then - it is gone and I cannot buy one for any money.

What worries me most: I cannot figure out what do I do right when I get it and what do I do wrong when I don't. It seems that I tip, flex and counter more or less the same both times (could also be an illusion). Can it be either timing or the turn shape? Apparently, I am losing track of something important.

I understand that it is hard to say anything without seeing it. I am working on my video. But maybe, some people have a similar experience? What are possible sources of bad inconsistency with rebound?

My usual sequence of drills while free skiing is: a few runs with hop edge changes, a few runs concentrating on counter/counter-balance (with boot-touching, etc.), a few runs getting feeling for float, and then moving to power release exercises. I am completely PMTS-self taught, I also take some racing clinics, which concentrate on tactics, with less attention to primary movements. Any suggestions on the drills?
Arkady
 
Posts: 101
Joined: Fri Aug 25, 2006 8:29 pm
Location: Minnesota

Postby jbotti » Tue Feb 27, 2007 11:23 am

What appears to be missing from your routine is some focus on fore/aft balance with pulling both feet back (at the start of the turn) and on free foot pull back during the turn. When I start a turn when I am slightly aft, I don't get early engagement, the turn is not nearly as tight and I don't get the same kind of snap and rebound out of it.

I have spent a lot of my time this seson working on the proper fore/aft balance. I agree with Harald that once you learn to pull the feet back and have that skill in your skiing you can allow yourself to get back some in certain stuations (to generate speed in a turn on more moderate terrain).

When it gets steep, and you want tight turns with rebound, getting back is pretty much a non starter for me. JB.
Balance: Essential in skiing and in life!
User avatar
jbotti
 
Posts: 1800
Joined: Fri Nov 28, 2003 10:05 am

Postby Arkady » Tue Feb 27, 2007 12:09 pm

Thank you, JB! It very well might be the case. Why did it not occur to me? I will give it a try. I already started adding fore/aft drills to the set. Anything you suggest in particular?
Arkady
 
Posts: 101
Joined: Fri Aug 25, 2006 8:29 pm
Location: Minnesota

Postby jbotti » Tue Feb 27, 2007 12:55 pm

Personally I think one of the most important skills to have in skiing is the ability to pull the feet back under the knees. I practice sucking back the feet often, both in a static position and when moving. This is quite simple and easy. There is a drill on this in The Essentials (don't have it in front of me so I don't have the page number)

What I have found harder is learning to ski in positon that allows me to get the earliest edge engagement and will set me up so that if I do everything else right I can carve the tighest arc. I think of this position as forward, with my shin pressed against the cuff of the boot and with my knees more forwrad than my ankles. For me in order to insure that my hips are not back, I also focus on keeping my sternum up so that if I had headlights on my chest they would face forward and would not shine on my skis.

For me, from this position, I can carve very tight short radius turns. if I get back at all, it deteriorates very quickly.

I spend multiple runs every time out focusing on excatly where my fore aft balance is and I am always asking whether I need to be more forward.

Every time I am carving in steeper terrain, I am very focused on sucking back the feet in every turn (easiest in the flexed position when the skis have released).

Free foot pull back can be added to all of this as well (as distinct from pulling back both feet). I generally start to pull back the foot in the upper part of the C. At speed and in steep terrain it requires some real force to pull it back. I also spend multiple runs each time out working on this in my skiing.

Harald's fore aft balance is incredible. I think he worked on it for years, but it is rarely (or maybe never) an issue in his skiing. I think it can take years to really find the right consistency with this.

I also think it is great to play with fore aft balance. Clearly on moderate terrain, getting back can be used to generate speed in carved arcs and it is fun and a valuable tool. Where it causes problems is when it becomes a skiers default position with no ability to get more forward when needed. When this is the case, carving steep terrain with any amount of control is essentially impossible. JB.
Balance: Essential in skiing and in life!
User avatar
jbotti
 
Posts: 1800
Joined: Fri Nov 28, 2003 10:05 am

Postby Max_501 » Tue Feb 27, 2007 2:41 pm

Keep in mind that the amount of rebound you get from a turn will be directly related to the speed at which you release. The faster you release the more rebound you will need to manage.
User avatar
Max_501
 
Posts: 4051
Joined: Thu Mar 10, 2005 7:39 pm

Postby Arkady » Wed Feb 28, 2007 7:38 am

Thanks for all the suggestions!

