Towers B/C

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Re: Towers B/C

Post by Balsa Man » November 16th, 2016, 6:21 am

I agree with the endpoint/conclusion that under this year’s rules balsa legs are more efficient than bass. We took a close look at the comparison this year, because back in 2012, we did tower with 3/32nds bass, won Regionals, won State…
Bottom line, for a B-Div tower, it should come out about 1.5gr heavier with the 3/32 bass (vs 1/8 balsa).

As we’ve discussed, for legs, its all about buckling under axial compression loading. So, let’s look at some more numbers than in the discussion above, for clarity and a deeper analysis.

The buckling strength range for bass, at 3/32”, falls within the buckling strength range for balsa at 1/8”. So how do these sizes in balsa and bass compare? Balsa at 1/8 runs (can be gotten) from about 0.7 to 4.7gr for a 36” stick, and 0.4 to 2.2gr for 3/32 36” stick. Bass is typically found in 24” sticks. But to compare apples to apples (i.e., at a 36” stick length), bass can be found from 2.10gr (=1.4gr at 24”) to 2.85gr (=1.9gr at 24”). The median weight we’ve seen in 24” bass sticks, btw, is about 1.55gr.

Buckling strength data from axial compression (buckling) testing over the years, again comparing at 36” lengths:

-3/32 bass ranges from 63gr (for 2.1gr at 36”) to 95gr (for 2.85gr at 36”)

-For 1/8 balsa the approximate stick weight to get a 36” buckling strength of 63gr is 1.22gr, and to get a 95gr buckling strength at 36” is approximately 1.71gr. A 36” buckling strength of around 60gr means that braced at 1/5 intervals, you’ll have a buckling strength around 4,800gr, which gives you a bit more than a 20% safety factor over the buckling strength needed to carry a 15kg load on the tower.

That gives you leg weights (for a B tower) in 1/8 balsa of 2.71gr, and in 3/32 bass of 4.52gr. Add bracing at 4.71gr for balsa, and 4.44gr for bass, we get total wood weights of 7.42gr (balsa) and 8.96gr (bass)- a difference of 1.56- factor in a bit less glue weight with the smaller wood size for the bass = about 1.5gr difference.

Based on these numbers, no surprise, we’re going with 1/8 balsa this year.

One other comment, again from significant testing, balsa is significantly more efficient in tension applications than bass

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Re: Towers B/C

Post by baker » November 16th, 2016, 7:50 am

Balsa Man... I hope people appreciate the work you do! Your research shows hours of testing, stuff I always wanted to do but never had time to. Sometimes hard to follow but I love the conclusion. Because of years of hands on building with different sets of students I can see your data follows our experience of test and rebuild and test again.

In 2012 we used the 3/23 sq Bass for the base legs as we kept breaking the 1/8 sq balsa. In the top we used 3/32 balsa which worked very well. This year we used 3/32 sq bass for legs but the mass of the tower came in just over 13 grams, too heavy. I like to have new students build a little heavy to get some build experience and an ego rush from success . From there we then start to bring the mass down. Different cross section or density.

So from the lasted data posted, I see legs - 1/8 x 1/8 sq, 10-12 # density balsa legs with bracing intersections at about 12 cm (5 inches).. Also, I know from past posts about how important bracing is, I'm interested in your opinion about which pattern you like best. 'X' or 'V' bracing.
Attachments
bracing .docx - Microsoft Word.jpg

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Re: Towers B/C

Post by Random Human » November 16th, 2016, 10:04 am

baker wrote:Balsa Man... I hope people appreciate the work you do! Your research shows hours of testing, stuff I always wanted to do but never had time to. Sometimes hard to follow but I love the conclusion. Because of years of hands on building with different sets of students I can see your data follows our experience of test and rebuild and test again.

In 2012 we used the 3/23 sq Bass for the base legs as we kept breaking the 1/8 sq balsa. In the top we used 3/32 balsa which worked very well. This year we used 3/32 sq bass for legs but the mass of the tower came in just over 13 grams, too heavy. I like to have new students build a little heavy to get some build experience and an ego rush from success . From there we then start to bring the mass down. Different cross section or density.

