Towers B/C

[retired1]

rule 3.a says that the tower is to span a 29 cm circle and may not touch the base inside of the circle.
Now you can put the legs just outside of a 21 cm chord and not have the lower horizontal brace at the exact bottom or it will touch the inside of the circle.
Notice that the magic word is span, not to remain completely outside of the circle. This is for a 4 legged tower. If you go with a 3 legged tower, then the chord is larger, but still smaller than 29 cm.

[parthpatel612]

Try both to be sure. My thought is that b is better as it will be more effective against twisting. (axiel movement).
Your diagram shows a pure rectangular tower. It should be angled to get the top to a desired size of 5 cm or less.

Yes B will be more effective against twisting, but won't it also cause more stress on certain joints? Does anyone else have insight or experience on this matter?

As far as my tower goes it's angled until 15cm then it's a pure rectangular prism.

[retired1]

You will have an easier to build and probably a stronger tower if you do not have that joint between the base and the chimney. Trust me, it is hard to make it perfectly symmetrical and perfectly plumb.
If the braces are an appropriate distance apart, and of a reasonable size you should have no problems with the joints. Do not use medium or gel superglue as the brace can pull off one grain line off of the leg. It just does not soak in so it is a weak joint even tho the glue itself is super strong.
If you use Duco, try thinning a small bottle by 50-50 with acetone. Use this as a primer and then use full strength Duco for the bond.

Your braces can be a lot smaller that you might guess.

[retired1]

Mmmm…, the diagonal of a 20cm square is the square root of 20 squared + 20 squared; sqrt of 800 = 28.28427. That’s 0.71573 cm less than 29.0 cm. So, splitting that, a 29cm diameter circle, with center aligned with center of a 20cm square will be outside the corners of the square by 0.357864 cm (say 3.6mm. So, if you set the insides of the legs say 6mm out from the corners of the 20cm square, your legs will be outside the circle by 2.4mm, a good working clearance….

Len, took a couple of readings to soak in, but that is a great idea if their hole is reasonably accurate.

[Balsa Man]

Thanks.

Having done a little more analysis after my post in the "bonus" thread, its clear to me that building for the bonus (leg ends outside 29cm circle, vs outside 20cm square) IS the way to go; what 2kg extra load credit gets you in points will be significantly more than what the extra wood for the wider stance costs you in points because of greater tower weight

[baker]

I drew a 29 cm circle on a piece of graph paper (cm scale) with a compass and it looks like... if you were to build a square base tower and set the legs at 22 cm apart you would be safely outside the circle. You might cut it to 21.5 cm but that might be too close for event super.

[Balsa Man]

That’s a nice, safe number, Baker.
When you calculate it, using a 29.6cm circle- that gives you 3mm clearance on each side, the chord – between adjacent legs – is 20.9272cm. so, if you set the dimension where it contacts the test base, between the insides of the legs at 21.0cm, you’ll have, when you set it up over a 29cm circle, 3+mm clearance on all 4 legs. 4mm less isn’t a lot, for sure, but if you’re seriously looking for all the ways to minimize the wood going into the tower, 4mm on four sides is 1.6cm at the bottom; maybe 0.2-0.3 gr lower bracing weight overall. You will need to make sure your leg contact points are really in a square, not a parallelogram; like within less than a millimeter. With a little care, that’s quite doable.

As to too close for event supervisors, a couple comments. I would hope in the vast majority of cases, for this particular measurement/confirmation that you meet the construction specs, they’ll be able… correctly handle this to….at worst, a millimeter. Reading the rules, and applying S.O. significant figures (sig fig) policy, “29cm” could be as big as 29.5 (or as small as 28.5). The circle they draw, and use to see if your leg-ends fall outside it, could be half a millimeter larger in diameter. In that case, with 21cm, you’d still have 2.5mm clearance on each leg. As long as your base is square, you’re good.

Having put the measurement tools/jigs together for…a number of State and Regional competitions for towers, bridges, and booms, and having help supervise a number, I always kept in mind that just as the rules (in this case, dimension rules + sig figs) apply to competitors, they apply to supervisors/judges. All supervisors/judges should understand and respect this. The nice thing about this particular rule/measurement/requirement is it’s a visual check against a line; easy/clear to see within a fraction of a millimeter; if you can see ‘daylight’ between all leg ends and the drawn circle, it clears. If, for whatever reason, you’re concerned, I’d suggest bring a good ruler, and a carefully (accurately) drawn circle. Get to the event room well before competition starts. Ask to have your base checked. If their circle is off enough your tower, that does in fact meet the rule, doesn’t clear their circle, explain, respectfully, you built, carefully, to the rules, you meet the rules, provide your carefully/accurately drawn circle, ask that they a) measure to confirm, and b) upon confirming, use it to assess all competitors. The key is to avoid a situation where one or more competitors have already tested, and going to a different confirmation basis would create a different standard for some competitors. Speaking of things to check ahead of time, with towers, its particularly important the test base be really level- front to back, and side to side. If it slopes to one side, the leg(s) to that side will have to carry more force/load than the high side. They should have a good level, and have used it, and should be checking periodically, to make sure a level base is being maintained.

[dholdgreve]

FWIW... Keep in mind that a 4 legged tower doesn't necessarily need to be 4 sides of EQUAL dimensions... Have you though about extending the "span" slightly more in one direction, but substantially reducing it in another? If all 4 sides are equal, each side needs to be something in the order of 21 CM... But if 2 sides were expanded to the full 29 cm, thus adding 8 cm x 2 sides = 16 additional cm, the opposing sides could (in theory) be reduced to whatever is still stable... Possibly something in the 10 CM range?... or less? This could result in a reduction of 11 CM on those 2 sides, or a savings of 22 CM at the base of the tower. The overall savings would conservatively be in the 6 cm of base perimeter range. Yes, your column angle would increase slightly, but is it worth what you are saving by shortening the amount of bracing required?

[SPP SciO]

FWIW... Keep in mind that a 4 legged tower doesn't necessarily need to be 4 sides of EQUAL dimensions... Have you though about extending the "span" slightly more in one direction, but substantially reducing it in another? If all 4 sides are equal, each side needs to be something in the order of 21 CM... But if 2 sides were expanded to the full 29 cm, thus adding 8 cm x 2 sides = 16 additional cm, the opposing sides could (in theory) be reduced to whatever is still stable... Possibly something in the 10 CM range?... or less? This could result in a reduction of 11 CM on those 2 sides, or a savings of 22 CM at the base of the tower. The overall savings would conservatively be in the 6 cm of base perimeter range. Yes, your column angle would increase slightly, but is it worth what you are saving by shortening the amount of bracing required?

Interesting idea - were you thinking more of a rectangular pyramid shape, or something that looks like an elevated bridge with a chimney on top? This picture seems interesting, particularly with the talk of rolled balsa tubes a few posts back: http://nexote.net/nexote/_sampleImages/ ... ltower.jpg

[retired1]

SPP-- any idea on how much it weighs, as pictured?