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One more REALLY important thing. I was rereading the rules, and in section 3.c it says that the center of the loading block measured horizontally from the face of the testing wall MUST be between 40 cm to 45 cm. Does that mean that the length of the boomilever, as in basically the compression cord in a standard tension boomilever MUST be 40 to 45 cm long?

Thanks Balsa Man.
I read your post several times and I think I understand most of it.
I tested my boomilever today, and it broke at 7.5 Kg. If I build box-beam compression members, with the same size as my solid members (as you mentioned above), will they be able to hold double the weight of the solid ones? Is there a way to figure out the size of my needed box-beam other than using softwares/ program?

UQOnyx wrote:One more REALLY important thing. I was rereading the rules, and in section 3.c it says that the center of the loading block measured horizontally from the face of the testing wall MUST be between 40 cm to 45 cm. Does that mean that the length of the boomilever, as in basically the compression cord in a standard tension boomilever MUST be 40 to 45 cm long?

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1) reading/re-reading rules; excellent= gold star. Rule # 1 is KNOW the rules. Only then can you begin to figure out how to maximize performance while staying within the rules.

2) It means - exactly what it says, center of load block betw,een 40 and 45. Pick your distance- anywhere in between.
So, think about it- read back a few posts. So are you better off at 40 or 45, or somewhere between, and why? 10 point open-book quiz.....

Whatever that distance is,btw, the compression member(s)/cord(s) need to be at least a bit longer than block centerline, just to hold the block, and that's fine, rule only speaks to distance to block center past the block center. For instance, ours is 1.5cm. You also want the tension member to join compression member(s) at, or VERY close to that "load distance".

jma wrote:Thanks Balsa Man.
I read your post several times and I think I understand most of it.
I tested my boomilever today, and it broke at 7.5 Kg. If I build box-beam compression members, with the same size as my solid members (as you mentioned above), will they be able to hold double the weight of the solid ones? Is there a way to figure out the size of my needed box-beam other than using softwares/ program?

You're welcome.
Absorbing new knowledge, for most folk, doesn't happen instantly/at first reading. Re-read, including the links more, you will understand more. Get a feel- take some balsa, put it together- feel the stiffness.

I assume your failure at 7.5 was compression member failure in the Y (vertical) plane, again? You change anything to go from 6.2 to 7.5?

Ok, to your question. Stiffness as I explained depends on both E & I. E depends on density; higher density, higher E. I is essentially the cross-section. So, assuming we're talking stiffness in the Y- plane. What increase in stiffness you will depend on the density of wood you use. If you do a box- same dimensions- with wood of the same density, it'll be a bit lighter, and almost as strong. If you then increase the density, weight goes up, but so does strength. (of the side walls), it'll weigh a bit more than solid, but be significantly stronger. What it'll take to double, you'll have to experiment. If you, as suggested, increase the Y- dimension, and the density, strength will go up significantly, weight up a bit.

You can do the calculations by hand- not easy. Yes, there are software packages that could do easier. Quickest way- start increasing E & I, and testing.

The link that had powerpoint slides on it gave a very detailed, complex answer to your question, I'll attempt to simplify.

As balsaman said, the load that the compression members can take is directly related to the 'configured stiffness' of the beam(the technical term is second area moment of inertia). For most simple beams with parts at right angles(not like circular members or tubes) they can be broken into parts that are simple rectangles(for example, an I-Beam is a central support column, two flanges at top and bottom, and two other areas of nothing).

Now let's look at your compression member...

So I won't worry about the horizontal buckling of the member, it's most likely strong enough in the regard. The vertical buckling however is something to look at...
(This is a vast simplification, in reality your current member would take a lot more math to figure out, but this should do for comparison.)

According to the PDF balsaman posted, the formula for calculating this stiffness that comes from the configuration of the beam(for rectangular pieces, like this) is When applied to your compression member(since the bracing serves only to connect the two members in the horizontal plane) We get(1.968 is the width of the beam in inches) The units really aren't that important, we're just comparing this member to a box-beam.

So your current compression member has a 'configured stiffness' in the y-axis of 0.0004883.

A box beam member is a little different beast... To calculate a box-beam, we can't simply use the equation like we did before, we have to subtract the part the doesn't actually exist. This gives us(Assuming the box is made of 1/16" side walls)

So the 'configured stiffness' of the same size box beam would be 0.002262, or 463% the strength of the solid member, because it's so much wider, and the width is part of the equation.

By Euler's buckling theorem, max force before buckling(the critical load) is proportional to not only the 'configured stiffness' but also the elastic modulus(E, which itself is related to density), and 1/exposed column length squared. In this case, the increase in 'configured stiffness' by switching to a box beam seems to be plenty to give you the increased max load(7.5 to 15kg), in fact, you could probably even use lower density(lower E) balsa and get away with it... but that'll take testing also.

