Towers B/C

[Random Human]

Our towers team is division B and we have 7 people on our team. Currently we ordered over 600 sticks in one order and this is only enough to last us a month. I personally think that you shouldn't buy it in one run but if you have to, do the math and your gonna lead a whole lot of sticks (around a couple thousand)

[HandsFreeCookieDunk]

Our towers team is division B and we have 7 people on our team. Currently we ordered over 600 sticks in one order and this is only enough to last us a month. I personally think that you shouldn't buy it in one run but if you have to, do the math and your gonna lead a whole lot of sticks (around a couple thousand)

Yeah..... It really isn't a choice otherwise I would. Can I ask what length of stick you're talking about? The ones we usually get are several feet long and I find it hard to believe that we'd be going through more than 50 in a month even at peak efficiency.

[Random Human]

Our team uses 1/8x1/8, 1/8x1/16, and 1/16x1/16. Each one of these sticks is 36 inches long or 3 feet long. For my previous tower, my efficiency was 1350 and I used 6 1/8x1/16 and 3 1/8x1/8 sticks so 9 sticks gone. Our team orders about 300 sticks of varying sizes every once and a while (maybe once every 2 or 3 months) and the 300 sticks only last us all half a month to one month. Our coach told us that we should get sticks money wise and the sticks the team orders are just "a bonus"
Therefore, I recommend ya'll get your sticks individually, and once and a while get a team bonus.

[Random Human]

Also, regionals are in a few months and if you only want to medal, you aren't going to have to be as aggressive as we are and only test a tower once every 2 or 3 weeks while we are going for placing at states or maybe even nationals so we test several towers every week. This affects how much wood you use so I think depending on your team size if you don't want to go too aggressively in this competition, you aren't going to need as much as us but still, throughout the year you are going to use a lot of sticks.

[Balsa Man]

So my school isn't big on spending money on the Science Olympiad team and they want us to just buy our balsa in one purchase at the beginning of the year. How much would you recommend we buy? For reference, we plan on building only with 1/8*1/8 and 1/16*1/16 and our goal is just to medal at a decent but not super competitive regional (so nothing too crazy). We have some experience working with balsa, but not with towers specifically.

To respond to your question, and some of the followup responses:
Just buying/ordering a bunch of sticks with unknown weights, and trying to deal with whatever you get (as in figure out what you can build with what you end up with) is a total crap shoot. As discussed in the previous pages of this thread, a) the buckling strength (which is what matters) depends on/varies with density (i.e., stick weight), b) the key design problem/decision is how you want to to do the trade-off between denser/heavier/stiffer legs with less bracing (fewer bracing points/longer bracing intervals), and lighter/less stiff legs with more bracing (more bracing points/shorter bracing intervals. So, in a bunch of 'grab bag' sticks, you'll have quite a variation in density/stick weights; maybe some will work, but who knows. The price per stick may be low, but most of the sticks will be a complete waste of money. While the price per stick for weight-graded wood will be more, the 'throw-away' sticks will be far less.

The efficient way to order wood is specifying density (stick weight). If you had a way to know about what stick weight(s) you wanted, you could order a relatively small number, and find the best (lightest with the buckling strength you want/need).

The attached graphs give you a way to do that, for 1/8 x 1/8 balsa. Notes for the graphs explain the source data. With this you can figure out target ranges for stick weights for 36" 1/8 x 1/8 sticks.

[Balsa Man]

Hmmmm...
For some reason, attachment didn't work
Tried again, get error message "failed to move uploaded file"
Admin, is something wrong w/ attachment function??

[baker]

Here is a Balsa Density Calculator. If you have a rough idea what density your looking for, put in the length, width, thickness and play with the mass to see your target weight of what a stick should be for your needs. Go shopping with a scale.
http://www.nclra.org/Programs/BalsaDensity.php

[Balsa Man]

Here is a Balsa Density Calculator. If you have a rough idea what density your looking for, put in the length, width, thickness and play with the mass to see your target weight of what a stick should be for your needs. Go shopping with a scale.
http://www.nclra.org/Programs/BalsaDensity.php

For sure go shopping with your scale. The calculator is a handy tool, for sure (give it stick weight, it will tell you density in pounds per cubic foot, and the other way around) but what you want is a way to correlate/tie density (expressed as stick weight) to buckling strength. The graph I'm trying to attach plots stick weights (at 1/8x1/8x36) vs buckling strength at 36". That, along with an inverse square table is what you need to get to a target stick weight for a given buckling strength.
For example, in a C-division tower (4 legs, spanning 29cm circle) with a 20% safety factor, each leg needs a buckling strength of 4,572gr. The 36" buckling strength of a 1.5gr stick, at 36" shows on the graph as 80gr. What this is saying is that if you take a number of 1.5gr sticks, you'll see their 36" buckling strength will trend around 80gr; there will be some with a little higher, some with a little lower.
Then you look at the inverse square relationship of length to buckling strength. If the buckling strength at 36" (91.6cm) is 80 gr, if you brace the legs (61.34cm long) at 1/3 intervals- i.e., braced at every 20.45cm (which is 0.223 of 91.6cm), the braced strength at that shorter length will be 1/0.223 squared, which is 20.77 times the buckling strength at 91.6cm; so 20.77 x 80 = 1,605gr. Not enough. So if you brace (that same stick) at 1/5 intervals (12.27cm), which is 0.134 of 91.6cm; 1 over 0.134 squared is 4,460gr- very close to our target of 4,572. If the 36" buckling strength is a bit more than 80gr (82, actually), then the braced buckling strength comes out at 4,571- you're there. What the graph tells you is that if you get.... some sticks between 1.5 and 1.6gr, you will almost certainly have some that test over 80gr buckling strength at 36", and if you use 4 of them (the lightest 4 that test over 82gr, and brace at 1/5 intervals, tower should work. Also suggests, for those looking for a more streamlined approach, that if you just order or find when you take your scale to the store sticks at 1.6-1.65 grams, any of them will likely work when braced at 1/5 intervals.....

[bernard]

Here is a Balsa Density Calculator. If you have a rough idea what density your looking for, put in the length, width, thickness and play with the mass to see your target weight of what a stick should be for your needs. Go shopping with a scale.
http://www.nclra.org/Programs/BalsaDensity.php

Another balsa density calculator, which allows setting any single parameter as unknown: http://scioly.github.io/freeflight/.

[hearthstone224]

Hey everyone, I'm going to put the numbers I got out there for verification. The "safe strength" column refers to the strength a stick of 36'' balsa must have to be braced at that certain interval and work out. I want to get this part right once and for all so that I don't need to think about this anymore.

Is there any disagreement? If not I think people can use these numbers.

I think only the colored ones will matter because the sticks usually don't reach 220 g in BS At least I don't have any.

So do you think the main composition will be bracing at 1/5th interval around 80 BS? Because I thought maybe we could be using 1/4th interval with sticks around 130BS or so. They are fairly heavy though.

And just out of curiosity, can we brace at a let's say 1/4.5 interval? I know it sound fairly weird but would that maybe have a positive tradeoff?