I am constantly asked whether Balsa is better than Basswood. Here are some of my notes comparing the two woods.
Balsa is cheaper and more available than Basswood.
This means you can buy more wood and build more bridges for the same amount of money. Hobby and craft stores seem to sell a larger selection of Balsa (Michaels, etc). If money is short but you want to build a lot of bridges or plan to constantly improve your designs as you see how they preform, you might lean towards Balsa. I see this as a minor factor, however.
Balsa is stiff, while Basswood will bend.
This is a broad generalization that I have found. Obviously, there are cases where isn’t true. But generally, Basswood is easier to bend than Balsa. More than that, I can create a sharper bend with Basswood. This makes Basswood a good choice if you are building an arch bridge. You can definitely still bend Balsa, but just not as easily as Basswood.
Because Balsa seems more stiff in general, you might consider using Balsa in certain cases where you want low mass/density but need the wood to be thicker (more cross section) to resist bending and eventually breaking under compression forces. Example: You can have two sticks of wood, one Balsa and one Basswood that are the same weight. The Balsa piece ends up being thicker than the Bass. This sometimes gives you an advantage.
Basswood won’t rip off at the joints as much as Balsa.
Sometimes the face grain of Balsa will tear away at the joint. This is especially the case the lower the density of the Balsa wood. You can compensate for this by the type of glue you use as well as increasing the surface area for the joint. But these changes might also add mass to your bridge, which in high level competition is a factor to keep in mind.
Balsa comes in a wide range of densities.
This is a pro and a con of Balsa wood. You don’t always know how strong the wood is that you are getting, but if you know what you want, you can build with very specific wood densities. Basswood usually comes in a much smaller range of density, which means you can be more consistent with Basswood. For beginners looking to have consistent results from building multiple but similar bridges, Basswood makes this easier. Balsa, because it is inconsistent, takes more work and careful inspection to get similar results from bridge to bridge.
Balsa is less likely to be the same strength throughout its entire length.
If you buy a piece 24 inches long, it may be stronger at one end then the other. Again, this points to the inherent inconsistency of Balsa wood, and it makes high level work with Balsa harder. You can hold up a sheet of Balsa to the light and see the changes in density (or potentially changes in thickness if the sheet was not manufactured well). You just have to be careful and make sure you know what you are working with. Basswood also has this problem, just not to the same degree.
Balsa will sand easier, but Basswood won’t crush.
You can squeeze a piece of Balsa, and totally deform it. You can also deform Balsa simply by cutting it with a dull knife. A “squeezed” piece of Balsa is weaker, because the internal structure is messed up. You need to be more careful when working with Balsa.
Balsa changes weight with changes in humidity more than Basswood.
Many times I have had a Balsa bridge lose weight after setting it in the sun for a couple of hours. Of course, the opposite is true. Balsa bridges will gain weight after humid days. Basswood does not change so much with the weather. Some people go as far as to put their Balsa bridge (or even Bass bridge) in an oven on low heat to dry it out. I can’t really recommend that practice, but it goes to show you that the weight of a bridge is affected by water, and sometimes those 0.01 grams really count. I definitely see no problems with using rice or silca gel packets to help prevent moisture accumulating in your bridge.
For the same mass, Basswood pieces must be smaller.
Usually, this means that a Basswood joint will have less surface area for glue than a Balsa joint. That means you may have to use stronger glue with Bass, which might add weight.
As you can see, there are many factors that play into which wood is better. But it comes down to this:
Both Balsa and Basswood are great choices for a strong and efficient model bridge. Each one has its strengths and weaknesses that must be harnessed in order to get maximum performance, but I don’t think one is inherently better than the other. I know folks on both sides, some who only use Balsa and some who only use Basswood. There are others who always mix them. Some might use Balsa for key compression members, or something like that.
I will say this, however: I think Basswood is easier for beginners who plan to build multiple bridges.
I will also say that it takes time to learn each wood, and you can take you level of care and wood selection very far. The most advanced builders in Science Olympiad spend most of their time choosing their wood, not designing or building.
Don’t forget about the other woods.
Birch, Spruce, Aspen, Pine, and others are all alternatives to Balsa and Basswood. But I guarantee that you won’t be able to find them like you can Balsa. They will cost more money. But don’t immediately rule them out, you may want to play around with them, and check out their qualities. Let me know what you find out.