So from the lasted data posted, I see legs - 1/8 x 1/8 sq, 10-12 # density balsa legs with bracing intersections at about 12 cm (5 inches).. Also, I know from past posts about how important bracing is, I'm interested in your opinion about which pattern you like best. 'X' or 'V' bracing.
The type of bracing you choose depends on the length between your 2 primary compression members. I they are narrow, I think that "z" bracing is a good option because using the x bracing is a lot of a waste- it will be strong enough. X bracing is probally the most strongest, so for the outer parts of the tower (where the 2 primary compression members are more seperate) x bracing is good. Most of my towers are using all x bracing and they do pretty well so what I have seen so far, X bracing is really good. But as I said, it really depends on your feel and keep in consideration that every gram counts and be as efficient as possible.

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Re: Towers B/C

Post by Balsa Man » November 16th, 2016, 11:42 am

baker wrote:Balsa Man... I hope people appreciate the work you do! Your research shows hours of testing, stuff I always wanted to do but never had time to. Sometimes hard to follow but I love the conclusion. Because of years of hands on building with different sets of students I can see your data follows our experience of test and rebuild and test again.

In 2012 we used the 3/23 sq Bass for the base legs as we kept breaking the 1/8 sq balsa. In the top we used 3/32 balsa which worked very well. This year we used 3/32 sq bass for legs but the mass of the tower came in just over 13 grams, too heavy. I like to have new students build a little heavy to get some build experience and an ego rush from success . From there we then start to bring the mass down. Different cross section or density.

So from the lasted data posted, I see legs - 1/8 x 1/8 sq, 10-12 # density balsa legs with bracing intersections at about 12 cm (5 inches).. Also, I know from past posts about how important bracing is, I'm interested in your opinion about which pattern you like best. 'X' or 'V' bracing.
Thanks, Baker.
I always enjoy your insights. Its a pleasure to share the.....basic data we've accumulated; helps everybody up their game, and do some real engineering, and less shooting in the dark.

On legs, 10-12#/cf translates, for 1/8x1/8 to 36" stick weights between about 1.45 and 1.75gr. At 1/5 bracing intervals for a C tower meeting 29cm circle specs (12.27cm which is I assume your "about 12cm), you're definitely in the right ballpark. Specifically, a 1.52gr (to the light end of the 10-12#/cf range stick calculates out to 4572gr buckling strength at 12.27 braced intervals (which is force on each leg, +20% safety factor, for a 15kg tower load).

On bracing, see attachment on my post on October 21 (pg 10 of this thread); provides a pretty detailed discussion of the "Xs and ladders" we use- your middle figure. What's different from most in implementing this pattern is the use of 1/16th wide 1/64th" strips, put on pre-tensioned, for the Xs. I'm not 100% convinced its the very lightest way to go, but it works......

On the 13gr 3/32nds bass tower, that weight sounds about right. With 1.52gr/36" legs, ladders and Xs, weight calculates out to a tad under 10 grams. We'll see, one of these days, if reality aligns with all the calculation.

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Re: Towers B/C

Post by baker » November 16th, 2016, 12:54 pm

X's and ladders are what we used in 2012 to counter what we found was a twisting action that would occur when loading. We wanted to try "V" bracing (figure 1) on our first tower to see if it works and if it saves any weight that X bracing might have.

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Re: Towers B/C

Post by Balsa Man » November 16th, 2016, 1:17 pm

baker wrote:X's and ladders are what we used in 2012 to counter what we found was a twisting action that would occur when loading. We wanted to try "V" bracing (figure 1) on our first tower to see if it works and if it saves any weight that X bracing might have.
I'll be curious if it does; good luck!
The pre-tensioned Xs are very effective preventing twisting, and its really hard to get precise/symmetrical enough leg alignment to not have some twisting moment induced under load.

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Re: Towers B/C

Post by BananaPirate » November 16th, 2016, 5:00 pm

Balsa Man wrote:
hearthstone224 wrote: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?
Nice. Essentially correct (I get a slightly different number in the "Proportion" column for 1/7th (0.96 vs your 0.91), which makes makes the number in the "Multiplier" column 109.26 (vs your 120.76) might want to re-check that.

Your conclusion that bracing at 1/5 intervals for a stick with a 36" buckling strength testing at 80 gr should hold is correct, and that a stick with a 36" buckling strength of 130gr should hold at 1/4 interval, and that to go to 1/3 interval, you'd need a stick testing at 220gr.

