Boomilever B/C

[scioly2345]

The 1/16 square basswood tension members I use can never seem to get full load. What density tension members have you guys seen get full load consistently? Also I remember reading something about using a luggage scale to test the tensile strength of tension members. Does anyone have any specifics on how to do that? Thanks

Where exactly on the boom where does your tension break?
I would say if they’re splitting (likely not bc bass tension at any size is really strong), but if that’s happening then go up to 1/8 by 1/16 bass tension but with a LOT of balsa lamination where the hook is. A common problem that comes with tension breaking is the lack of lamination where the piece at the hook and the two tension pieces are connected. I recommend gluing the actual two tension pieces onto the piece by the hook with diluted wood glue (green titebond bottle), then after letting that dry for at least an hour, further laminate the area with balsa and CA. You wanna give the tension more wood to hold onto by laminating. I definitely recommend switching to 1/8 by 1/16 bass though. As for strength testing, I wouldn’t worry about density. Just make sure the two tension members weigh the same and you should be fine.

[Tendan]

The 1/16 square basswood tension members I use can never seem to get full load. What density tension members have you guys seen get full load consistently? Also I remember reading something about using a luggage scale to test the tensile strength of tension members. Does anyone have any specifics on how to do that? Thanks

To test strength of the tension members, I would assume you build the top as usual, meaning the hook mount and tension arms, but instead of gluing the arms to the legs, clamp that end down to a table or something. Then hook the luggage scale to where the normal mounting hook goes and pull until the tension members break, watching what the reading on the scale reaches before it breaks.

[dholdgreve]

I suppose there are ways of getting really scientific and testing the tensile strength of bass, but the problem comes in actually attaching a quantifying device such as luggage scale. With 1/16" material this can very difficult without either mashing or bending the piece which adds shear forces to you test. After 20 years of trying to pretest the material, the best way we have come up with is for each builder to grab an end of a 36" stick be between their thumb and index finger and lean back, being very careful not to bend the piece or let it slip from their hand. If they hit the floor, the piece is not strong enough. (LOL)! Seriously, probably 19 of 20 pieces tested are more than strong enough to carry the forces applied by Division B, and probably 9 of 10 in Div C. The problem is very rarely shear parallel to the grain, which is when the stick literally just shreds and pulls apart in the middle of the stick. Far more often, the tension rod will separate at its ends from either the mounting block by the J hook, or from the compression beam at the distal end. Rarely is it the fault of the wood. Most of the time it is a matter of not creating enough gluing surface, using the wrong glue, or inaccurate construction methods that may cause the tension rod to incur shear perpendicular to the grain and cause it to snap. These breaks are usually clean breaks. Shear parallel to grain (due to wood failure) will usually have longer, jagged break ends that look like it has just been pulled apart. Just my thoughts.

[JonB]

I suppose there are ways of getting really scientific and testing the tensile strength of bass, but the problem comes in actually attaching a quantifying device such as luggage scale. With 1/16" material this can very difficult without either mashing or bending the piece which adds shear forces to you test. After 20 years of trying to pretest the material, the best way we have come up with is for each builder to grab an end of a 36" stick be between their thumb and index finger and lean back, being very careful not to bend the piece or let it slip from their hand. If they hit the floor, the piece is not strong enough. (LOL)! Seriously, probably 19 of 20 pieces tested are more than strong enough to carry the forces applied by Division B, and probably 9 of 10 in Div C. The problem is very rarely shear parallel to the grain, which is when the stick literally just shreds and pulls apart in the middle of the stick. Far more often, the tension rod will separate at its ends from either the mounting block by the J hook, or from the compression beam at the distal end. Rarely is it the fault of the wood. Most of the time it is a matter of not creating enough gluing surface, using the wrong glue, or inaccurate construction methods that may cause the tension rod to incur shear perpendicular to the grain and cause it to snap. These breaks are usually clean breaks. Shear parallel to grain (due to wood failure) will usually have longer, jagged break ends that look like it has just been pulled apart. Just my thoughts.

