GrimSurfer wrote: ↑Wed Jan 18, 2023 4:19 pm
TallGrass wrote: ↑Wed Jan 18, 2023 4:04 pm
@GrimSurfer Why can a cable NOT
trans-mit force from the human to the ski, via however many intermediaries (including socks)?
Because the cable is, in effect, attached to itself through the immovable binding plate. Thus, all the tension is internal.
I'll need that fleshed out to follow you. Internal to 'what'?
Perhaps a hang-up is "immovable", ala static, non-dynamic.
Truth is everything is dynamic, has give, movement, though may be relatively-static.
This is important in structural failures, i.e. Yield Point, Failure Threshold.
Like a stress-gauge indicates (needed because it's hard-to-impossible for the human eye to detect), materials stretch, bend, deflect BOTH throughout their medium (e.g. steel binding plate) and those they're are connected to (screws, wood glue, top sheet, wood ski core, ...).
One can observe this by unscrewing the binding plate from the ski and mounting the boot and cable. The system will continue to function, independent of the ski. Why? Because it is a closed system. All of the tension is internal to the binding, cable, boot. This is what tension means: equal and opposing forces.
What "system" (to be clear on which parts are in it and not in it)?
If you have a boot-cable-binding, you'll never see it get into a configuration of (boot) 'flex' without a human applying force one direction and the ground (resistive force of ground to snow to ski ...) counteracting it.
This is different than an aircraft cable, where one end is attached to a stick and the other to a moveable element (like an elevator). In that case, the pilots arm applies force to the stick, which moves a lever and pulls a cable to another lever which activates a moveable element (the elevator).
Not really, because the more wind resistance the more force the pilot has to apply, AND at some point that force can exceed what the cable (or other object such as a lever, rod, pivot... in the system linking pilot to control flap) can take and !SNAP! (energy stored in tension released as sound... vibrations in air... and localized damage to other parts).
A looped cable doesn’t work this way. Whatever force is applied by the heel is countered by the attachment point of the binding. The forces cancel out. So there is nothing to impart force onto the ski.
Ever use a clothespin? The steel spring can store (potential) energy. So can cables.
The cable, binding, and flexpoint of the boot make a triangle, and under tension can likewise store energy, given amounts for given angles, which the skier can change like a water valve/faucet to determine how much passes through.
The cable tension acts on the boot, and only on the boot. It makes the boot harder to move. In other words, it imparts a controlling force on the boot. The ski only “sees” whatever force the skier applies.
Um, were that true, a skier standing on his skis would fall through to Earth's molten core. In contrast, the ski "sees" the "force" of the skier from above, and resistive force of the earth (ground) below.
When skier > earth, the skier "sinks in" ala into powder or mud until balance is achieved.
When skier < earth, the skier rises like when hitting a bump, and if he "gets air", the energy is not lost but converts to kinetic energy (upward) which transitions (0-100, 10-90, 20-80...) eventually to entirely potential energy (100-0) at the apex, then starts to fall (potential --> kinetic, 90-10, 80-20, ...) and so on.
Once you get past the control force (which is internal to the binding and boot), the only forces acting against the ski are exerted by the skier and gravity.
This omits both the storage of energy (in various ways) by the materials between the skier and the earth, and the way (location) forces are transmitted, same as moving a flap on a plane causes one side's force encountered to increase and the other side decrease (i.e. imbalance) which changes the attitude of the aircraft.
The phrase "control force (which is internal to the binding and boot)"... a person's foot is internal to the boot, yes, but the force is NOT restricted to it, as it can be passed through the sock to the boot, boot to binding, binding to screws, screws to ski, ski to earth/terrain.
It appears from the above that you're treating the foot-boot-binding like three links and a chain of their own and in isolation, but they are connected on both ends to other things which play a part.
The whole point of skiing and control is creating imbalances that upset the momentum to "remain still" into movement down the hill and or direction and or speed (vector: speed+direction).
tkarhu wrote: ↑Wed Jan 18, 2023 4:48 pm
I like this better than the other diagram with arcs. Only explicit thing it leaves out is the short "fixed" leg of the triangle between the blue and purple ends at the plate/ski.
Also, I would not put the blue focus (pivot) at the boot tip, rather under the ball of the foot along the mid-line of the boot-sole, a couple inches back. The shorter that "invisible fixed leg" is, the flex a human can get out of it, and the longer the more rigid it becomes.
The blue and or purple have to give appreciably or it would act like an Alpine Binding.
Alternatively, if blue and purple pivoted at the same point on the binding, you would effectively have a 3-pin (toe-only) as the resistance would be from the flexing of the boot sole alone.
And if the blue and purple swapped places...
With the triangle, it becomes easier to see the Lever Type in play. (The pliers is a confusing example because it's two CROSSING levers sharing a fulcrum, at least color some arrows red and others blue diagonal from each other to conform with illustration to its left. If you've used circlip pliers that spread-to-open you have non-crossing levers sharing a fulcrum, and arrow colors on same side.)
Even if the cable does not noticeably deform, the boot does. "Elasticity" ...
There's the basis for a sit-com episode here...
