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inside/outside skis load distribution - Page 3

post #61 of 86
Bingo!
post #62 of 86
GOD I wished it was snowing so i could try this stuff out!!!
post #63 of 86
Quote:
Originally Posted by sfdean View Post
Let me try again, Ghost--

Are you talking about, rather than a progressive rolling of the skis to engage the sidecut to carve a turn (e.g., a racing turn that is mostly carved), instead throwing the skis a little sideways to initiate a skidded turn and then standing against the edge in a hockey stop power slide/skid, where you're balancing against the G forces, but using a pivot entry specifically to create a larger divergence from the original direction than can be maintained in a carved turn using the sidecut. (The approach that is taken by most recreational skiers in most turns.) If so, then I do get it.
Well doing the above will certainly help induce a skid, but that is not what I'm talking about. All you have to do to induce a skid on ice at say 27 mph is tip the skis enough, smoothly or otherwise, so that the sidecut would dial up a slalom turn. The skis will be skidding if they are on ice. At lower speeds skis stiffness helps dislodge the skis more when you dial up a tighter turn, even if the the sidecut tipping angle - turn radius function would permit (slightly) higher speeds at the shorter turn radius.

Consider the following separately for a ski tipped on edge

1. The angle you need to keep the skis tipped at in order that the force vector of centripetal force (M * ((V m/s) ^2 )/ radius m) and gravity gM combine to push at an angle that drives the skis into the groove instead of pushing the ski out of its groove. As speed increases for any given R this angle gets steeper.

2. The angle you need to tip the skis to get a given radius turn.

Tipping the skis more allows you to carry a greater speed without pushing the skis out, but no matter how much you tip, you still can't exceed about 25 or 26 mph on ice before the critical angle will be exceeded by the g-forces.
post #64 of 86
Quote:
Originally Posted by Ghost View Post
All you have to do to induce a skid on ice at say 27 mph is tip the skis enough, smoothly or otherwise, so that the sidecut would dial up a slalom turn. The skis will be skidding if they are on ice. At lower speeds skis stiffness helps dislodge the skis more when you dial up a tighter turn, even if the the sidecut tipping angle - turn radius function would permit (slightly) higher speeds at the shorter turn radius.

Consider the following separately for a ski tipped on edge

1. The angle you need to keep the skis tipped at in order that the force vector of centripetal force (M * ((V m/s) ^2 )/ radius m) and gravity gM combine to push at an angle that drives the skis into the groove instead of pushing the ski out of its groove. As speed increases for any given R this angle gets steeper.

2. The angle you need to tip the skis to get a given radius turn.

Tipping the skis more allows you to carry a greater speed without pushing the skis out, but no matter how much you tip, you still can't exceed about 25 or 26 mph on ice before the critical angle will be exceeded by the g-forces.
OK. I think I've got it now. (You haven't been unclear. I've been dense and resistant to understanding.)

As I further translate into the practical, the issue is in part that (1) in varying snow conditions even at a very high edge angle, lateral force from change of momentum of over 2 Gs x 190 pounds of skier and equipment will push the ski laterally out of its groove (skidding) rather than permitting continued carving with a tighter turn radus caused by increased bending (decambering) of the ski. And (2) in icier/hardpack conditions, it takes less lateral force to cause the ski to break loose and start skidding than it does in softer snow, where you just end up compacting more of the snow under foot as you increase the lateral Gs.
post #65 of 86
Quote:
Originally Posted by Ghost View Post
Well doing the above will certainly help induce a skid, but that is not what I'm talking about. All you have to do to induce a skid on ice at say 27 mph is tip the skis enough, smoothly or otherwise, so that the sidecut would dial up a slalom turn. The skis will be skidding if they are on ice. At lower speeds skis stiffness helps dislodge the skis more when you dial up a tighter turn, even if the the sidecut tipping angle - turn radius function would permit (slightly) higher speeds at the shorter turn radius.
You seem to be suggesting that the tips and tails will lift the edge that the skier is standing on out of it's groove. With all due respect, Ghost,I don't think so.
post #66 of 86
I'm suggesting that if you tip too far, say dial up a 1-m turn radius (which the ski will not bend into), the ski's longitudinal flex will conspire with the centrifugal force to dislodge tip and tail, even in a torsionally rigid ski.

