tap

It’s been over a decade since I’ve used rubber boots, but I recall liquid laundry detergent being pretty awesome. Much more slippery and washed out much faster then dish soap, also always available in the cabinet.

Pretty sure a shock tube will do absolutely nothing to stop a handle from flying into the boat. They only help cut down on the rope near the pylon from lasso’ing someone.

We did convert this to angular velocity, which showed what I found to be the most interesting part of the data. I don’t fully understand how he does it, but Adam was able to increase his angular velocity coming off the second wake in a sort of slingshot effect. It was clear and evident in the angular velocity plots. The data from when I was skiing had no evidence of that whatsoever, my angular velocity progressively declined after center. It was cool to see the data clearly show the effects of better technique.

I’d have to pull the data back up to see exactly what we did. I may have done a simple moving average filter to take some of the chatter out, but to me that plot looks like it could be raw data. We were in a bit of a hurry so I did a very quick force calibration with a scale on the pylon. As Adam stated, it is liable to be off a bit, but should be close. I don’t think Adam needed to put the hammer down until the rope got shorter. The data was generated with two strain gauges at the base of the pylon positioned 90 degrees to each other. At the time I thought this was going to be an elegant solution as the total force vector (magnitude and direction) can be resolved. In practice however, when the rope load got too small the signal to noise ratio dropped off to much to be useful. That is why the angular position data is very choppy at the higher angles, i.e. no rope load to measure and derive the angle from. When Adam was pulling hard on the rope the data was very clean. The peaks of the force curve are not ‘chopped off’, that is actually the most accurate portion of the data as the signal to noise ratio was the highest. The leveling off of the force peaks shows that Adam was sustaining the peak load for a short duration as opposed to just spiking it.

Two quick thoughts. 1. I like the figure, I think it’s important, but I also think this is the obvious part of the kinematics and is fairly easy to grasp and understand. The overhead view on the other hand I think is much more interesting particularly as the kinematics play out over time and can better identify effective loading of the line vs just trying to slow the boat down. And then combine the two orthogonal frames of reference for the whole picture. 2. Maybe a quick refresher of vector addition is in order.

I think I counted somewhere around 56 skis. At ~$1,500 per ski that’s $84,000 sitting there.

@VONMAN you are assuming core failure is what kills a ski. I agree that is a reasonable guess, but I'm not fully convinced. Or at least I'm not convinced that core failure is always the cause, not anymore. Back when skis were made from earlier generations of polyurethane core I would probably agree with you. However, with the better core materials being the standard nowadays for the high end skis (PVC's, SAN's, PMI's, etc.) I think there's room to argue that the carbon/epoxy may be the first to fail depending on the specific ski, materials used, load case, etc... I could even argue that the laminate is the dominate point of failure. Here's my logic... ski designers are always weight conscious, all else being equal. Ski design is by and far an evolutionary design process, meaning that most design changes are field tested by trial and error. The desire to pull out weight affects the laminate as well as the core. There is a reason a ski has 5 or 6 lb/ft^3 PVC foam in it, most likely because someone built one with 3 or 4 lb/ft^3 core and it broke. I suspect the very same holds true for the laminates used, meaning at some point in time somebody tried to make a ski with less laminate and it broke. So with that logic in mind, both the laminate and the core are being pushed to an appropriate limit of the materials respective strength. And, here's the kicker... the higher end foams have a much higher strain to failure than carbon/epoxy. At least that's true for PVC and SAN. I'm not sure about PMI, it may as well I just don't know to much about it. I'm generalizing as the fiber orientations and the direction of strain has a big effect on the carbon/epoxy, but we're talking order of magnitude difference here. So, in the right loading condition, the core may just keep "stretching" thereby forcing the laminate to pick up a higher percentage of the load, potentially all the way to failure. My comments here are purposefully general. Your comment seems to be specific to 'core crushing'. Is core crushing really a problem with slalom skis? I have no idea. I know it is with trick skis as they get heel dented quite often from flips, but I've never seen a dented slalom ski, other than a handle pop. If it is a problem then I would think a slightly higher density foam insert under the foot would also be a very appropriate solution. Don't get me wrong here... you can make excellent ribbed skis as well as non-ribbed skis. I guess I'm just suggesting that structural design is very complicated and takes a fair bit of effort and testing to get it right, and some of the generally accepted assumptions may not be correct.

Yeah well... I thought this was a normal conversation

@adamhcaldwell I'm pretty sure we've had this conversation before, but out of curiosity... completely thinking out loud here and only half baked... I wonder if that feeling of chatter is a function of the wave slap impulse passing through the thickness of the ski more so than the ski having less flexural damping. In other words, with a series of very stiff ribs directly under foot tying the top and bottom skins together you may feel much more of the wave slap hitting the bottom of the ski. A foam core ski will behave a bit more like an isolator for the impulse traveling directly through the thickness. I don't know if you could tell the difference, but do you recall feeling excessive bending vibration or just 'slap' for lack of a better word. If I had a ribbed ski I'd be happy to measure it and compare against an all foam core ski. And so as not to contradict myself from earlier... yes, all skis have sidewalls which are ribs, but sidewalls are not directly underfoot. So that is a unique distinction.

