John E Posted July 30, 2007 Author Report Share Posted July 30, 2007 ~tb - I understand roughly what you are talking about. However, I would think that the fundemental frequency of any snowboard would be simple bending with the tip & tail being opposite the waist. This should be a fairly low frequency & if the accelerometers were placed near the tip & tail, they should not be at a null at the lowest frequency. I understand that the amplitude board-to-board could be quite different. Maybe, as Bruce stated, there would be so much variation between "identical" boards that such measurments would be meaningless. Since the stiffness of a beam is proportional to the cube of its thickness, a very slight variation in thickness can result in wildly different stiffness. Quote Link to comment Share on other sites More sharing options...
Bruce Varsava Posted July 31, 2007 Report Share Posted July 31, 2007 If you ever try a board and really like it, you're best off to buy that actual board. Duplication is tough but not impossible to get it close enough for our purposes Quote Link to comment Share on other sites More sharing options...
John E Posted July 31, 2007 Author Report Share Posted July 31, 2007 Because the stiffness of a beam is proportional to the cube of the thickness, if the thickness of the core were to vary by +/- 10%, this could result in a variation in stiffness of 73% from the weakest to stiffest board ALL ELSE BEING EQUAL. I don't know what kind of tolerances board makers hold but +/- 10% does not seem out of line. That's only .025" on a .25" thick core. Quote Link to comment Share on other sites More sharing options...
Bruce Varsava Posted August 1, 2007 Report Share Posted August 1, 2007 I pretty much try to hit measurements to within 1/10mm or .004". Since the tip/tail are a lot thinner than the mid section if you miss by a couple tenths in the nose or tail where the board is real thin, thats when the trouble starts. Missing by a bit in the middle isn't as bad as your percentage of the actual measurement is much lower given the max thickness. If I missed a thickness anywhere by .025 its not going to end up being the board it was originally designed to be and would be planed down to a smaller size. Quote Link to comment Share on other sites More sharing options...
BlueB Posted August 1, 2007 Report Share Posted August 1, 2007 Great to have Bruce in this discussion! Quote Link to comment Share on other sites More sharing options...
John E Posted August 1, 2007 Author Report Share Posted August 1, 2007 I recently toured a local (non-alpine) board manufacturer (Never Summer). I don't know if their process is typical but it would surprise me if they could hold a .004" thickness tolerance. That's impressive. Quote Link to comment Share on other sites More sharing options...
pebu Posted August 1, 2007 Report Share Posted August 1, 2007 .025 is easy to hold. Under controlled conditions .004 shouldn't be too hard, but without tightly controlled conditions it seems like wood would be a difficult thing to hold too closely. Quote Link to comment Share on other sites More sharing options...
~tb Posted August 3, 2007 Report Share Posted August 3, 2007 here is an animation of one of the modes. www.ourwalden.net\119hzfree.avi oldacura, you can see that in this video, at this frequency, the board has a null near the tip, as well as 5 others along the length. An accelerometer placed in any of these locations will return very low values at this frequency, even though there is ALOT of energy of the board. What happens at the fundamental frequency is important, however, I personally believe that there are other frequencies, and modes (and mode shapes) that are probably more important when it comes to how a board feels. I can show mode shapes that leave just about any point on the board as a null. guess that is all for now. Quote Link to comment Share on other sites More sharing options...
John E Posted August 3, 2007 Author Report Share Posted August 3, 2007 ~tb - I was just thinking that the fundemental frequency (not necessarily the mode shape) would be an indicator of stiffness/weight ratio. Maybe a simple, objective way to compare one board with another. The higher the weight, the lower the frequency. The higher the stiffness, the higher the frequency. I have no idea what harmonics get excited in real world riding. My guess is that the higher harmonics would come into play at higher speeds & harder snow. Quote Link to comment Share on other sites More sharing options...
~tb Posted August 3, 2007 Report Share Posted August 3, 2007 I agree that the fundamental coorelates well to the stiffness VS weight phenomenon. I can show though that the position of the maximum and nulls of a board (as well as fundamental frequency) can move as a function of taper profile. different manufacturers use different taper profiles. The amplitude measurments taken with an accelerometer would vary greatly because of this. here is an animation of the fundamental natural frequency of a soft alpine board. www.ourwalden.net\20hzfree.avi I can manipulate the taper on this board to shift the null in blue forward and aft. by moving the null forward, the vibration level at the nose goes down, but under the feet, it increases. I can shift the null towards the center of the board, but then the tip energy goes up. The total energy in the system stays the same. Even if you pick a standardized location for accelerometer placement, it will never be a good board to board comparison (unless you take data at MANY locations along the length, and width). What do you think? ~tb Quote Link to comment Share on other sites More sharing options...
cail Posted August 3, 2007 Report Share Posted August 3, 2007 you can add a dynamic excitation to the board that varies over frequencies starting from below the first resonance of the board. measuring excitation force through the board and displacement across it simultaneously as functions of frequency allows one to calculate a complex stiffness function (force per unit displacement at a given frequency)... real component = stiffness imaginary component is associated with damping typically complex stiffness is represented by the real component and a loss function (loss fcn. = Im[cmplx stiffness] / Re[cmplx. stiffness]) Quote Link to comment Share on other sites More sharing options...