This is what happens when I write long things.. I kinda loose track of what I meant to do... but I hope that more thoroughly answers your original question, and I hope you understand the math... I can be pretty bad at explaining things at times Just remember that we don't have enough time or calculus skills to do the exact math, so this is all just rough comparisons...

Oh, and if someone could look over the theory and make sure this all makes sense... first time I did it I assumed his member was a solid piece 1.968x.25 inches in cross section, and I got that the box-beam was 88% percent the strength of the solid member, which makes sense, but, it doesn't make sense to model his member as a solid one and the box member as a tube, so I calculated his as a tube with a wall thickness on the long sides of 0(which is the same as 2 solid members on the sides), and got that the box beam was significantly stronger... so I don't know what I'm doing wrong(if anything at all)... Oh, and for some reason the math tags aren't working...

Sorry for double posting, but has anyone else had trouble getting the proper washers? None of the contests I've been to have had 3/4" od washers, but I'm getting my own testing rig ready and went to the local big box hardware store and the standard 1/4" flat washer is 3/4" od...

iwonder wrote:Sorry for double posting, but has anyone else had trouble getting the proper washers? None of the contests I've been to have had 3/4" od washers, but I'm getting my own testing rig ready and went to the local big box hardware store and the standard 1/4" flat washer is 3/4" od...

I have heard that there was a rule clarification or FAQ the last time this event was run that stated something about a different size washer being ok. Does anyone know if this is true or has more information on it?

Also, can we bring our own washers to the competition? I know the rules state that the event supervisor must provide all the materials needed for testing, but a smaller washer size can significantly change the strength of the connection.

soccerkid812 wrote:

iwonder wrote:Sorry for double posting, but has anyone else had trouble getting the proper washers? None of the contests I've been to have had 3/4" od washers, but I'm getting my own testing rig ready and went to the local big box hardware store and the standard 1/4" flat washer is 3/4" od...

I have heard that there was a rule clarification or FAQ the last time this event was run that stated something about a different size washer being ok. Does anyone know if this is true or has more information on it?

Also, can we bring our own washers to the competition? I know the rules state that the event supervisor must provide all the materials needed for testing, but a smaller washer size can significantly change the strength of the connection.

Wow, I don't remember anything like that. I think I would, because our design used/depended on that dimension.....
Because the rules clearly, explicitly say "ES will provide... 3/4" washer, I would a) first thing, as soon as check-in opens- before anyone tests(!), go see, check, make sure there were 3/4" washers there to be used, b) have, bring a few washers meeting the rule c) if the ES did not have washers meeting the rule, point out the rule, state your team had designed to it, give them the correct washers, and confirm you and anyone else who wanted to could use them, and d)(politely) scream bloody murder if an ES didn't have washers meeting the rule, had some other size, and said you you had to use them instead of washers meeting the rules... Actually, tell ES you're protesting- going immediately to the tournament director -explain ES is clearly and absolutely in error, you've provided washers that do meet the rule, nobody has run yet (cutting off the"well, some have already run with what I brought" argument . You will prevail. Showing up with washers that don't meet the rule size would be lust like showing up with 1/2" bolts, or a 3.5cm x 3.5cm load block.....

Off my soapbox....

Balsa Man wrote:

soccerkid812 wrote:

iwonder wrote:Sorry for double posting, but has anyone else had trouble getting the proper washers? None of the contests I've been to have had 3/4" od washers, but I'm getting my own testing rig ready and went to the local big box hardware store and the standard 1/4" flat washer is 3/4" od...

I have heard that there was a rule clarification or FAQ the last time this event was run that stated something about a different size washer being ok. Does anyone know if this is true or has more information on it?

Also, can we bring our own washers to the competition? I know the rules state that the event supervisor must provide all the materials needed for testing, but a smaller washer size can significantly change the strength of the connection.

Wow, I don't remember anything like that. I think I would, because our design used/depended on that dimension.....
Because the rules clearly, explicitly say "ES will provide... 3/4" washer, I would a) first thing, as soon as check-in opens- before anyone tests(!), go see, check, make sure there were 3/4" washers there to be used, b) have, bring a few washers meeting the rule c) if the ES did not have washers meeting the rule, point out the rule, state your team had designed to it, give them the correct washers, and confirm you and anyone else who wanted to could use them, and d)(politely) scream bloody murder if an ES didn't have washers meeting the rule, had some other size, and said you you had to use them instead of washers meeting the rules... Actually, tell ES you're protesting- going immediately to the tournament director -explain ES is clearly and absolutely in error, you've provided washers that do meet the rule, nobody has run yet (cutting off the"well, some have already run with what I brought" argument . You will prevail. Showing up with washers that don't meet the rule size would be lust like showing up with 1/2" bolts, or a 3.5cm x 3.5cm load block.....

Off my soapbox....

The whole different washer size thing is probably not true then. I just wanted to double check. Thanks!

EDIT: It might have been a local clarification.

I didn't get it. How is a 3/4" washer supposed o fit around a 1/4" bolt?