Max, I need a clarification.

Keep in mind that the amount of rebound you get from a turn will be directly related to the speed at which you release. The faster you release the more rebound you will need to manage.


I tend to read it in three different ways:

1. going faster (mph) at the moment of transition
2. changing edged faster
3. changing edges earlier in a turn

I totally understand (1) - speed really helps. However, I have questions about (2) and (3): I seem to have a better success when I let myself finish the turn and flex at transition a litte slower. On the contrary, I seem to "choke" the rebound if I change edges early and fast. Is it completely counterintuitive?[/quote]
Arkady
 
Posts: 101
Joined: Fri Aug 25, 2006 8:29 pm
Location: Minnesota

Postby Max_501 » Wed Feb 28, 2007 9:06 am

Arkady wrote:I seem to have a better success when I let myself finish the turn and flex at transition a litte slower. On the contrary, I seem to "choke" the rebound if I change edges early and fast.


When you move from one turn to the next you are using the force (momentum) to move your center of mass into the new turn. The speed at which you release the turn has a large effect on the forces you feel. If you need to get from one turn to the next very quickly (for example a slalom turn) you can aggressively retract (lift) the outside leg and tip. In this case the movement of the CM is so fast that you need to suck that outside leg way up and be in excellent early counter balance or you are going to fall to the inside of the new turn. Its takes alot of practice to be able to manage the forces of very short turns without getting thrown around.

Does this make any sense?
User avatar
Max_501
 
Posts: 4051
Joined: Thu Mar 10, 2005 7:39 pm

Postby Max_501 » Wed Feb 28, 2007 9:21 am

serious wrote:The amount of rebound you get from the ski depends only on how decambered the ski is at the moment of release.


Is the rebound from the ski or from the turn forces? I suspect its mainly from turn forces.
User avatar
Max_501
 
Posts: 4051
Joined: Thu Mar 10, 2005 7:39 pm

Postby stikki987 » Wed Feb 28, 2007 9:36 am

My guess is the ski allows the platform for the rebound. A stiffer ski creates a better platform for the body to load forces whereas a soft ski doesn?t. Sort of like shocks in a car, stiff shocks don?t create much rebound themselves in a corner but instead allow the forces to load to chassis, etc. better.

Also, if you can generate rebound on inline skates in a corner then I suspect it's from turn itself as skates have no flex.
stikki987
 
Posts: 38
Joined: Mon Jan 15, 2007 10:33 am

Postby Arkady » Wed Feb 28, 2007 4:36 pm

I was skiing today for a short while (both free skiing and NASTAR), and I think that I have the answer (or AN answer) to my rebound problem. The weather was warm but windy, snow was sticky and slow. It seems like the prescription (see the thread above) is working:

1. When I stay forward and pressure the tips (in order to do it, I think of PMTS instruction: pull free foot back), the ski bends, and the rebound is there!
2. When I try to speed up (let the skis go), I fall behind momentarily and all the rebound is gone.
3. Surpisingly (or not), I end up going faster the first way, not the second.
4. One additional point: it seems like due to bad Hi-C turn management I get the angles on my new edges late, so in order to get the ski decambered and get a meaningful rebound, I have to wait longer on the Low-C part of the turn to make a transition. If I make a transition earlier, there is not enough energy from the Hi-C part of the turn, and the rebound is weak. So there is an additional issue with my Hi-C turns.

Thank you for all the feedback! The forum works really great for me in absence of face-to-face PMTS training. I am looking forward for the first opportunity to go to a camp (must be December). By the way, when do you usually sign up for the next season's camp?
Arkady
 
Posts: 101
Joined: Fri Aug 25, 2006 8:29 pm
Location: Minnesota

Postby dewdman42 » Wed Feb 28, 2007 4:42 pm

sorry Tom, can't agree with you about that. At least not strictly speaking the way you have phrased it. Your decambered ski will not "rebound" you like a bow and arrow of any significant amount. The lion share of energy that you feel is from capturing centripital G-forces through solid carving skills. As you finish the turn, if you have not allowed those G-forces to bleed off through skidding or perhaps excessive flexion, you can release suddenly as Max has described, which will give you a feeling of explosion, as all that saved up G-force energy is released towards the new turn. I do not even like to use the word rebound here, because rebound really implies a force that is bouncing back in the opposite direction it was initially going. The release Max described is not really a rebound. It does not bounce back. It is "released".

