The concept of stiffness is one I have been delving into this year. The stiffness of the wood you use directly translates into the strength of your bridge. Density no longer matters much, and is almost not worth calculating unless you can find a relationship between density and stiffness. I have not been able to yet. There are too many other factors.
The ideal piece of wood has a high stiffness to weight ratio. It really does you no good to have a very heavy piece of wood that also happens to be very stiff. For almost every competition I have been in, one needs to build light while keeping strength.
Another interesting number is the stiffness coefficient of your wood. Basically this is a comparison number. Generally accepted comparisons are:
A stiffness coefficient of:
<90 = poor
90-100 = average
110 = good
>120 = super
The calculations of the stiffness coefficient are slightly complicated, so I will not explain them here. Thankfully, brilliant people have done the work for you and created programs that will make the calculations automatically. You can find a calculator you can download to your computer.
Here are some more good sites about Balsa stiffness:
Testing the Stiffness
The accepted method of testing the stiffness of wood is to take one end of a balsa sheet, and push it down onto a scale. You stop pushing when the sheet when the numbers on the scale stop going up. I have made a video tutorial so you can watch this process:
Now you can do your own stiffness testing. Have fun 🙂
Buying Balsa Wood Sheets
Balsa Sheets from Midwest Products
Midwest Products has a wide range of Balsa wood sheets available at amazon. You can buy their sheets in lots of sizes and in different amounts.