Designs

[LKN]

thsom, I can't be exact because I haven't used any basswood at all this season. But, from what I can recall from last season is definitely over 2g, probably closer to 3g. No one has answered, so I thought I would give you an idea. Don't take my word for it though! If you have bass now, what is your 24" bass sticks weighing in at? Bass is very consistent compared to balsa, so I would say the average of the sticks you have would be very close to "baseline".

[Balsa Man]

OK, here are some numbers, and a bit of context.

The short answer for 24” 1/8 x 1/8 bass is in the range of 2.6 to 2.7 grams. For 24” 1/8 x 1/8 balsa, in the range of 1.27 to 1.33.

The simple “average” weight is pretty meaningless, in terms of design. These #s above are for the most common – the midpoint of the “common range.”
The range of density of balsa is, as LKN notes, much wider than that of bass. The Specialized Balsa website shows the weights of 1/8 x 36” they sell. Adjusted to 24”, that range is from 0.5 to 3.1 gr. If you look at the prices per stick, you see that at the high and low ends a significant price jump. What that reflects is the rareness of the really dense, and really light pieces. Out of any log, most will fall in a narrower range, and as you get toward the extremes, the number you’ll get falls way off. The mid zone (for 36” sticks) is from 1.3 to 1.6, which for 24” sticks is from 0.87 to 1.73, and the middle of that zone is 1.9 to 2 (for 36”); 1.27-1.33 for 24”

Nobody that I know of sells weight-graded bass; the only way to get specific densities is hand picking through a bunch of sticks. As I’ve posted before, living 20 miles from Specialized Balsa, I have the luxury of being able to go down and do that. Have picked through and weighed….a few hundred sticks. What I’ve seen, adjusted to 1/8th x 24” is this. 75 to 80% will fall between 2.5 and 2.8gr, with the majority between 2.6 and 2.7. The outer limits go from 3.3 down to about 2.2- two or 3 sticks out of a hundred getting out to the limits.

So, what does this mean for a tower? Column strength, at a given cross-section, is a (roughly linear) function of density, and of cross section, and an inverse square function of exposed column length – the braced interval.
So, the design question, for balsa or bass, is what density, at what cross section and braced length gets you the strength needed. 1/8 bass is…..gross overkill in terms of use in a tower- much heavier and stronger than needed; no way it can be used to get to a competitive weight.

3/32nds bass, on the other hand, is a competitive option. The cross section of 3/32nds is 9/16ths of 1/8 x 1/8th, so the range in weight of 3/32nds bass at 24” is from about 1.3 to 1.85gr. The middle zone is 1.4 to 1.6gr. As I’ve posted before, we have column strength test data on 3/32nds. At the upper end, the heaviest 3/32nds stick I’ve found (at 1.95gr at 24”) carried 11 kg at a length of 8.25cm; a piece at 1.4 carried 7.25kg, and a piece at 1.5 gr carried 8kg.

In a 4-legged C-Div tower, the chimney legs need to each carry 3.75kg; in the base, its about 4.11kg; applying a 15% safety factor, that’s 4.32 kg and 4.73kg. Setting up an inverse square table from these data points, we see that for a chimney, 1.4gr/24” at a braced interval of 11 cm (and 5 11cm sections get you a 55cm chimney) a strength of 4.75kg. Looking at the base, at 17cm, 1.5gr/24” carries about 2kg (way under what you need); putting one ladder in at midpoint, strength jumps to about 8kg (way over what you need). So, light 3/32nds bass is quite viable for a chimney, and even the lightest bass is overkill in the base….

So, what about 1/8th bass in the base? Could it work with no column bracing? No test data, but the cross section of 1/8th is 4/3 of 3/32nds, which suggests, for 1/8th bass at the density of a piece of 1.5gr/24” (which would weigh 2.7gr/24”), a strength of about 2.8kg at 17cm; even going to the density of 1.9+ gr/24 (which would be 3.3gr+/24” for the 1/8th) we only get to 2.75kg- well short of what’s needed.

[thsom]

OK, here are some numbers, and a bit of context.

The short answer for 24” 1/8 x 1/8 bass is in the range of 2.6 to 2.7 grams. For 24” 1/8 x 1/8 balsa, in the range of 1.27 to 1.33.

The simple “average” weight is pretty meaningless, in terms of design. These #s above are for the most common – the midpoint of the “common range.”
The range of density of balsa is, as LKN notes, much wider than that of bass. The Specialized Balsa website shows the weights of 1/8 x 36” they sell. Adjusted to 24”, that range is from 0.5 to 3.1 gr. If you look at the prices per stick, you see that at the high and low ends a significant price jump. What that reflects is the rareness of the really dense, and really light pieces. Out of any log, most will fall in a narrower range, and as you get toward the extremes, the number you’ll get falls way off. The mid zone (for 36” sticks) is from 1.3 to 1.6, which for 24” sticks is from 0.87 to 1.73, and the middle of that zone is 1.9 to 2 (for 36”); 1.27-1.33 for 24”

Nobody that I know of sells weight-graded bass; the only way to get specific densities is hand picking through a bunch of sticks. As I’ve posted before, living 20 miles from Specialized Balsa, I have the luxury of being able to go down and do that. Have picked through and weighed….a few hundred sticks. What I’ve seen, adjusted to 1/8th x 24” is this. 75 to 80% will fall between 2.5 and 2.8gr, with the majority between 2.6 and 2.7. The outer limits go from 3.3 down to about 2.2- two or 3 sticks out of a hundred getting out to the limits.

