Designs

[LKN]

Balsa Man

How did you get ahold of 1/64 x 1/16 inch balsa? Specialized Balsa doesn't offer those strips, but they do have 1/64 inch sheets. Did you buy 1/64 inch sheets and cut them? On that note, what is the most precise way to cut thin balsa sheets? I usually use a ruler and a razor blade, but I never seem to be exact as I want to be and end up abandoning the idea to use hand-cut balsa strips for my X-bracing for a chimney. Because of this I have been using 1/16 the lowest square balsa (much cheaper than online, but takes time to individually weigh each stick) at my local hobby store.

[Balsa Man]

Balsa Man

How did you get ahold of 1/64 x 1/16 inch balsa? Specialized Balsa doesn't offer those strips, but they do have 1/64 inch sheets. Did you buy 1/64 inch sheets and cut them? On that note, what is the most precise way to cut thin balsa sheets? I usually use a ruler and a razor blade, but I never seem to be exact as I want to be and end up abandoning the idea to use hand-cut balsa strips for my X-bracing for a chimney. Because of this I have been using 1/16 the lowest square balsa (much cheaper than online, but takes time to individually weigh each stick) at my local hobby store.

Yup, cut from 1/64th sheet. Done with a razor blade and ruler. A few tips on doing that:
Don't try to cut them too long; cut the sheet down into sections a bit longer than you need the strips to be, and strip from them.
Get a ruler with ....a nice hard edge. We use one from a square - one of those squares with a 90 degree & 45 degree angle, cast metal piece with a slot in the bottom that the ruler fits into/slides along. The ruler is steel, and the edges are hard/sharp. When you press it down on a piece of sheet, it doesn't slip around.
Use a surface to do the cutting on that doesn't let things slide around too easily- a plastic cutting board works well.
Use a "guide piece" to line up the ruler; cutting 1/16th wide strips, use a little piece of 1/16th square; work at both ends of the ruler/sheet, putting the guide piece down, aligned with the exposed edge of the sheet, move it back and forth, adjusting the ruler till you have it lined up- parallel to, and a 16th back from the sheet edge.
Use two fingers spread well apart to get the ruler firmly held in-place.
Don't try to make the cut in one pass- make the first couple passes with very light blade pressure, then gradually increase pressure with each pass. With high-density wood, the grain structure can be quite solid, and will tend to deflect the blade off a straight cut line.
Use a sharp blade; buy a big box of blades; we actually sharpen from out-of-the box (w/ a really fine sharpening stone)
Don't expect 100% success; of the strips you cut, most should be keepers, but some go to the scrap pile
Before you strip, look at your sheet carefully; hold it up to the light; mark areas (w/ a magic marker/highlighter) that are.....suspect- visible holes, areas that are much more transparent when held up to a light; areas where you see significant diagonal grain structure. Don't use strips that have marked areas in the middle of them.

That's pretty much the set of tricks for cutting useable strips; have fun!

[silverheart7]

Do you think a tower is better with a longer chimmney or a shorter one?

[Balsa Man]

Do you think a tower is better with a longer chimmney or a shorter one?

Well, that depends on what you mean by better....
The way the scoring is set up, taller is better - you get a significant scoring bonus for going from the minimum of 40cm up to the max of 70cm - and that bonus, best I can calculate, is bigger than the cost in weight/efficiency that building taller costs you.
The rules define where the base stops, and the chimney begins - where the tower has to fit inside the 8cm circle. The base is up to 30 cm (in a B-tower); the chimney is the skinny part from there up. So, to get a taller tower means a longer chimney, and circling back to the start, a longer chimney/taller tower scores more points.

Now, that all assumes that the taller and shorter towers both hold the same load. If because you don't/can't build with enough precision to get a 70cm tower to hold full weight, or about the same weight as you could hold with a shorter tower, you'll be better off with a shorter one.

Does this answer your question?

[baker]

For those that were calculating downward load forces... If the chimney had a 4 cm square top and a 5 cm square bottom (where it mates with the base), how much of a load change is there on the mains? Is it better to remain vertical or have a little angle? Thanks..

[Balsa Man]

Load increase from vertical is a function of 1/cosine of the angle from vertical. The lean you describe would be <2 degrees. 1/cos 2.3 degrees is 1.0008- for all practical purposes, no difference. 1/cos doesnt get to 1.01 until 8 degrees.

Having a bit of lean in is good - not a strength/load issue- an overall structural stability/ability to tolerate bucket sway/off-center loading.

[silverheart7]

Do you think a tower is better with a longer chimmney or a shorter one?

Well, that depends on what you mean by better....
The way the scoring is set up, taller is better - you get a significant scoring bonus for going from the minimum of 40cm up to the max of 70cm - and that bonus, best I can calculate, is bigger than the cost in weight/efficiency that building taller costs you.
The rules define where the base stops, and the chimney begins - where the tower has to fit inside the 8cm circle. The base is up to 30 cm (in a B-tower); the chimney is the skinny part from there up. So, to get a taller tower means a longer chimney, and circling back to the start, a longer chimney/taller tower scores more points.

Now, that all assumes that the taller and shorter towers both hold the same load. If because you don't/can't build with enough precision to get a 70cm tower to hold full weight, or about the same weight as you could hold with a shorter tower, you'll be better off with a shorter one.

Does this answer your question?

Yes, thank you!

[Vizard007]

Hi guys! I was wondering, what do you think the strengths/weaknesses there would be with a square base or a rectangle base? Because I've been trying to figure out which would be more efficient, but I can't seem to determine it with my team. =/ Sorry if this is an ignorant question. Thanks for your help guys!

[Balsa Man]

Hi guys! I was wondering, what do you think the strengths/weaknesses there would be with a square base or a rectangle base? Because I've been trying to figure out which would be more efficient, but I can't seem to determine it with my team. =/ Sorry if this is an ignorant question. Thanks for your help guys!

The "efficiency" question is which would require more wood (weigh more). Well, this is another one of those geometry questions that says grab some graph paper and draw things out; leg lengths and bracing lengths- for the base (the chimney will be the same). When you do that, you’ll see its pretty much a wash for the “best case” (rectangle and square that use the least wood). The tops of the base legs, in both cases have to come into a square that fits inside a 8cm circle (roughly 5cm x 5cm). With a square, the legs come in from the midpoints of the sides of the clearance hole in the test platform. With a rectangle, you have 2 legs coming in from each of 2 opposite sides, with their bottoms positioned 2.5-ish cm either side of the midpoints of those 2 opposite sides. You essentially have a truncated pyramid w/ the square, and a bridge w/ 2 parallel sides (separated by ~5cm) w/ the rectangle. Leg angles/loads are about the same. If you move the bottoms of the leg ends out, so your "bridge sides" lean in toward each other, the leg lengths (and loads) go up.

However, the next question is which is going to be more stable with a big tall chimney (with a hung load) sitting on top of it. Here the square, IMHO, wins hands down. With any bucket sway, off center loading, distortion in the chimney- the square gives you strong, equal bracing/support on two perpendicular axes; with the rectangle, strong along one axis – the long axis; that of the “span, and very little on the short axis, perpendicular to the span… It will take very little off-center loading, when its off-center on that short axis, to take the tower over/out. Some have noted in the past its much easier to build a rectangular one - make two sides on a flat jig; join those sides with equal length ladders, but, it seems to me that ease comes with a big price.

[mrsteven]

Here's a question I just thought of...
When you build bridges and test them, if it holds all the weight and your totally happy about its weight, do you use it again? Or because you put force onto it, its weaker than before?