Balsa or Bass
Posted: September 16th, 2010, 7:53 am
Should me and my partner use balsa or bass? We're thinking of bass but we would still like some advice.
Science Olympiad Student Center
To this is why, as has been said before, you must be very careful in massing your pieces, checking grain, strength, stiffness, straightness, and all other aspects. Essentially: make sure you've got good pieces before you start building your competitive structures! As the density increases you'll likely find less variation, but once you get down to the extremely thin, small and light ones you'll have to carefully inspect every one! Good preparation and materials is essential to creating a good structure.old wrote:My point is that you can't simply go out and buy a certain density of balsa and think that it is going to be even remotely close in strength to other similar density pieces. You will even find that different portions of a single piece of balsa will vary in significantly in strength.
Absolutely! We have found that the density range in a single stick of balsa can vary by 2:1 or more and the tensile strength and modulus can vary much more than that. If you cut your sticks from boards/planks the variation can be even greater. Pieces cut from one part of a board, especially lower density boards, can vary through almost the entire range of densities of balsa wood (<4 to >20 lbs/ft cu). When we (my partners and I) were building bridges and we decided that we wanted to win at nationals, we had to weigh and measure the significant characteristics (depending on whether the piece was in tension, compression or shear) of every single piece we used. We threw out 20 pieces for every one that we used. Some pieces were like magic, they were so strong and light, while other pieces were almost completely without strength (sometimes almost crumbling in our hands). We cut all our pieces from boards, to save money, so the cost of individual pieces was extremely low. I would guess that we paid less than $100 for all the wood we used for 4 years of SO structures (dozens of them), but the work was oppressive. I don't even want to think about how much of my time was spent sorting through mountains of sticks or balsa. One thing that you really need if you want to be competitive is a very sensitive scale so that you can see the difference in mass of very small pieces. 0.01 gram sensitivity is the minimum that is useful but 0.001 or even better would be useful. The only problem with the extremely sensitive scales is that they require an environment totally free of air currents to read below about 0.01 gram differences. Even at 0.01 gram sensitivity you will easily see the air currents from people gently moving around the room. Using such a scale in a room with an A/C going would render it useless. Most of these scales come with a glass enclosure to keep out air currents but often your pieces will not fit inside so you have to find a way to minimize the air currents around the scale.lllazar wrote:Ive always wondered - is there a significant variance of characteristics in a single stick of balsa - for example, can the density in a 3ft x 1/4 in x 1/4 in stick of balsa be significantly different from region to region?