@kurtis500 sure. communicating via short blurbs of text is not exactly ideal. The "tuning fork" comparison was just to imply that the inherent structure of a ski is not a highly damped structure. Granted "highly" is a very relative term. I was trying to stay within the context of this discussion which was specific to the structure and materials of a modern ski and how "flex" and "rebound" are or are not tied together. Somehow damping got thrown into the mix and everything went sideways. If you are just talking about the ski itself and the materials and design, then you would want to measure it by itself. No skier, no water, no bindings, just the ski floating in air. Best I can do is to hold it in such a way as to try and not interfere with its vibration. I just grabbed a modern high end ski off the rack, no bindings, gave it a whack and held it up to a microphone... here's what we get: For perspective, that's a ~47 Hz signal. At 47 Hz it only takes about .02 seconds per cycle, so that entire graph is roughly 0.8 seconds long. Not particularly important, just context. What is important is that the signal does not decay anywhere near something that is 'critically damped'. So... I'm suggesting that the structure of a ski is much closer to a tuning fork than a shock absorber. Tuning forks are better designed for radiating noise, but both tuning forks and skis resonate fairly well. Note, if you hold the ski in the wrong spot it won't do anything... the boundary conditions are hugely important. That's why tuning forks have a nice little handle built in at the perfect location. All that said... @BG1 put it all into context in a much more straight forward way. The inherent damping of the ski is almost meaningless by comparison to the effects of the water and the skier bolted on top which do provide an abundance of damping to the "system". I'm not a ski designer. But I would suspect that the characteristics of importance are dominated by shape, then stiffness (flex) as a close second, then weight as a distant third, and damping is just a fun thing to talk about but not actually important as the water and the skier will take care of that. I say weight is a distant 3rd in the context that a 3.0 lb ski and 3.5 lb ski are still both extremely light relative to the system of the ski + water + skier. Obviously you don't want a 20 lb. ski. I'm not saying someone couldn't eek out that last 1% of performance by going after internal damping within the ski, but I'd be pretty impressed if it was done well and without taking away from the other more important characteristics.

@chrislandy your graph is misleading. Your ‘hard’ and ‘critical’ damping signals both start at time t=0 with a peak amplitude while your ‘light’ damping signal starts with an amplitude of 0 at t=0. These signals are not in phase, so the visual comparison of “rebound” is wrong. the statement that “critically damp is ideal” is a stretch and unfounded. There are many many applications where that would be a very bad thing. In the discussion of ski rebound, a critically damped ski will take longer than a lightly damped ski. skis are no where near critically damped, they hum like a tuning fork.

Nice!! I completely agree with Horton’s disclaimer and sentiment above. Keep at it and you will get hurt. That said... do you at least use a binding plate that runs under both feet? One single plate, not two. The stiffness discontinuity between two separate binding plates just adds to the risk factor here where a single plate can buy back some margin. I’ve done the same, and probably worse, but I’ve also added more laminate back to ski when appropriate to make up for the damage. And I accepted the risk involved. If you’re determined to proceed, I would suggest a proof test. Set it on some boards and bounce on it a few times. A ski should easily handle a few hundred pounds of force without breaking. If you snap your ski then you just saved yourself tons of pain and medical bills.

@Horton I agree, carbon ribs are interesting for sure, certainly opens up the design space a bit more than foam alone and that’s pretty neat from an engineering perspective, but they’re not magical. Fact is, all skis have carbon (or glass) ribs, they’re called sidewalls. We could probably fill a couple hour conversation on the mechanical effects of adding more ribs, good and bad, but I doubt anyone cares that much. I started typing a bunch of stuff, but my thumbs started to hurt. One thing is for certain, adding more ribs is not more economical. There’s an infinite way to configure a composite rib, everything depends on the details. Stiffness, strength, failure modes, fatigue life, weight, cost...

@Horton did you just ask if you can make a ski softer by making it more rigid? No. Stating the obvious here, but you can make one part of a ski softer and another part stiffer (faster rebound). Or make bending softer and torsion stiffer. But you can’t make the same part and the same mode of deflection softer and stiffer (faster rebound) at the same time. You’re only other option is to pull out mass, and I’m not sure there’s any mass left to pull out.

@Gloersen i feel like you’re baiting me. Have any of you ever taken the bindings off a ski and tapped on it. Modern skis are tuning forks, not shock absorbers. The argument that all of this is simply explained away by damping is just plain wrong. If by “rebound” you mean the time it takes for the ski to cycle through a forced deflection and back, then to me you are describing the frequency of vibration in some form or another. For any physical system, the frequency response is proportional to the stiffness over the mass. If stiffness goes down, so does frequency, aka rebound. Yes damping plays a role, but we’re not even in the right ballpark here. Stiffness and mass are king. And... by mass I mean mass of the system, i,e, bindings, water, skier all contribute. If all you care about is how fast the tip of your ski vibrates while you wheelie out of one ball then I’ll concede it’s mostly just the ski in play, but that’s still not the answer you’re looking for. Think in terms of a mass on a spring. Now pull it down and let it go. If you want it to “rebound” faster you need to take away mass or use a Stiffer spring, not softer. And if added a damper it’s just going to rebound even slower. The only positive assumption I can make is that the statement is a description of perception and not anything based in physics.