Nationals Predictions?

[dholdgreve]

I'm going to guess that of the top 5 towers at Nationals, none will go qualify for the bonus. All will be just barely outside of the 20 CM box, all will weigh less than 5 grams, the top 3 will carry full load, 4th place will carry 12.65 KG, 5th place will carry 13.7 KG, but will be heavier than 4th place. We may well see one of these towers under 4 grams. I think 2 of these towers will score above 4,000, the other 3 will be over 3500.... My crystal ball just blew a fuse...

[Crtomir]

Div-B: Over 4000 is certainly possible. I believe it will be a no-bonus design and probably 11 X-braces (only tension members, no horizontal members) on each side. The density of the main (1/8" x 1/8") columns will probably be around 5.4# to 6.0#. The density of the (1/16" x 1/32") X-braces will probably be around 4.2# to 5.5#. Using the lowest density values, the score could reach as high as 4600.

[Balsa Man]

Div-B: Over 4000 is certainly possible. I believe it will be a no-bonus design and probably 11 X-braces (only tension members, no horizontal members) on each side. The density of the main (1/8" x 1/8") columns will probably be around 5.4# to 6.0#. The density of the (1/16" x 1/32") X-braces will probably be around 4.2# to 5.5#. Using the lowest density values, the score could reach as high as 4600.

That sounds like a knowledgeable ...guess. Thanks for the insights.
Do you think the legs will be straight, or with a slight outward bowing, to help put/keep tension loading on the Xs??

[Crtomir]

Crtomir wrote:
Div-B: Over 4000 is certainly possible. I believe it will be a no-bonus design and probably 11 X-braces (only tension members, no horizontal members) on each side. The density of the main (1/8" x 1/8") columns will probably be around 5.4# to 6.0#. The density of the (1/16" x 1/32") X-braces will probably be around 4.2# to 5.5#. Using the lowest density values, the score could reach as high as 4600.



That sounds like a knowledgeable ...guess. Thanks for the insights.
Do you think the legs will be straight, or with a slight outward bowing, to help put/keep tension loading on the Xs??

Our girls tried the outward bowing technique, but it didn't seem to work too well. The 4 legs (or columns) were bowed outward slightly on the jig, so that puts some pre-tension on the tower. When the tower was lifted off the jig, the stronger sides pulled the weaker sides inward and the tower ended up slightly bowed in one direction. I'm guessing the legs should be as straight as possible. However, a lot of (1/8" x 1/8") sticks are slightly bowed to begin with. You could maybe orient those so the natural bowing was outward (facing away from center) on the jig.

[dholdgreve]

Div-B: Over 4000 is certainly possible. I believe it will be a no-bonus design and probably 11 X-braces (only tension members, no horizontal members) on each side. The density of the main (1/8" x 1/8") columns will probably be around 5.4# to 6.0#. The density of the (1/16" x 1/32") X-braces will probably be around 4.2# to 5.5#. Using the lowest density values, the score could reach as high as 4600.

That sounds like a knowledgeable ...guess. Thanks for the insights.
Do you think the legs will be straight, or with a slight outward bowing, to help put/keep tension loading on the Xs??

If the legs were forcibly bowed out while on the jig, then removed from the jig, would the legs revert to a straight position, thus the opposite of pre-tensioning?... Pre-loosening?

[Crtomir]

Div-B: Over 4000 is certainly possible. I believe it will be a no-bonus design and probably 11 X-braces (only tension members, no horizontal members) on each side. The density of the main (1/8" x 1/8") columns will probably be around 5.4# to 6.0#. The density of the (1/16" x 1/32") X-braces will probably be around 4.2# to 5.5#. Using the lowest density values, the score could reach as high as 4600.

That sounds like a knowledgeable ...guess. Thanks for the insights.
Do you think the legs will be straight, or with a slight outward bowing, to help put/keep tension loading on the Xs??

If the legs were forcibly bowed out while on the jig, then removed from the jig, would the legs revert to a straight position, thus the opposite of pre-tensioning?... Pre-loosening?

Possibly. We only tested this once, but when we took the tower off the jig, one side bowed outward even more and the other side bowed inward. I'm not sure if they just were not that careful in building the X-braces or what. It just seemed that the tower shifted after taking it off the jig. Maybe if I sent a picture of our jig, it would be more clear that this could happen. How do you send images or files on this forum?

[dholdgreve]

This is really splitting hairs, but when installing the X bracing, it will hold more symmetrical if one face is applied, then the opposite face is applied, as opposed to working your way around the tower, adjacent face to adjacent face... Doing it like hat will cause the tower to twist.

[Crtomir]

This is really splitting hairs, but when installing the X bracing, it will hold more symmetrical if one face is applied, then the opposite face is applied, as opposed to working your way around the tower, adjacent face to adjacent face... Doing it like hat will cause the tower to twist.

That's true. Our two girls that built towers together always worked on opposite sides of the tower to install the X-braces simultaneously. By the way, how does one submit photos or documents to this forum?

[cheese]

This is really splitting hairs, but when installing the X bracing, it will hold more symmetrical if one face is applied, then the opposite face is applied, as opposed to working your way around the tower, adjacent face to adjacent face... Doing it like hat will cause the tower to twist.

That's true. Our two girls that built towers together always worked on opposite sides of the tower to install the X-braces simultaneously. By the way, how does one submit photos or documents to this forum?

Use the "Img" button.

[Crtomir]

The "img" button only lets you post a link to an image, not the image itself. No matter. I'm posting a link to a Google Drive folder with some (hopefully) useful Tower stuff: https://drive.google.com/open?id=0B-oLt ... lN5eUtQdUE

Towers.docx, Towers.pdf <--- Document describing some of the key points we learned about towers this season (mostly just before State)
Youngs Mouduli of Balsa 1956.pdf <--- Published document reporting measurements of the Modulus of Elasticity of balsa (different grains, different densities)
Tower_Jig.jpg <--- Photo of our last jig before State for a design not attempting the bonus
TowerDesignOptimizer.xlsx <--- Spreadsheet for estimating the weight/score of simple towers with only evenly-spaced X-braces on each side given the density and cross-sectional dimensions of wood. It also allows you to calculate the number of X-braces you need based on the Euler critical length (from the "Youngs Mouduli of Balsa 1956.pdf" data with a fudge factor to bring the numbers in-line with our own crude measurements)

Hope this can help some teams build some really good towers. The secret is science + hard work, but it helps to be brave enough to choose low density wood.