Ice being hard forces the ski to decamber where as soft snow will allow a little deviation from the shape that must occur to keep the edge flat on the snow.

If the skis are tipped and the surface hard and the tip and tail do not adopt that super short turn because their flex will not permit it, the tip and tail must lift the centre out of the ice; it's geometry.
post #67 of 86
Don't forget too that the forces you apply to the ski will become more and more parallel to the plane of the hill as you bend the ski further and further. Less force will be digging the ski into the snow pack.

Slippage happens well before the ski lifts the skier.
post #68 of 86
Wouldn't the amount of side bevel come into play in this topic?

Ever skied with a 6 degree side bevel?? bob barnes has a great illustration of this too.
post #69 of 86
Quote:
Originally Posted by bud heishman View Post
Wouldn't the amount of side bevel come into play in this topic?
There are lots of variables like side bevel, plate height, boot stiffness, counter balance (angulation) ability that will all influence the skiers ability to hold an edge on ice.
post #70 of 86
Sharper edges and a more acute edge due to side bevel will definitely aid in cutting the groove, but it's the base that gets pushed against the ice by the centrifugal force. When the centrifugal force gets too great for the angle at which the skis are tipped, they will get pushed out of , not into, the groove.
post #71 of 86
Quote:
Originally Posted by Ghost View Post
When the centrifugal force gets too great for the angle at which the skis are tipped, they will get pushed out of , not into, the groove.
Sounds like the solution is to go slower than that critical speed.
post #72 of 86
Ghost,

If the angle is 90 degrees or less to the resultant force, what causes it to get pushed out? ...if it is less, wouldn't it have the tendancy to dig in more? ...if the angle is already greater than 90 degrees, why does it hold in the first place?

You've stated several times that as the force increases, you have to tip more to continue to hold the edge (with the penalty of a smaller turn radius being created), however, does this not assume that you were at critical edge angle to begin with ...and are now tilting the resultant force vector more towards parallel to the slope, requiring ski to snow angle change? If, on the otherhand, you have created an acute angle to the resultant force to create your desired turn radius, would you not then be able to tip more inside (up to critical edge angle) and maintain the same ski to snow angle without dislodging? (so in BB's animation you linked to, freeze the ski and move the vector towards the snow instead)

Chris
post #73 of 86
Consider, for the moment, making a turn on a horizontal piece of skating rink ice. The direction of the force exerted by your cm on the ski's edge will depend on the centrifugal force and the gravity force. For balance, you can angulate at lower speeds to keep the force directed from the cm through the edge . As speed increases for the same size turn, the centrifugal force increases but gravity remains the same, so the angle of the force from the vertical increases (in the extremes at 0 m/s you are right over the edge and all the force is straight down, at the other limit as you approach infinite velocity the net force is 90 degrees from straight down).

Compare this angle to the angle the ski is tipped at. If you are in a pure banked turn (no angulation), then this force is directed straight down into the plane of the ski. If you go past banking the force being applied to the ski has a component pushing the ski up out of the groove.

The turn radius that the ski decambers to is equal to the sidecut radius of the ski times the cosine of the tipping angle (angle between ski and ice).

The ratio of gravity force to centripetal force for a pure-banked turn is the tangent of the angle between the ski and the vertical (90-tipping angle, I'll call this the banking angle). Assigning a value of 1g for the gravitational acceleration and working in terms of accelerations, since the centripetal acceleration required for the turn will be a= V^2/R, the velocity of a pure banked turn can be worked out for the tipping angle, as the square root of (9.81 * R cosine (tipping angle) * 1/(tan(90-tipping angle))

For a 13 m sidecut ski:
(Tipping angle in degrees, Banking Speed in mph)
10,10
15,14
20,18
25,19
30,20
35,21
22,23
40,24
45,24
50,24
55,25
60,25
65,25
70,25
75,25

On snow the egdes can cut into the snow a little more than the midsection, and the tipping-angle turn radius function is not as strict, so we can carve much faster, but on hard ice we can only carve so fast.
post #74 of 86
Quote:
Originally Posted by Ghost View Post
Sharper edges and a more acute edge due to side bevel will definitely aid in cutting the groove, but it's the base that gets pushed against the ice by the centrifugal force. When the centrifugal force gets too great for the angle at which the skis are tipped, they will get pushed out of , not into, the groove.
That is true, but don't forget about the "spoon shaped" blade at the tip of the ski, right at the widest point of the running surface and the beginning of the upward curve of the tip. When pressured and tipped it cuts into the surface and then the tiny hook where the edge turns upward shaves a rut in the surface that the remainder of the decambered running edge continues to cut out.

