## Towers B/C

thegreaterhawk
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### Re: Towers B/C

does anyone have the tower score sheet for Grayslake invitational?

Tesel
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### Re: Towers B/C

Hey Balsa Man, I have one last question.
I've heard you mention 3/32" x 3/32" wood. Would you recommend that for my ladders instead of 1/8"? I can see that having benefits with accuracy, but would it be better to just have the consistency in your experience?
I only ask because this density wood is only available in a store literally 2 hours+ away, and I'd like to buy everything during this trip, I'm just curious if I should pick some of that up too.

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Balsa Man
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### Re: Towers B/C

Ladders have to have sufficient buckling strength to prevent adjacent legs buckling in toward each other. As previously discussed, experience says that a buckling strength of about 1kg will be sufficient.

Back on page 30, about 2/3 of the way down, is description of how to calculate what's needed, in terms of measured 36" buckling strength (one finger push-down test.

Buckling strength is a function of cross section, and inherent stiffness, which is a function of density. So 3/32 would have to be of higher density than 1/8 to have the same buckling strength. When you do all the calcs, it really doesn't gain you much at all, and its much easier to get 1/8 ladders aligned w/ 1/8 legs (as in centered on) than centering 3/32 on 1/8.
This help?
Len Joeris
Fort Collins, CO

Tesel
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### Re: Towers B/C

Balsa Man wrote:Ladders have to have sufficient buckling strength to prevent adjacent legs buckling in toward each other. As previously discussed, experience says that a buckling strength of about 1kg will be sufficient.

Back on page 30, about 2/3 of the way down, is description of how to calculate what's needed, in terms of measured 36" buckling strength (one finger push-down test.

Buckling strength is a function of cross section, and inherent stiffness, which is a function of density. So 3/32 would have to be of higher density than 1/8 to have the same buckling strength. When you do all the calcs, it really doesn't gain you much at all, and its much easier to get 1/8 ladders aligned w/ 1/8 legs (as in centered on) than centering 3/32 on 1/8.
This help?
Yeah, a lot. Sounds like there are insignificant benefits if any to doing something more complicated.
My plan is to stick to 1.2-1.3g sticks for the legs, 0.75-0.85g sticks for compression ladders, and 0.2-0.3g 1/16" sticks for tension members. That sound reasonable?

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Balsa Man
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### Re: Towers B/C

Tesel wrote:
Balsa Man wrote:Ladders have to have sufficient buckling strength to prevent adjacent legs buckling in toward each other. As previously discussed, experience says that a buckling strength of about 1kg will be sufficient.

Back on page 30, about 2/3 of the way down, is description of how to calculate what's needed, in terms of measured 36" buckling strength (one finger push-down test.

Buckling strength is a function of cross section, and inherent stiffness, which is a function of density. So 3/32 would have to be of higher density than 1/8 to have the same buckling strength. When you do all the calcs, it really doesn't gain you much at all, and its much easier to get 1/8 ladders aligned w/ 1/8 legs (as in centered on) than centering 3/32 on 1/8.
This help?
Yeah, a lot. Sounds like there are insignificant benefits if any to doing something more complicated.
My plan is to stick to 1.2-1.3g sticks for the legs, 0.75-0.85g sticks for compression ladders, and 0.2-0.3g 1/16" sticks for tension members. That sound reasonable?
The thing that would worry me on 1/16 x 1/16 for Xs at 0.2gr/0.3gr/36” is the very low density, and the potential for shearing failure- a thin layer of wood peeling/shearing off. You’re looking at 4lb/cu ft for 0.2gr/36, 5.5 lb/cu ft for 0.3gr/36.