The (1/8 x 1/8 x 36") stick weights you should expect to get those buckling strengths are (from the table I'm having trouble attaching) are:
1/5 interval- 1.5-1.6gr range
1/4 interval- 2.2-2.3gr range
1/3 interval- 3.5-3.6gr range. I'm not surprised you haven't seen any this heavy, it's pretty uncommon/pretty high density. They do exist; in fact, you can order sticks up to 4.7gr from Specialized Balsa . They start to get pricey as you push past 3gr. Bottom line, 1/5 and 1/4 bracing intervals should be achievable with common/typically available balsa.
Hi everyone! It's my first time posting this year!

So the question I have is about the wood...are the weights you're listing (1.5-1.6g, 2.2-2.3g) for the full 36" stick or the piece after it has been cut down to leg length (61 cm or so)? From the wording it seems like you mean for the full 36", however, after some testing, I've never found any 1/8 x 1/8 x 36 pieces to have a buckling strength of anywhere close to a buckling strength of 80 grams :/

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Re: Towers B/C

Post by Balsa Man » November 16th, 2016, 5:34 pm

BananaPirate wrote:Hi everyone! It's my first time posting this year!

So the question I have is about the wood...are the weights you're listing (1.5-1.6g, 2.2-2.3g) for the full 36" stick or the piece after it has been cut down to leg length (61 cm or so)? From the wording it seems like you mean for the full 36", however, after some testing, I've never found any 1/8 x 1/8 x 36 pieces to have a buckling strength of anywhere close to a buckling strength of 80 grams :/
Yup, these are buckling strength at 36". You should see about that buckling strength in 1/8 x 1/8 at about 1.5gr (36") stick weight. 1.4 at about 75.. 1.6 at about 85gr. What can I say, that's what our testing's seeing.....

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Re: Towers B/C

Post by BananaPirate » November 16th, 2016, 9:07 pm

Balsa Man wrote:
BananaPirate wrote:Hi everyone! It's my first time posting this year!

So the question I have is about the wood...are the weights you're listing (1.5-1.6g, 2.2-2.3g) for the full 36" stick or the piece after it has been cut down to leg length (61 cm or so)? From the wording it seems like you mean for the full 36", however, after some testing, I've never found any 1/8 x 1/8 x 36 pieces to have a buckling strength of anywhere close to a buckling strength of 80 grams :/
Yup, these are buckling strength at 36". You should see about that buckling strength in 1/8 x 1/8 at about 1.5gr (36") stick weight. 1.4 at about 75.. 1.6 at about 85gr. What can I say, that's what our testing's seeing.....

Len Joeris
Ft. Collins CO
That's interesting...just to confirm, you are pushing down on a scale until around a 0.5 cm bow? Perhaps my sample size is too small, I will do some more testing as well. The only other thing I can think of is that the balsa sticks I am testing with (from Menards and Hobbytown), are lower quality than ones bought from another retailer like specialized balsa. This may result in same density but different buckling strength? Thanks for the quick response!

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Re: Towers B/C

Post by HandsFreeCookieDunk » November 17th, 2016, 3:35 am

BananaPirate wrote:
Balsa Man wrote:
BananaPirate wrote:Hi everyone! It's my first time posting this year!

So the question I have is about the wood...are the weights you're listing (1.5-1.6g, 2.2-2.3g) for the full 36" stick or the piece after it has been cut down to leg length (61 cm or so)? From the wording it seems like you mean for the full 36", however, after some testing, I've never found any 1/8 x 1/8 x 36 pieces to have a buckling strength of anywhere close to a buckling strength of 80 grams :/
Yup, these are buckling strength at 36". You should see about that buckling strength in 1/8 x 1/8 at about 1.5gr (36") stick weight. 1.4 at about 75.. 1.6 at about 85gr. What can I say, that's what our testing's seeing.....

Len Joeris
Ft. Collins CO
That's interesting...just to confirm, you are pushing down on a scale until around a 0.5 cm bow? Perhaps my sample size is too small, I will do some more testing as well. The only other thing I can think of is that the balsa sticks I am testing with (from Menards and Hobbytown), are lower quality than ones bought from another retailer like specialized balsa. This may result in same density but different buckling strength? Thanks for the quick response!
It is very likely that your sample size is too small. In general, cheaper, lower quality balsa is lower quality not because its weaker, but because there is far more variability in strength and density.

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