Back to Weem's platform ... if the forward part of the ski hasn't cut enough of a "trench" for the rest of the ski to follow and cut out a platform (centripetal force) to oppose the centrifugal force, the ski will wash out. It is a continual process.

For example, take a router and start routering a groove in the middle of a board. Depending on the down force used and the shape of the bit, as you push forward, there will be a permanent rut left in the board. At the end of the rut (which was actually the start of the rut to begin with) there will be a "ramp" down into the wood. From then on, the rut will conform to the shape of the bit as the bit moves forward through the wood. If downward pressure on the router is relieved, the bit might kick up and the depth of the rut will decrease. If enough pressure is released, the bit might jump completely out of the groove and start a new line. Same with a ski tip.

The ski has a much longer contact surface with the snow, so there is additional cutting going on as the ski passes over the rut created by the tip, but the action going on at the tip is essentially just like a router bit. (Or the sculpting tool that BigE suggested.)
post #75 of 86
Quote:
Originally Posted by Ghost View Post
On snow the egdes can cut into the snow a little more than the midsection, and the tipping-angle turn radius function is not as strict, so we can carve much faster, but on hard ice we can only carve so fast.
Ghost, I'm pretty skeptical that the simple model you propose really fits the situation. At least once thing that seems to be ignored is that ice is not a perfectly hard surface. I suspect that a ski (especially in high end racing) can cut into the ice and change the situation considerably by producing a groove that adds considerable hold for the ski, like softer snow but to a lesser degree.
post #76 of 86
Yes the ski can cut the groove, but the platform it cuts does not compress, so the radius of the cut platform is pretty close to the sidecut radius times the cosine of the tipping angle, and the angle of the platform being cut is pretty close to the tipping angle. The ice does not compress under the load like snow. Add in a base bevel and things get worse.
post #77 of 86
I would guess the ice melts and in that way is more accommodating.
post #78 of 86
I think you guys are speaking about vanishingly small inaccuracies in the model. In doing so, the thread is going down the rabbit hole, and won't return.

Folks,

the key issue is that critical angle is speed related. The critical angle must become more acute as speed rises. This should be clear to anyone that has ever skied!

Thus the problem -- I need more angle, but I'm moving too fast to maintain a carve.

The error here is that you were moving too fast! It is one of tactics. Racers do not try to go as fast as humanly possible through each gate, because at some point the speed will have to be scrubbed to make the turn. They manage their speed through the course so that they'll have the highest AVERAGE speed.

Bode did it at Solden, when he slipped on turn and carved the next. He himself said he could not arc through the entire course, because there would be places he would be going too fast, and would need to dump too much speed.

Ghosts issue for racers is PROOF that even racers need to keep a lid on it.
post #79 of 86
No argument with the main concepts Big E. The thing I'm concerned about is whether the quantitative estimates can be believed.
post #80 of 86
Again, all other things being equal, a more acute side bevel will allow the ski engage deeper in the surface (ice) thus creating more grip or resistance to breaking loose. Imagine a cross section view of a ski engaged in hard snow. Bob Barnes has a diagram that compares a ski with a 90 degree angle and one with a more acute angle. When the side wall of the ski is 90 to the base the ski easily lifts out of the groove as edge angle increases because it is leveraged up off the vertical (90) side wall. Conversely, with a 6 degree side bevel (84 degree edge angle if base is flat) the ski can be tipped higher before the side wall contacts the snow surface thus maintaining a stronger edged engagement by not lifting the edge out of the surface. Of course the down side of increasing side bevel angles, the skis become less forgiving and more difficult to get off the edge so movements better be more precise. For most people there is a happy median. I prefer 3 degrees on all my carving skis and 2 on off piste skis. If I were running Nastar type courses or Pro type radius courses I would probably run closer to 6 degrees. All other things being equal.