The tensile strength is way more than enough; 4kg for 0.2gr/36, 5.3kg for 0.3gr/36. But that assumes the entire cross section of the stick is….working; carrying the load. As discussed before, experience suggests that a tensile strength of around 1kg will work (prevent outward buckling). But with glue only on the face of the X stick contacting the leg, only a small portion of the stick is carrying the load, maybe 1/100 of an inch. That’s roughly 1/6 of the cross section of the stick, so instead of 4kg, looking at maybe 0.76kg, instead of 5.3kg, looking at maybe 0.88kg. Might work, but very well might not. Adding glue on sides and top of the stick will…..bring the entire cross section into play/use, but at a significant weight increase (not a lot more in any single X to leg joint, but a lot of X to leg joints). To have any reasonable expectation of them working, I think you’ll want glue catching/on the sides of the sticks.

This is one of the reasons I’ve been…..advocating strips cut from 1/64” sheet for Xs. They can be at significantly higher density, and be significantly lighter than 1/16” sticks. An 8gr 3 x 36” sheet is about 10 lb/cu ft. 1/16” wide strips give you a tensile strength of about 1.4kg, and 3/32” wide strips give you about 1.9kg. At 1/64” thickness, (and glue just on the face contacting the leg), the entire cross section is in play.

So, how do weights compare? For a C-tower (meeting the circle bonus), with bracing at 1/5 intervals, total length of Xs is about 802cm. With 0.2gr/36” sticks, total X weight is 1.75gr, with 0.3gr/36, up to 2.63gr. With 3/32” wide 1/64 strips. 2.1gr; with 1/16” wide strips, 1.36gr. Less glue for the 1/64th options; I'd guess 0.3, maybe 0.4gr less.

So, that’s how the numbers shake out; hope these insights help.
Len Joeris
Fort Collins, CO

Tesel
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### Re: Towers B/C

Balsa Man wrote:
Tesel wrote:
Balsa Man wrote:Ladders have to have sufficient buckling strength to prevent adjacent legs buckling in toward each other. As previously discussed, experience says that a buckling strength of about 1kg will be sufficient.

Back on page 30, about 2/3 of the way down, is description of how to calculate what's needed, in terms of measured 36" buckling strength (one finger push-down test.

Buckling strength is a function of cross section, and inherent stiffness, which is a function of density. So 3/32 would have to be of higher density than 1/8 to have the same buckling strength. When you do all the calcs, it really doesn't gain you much at all, and its much easier to get 1/8 ladders aligned w/ 1/8 legs (as in centered on) than centering 3/32 on 1/8.
This help?
Yeah, a lot. Sounds like there are insignificant benefits if any to doing something more complicated.
My plan is to stick to 1.2-1.3g sticks for the legs, 0.75-0.85g sticks for compression ladders, and 0.2-0.3g 1/16" sticks for tension members. That sound reasonable?
The thing that would worry me on 1/16 x 1/16 for Xs at 0.2gr/0.3gr/36” is the very low density, and the potential for shearing failure- a thin layer of wood peeling/shearing off. You’re looking at 4lb/cu ft for 0.2gr/36, 5.5 lb/cu ft for 0.3gr/36.

The tensile strength is way more than enough; 4kg for 0.2gr/36, 5.3kg for 0.3gr/36. But that assumes the entire cross section of the stick is….working; carrying the load. As discussed before, experience suggests that a tensile strength of around 1kg will work (prevent outward buckling). But with glue only on the face of the X stick contacting the leg, only a small portion of the stick is carrying the load, maybe 1/100 of an inch. That’s roughly 1/6 of the cross section of the stick, so instead of 4kg, looking at maybe 0.76kg, instead of 5.3kg, looking at maybe 0.88kg. Might work, but very well might not. Adding glue on sides and top of the stick will…..bring the entire cross section into play/use, but at a significant weight increase (not a lot more in any single X to leg joint, but a lot of X to leg joints). To have any reasonable expectation of them working, I think you’ll want glue catching/on the sides of the sticks.

This is one of the reasons I’ve been…..advocating strips cut from 1/64” sheet for Xs. They can be at significantly higher density, and be significantly lighter than 1/16” sticks. An 8gr 3 x 36” sheet is about 10 lb/cu ft. 1/16” wide strips give you a tensile strength of about 1.4kg, and 3/32” wide strips give you about 1.9kg. At 1/64” thickness, (and glue just on the face contacting the leg), the entire cross section is in play.

