Scioly.org

Umm quick question, I was looking at the Boomilever wiki, and I read this:

balsa wood is better at compression then tension.

I remember Balsa Man (May he rest in peace) and a couple of others talking about how balsa wood is much better in tension than compression last season (2017-2018). So after seeing that on the wiki... I'm kind of confused...

Is balsa wood better at tension than compression, or is it the other way around?

Any material is better in tension than in compression. I think that the main reason is that it can buckle in compression but it cannot in tension.
For boomilever, you need to have good joints when in tension.
For a brace that is in compression, something like a square cross section is the best for a given weight of balsa. For a brace in tension, I like to use a thinner-wider piece for better gluing strength.

I competed in boomilever back in middle school and was typically getting 1300-1400 with a "normal" boomilever design, the score dropped to about 1200 at States, but it was still much higher than 2nd place. The score dropoff that Windu mentioned at nationals was also similar to most state competitions, though not as drastic. Hopefully, there are changes to the rules to make the event a little more interesting and competitive at the state and regional levels as there was pretty big learning curve for boomilever that wasn't really there for towers or bridges. Boomilever was a fairly complex problem the first time around so I'm hoping there's a twist this year and not just the standard boomilever rules.

As for winning scores, it really depends on what changes there are to the rules, though based on the trend for bridges/towers, the first year in rotation has been a standard structure with a significant change the second year. If the rules stay the same as 2014, I think scores at nationals will have a similar dropoff with a few teams possibly scoring much higher than the rest. Scores might be higher in general though as there are probably other people like myself who competed in boomilever in Div B and are now in Div C.

What design would u recommend? simple or trianglular?? also, if triangular, how would u support the bolt?

And one more thing: r u guys div B, C, or grad? This is my first boomilever stuff, but it seems that with the stuff u guys r saying im okay at it.
in my opinion, this is like 50% easier than towers.

triangulator wrote:What design would u recommend? simple or trianglular?? also, if triangular, how would u support the bolt?

And one more thing: r u guys div B, C, or grad? This is my first boomilever stuff, but it seems that with the stuff u guys r saying im okay at it.
in my opinion, this is like 50% easier than towers.

To clarify, I'm in Division C, but I competed in Boomilever 4-5 years ago when I was in Division B. The difference between div B and C was fairly standard. Div B had 5 cm of extra height to work with, one of the major factors in the strength of cantilever bridges.

When I competed, I only used the standard boomilever style, though that's probably because I've never seen what a triangular prism boomilever looks like. However, it does seem as though there are references to this design in the 2014 boomilever forums (haven't found pictures yet though).

Trying to compare the easiness of towers and boomilever is fairly trivial. Boomilever is much easier to construct as every joint is flat and flush 90 degrees unlike towers, where issues in construction will be the primary downfall.

However, as someone who enjoys designing more than the actual construction of structures, I have mixed opinions on both events.

Towers:
- All competitive towers basically look the exact same, so its really easy to start off with a good efficiency.
- The forces that act on a tower are extremely simple, so its extremely easy to calculate the forces on certain parts of the tower, making it extremely easy to modify designs with consistent increases in efficiency.
- long build time, around 5-6 hrs for this years competition, 3-4 hours for 2016-2017
- I typically only changed bracing patterns once I found the optimal measurements to barely pass the bonus circle and 8cm circle, so I only had to build one jig.

Boomilever
- Because boomilever does have a relatively more complex group of forces acting on it, it is much harder to come to a conclusive "optimal"
design, which results in people coming up with drastically different solutions and with different types of wood.
- I changed the general structure and measurements of the compression chord fairly frequently, so I had to make a new jig for each fundamentally different design. (On some designs I experimented with would thicker than 1/8 x 1/8)
- much simpler to construct, while proper construction techniques are crucial (joints), not as much effort is required to ensure a good build.
- definitely much less build time around 2-3 hrs for a competition boom
.

Here is a hint for next year's boomilever.
Devthane 5 (not 3) is a fantastic adhesive for connecting the tension members to the top "bar". It is moderately flexible but rather more difficult than quick set epoxy. It is very sticky and getting a small amount in just the right places is no fun. But that joint will hold nearly any load and flex that you can put on it.
It is expensive and not readily available except on line. Amazon or Ebay had the small double tube for 8 or 9 dollars which is like half price. A "gun" would be nice but it is not cheap for a 1 or 2 year working.
I refrigerate it and it has lasted well until I threw it away this year.
It is a 2 component polyurethane.

In a simple boomilever, where the piece in compression is perpendicular to the testing jig, isn’t the force of compression on the compression piece twice that of the force downward applied by the bucket? So 30kg at full load?

Also for the tension pieces would you guys normally make those out of dense balsa or basswood or regular balsa... what affects the strength of wood under tension?

pajobubo wrote:In a simple boomilever, where the piece in compression is perpendicular to the testing jig, isn’t the force of compression on the compression piece twice that of the force downward applied by the bucket? So 30kg at full load?

Also for the tension pieces would you guys normally make those out of dense balsa or basswood or regular balsa... what affects the strength of wood under tension?

I believe the compressive force (as well as the tensile force) will be determined by the angle between the 2 members and the load imposed.
Think of the extreme case. If you had a boom with compression beams extending to a wall, and tension chords at 89 degrees to it, connect to... lets say a cloud, there would be very little compression force. nearly all of the load will transfer at nearly 100% to the tension rod.

Now think of the opposite extreme... Compression beam back to the wall, but the tension cord is now a cable set at 5 degrees to the beam, anchored to the wall and around a pulley at the end of the beam then continuing down to the load. You will have immense pressure on both the compression beam and the cable.

What would a jig for boomilever look like?

Can anyone tell me the format of a build report for Boomilevers. My school is having testoffs and require a build report. It would be a great help.