WhatScience? wrote:Where do you guys buy your wood? Most places I see the wood is to thick/wide.
It really comes down to how competitive are you trying/willing/able to be, and are we talking engineering or art?
I’m really curious how you came to the conclusion that the wood you saw was too thick/wide? It suggests you have a sense of what is… just right, and I’m wondering what that sense is grounded in. Folk who succeed in being seriously competitive (let’s say medaling at State) ground it in engineering. Others, who do not yet have an understanding of the basic engineering involved create (sometimes) a really cool looking structure that scores a fraction of what the ‘good’ structures do.
The American Engineers' Council for Professional Development (ECPD) has defined "engineering" as “the creative application of scientific principles to design or develop structures, machines, apparatus, or manufacturing processes, or works utilizing them singly or in combination; or to construct or operate the same with full cognizance of their design; or to forecast their behavior under specific operating conditions; all as respects an intended function,
Folk who succeed in being seriously competitive have done enough study to understand the key ‘scientific principles’ that allow you to design a ‘good tower.’ Or perhaps they are fortunate enough to have a coach/teacher who understands those principles, and has taught them. The Scioly board is a resource that provides the largest…library of information on the science and engineering of Science O towers (and bridges and boomilevers). If you go back to the 2017 archives for towers -
https://scioly.org/forums/viewforum.php?f=243– you’ll find over a hundred pages of information. The first topic just happens to be two pages on ‘Wood Orders’… I strongly urge you, and anyone who hasn’t yet gotten to an understanding of how to ‘engineer’ a good tower design, to take the time to read through this unique library of information.
You’ll find that the vast majority of folk with competitive towers used 1/8” x 1/8” wood for legs; some used 5/32”, some used 3/32. For bracing pieces, the picture is more diverse, and depends on the bracing configuration (e.g., ‘ladders and Xs’, ‘all Xs’, ‘Z-bracing’)
OK, on to where to get wood. I and many others have come to the conclusion that if you are or want to be seriously competitive, Specialized Balsa in Loveland, Colorado is the best place.
http://www.specializedbalsa.com/products/Why? Because you can get sticks in 1/10gram weight increments (and sheets at specific weights). Why does that matter? Because to build a competitive tower, you’re looking for/you need the very lightest wood that are strong enough to carry the forces produced in the pieces/members that make up the tower.
Summarizing very briefly:
First, you need to know the forces that the legs will see under a 15kg tower load (if you go to the “Towers B/C” thread, you’ll see where I posted a table that provides those forces for the upper and lower leg segments for towers meeting this year’s rules (and meeting the 29cm circle bonus).
Then you need to figure out the bracing interval you want to use. This critical aspect/step of the design process is looking at a basic trade-off; is it better (more efficient) to use lighter, ‘floppier’ legs with more bracing- a tighter bracing interval, or heavier, ‘stiffer’ legs with less bracing- a wider bracing interval. To do this, you need to understand how buckling works, how the buckling strength of a long piece (a leg, or in this year’s case a leg segment (upper, ‘chimney’ section, and lower, ‘base’ section) is increased by bracing (the braced points turn the long piece into a set of shorter “stacked” pieces). The increase in buckling strength has an ‘inverse square’ relationship to length.
What does that mean? If you have a stick of x length that has a buckling strength of y, and you brace it at the midpoint (cutting the ‘braced length’ length to ½x), the buckling strength of the two halves (and therefore the entire stick) will be 4 times y (1 divided by ½ squared). If you brace it at the 1/3 points (cutting the braced length to 1/3x), the buckling strength of the braced segments, hence the entire stick will be 9 times y (1 divided by 1/3 squared). There’s very detailed discussion in past posts of exactly how to test buckling strength of balsa sticks, and apply those measurements to get to the “design” buckling strength for legs/leg segments you want to use.
A piece of very important information that is in the ‘Wood Orders’ thread is a graph of buckling strengths vs 36” stick weights. What you’ll see is that there is a clear relationship between density (stick weight) and buckling strength- if you double the density, the buckling strength is roughly doubled (actually, doubling density bumps buckling strength by about a factor of 2.25). But the other thing you can see is there is ‘variation about the mean’ in the density vs buckling strength relationship- at a given stick weight, buckling strength will vary. What you’re looking for, for a really competitive tower is a set of sticks/legs that are the very lightest that have the ‘design buckling strength’ you need.
Lots of words to get to the why ordering from Specialized makes sense if you’re seriously trying to produce a ‘winning’ tower. It’s a matter of time and economics. With enough time, by going to local stores with a scale, you might eventually get a few sticks that are close to your design buckling strength. By ordering from a place that uses …fairly broad increments of sorting (e.g., between 6 and 8 pounds per cubic foot density), if you order and sort through enough sticks, you will find a few that meet your design buckling strength, and if you’ve ordered enough, of those that meet your design buckling strength, a full set, or sets will be significantly lighter than the others; it’s a statistical game. However, if you know, to a 1/10 of a gram the stick weight likely to have the buckling strength you need, you can order to that narrow range. Example, from the graph on 1/8” stick weight vs buckling strength, you see an average 1.5gr stick has the buckling strength you want, and some 1.4gr ones do…..order a reasonable numbers of 1.4s.