So, how do weights compare? For a C-tower (meeting the circle bonus), with bracing at 1/5 intervals, total length of Xs is about 802cm. With 0.2gr/36” sticks, total X weight is 1.75gr, with 0.3gr/36, up to 2.63gr. With 3/32” wide 1/64 strips. 2.1gr; with 1/16” wide strips, 1.36gr. Less glue for the 1/64th options; I'd guess 0.3, maybe 0.4gr less.

So, that’s how the numbers shake out; hope these insights help.
It actually looks like I can get access to at least 1/16x1/32" sticks, even if the 1/64" sticks aren't attainable. Are you suggesting a similar weight (0.2-0.3g), but with thinner sticks to have higher density and better tensile strength?

Sorry if it seems like I'm ignoring your suggestions, but I don't really have access to the type of equipment to cut sticks out of 1/64" sheets or make a polycarbonate jig, I did redo the other jig and get some better dimensions and I'll be sure to post that when it's completely done. Thanks to your posts and help, my goals have shifted a lot, I'm really hoping to get first in my region and upset Grand Haven for once and I think that's possible now.

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Balsa Man
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### Re: Towers B/C

Yeah, if you can get/have 1/32 x 1/16 in the 0.2-0.3gr/36 range, you'll be significantly better off (more likely to hold) than with 1/16 x 1/16 in that weight range (twice the density).

BTW, all it takes to cut strips from 1/64" sheet is a single edge razor blade, a metal ruler/straight edge, a stick the width you want to cut (1/16 or 3/32) some masking tape, and a cutting surface (and, of course, 1/64" sheet). You might want to get some for State..... order from Specialized Balsa 3 x 36 sheet, sheet weight between 7 and 8gr). If/when you do, cut sheet into 3 ~12" pieces, tape piece to be stripped from down on cutting surface (masking tape, covering about 1/8" on each end). Put ruler down, with guide stick, align edge of guide stick with edge of sheet, hold ruler down in place, lift the guide stick off. Keeping razor blade vertical, and ruler held firmly in place, with slight pressure of blade against the ruler, gently run the blade down the ruler, make a second pass if it didn't cut through all the way, peel the tape back/off one end, pull the strip free, and repeat for more.

Good luck on your new goal; wish you all the best; glad to hear I've been of some help; looking forward to hearing how you do.
Len Joeris
Fort Collins, CO

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### Re: Towers B/C

Balsa Man, hearing that a team got to 3430 and a team got to 3050 (div b), how do you think nats would look (for medal and winning)?
Thanks
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Balsa Man
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### Re: Towers B/C

I have no idea.

Some time ago dholdgreve said he wouldn't be surprised to see Nats winner (for B, I believe) in the 3200 to 3500 range. I thought at the time that was a little high, because I couldn't come up with any scenario/calculations (with the data I have on wood density vs strength) on a configuration that would weigh significantly below 5gr, and have the strength to carry 15kg. Obviously, it is doable.

A factor I have no handle on at all is what teams will likely make it to Nationals, and how those teams are doing on towers. Obviously, if teams that have scored in this range get to Nationals, there’s a good chance they’ll score similarly.. How many teams that have figured out what it takes to get to this range, and that will make it to Nationals… no idea. As I’ve said before, typically at Nats you’ll see one or two teams w/ amazing scores, and a pretty sharp fall-off as you go down from the top.

The only info I have access to as to who is doing how well is what gets posted here; not enough to make any guesses.
Len Joeris
Fort Collins, CO

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### Re: Towers B/C

I am not sure but my sense is that to be able to do 3200-3500 score range, you need a big campaign of finding the one in a hundred or maybe a thousand pieces of the 4 legs. You can setup a test and bin procedure to test a vast number of balsa sticks and acquire 4 exceptional few of the pieces that have exceptional buckling resistive strength. It is not the design that matters that much at this score range since the braces are going to be super flimsy, it is the material that matters at this point. JMO.

PS: And also I think baking trick also most likely comes into play in this score range too.

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