John E Posted August 3, 2007 Author Report Share Posted August 3, 2007 ~tb - I don't have the Windows Media Player here at work so I can't view the video. The system would be board & rider. Addition of a 150# - 200# load at the binding location would completely change the dynamics. This would not really be just a mass but a mass/spring/damper. This may be easier to do as an FEA than a real lab setup. If someone had lots of time & money to spend on this, they could fix accelerometers to their board & get a backpack data acquisition system & take data while riding. Quote Link to comment Share on other sites More sharing options...
cail Posted August 3, 2007 Report Share Posted August 3, 2007 i agree with doing FEM simulations. only as bruce mentioned earlier, grain structure, flex patterns, etc would cause different results, so either assume those are constant, or get really creative. Quote Link to comment Share on other sites More sharing options...
~tb Posted August 3, 2007 Report Share Posted August 3, 2007 you can add a dynamic excitation to the board that varies over frequencies starting from below the first resonance of the board. measuring excitation force through the board and displacement across it simultaneously as functions of frequency allows one to calculate a complex stiffness function (force per unit displacement at a given frequency)...real component = stiffness imaginary component is associated with damping typically complex stiffness is represented by the real component and a loss function (loss fcn. = Im[cmplx stiffness] / Re[cmplx. stiffness]) agreed . . . but I still believe that you would have to take data at many locations. as the nulls move around the surface at different freauencies. Quote Link to comment Share on other sites More sharing options...
~tb Posted August 3, 2007 Report Share Posted August 3, 2007 ~tb - I don't have the Windows Media Player here at work so I can't view the video. The system would be board & rider. Addition of a 150# - 200# load at the binding location would completely change the dynamics. This would not really be just a mass but a mass/spring/damper. This may be easier to do as an FEA than a real lab setup.If someone had lots of time & money to spend on this, they could fix accelerometers to their board & get a backpack data acquisition system & take data while riding. Download a player and take a look. I have run the two extreme cases, a completely unconstrained board, and a board that is rigidly fixed to ground at the reference stance. I was surprised by how similar the modal frequencies, amplitudes and shapes were. based on the dynamic loading of a board, II think that the unconstrained case is actually more telling. Quote Link to comment Share on other sites More sharing options...
tinkler Posted August 3, 2007 Report Share Posted August 3, 2007 This subject is one that I gave a lot of thought to seventeen years ago. Bruce is correct in everything he said. As a board builder there are many variables that can affect a board’s outcome. When it comes to numbers, I played that game at K2 and Solomon. We were testing my Personal Flex Control System aka Snow Stix. We tested longitudinal flex, torsional flex, pressure distribution, camber affect and vibration damping. We even got Boeing involved and produced even more numbers. The point was to see how effective my system was at changing flex and damping numbers, by simply tensioning my system. Turned out it was very effective. Then K2 licensed my design then kept it off the market and away from Solomon who had a similar design called the ProLink, but it was not interactive like mine. K2 Marketing felt the market was not ready for above board external systems no matter how good. Its interest like this that gives me hopes that some riders think that generating personal flex numbers is important. The point is it’s not about the numbers in the lab. The ultimate board is one where you can control the numbers on the mountain to find your own flex numbers. I don’t think riders know their numbers until they have felt it. To have a board that can morph into many feels is a big advantage. I know that in the morning when I am feeling aggressive and ready to get after it I need my board to have a lot of torsional resistance. So I start tensioning my board accordingly to find my number or feel that works for the conditions and how I feel; but after lunch when the conditions have changed and I feel tired, I want to cruise a bit more so I start releasing the tension that I was using in the morning . Many adjustments can be made. Toe turn tip torsionally stiffer than toe side tail. Heel side tip softer than heel side tail or any combo for goofy or regular. You have four corners that you can alter camber and torsional resistance to find the right feel. When it comes to damping vibration an interactive external system such as mine has been proven in the lab to be very effective and when used on the mountain can be dialed in to control a misbehaving board. The Snow Stix tip slides in a urethane rubber slot that through friction damping and a downloading of the tip through tension controls vibration .Again the point is numbers don’t mean much if you don’t know what your numbers are . And the only way to find out is to try different feels. Snow Stix were developed to put custom on the mountain and literally in the hands of the rider for any given day and condition. Anyway, I thought I should jump in with some of my thoughts on flex numbers, and maybe some information on why I came up with Snow Stix. If any of you need any more information or have comments you can e-mail me directly. Tinkler 1 Quote Link to comment Share on other sites More sharing options...
www.oldsnowboards.com Posted August 3, 2007 Report Share Posted August 3, 2007 Personal Flex Control Systems / If you are not familiar with Mike Tinkler's work. Here are some examples. Thanks for building "Duo-Construction" boards that allow me to "Tune" on the fly. Good to see your post Mike! Welcome to Bomber Original Gallery on OSB 1 Quote Link to comment Share on other sites More sharing options...
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