which boom to bring -- tested or identical new

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tweedle
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which boom to bring -- tested or identical new

Post by tweedle » February 11th, 2020, 8:57 am

So we've built the boom we want -- tested it with max load. Should bring that one to the competition or a new identical one. The concern is on the tested one -- the wood is stressed (and may not perform quite as well as in the initial test. Conversely the identical new would not have been tested

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Re: which boom to bring -- tested or identical new

Post by MadCow2357 » February 11th, 2020, 9:21 am

tweedle wrote:
February 11th, 2020, 8:57 am
So we've built the boom we want -- tested it with max load. Should bring that one to the competition or a new identical one. The concern is on the tested one -- the wood is stressed (and may not perform quite as well as in the initial test. Conversely the identical new would not have been tested
I always build new ones even if the old ones held full load

Not sure what other people do though
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Re: which boom to bring -- tested or identical new

Post by dholdgreve » February 11th, 2020, 9:35 am

We pretest every boom we take to a competition to full load and immediately remove the load. The bonus is just too critical to leave to chance. IMHO, there is no such thing as "identical." They may share similar designs but the grain structure, density, stiffness cannot possibly be identical. Balsa is by nature heterogeneous. I believe the critical factor about boom is not haw many times it is tested, but over how long of a time span. Super Glue gets brittle after 60 to 90 days. We never use a boom in competition if it is over 90 days old.
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Re: which boom to bring -- tested or identical new

Post by michiganmarc » February 11th, 2020, 10:34 am

dholdgreve wrote:
February 11th, 2020, 9:35 am
We pretest every boom we take to a competition to full load and immediately remove the load. The bonus is just too critical to leave to chance. IMHO, there is no such thing as "identical." They may share similar designs but the grain structure, density, stiffness cannot possibly be identical. Balsa is by nature heterogeneous. I believe the critical factor about boom is not haw many times it is tested, but over how long of a time span. Super Glue gets brittle after 60 to 90 days. We never use a boom in competition if it is over 90 days old.
Dan, what are your thoughts on pre-loading multiple times? My experience has been that a single load test can damage the device, without actually failing, if it's close to its maximum capacity (90%+). In years past, we'd pre-load to about 75-80% of the "known" maximum capacity for a given design/build. We'd typically work our way up to 3 times if possible.

This year, holding the entire 15kg is critical, so it will be somewhat of a gamble to get close to the limit as you'd really need to design for ~17-18kg to safely test to 15kg. FWIW, we tested our Centerville devices to 13.5 kg 2x. The lighter one held 15 kg and the heavier one didn't. We didn't have enough devices built by the proper team members to test them to 15kg. I have personally built some boomis that have pre-tested to 15kg x2 and were lighter than what we brought, so it's definitely possible and there is room for improvement.

Another pre-testing note, when we pre-test, we "cheat" and use a level to make sure it's aligned perfectly as we want to test the device and not introduce any setup variation. That can always be an issue during actual competition day as I'm sure you know.

I like the rules this year, it definitely makes things more challenging!

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Re: which boom to bring -- tested or identical new

Post by dholdgreve » February 12th, 2020, 1:13 pm

Over the last few invitationals, I have seen so many booms carry 13.5 Kg to 14.5 Kg. At Centerville we even had one that broke at 14,983 grams... Ya... 17 grams short of a full load. I've also seen one break at Hudson at 14,940 grams. Pretesting to 80% is probably better than nothing, but I feel the risk is much less if it has been pretested to 15 kg, and then reloaded, than if it were pretested to 13 kg once, and unproven for the remaining 2 kg. Just 2 different schools of thought I guess.

I think we have 5 booms sitting on the table in my basement that have been pretested and carry full load, so it is much easier to say pretest to full load with that kind of back-up. If we didn't have those, I might well feel as you do.

