Tower Base

[Unome]

I work primarily on the construction side of the tower, and am fairly confident in my team's ability to construct an adequately balanced and effective three-sided base, which we will probably continue to work with for now until we create a new clean-sheet design for a four sided tower.

That being said, I've lightly looked through the forums from this year and last, finding that people have had success in the past with three-sided towers. My main question is how did you build the top section (this is assuming a pyramidal bottom section with a top piece more or less stacked on top)? I'm concerned by the gluing at 60 degree angles that would need to occur.

Or, are successful three-sided towers just a single pyramid shape as opposed to a pyramid topped by a triangular prism of sorts?

A three-sided tower can be built successfully, but if you can build a good three-sided tower you can probably build a significantly better four-sided tower (I believe Balsa Man has explained why in previous posts, I'll not try to reiterate and risk misinterpreting).

[Balsa Man]

I work primarily on the construction side of the tower, and am fairly confident in my team's ability to construct an adequately balanced and effective three-sided base, which we will probably continue to work with for now until we create a new clean-sheet design for a four sided tower.

That being said, I've lightly looked through the forums from this year and last, finding that people have had success in the past with three-sided towers. My main question is how did you build the top section (this is assuming a pyramidal bottom section with a top piece more or less stacked on top)? I'm concerned by the gluing at 60 degree angles that would need to occur.

Or, are successful three-sided towers just a single pyramid shape as opposed to a pyramid topped by a triangular prism of sorts?

A three-sided tower can be built successfully, but if you can build a good three-sided tower you can probably build a significantly better four-sided tower (I believe Balsa Man has explained why in previous posts, I'll not try to reiterate and risk misinterpreting).

I'll simply re-say what I've said a number of times. For multiple, unavoidable reasons, a... well-engineered 4-leg tower will always beat a well engineered 3-leg tower. Yes, people have "had success" to a limited extent with 3-leggers; gotten them to hold full load, with a tower weight that maybe got into top 10 in Regionals, maybe State. There are a number of... good builders that have tried 3-leggers over the years, but then concluded they can't beat what they could do with a 4-legger. Been there, done that, have the T-shirt.

Key reasons-
1) 4 legs give you parallel leg faces to glue bracing onto (3 legs give you only diagonal edges).
2) If you go to triangular cross section legs to get parallel leg faces to glue bracing onto, you end up putting non-axial loading onto the bracing pieces, because of where the centroid of a triangle is, and the direction from it to the leg face (the shortest cross section; the direction it will buckle). With any force onto the bracing piece, it starts to bend immediately. It bows/buckles/breaks at much lower loading than it would if axially loaded (which is what would happen w/a brace in a 4-leg tower).
3) To meet 29cm circle bonus, the distance between bottom ends of the legs has to be significantly longer than in a 4-legger. Remember the inverse square relationship of length to buckling strength. The lower ...2-3 bracing sets are one of the big design challenges- how to get then strong enough without excessive weight. A bracing piece only 10% longer would have only about 82% of the buckling strength....
4) The challenge of getting equal loading onto the legs. While it can be done, getting the eye block in the load block aligned with center of the tower is ...trickier than you might think.

[Unome]

Yes, people have "had success" to a limited extent with 3-leggers; gotten them to hold full load, with a tower weight that maybe got into top 10 in Regionals, maybe State.

My team used a 3-legger last year for most of the season, scoring a little over 2000; I wonder how they would have done with a 4-legger.

[Snarknado]

I'll simply re-say what I've said a number of times. For multiple, unavoidable reasons, a... well-engineered 4-leg tower will always beat a well engineered 3-leg tower. Yes, people have "had success" to a limited extent with 3-leggers; gotten them to hold full load, with a tower weight that maybe got into top 10 in Regionals, maybe State. There are a number of... good builders that have tried 3-leggers over the years, but then concluded they can't beat what they could do with a 4-legger. Been there, done that, have the T-shirt.