So, what does this mean for a tower? Column strength, at a given cross-section, is a (roughly linear) function of density, and of cross section, and an inverse square function of exposed column length – the braced interval.
So, the design question, for balsa or bass, is what density, at what cross section and braced length gets you the strength needed. 1/8 bass is…..gross overkill in terms of use in a tower- much heavier and stronger than needed; no way it can be used to get to a competitive weight.

3/32nds bass, on the other hand, is a competitive option. The cross section of 3/32nds is 9/16ths of 1/8 x 1/8th, so the range in weight of 3/32nds bass at 24” is from about 1.3 to 1.85gr. The middle zone is 1.4 to 1.6gr. As I’ve posted before, we have column strength test data on 3/32nds. At the upper end, the heaviest 3/32nds stick I’ve found (at 1.95gr at 24”) carried 11 kg at a length of 8.25cm; a piece at 1.4 carried 7.25kg, and a piece at 1.5 gr carried 8kg.

In a 4-legged C-Div tower, the chimney legs need to each carry 3.75kg; in the base, its about 4.11kg; applying a 15% safety factor, that’s 4.32 kg and 4.73kg. Setting up an inverse square table from these data points, we see that for a chimney, 1.4gr/24” at a braced interval of 11 cm (and 5 11cm sections get you a 55cm chimney) a strength of 4.75kg. Looking at the base, at 17cm, 1.5gr/24” carries about 2kg (way under what you need); putting one ladder in at midpoint, strength jumps to about 8kg (way over what you need). So, light 3/32nds bass is quite viable for a chimney, and even the lightest bass is overkill in the base….

So, what about 1/8th bass in the base? Could it work with no column bracing? No test data, but the cross section of 1/8th is 4/3 of 3/32nds, which suggests, for 1/8th bass at the density of a piece of 1.5gr/24” (which would weigh 2.7gr/24”), a strength of about 2.8kg at 17cm; even going to the density of 1.9+ gr/24 (which would be 3.3gr+/24” for the 1/8th) we only get to 2.75kg- well short of what’s needed.

Funnily enough, you knew exactly what I wanted to try to do with the basswood which was using it in the base with barely any bracing. Well, there goes that idea!

[Balsa Man]

Yeah, figured that must be where you were going. Considered & ruled out early in the design process; now you know why.

[LKN]

Since we are on the subject of of using bass wood verses balsa, can bass be as efficient or more efficient than balsa in a rectangular base with T-shaped legs? I have yet to finish the base and test it, but I am using 1/32" x 3/16" bass and it definitely feels stronger than the balsa T-legs I have been working with.

The specs
1/32" x 3/16 cross section
24" length
All of the bass used in the T-legs weigh 1.0gm per 24" strip

Any thoughts? Has anyone else tried this? It appears that the bass strip (the vertical component of a T shape) on the inward side towards the bracings are much stronger than the preloading made by the T-leg in the longer plane. This is somewhat problematic, because I am using 3-alternating Z bracing on the narrow side of the base, and the stronger component is in direction of this pattern. This would not eliminate too much weight, because I don't feel comfortable with only 2 tension diagonals on the base.

[Balsa Man]

Sorry, LKN, no data or experience, on either composite T configuration, or Z-bracing...... The modulus of elasticity of bass is higher than balsa, and work with solid bass indicates a slight weight advantage (bass to balsa) in compression (column) loading. Interested to hear how it works

[LKN]

I'll let you know how it works out. For balsa, How much weight savings were you getting when you use 1/16x1/64 for the tension bracings? Is it about 1/4 the weight of 1/16^2 that you used for "higher density" bracings? I plan on using it in the chimney and the "loop" around the base, and I was wondering if you have a ballpark guess what weight savings should be. We are cutting our 1/64th strips later today.

[Balsa Man]

Yup. The "right" density range is around that of 0.9gr/36 1/16th square, so at 1/64th, a quarter of that; ~2.25gr/36"

[thsom]

Yup. The "right" density range is around that of 0.9gr/36 1/16th square, so at 1/64th, a quarter of that; ~2.25gr/36"

However, at normal density isn't 1/16 squared around .3-.5 grams?

[mrsteven]

Yup. The "right" density range is around that of 0.9gr/36 1/16th square, so at 1/64th, a quarter of that; ~2.25gr/36"

However, at normal density isn't 1/16 squared around .3-.5 grams?

you have to specify length... I think you mean 24'' though
Ya I mostly find .3-.5 with occasional .1 or .2 and a few dozen .6+