Towers B/C

[jgrischow1]

So we ordered some 1/8 by 1/8 36" sticks from Specialized Balsa. Light density, which they advertise as 6-10 lb/ft^{^3}. We ordered 10 sticks, and got the following weights in grams:

1.02
1.11
1..12
1.14
1.40
1.36
2.14
2.24
2.28
3.44

Is that much variance to be expected?

[waffletree]

So we ordered some 1/8 by 1/8 36" sticks from Specialized Balsa. Light density, which they advertise as 6-10 lb/ft^{^3}. We ordered 10 sticks, and got the following weights in grams:

1.02
1.11
1..12
1.14
1.40
1.36
2.14
2.24
2.28
3.44

Is that much variance to be expected?

I also ordered some wood from Specialized Balsa, but haven't received it yet. I'm not sure, but I think they have a general range for each category. Like extra light is probably <1, light is probably 1-3.5, medium is 3.5-5, and so on. It's pretty hard to get them all to be really close.

[MadCow2357]

I have a tower that keeps breaking in the middle, at the place where the two halves of the tower are glued together. The top kinda falls through sometimes, and I'm pretty sure the top just fell over once. I've been making sure that the legs match up, and using a level to make sure it's straight. Does anyone have any idea how to fix this problem?

Regarding the lower section of a tower, the top and bottom are under totally different reactions. The very bottom of the columns are being forced out. away from the center of the tower. To resist this, a "tension band needs to be wrapped around the tower near the bottom. It can be in the form of a horizontal band, or a low profile X brace. Due to the lean of columns, the faces of the columns are not co-planar... they angle in slightly. Because of this, along with the substantial amount of outward thrust on the column bottoms, you may want to increase from 1/32" thick material... maybe 1/20 or even 1/16" just at the bottom. The thrust is greater on the bonus towers than on the non bonus towers. Remember, this will put TENSION on the bottom braces, not compression. As such, they should be glued to the sides of the columns, not butt jointed, like a ladder would be. Because the column faces are not co-planar, as previously mentioned, there are also some shear forces as work here as well, which is why I'm recommending an increased thickness on these braces.

As for the top of the lower section, the upper columns bearing down on the tops of the lower columns will force them inward (not outward like the bottom).. To resisting this, insert a horizontal ladder between the tops of the lower columns. This will be dealing with COMPRESSIVE forces, so it should be placed between the column tops and glued as butt joints on each end. Because it is resisting compression, it needs to be larger than the bottom members, even though it is substantially shorter. At a minimum, it should be at least 3/32" square, or even medium density 1/8" square. Once the joints between upper and lower columns are stabilized, there is still a problem. When a tower is under substantial load (say 10 kg or greater) to top section can "punch" right through the top of the bottom section. I've seen towers where the chimney remains totally intact, but is now inside the lower section of the tower after testing. To compensate for this, consider placing additional ladders just down from the top of the lower section, and make the top tier of bracing a little shorter. Once the forces are redirected around the angle of upper to lower columns, you'll be ok, but getting them to redirect due the vector change can be a real bugger!

Thanks for the help! I have been using 1/16 on the bottom, but I've been using a few butt joints.

Remember to use lap joints whenever possible, and gussets in key places where you must use butt joints.

[MadCow2357]

So we ordered some 1/8 by 1/8 36" sticks from Specialized Balsa. Light density, which they advertise as 6-10 lb/ft^{^3}. We ordered 10 sticks, and got the following weights in grams:

1.02
1.11
1..12
1.14
1.40
1.36
2.14
2.24
2.28
3.44

Is that much variance to be expected?

I also ordered some wood from Specialized Balsa, but haven't received it yet. I'm not sure, but I think they have a general range for each category. Like extra light is probably <1, light is probably 1-3.5, medium is 3.5-5, and so on. It's pretty hard to get them all to be really close.

Everybody's tower designs, of course, will vary, but the weights of each stick are very important when building competitive towers. I ordered 1.2g per /8 stick, and my tower weight came to be around 8.5 grams (not too competitive). I would consider anything above 2 as overkill, but please correct me if I'm wrong.

[Cow481]

I definitely agree that over 2 grams is overkill adn anything over 1.5 is overkill for competetive towers. I personally was at 1.8 at the start if the season and am now down to 0.9-1 grams pieces.

[TowerPro+2500]

Does CA glue brittle and weaken over time? if so, how long would it take for that to happen? I have had better experiences with towers that are built right before testing, and joint failure on towers not tested for a few days. I have also heard that accelerators (which i don't use) weaken the joints.

[waffletree]

Does CA glue brittle and weaken over time? if so, how long would it take for that to happen? I have had better experiences with towers that are built right before testing, and joint failure on towers not tested for a few days. I have also heard that accelerators (which i don't use) weaken the joints.

I assume the towers would be best within 3-4 days of building it, I recently tested a tower that I build a week before, and it was crap.
Also, accelerators definitely weaken the wood. I try to avoid using accelerators even on my wright stuff planes.

[TowerPro+2500]

Does CA glue brittle and weaken over time? if so, how long would it take for that to happen? I have had better experiences with towers that are built right before testing, and joint failure on towers not tested for a few days. I have also heard that accelerators (which i don't use) weaken the joints.

I assume the towers would be best within 3-4 days of building it, I recently tested a tower that I build a week before, and it was crap.
Also, accelerators definitely weaken the wood. I try to avoid using accelerators even on my wright stuff planes.

I am going to test at competition and was wondering if I should freshen the glue just to be safe... would prob only add ~0.03 or less gs of weight, but since I only built 4 days ago i will prob only do the tension belt joints...

[waffletree]

Does CA glue brittle and weaken over time? if so, how long would it take for that to happen? I have had better experiences with towers that are built right before testing, and joint failure on towers not tested for a few days. I have also heard that accelerators (which i don't use) weaken the joints.

I assume the towers would be best within 3-4 days of building it, I recently tested a tower that I build a week before, and it was crap.
Also, accelerators definitely weaken the wood. I try to avoid using accelerators even on my wright stuff planes.

I am going to test at competition and was wondering if I should freshen the glue just to be safe... would prob only add ~0.03 or less gs of weight, but since I only built 4 days ago i will prob only do the tension belt joints...

Super glue is pretty heavy - I don't know if it's a good idea to put on extra glue...

[dholdgreve]

Does CA glue brittle and weaken over time? if so, how long would it take for that to happen? I have had better experiences with towers that are built right before testing, and joint failure on towers not tested for a few days. I have also heard that accelerators (which i don't use) weaken the joints.

Its been my experience that CA will remain relatively stable for 60 to 75 days. After that, it will indeed get brittle and lose strength. I would not recommend "freshening" Glue. The actual bond would still be the old glue, so all you would be doing is adding weight with no appreciable gain in strength.

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