Historically, the term rebound was used to refer to certain ski moves which involved a hard edge set followed by a release. Personally I do not think its an appropriate concept to use in modern skiing, but that's just me.

Now you may be asking yourself why some skis feel like they have a lot of energy and some feel more dampened. One could draw the conclusion that a stiffer ski is somehow able to sling you like a bow and arrow while a softer ski would be like a low powered bow that has no power to do so. However, that is not really how it works. Some skis have characteristics which will enable you to conserve the G-forces of the turn better than others, at different speeds. A soft, damp ski is probably bleeding off some G forces, thus you don't feel the zip factor. A ski with a stiffer tail allows you to finish the turn and conserve those G forces right up until the very last instant, giving you a bigger and more explosive "release". Modern skis have changed so much. Now they can make skis that are torsionally very rigid, yet quite soft to flex. They are not necessarily bleeding off G forces, even when they are highly bent. So what makes a ski feel more energetic vs damp is more complex then simply the stiffness of the ski. But suffice it to say, that the skis do not sling you like a bow and arrow. They conserve the G-force energy of the last turn more effectively and allow a bigger and more explosive release to occur.

Technique is also a factor.
dewdman42
 
Posts: 511
Joined: Thu Jan 19, 2006 11:52 pm

Postby dewdman42 » Thu Mar 01, 2007 11:03 am

I do not agree Tom, sorry. the forces contained in your decambered ski are not significant enough to do what you are imagining. But ask yourself a more important question. Let's say hypothetically they were strong enough. Why would you want that to happen?
dewdman42
 
Posts: 511
Joined: Thu Jan 19, 2006 11:52 pm

Postby Arkady » Thu Mar 01, 2007 2:52 pm

If I understand it right, the "rebound" or whatever you call it is the response to your pressure coming momentarily from your skis toward your feet in transition (ski is bent, and direction is not up or down) and if you flex, provides a way (other than up-unweighting or down-unweighting) to release efficiently and fast (power release).
Arkady
 
Posts: 101
Joined: Fri Aug 25, 2006 8:29 pm
Location: Minnesota

The Force

Postby geoffsep1963 » Thu Mar 01, 2007 4:04 pm

I agree with the Dude, but wanted to try to quantify an example for my own interest.

Please don't read further if you aren't interested in the maths, it's just a different way to learn for us dorky engineers. :-)

Think of how easy it is to put a ski tail on the ground hold the tip and bend it into reverse camber with your hand/arm over the time it takes to complete even a short turn. Not a lot of energy required or therefore stored.

To convince myself of the force required to hold a ski in reverse camber, I just stood on some bathroom scales and measured an increase of 40kg to bend my skiis 50mm (2 inches) at the binding with tip and tail supported under two adjacent shelves. So the force is, F=ma= 40x9.81=392N, (acceleration due to gravity approx 9.81)

Now think of trying to stop a skier with the momentum, lets say 100kg for convenience doing 30 km/hr , (8 m/s) at the end of the turn. Try to stop that with your arm! So intuitively, I suspect that the bent ski does not contribute much to "The Force" at release.

A bent, edged ski is storing an amount of energy, but it is transferring the force necessary to hold all force created by the moving skier, (force due to both gravitational acceleration and centripetal acceleration), to the snow/ground.

F(gravity) =ma = 100x9.81=981N
(acceleration due to gravity approx 9.81 m/s ^2)

F (centrifugal) = ma(centripetal)
a(centripetal) = v(tangential)^2 / r
Lets say the turn radius is 15m
and the tangential velocity is 8 m/s
a(centripetal) = 8 ^2 /15 = 4 m/s^2
so F (centrifugal) = 100 x 4= 400N

Do the vectors, gravity is straight down, centripetal acceleration is up the fall line. The slope of the hill? say 40degrees from horizontal. We have force due to gravity straight down to the centre of the earth and force due to the turn straight down the fall line.

I get total force is approx 1236 N at 76 degrees below the horizontal.

This means the ski must be tipped 14 degrees above horizontal, or if we add the slope of the hill, 54 degrees above the slope.