Here's another thought or argument for proper boot alignment. The less base bevel the better the grip too. People who are overcanted tend to like more base bevel to compensate for the overcanted boots thus compromising edge grip. Moral of the story, if your boots are properly aligned you can ski with little or no base bevel. Sure, I know that speed event skiers want a bit more base bevel, but this is done to improve glide not edge hold. It is a compromise.
post #81 of 86
Quote:
Originally Posted by bud heishman View Post
The less base bevel the better the grip too.
To think of it another way, as you increase the base bevel you have to increase the tipping angle to get the same grip as a less beveled base.
post #82 of 86
Yes agreed, I should have qualified that statement with given the same tipping angle.
post #83 of 86
What happens with my skis when I go faster than the "pure carving limit" or try to turn sharper than a pure carve can, is that the tips shave along the ice, but the centre portion seems to cut in a longer radius turn than the tips are bent into. I notice the tails are not as wide as the tips. The ski's side cut is not a pure arc. Sure it's nicer when the tips cut the grove and everything follows, but the skis will still turn at higher speeds as long as you tip them.
post #84 of 86
Thread Starter 
Quote:
Originally Posted by Ghost View Post
.. is that the tips shave along the ice, but the centre portion seems to cut in a longer radius turn than the tips are bent into. ..
i suspect that its because tip, center and tail are not in the same force plane. otherwise there should be no sideway force to effect the shaving. it could have to do with fore/aft balance and its normal?

Quote:
Originally Posted by sfdean View Post
Thanks, BigE, that makes sense.
So, in steeper hardpack sections on slalom skis, the problem could come from any of the following (any resemblance to actual skiing of SfDean is really darned unfortunate)
1. Not being forward enough at turn initiation. The whole turn gets delayed from insufficient early forward commitment, and the skier tries to compensate by hanging onto the turn longer. (Coach to SfDean, only partly in jest: "I think we should try putting tacks in the heel of your boots...")

2. Fear (or, er, let's just say interesting line choice) -based trying to hang onto the turn too long to go more across the hill (skiing the slow line fast) to reduce vertical (top to bottom) speed.

3. Failure to manage forces properly later in the turn. Instead of releasing (or at least relaxing the old outside leg somewhat) in the final phase of the turn, skier, thinking "Must...Stick...Turn...On...Ice..." just continues to resist forces to the maximum extent at the end of the turn. (SfDean not having yet located "feel for snow in different conditions" in somewhat smallish bag of tricks.*)

SfDean.
...
I ll keep these in mind next time i ski icy steep.
Quote:
Originally Posted by HeluvaSkier View Post
The answer is no. When manipulate pressure management to be a certain distribution you are no longer in balance and as limiting the turn you can make.
i understand now why inside leg is not suitable to take too much load. but may i ask if i were to manipulate loading distribution what mechanism make me ruin the balance?
post #85 of 86
Quote:
Originally Posted by carver_hk View Post
i understand now why inside leg is not suitable to take too much load. but may i ask if i were to manipulate loading distribution what mechanism make me ruin the balance?
Anything that you do to forcefully change the pressure distribution between your legs from what it wants to be naturally for the turn you are making at any part of the turn you are making will adversely affect your balance. Think of it like this: In high edge angle carved slalom turns if you were to deliberately reduce the pressure on the outside ski it would end up breaking from the carve. You can also over pressure the outside ski and break it from a carve if you are trying to force pressure on to that ski when it is not needed and you actually should be using your inside leg to support some of your weight. At the end of it all however, your balance is in relation to the amount of pressure that your outside leg/ski needs at any given time. Sometimes it may require huge amounts of pressure, and other times it may require less pressure depending entirely on the turn that you are making.

Later

GREG
post #86 of 86
Thread Starter 
Quote:
Originally Posted by HeluvaSkier View Post
....your balance is in relation to the amount of pressure that your outside leg/ski needs at any given time.
GREG
i been wondering if there would be an tolerance? within that tolerance the skis could still maintain the same traction?
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