Everything dholdgreve is right on. Once you figure out gluing surface area, angles of the tension pieces, connections, etc... then you can start figuring out what the minimum size of that tension piece can be. Although it seems that there are some discrepancies in the wood itself from piece to piece, bass is relatively consistent, all things considered. Look at where the bass is breaking. There is most likely a reason that it is breaking there and it is not usually due to the strength of the wood. Figure out why there is shear in this part of the tension piece, minimize/eliminate it, and you have fixed the issue (until the next one comes up).

[JZhang1]

I suppose there are ways of getting really scientific and testing the tensile strength of bass, but the problem comes in actually attaching a quantifying device such as luggage scale. With 1/16" material this can very difficult without either mashing or bending the piece which adds shear forces to you test. After 20 years of trying to pretest the material, the best way we have come up with is for each builder to grab an end of a 36" stick be between their thumb and index finger and lean back, being very careful not to bend the piece or let it slip from their hand. If they hit the floor, the piece is not strong enough. (LOL)! Seriously, probably 19 of 20 pieces tested are more than strong enough to carry the forces applied by Division B, and probably 9 of 10 in Div C. The problem is very rarely shear parallel to the grain, which is when the stick literally just shreds and pulls apart in the middle of the stick. Far more often, the tension rod will separate at its ends from either the mounting block by the J hook, or from the compression beam at the distal end. Rarely is it the fault of the wood. Most of the time it is a matter of not creating enough gluing surface, using the wrong glue, or inaccurate construction methods that may cause the tension rod to incur shear perpendicular to the grain and cause it to snap. These breaks are usually clean breaks. Shear parallel to grain (due to wood failure) will usually have longer, jagged break ends that look like it has just been pulled apart. Just my thoughts.

Everything dholdgreve is right on. Once you figure out gluing surface area, angles of the tension pieces, connections, etc... then you can start figuring out what the minimum size of that tension piece can be. Although it seems that there are some discrepancies in the wood itself from piece to piece, bass is relatively consistent, all things considered. Look at where the bass is breaking. There is most likely a reason that it is breaking there and it is not usually due to the strength of the wood. Figure out why there is shear in this part of the tension piece, minimize/eliminate it, and you have fixed the issue (until the next one comes up).

Thanks for the advice, i figured it might be bad angling that's causing the problem.

[MmmBddRS1234]

I suspect that a lot of us smaller, lesser known teams will have a harder time (like me) because now they are restricted to width and not just height like they're used to. And I know that you bigger teams cope better with change but personally I will be happy to see this event go even though I like it but it has given me much stress.

There is certainly a steep learning curve to boomilever, and all balsa events in general. I came from a small team, and it did require significant time and resources to even get on the playing field with traditionally stronger teams. However, mostly anyone can learn to become good at this event if they put in the effort.

Ok. Thank you!

[dholdgreve]

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Thanks for the advice, i figured it might be bad angling that's causing the problem.
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Ya, the whole primary key is to keep that tension rod as straight as possible from mounting block to compression beam... No slight curves or angles anywhere, or they will turn into sharp curves and angles under load, which will turn into shear perpendicular to grain.

Secondary key is to design a method of connecting the T-rod to the mounting block without adding significant weight to the structure, and figuring out how you can create enough surface area between a 1/16" piece of bass and balsa compression beam to carry the loads imposed.

[Lorant]

How are scores looking so far?

[dholdgreve]

How are scores looking so far?

We've had a couple right around 2000 in Div B

[musicalwhang]

Hey guys,

I will be buying all my wood for the season through specialized balsa and was working out dimensions for this year's boomilever design. I believe I am going to start with a simple double beam compression boomilever. For the compression members, I'm thinking 3/8 x 1/8 and for the bracings, I will probably do an x design on both sides with 1/16 x 1/32. I have questions however about the bracing between the compression members and the tension members. What design are you guys using and what dimensions are those braces as well as what dimensions are you guys using for the tension members. For the vertical bracing, I was thinking 1/16 x 1/8. For the tension members I was thinking about going with bass wood with dimensions 1/8 x 1/16 as well. Thoughts on bass vs balsa and thoughts in general of the overall dimensions of my design. I will be aiming for the bonus. Thanks guys.