Just a few personal opinions:
I believe in keeping the design stupid simple... Fewer pieces = fewer joints. Fewer joints = fewer opportunities for failure
Fewer pieces allows for more accurate duplication of successful designs.
I believe that superglue, once placed on a joint, has a finite lifespan. We allow 90 days. After that, the glue becomes brittle and may fail. Within those 90 days, test it as often as you'd like.

20 years ago, my oldest son competed in booms. He had one he named Goliath (although it was not that heavy) He tested it in competition 7 times that season, and it carried full load every single time. Tried desperately to improve on it the weeks before State, but never could. Ended up using Goliath at Ohio State and placed 3rd. This was right at the far reaches of that 90 day limit.

If there are pieces within your boom that can be crushed by loading (directly under the loading block, or the mounting block behind the J hook) I'd make an exception. In those cases I'd agree that structural damage may occur if pretested. We have modified our designs over time, so these areas are made of more stable material that will not take damage by pretesting.
Dan Holdgreve
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"For the betterment of Science"

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Re: which boom to bring -- tested or identical new

Post by michiganmarc » February 12th, 2020, 2:00 pm

dholdgreve wrote:
February 12th, 2020, 1:13 pm
Over the last few invitationals, I have seen so many booms carry 13.5 Kg to 14.5 Kg. At Centerville we even had one that broke at 14,983 grams... Ya... 17 grams short of a full load. I've also seen one break at Hudson at 14,940 grams. Pretesting to 80% is probably better than nothing, but I feel the risk is much less if it has been pretested to 15 kg, and then reloaded, than if it were pretested to 13 kg once, and unproven for the remaining 2 kg. Just 2 different schools of thought I guess.

I think we have 5 booms sitting on the table in my basement that have been pretested and carry full load, so it is much easier to say pretest to full load with that kind of back-up. If we didn't have those, I might well feel as you do.

Just a few personal opinions:
I believe in keeping the design stupid simple... Fewer pieces = fewer joints. Fewer joints = fewer opportunities for failure
Fewer pieces allows for more accurate duplication of successful designs.
I believe that superglue, once placed on a joint, has a finite lifespan. We allow 90 days. After that, the glue becomes brittle and may fail. Within those 90 days, test it as often as you'd like.

20 years ago, my oldest son competed in booms. He had one he named Goliath (although it was not that heavy) He tested it in competition 7 times that season, and it carried full load every single time. Tried desperately to improve on it the weeks before State, but never could. Ended up using Goliath at Ohio State and placed 3rd. This was right at the far reaches of that 90 day limit.

If there are pieces within your boom that can be crushed by loading (directly under the loading block, or the mounting block behind the J hook) I'd make an exception. In those cases I'd agree that structural damage may occur if pretested. We have modified our designs over time, so these areas are made of more stable material that will not take damage by pretesting.
I agree 100% with the keep it as simple as possible wrt to the design. I believe our current design takes me about 90 min to build from start to finish and the kids take a bit longer than that, but it's still on the simple side compared to some I've seen.

I think pre-testing to the near limit is somewhat of a crap shoot. I have definitely seen some cases where a single successful pre-test has damaged the device, not the glue joints, but the balsa itself. We try to listen very carefully for any cracking during the testing. I agree with getting to 15 kg pre-test for this year though. Our team has another Invitational this weekend, so that put an additional stress on our "stock". We have one that could be better competing on Saturday, so we'll see! (FWIW, it's also been my experience that a properly executed CA glue joint will never be a point of failure for these devices. You may shear the wood itself at the joint, but the CA is plenty strong enough. That takes some practice and patience with good technique which can be especially challenging for Div B kids).

For your boomis that you test to 15kg, do you ever test them to failure? I'd be curious to know if you are generally building devices that hold 17-18+kg or more like 15.25-15.5kg (if you care to share that info). Thanks!

I'm also curious to test your 90 day CA glue theory a bit. I have about 10 boomis from last season, both Div B and Div C that were all pre-tested but never used in competition. I will test them at some point this year and see if any fail lower than their pre-test level from last year, which would agree with your glue theory. If they all hold more than their pre-test, maybe something else is at play.

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