Key reasons-
1) 4 legs give you parallel leg faces to glue bracing onto (3 legs give you only diagonal edges).
2) If you go to triangular cross section legs to get parallel leg faces to glue bracing onto, you end up putting non-axial loading onto the bracing pieces, because of where the centroid of a triangle is, and the direction from it to the leg face (the shortest cross section; the direction it will buckle). With any force onto the bracing piece, it starts to bend immediately. It bows/buckles/breaks at much lower loading than it would if axially loaded (which is what would happen w/a brace in a 4-leg tower).
3) To meet 29cm circle bonus, the distance between bottom ends of the legs has to be significantly longer than in a 4-legger. Remember the inverse square relationship of length to buckling strength. The lower ...2-3 bracing sets are one of the big design challenges- how to get then strong enough without excessive weight. A bracing piece only 10% longer would have only about 82% of the buckling strength....
4) The challenge of getting equal loading onto the legs. While it can be done, getting the eye block in the load block aligned with center of the tower is ...trickier than you might think.

Thanks! We'll definitely look into it in more depth.

Yes, people have "had success" to a limited extent with 3-leggers; gotten them to hold full load, with a tower weight that maybe got into top 10 in Regionals, maybe State.

My team used a 3-legger last year for most of the season, scoring a little over 2000; I wonder how they would have done with a 4-legger.

We're not at that score yet, but we're still hoping to go with our existing three sided design so we can have decent tower done in the next two weeks.

[Balsa Man]

Thanks! We'll definitely look into it in more depth.

My team used a 3-legger last year for most of the season, scoring a little over 2000; I wonder how they would have done with a 4-legger.

We're not at that score yet, but we're still hoping to go with our existing three sided design so we can have decent tower done in the next two weeks.

You're quite welcome. I know it may not be what you want to hear, but it comes from many years of experience....
C tower I worked with last year (4 legs) got to 2246

[Chameleon02]

Has anyone achieved success with a rectangular bonus tower, and is it a viable option this year?
I am curious because my first two towers this year have been rectangular bonus and scores have been about 1500.
The downside is the 28 cm gap between the two long sides, but I was able to counter that in my second tower with compression pieces- it broke from the short sides

[Balsa Man]

Has anyone achieved success with a rectangular bonus tower, and is it a viable option this year?
I am curious because my first two towers this year have been rectangular bonus and scores have been about 1500.
The downside is the 28 cm gap between the two long sides, but I was able to counter that in my second tower with compression pieces- it broke from the short sides

You hit on one of the two limitations/challenges with a rectangular base, those LONG lower braces. The other is that it is very tricky/demanding to put together a high-precision jig, where it is very easy to do that with a square base. Getting precise 'shape control' is SO important.....

[Unome]

The other is that it is very tricky/demanding to put together a high-precision jig, where it is very easy to do that with a square base.

Really? I'd imagine that aligning two flat pieces would be much easier than trying to put together a 3D jig (which I've yet to successfully visualize how to do in a manner that would orient the legs properly).

[Random Human]

The other is that it is very tricky/demanding to put together a high-precision jig, where it is very easy to do that with a square base.

Really? I'd imagine that aligning two flat pieces would be much easier than trying to put together a 3D jig (which I've yet to successfully visualize how to do in a manner that would orient the legs properly).

Agreed, rectangular offers easy precision. Do some calculations, the further distance of the legs angle the legs to reduce the structural strength of the legs. You really have to weigh and compare the two designs. Rectangular designs have a lower perimeter than square designs, but you really have to analyze on your own. Build some towers do some tests. Last years winning designs were all square and generally, squares offer more stability. There have been in some cases rectangular towers which have fell over due too bad building.

[Balsa Man]

The other is that it is very tricky/demanding to put together a high-precision jig, where it is very easy to do that with a square base.

Really? I'd imagine that aligning two flat pieces would be much easier than trying to put together a 3D jig (which I've yet to successfully visualize how to do in a manner that would orient the legs properly).

See recent post in "towers" thread. With a cruciform jig it is cake to get square legs lined up- with diagonal cross section lines pointing to the vertical centerline of the tower (which gives you adjacent leg faces parallel) I can't visualize a jig configuration you can easily build that will get comparably precise alignment. I can visualize some...significantly more complicated arrangements, but the more complicated it gets; the more pieces, and the more angles you need to get precise and symmetrical,the more 3-d precision and symmetry gets degraded. Is there something simple I'm missing/not seeing??