Objective: To build a boomilever with the greatest efficiency.
Basic Design:
There are two basic types of boomilevers. First is the tension boomilever, where the tension chord is longer than the compression chord.
Second there is the compression boomilever, which is the opposite of the tension.
It is generally accepted that the tension boomilever is inherently better than the compression. This is because the strength of wood in compression decreases per length unit the longer the piece of wood is. However, wood holds the same strength in tension no matter the length of the piece. In the compression boomilever, the compression chord is longer than in the tension boomilever.
Height:
The “taller” the boom, the less force is put onto the top and bottom chords. This allows you to decrease the weight of these chords. However, the taller the boomilever, the longer the truss pieces must be, which increases the weight. The goal is to find a balance. My suggestion is to not go any shorter than 3-4 inches. I do not, however, believe you need to use the entire 20cm. Plug your design into the Bridge Designer and see how the numbers change as you increase the height of the boomilever.
Truss:
Most boomilevers, unlike the two demo examples, have what is called a truss. There are three standard trusses, the Warren, Pratt, and Howe. Each of these trusses have different ways of spreading out the load, and it is worth playing with the bridge designer to see how each of these work. Remember the KISS principle, and keep the truss simple.
The basic idea behind a truss goes back to the tension verses compression boomilevers. A piece of wood loses strength as the length increases. Thus the truss serves to divide the compression chord up into smaller sections, greatly increasing the amount of load before buckling. Basically the truss keeps the compression chord from bending and twisting. As a starting place, I would try and break the bottom chord into sections 2-3 inches long.
Base:
Most boomilever bases I have seen through the years are way overbuilt. Anybody should be able to make a base under a gram, and less than .5 of a gram if you are good. Obviously, you will not be able to achieve this if you are using both bolts to attach your boomilever to the testing wall. I honestly cannot see any reason to use both bolts. If you are worried about stability, I say look at the bridges of the past couple years. Most were no wider than 5cm and had no problem with stability. You will cut a lot of weight by only using one bolt, not only on the base but also with the lateral bracing.
Lateral Bracing:
Don’t let the term turn you off. Lateral bracing is simply a horizontal “truss” connecting the compression chords of your boomilever. Like any normal truss, there are various designs you can use. And the goal is the same for lateral bracing as is a normal truss, to keep the compression chords from bending and twisting.
You generally do not need a lateral truss connecting the tension chords of your boomilever. The tension chords do not bend, and only twist under very extreme force. If anything, you can use a couple perpendicular pieces to connect them.
Glue:
Glue weight can be a major weight factor, but it doesn’t have to be. You can easily use 1-2 grams of glue (or less) on a 10-gram boomilever. A lot of people use CA glue, or ambroid. I use an interesting glue called Weldbond found at Ace
Hardware. Find a glue you can use well and stick with it. Make sure not to use too much.
Testing:
It is generally a good idea to test your boomilever before competition. Of course, you never test unless you have enough time to build another one before competition. Testing a boomilever does not weaken the structure in most cases. The key to know if your boomilever is damaged is to listen while you test. If you hear any pops, cracks, or groans then the boom is probably in need of repair. However, if you do not hear anything, your boomilever should be as good as new.
One method is to test the boomilever to the point where you like the efficiency score and stop without breaking the boomilever. I definitely think it is a good idea to test some boomilevers to failure, but you can use this method if you are nearing a competition. Testing to failure lets you know where the structure is weak, and gives you the knowledge to make the boomilever better. Typically, the weak spot on boomilevers is the connection to the base.
Boomilever is a fun event; make sure you don’t get caught up in trying to win so hard that you stop enjoying it. Build a lot, test a lot, and document everything.
Do you have any tips for ensuring the connection at the top of the boomilever to the test wall is strong enough?