1/20 vs. 1/32 bracing

[SOCoach]

I have a question about bracing. My kids are using 1/8 for the legs and 1/32 strips for crossbracing (X design). It seems our ceiling with this design is right around 10 kg. They are putting a slight bow in the bottom and top to insure the bracing is in tension. We are getting towers in the 6 to 7 gram range. It seems the bracing is not holding up . . . we thought about going to 1/20 thick bracing on the bottom only but the kids are afraid that is going to push the total weight up too much. Is the 1/32 bracing the way to go, they just need to improve technique or design, or should I encourage them to go to 1/20?

[SOCoach]

One additional question. I have been reading about the tension strip along the bottom of the base of the tower. My team is not using that strip . . . . they are just going with the X bracing. Does that tension strip make a big difference?

[Balsa Man]

As I've posted a few times, I think a bottom tension band is very important. It is quite possible that just adding this to what you've been running will bump the load carried, significantly. Those base leg ends are pushing apart w/ a force over 2kg.

As to going from 1/32 to 1/20 on base Xs, depends on how current Xs are failing. If they're failing in compression (bending, allowing movement of braced point, that extra cross section will add buckling strength (and maybe more efficiently than staying at 1/32 and increasing density). If they're failing under tension (either tension failure in the strip, or shear failure in the X to leg joint), won't help.

[SOCoach]

Thanks!! I'll have them try the tension strip along the bottom first and see how that works. I may have them use 1/20 for just the bottom two X bracing on the base also.

[dholdgreve]

We are currently experimenting with bumping the bottom X only to 1/20, keeping the rest at 1/32. Will report back

[DarthBuilder]

So a few hours ago there was this tower with five total bracings on each side. It held everything. Was it light or heavy wood? It seems nearly impossible for success with light wood.

[SOCoach]

dholdgreve . . . I'd be interested in hearing how it does.

[Balsa Man]

So a few hours ago there was this tower with five total bracings on each side. It held everything. Was it light or heavy wood? It seems nearly impossible for success with light wood.

As we have discussed before - this goes back to the basics of how bracing works- with wider bracing intervals, the leg wood has to be stronger (hence heavier); with tighter bracing intervals, leg strength can be less (lighter). What matters in picking the optimum within this spectrum is the combined weight of leg wood and bracing wood. Its pretty easy, using the inverse square table and graph of buckling strength vs stick weight, to roughly calculate approximate tower weight at different bracing intervals.

I'm curious, was it a B or C tower? Wonder how much it weighed.....

[SOCoach]

Ours was a B tower, no bonus. 1/8 legs and 1/32 bracing. X bracing, 5 X's on the base and 5 on the top. The kids have been getting anywhere from low 6 to low 7 gram towers holding 10 to 11 kilograms. Decent score, but it probably won't win our region. They haven't been using the tension pieces along the bottom though so I'll encourage them to try that. Do you think we should go 1/20 or 1/32 for that tension piece?


Are those buckling tables you spoke of in the previous post somewhere in this forum? I'd like to look at them.

[Balsa Man]

Ours was a B tower, no bonus. 1/8 legs and 1/32 bracing. X bracing, 5 X's on the base and 5 on the top. The kids have been getting anywhere from low 6 to low 7 gram towers holding 10 to 11 kilograms. Decent score, but it probably won't win our region. They haven't been using the tension pieces along the bottom though so I'll encourage them to try that. Do you think we should go 1/20 or 1/32 for that tension piece?


Are those buckling tables you spoke of in the previous post somewhere in this forum? I'd like to look at them.

The inverse square table and plot of BS vs stick weights are in a post on pg 6 of the Towers B/C thread.

As posted before, most weight efficient approach to the tension strips is 1/64th x 1/16 from fairly high density sheet (8-9gr 3x36x1/64). Higher density helps prevent shear failure in the joint (whatever size).

Increasing X (buckling) strength can be done by either increasing density, increasing thickness, or by adding stiffener strips at the middle of the strips. Its just a question of which approach is lightest...

The big thing that jumps at me in your description is not going for the 29cm circle bonus. At 5kg bonus, it is.....so worth it. 6.2 carrying 11kg = 1774.... As an example, we just did first build test (B tower, circle bonus); 6.77gr, carried 11.3kg)=2407.