So: we have the ski requiring a force of 392N to keep it bent, but transferred through the ski to the snow, and with stored energy available to re-camber the ski at release.

I think that the energy in the bent ski simply helps the stance leg retract at the release/transfer. As the ski is no longer on the snow resisting the forces, the body is propelled down the hill in the direction of the forces at an acceleration of this force divided by the mass of the skier.

a=F/m= 1236/100 = 12.36m/s^2 or about 1.25 g's.

No wonder we need to re-centre during this part of the turn. Of course if we tighten the radius, increase the tangential velocity or increase the mass of the skier this number quickly increases and so does the required tipping of the ski.

If I have this right, The "Force", is mostly the combined forces on the skier mentioned above and the force used to bend the ski simply helps the retraction of the stance leg at release.
geoffsep1963
 
Posts: 58
Joined: Wed Dec 13, 2006 7:10 pm
Location: Queensland, Australia

Postby h.harb » Thu Mar 01, 2007 4:29 pm

If you look back at the equation I posted in another thread, for centripetal acceleration and relate the amount of rebound to the centripetal force created, you will be able to determine how the ski relates to what we feel is rebound. A stiff ski doesn?t necessarily provide more rebound , it may reduce the rebound feel if it can?t create the radius or hold the radius with speed. Which is directly related to the amount of ski bending under the foot. A ski that is bending may give the skier more confidence to lay the body down and therefore create a tighter radius resulting in more centripetal acceleration. If we say that a ski bends, by pressuring it, will it not, in the release phase de-camber quickly, and that may provide what we feel as rebound. So skiers often conclude that a bent ski de-cambers more and helps with rebound.


So what comes first, the ability to carve a tight arc with high speed, or does it depend on a ski that gives you confidence by allowing the skier to feel more grip, platform and edging. I know I can carve on skis I don?t like, but I can?t get the same rebound feel out of those skis. Are we asking a ski to provide more rebound or are we getting more rebound feel because the ski encourages and assists us in achieving more angles, and grip so we create more rebound feel. Rebound feel is important whether it comes from the ski or from a combination of centripetal force and gravity.


I use the rebound feel to evaluate how I should adjust my technique for improved performance in an arc. When I don?t get the rebound I want, it has to do with the quality of the edge at the release. So the amount of rebound has to do with edge angle and edge hold. In these situations when I watch my video or review my photos of the turns, I associate a turn I like with a ski in a bowed revered camber situation, just before release.

The turns I like have two thin pencil lines as tracks. So I conclude more ski bend more rebound, but it?s really more centripetal acceleration more rebound. Ski bend with pencil tracks are just a necessity for achieving strong rebound, so they do co-exist. The question is does the ski give you less or more rebound then we conceive? Let?s look at skis. If you park yourself between two bumps and suspend you feet approximately 12 inches off the ground and begin to bounce will the ski rebound or spring you back up? Of course it will. How much push do you have to put into the ski to get a good rebound springing action. In my experience you have to jump up and down on the ski to get it to really spring.

In a carved arc the ski is never bent to this extreme. But the force on the ski in a high speed tight radius is more than the jumping force or your body weight. The ski carving on hard snow can?t bend very far. So conclude from this that the ski doesn?t provide much spring, it isn?t really bent very far. If we look at the turn arc with a vector diagram the largest loads are at the bottom of the C. Yet that?s not where I get my best rebound. My best rebound comes in the arc later then where the greatest gravitation and centripetal forces are measured. Why, because I can shorten the radius, right after the maximal loaded point. As we know from the acceleration equation, shortening the radius increases centripetal acceleration. Therefore, I get more rebound feel as I shorten the radius. From a practical stand point releasing is only valuable if you can control the forces. The extra radius after the maximal loading point gives my skis the direction I want them to go after the release. If the release is high energy, this is critical to success.

Although the skis can?t be the provider of rebound, the way we want or expect it, the skis bend is an integral part of using, feeling and determining release, rebound and transition.
User avatar
h.harb
 
Posts: 6772
Joined: Sat Feb 03, 2007 2:08 pm
Location: Dumont, Colorado

Next

Return to Primary Movements Teaching System

Who is online

Users browsing this forum: MarcS and 2 guests