Bridge B/C

[mklinger]

Great discussion. I definitely think with the 5,000 g bonus it's worth pre-testing to full load.

One thing we haven't done is test to failure. Harder to do when you don't have a custom-built dynamic load cell We will try that more for next season so we have a better idea how much of a safety factor we have built in. It's possible his bridge was just barely hanging on and a second test did it in and if we had tested more to failure we would have a better idea of that model's limits.

It's definitely useful to test to failure for a given design/build if you can so you can understand the weak points and the general factor of safety (overdesign) you are dealing with. The thing that really helps is high speed video. I wish I had had the Chronos while coaching, but that is a recent acquisition. Most phones these days do a decent job of high speed video, so try to do that while testing to failure to see what breaks first. Sometimes is really hard to tell or even impossible to tell what failed first from just the debris after the fact.

If you only have 15kg of sand, don't be afraid of using something like 5 to 10kg of steel weights in the bucket first. Before we had access to an auto-loader of any kind, for pre-load testing we'd often just use steel weights as it was much quicker and less messy than dealing with sand.

If the device can hold close to 15kg, it will be no problem at all to place 5-10kg of weight in the bucket (gently) before adding sand to get the final result.

Marc

[jgrischow1]

Great discussion. I definitely think with the 5,000 g bonus it's worth pre-testing to full load.

One thing we haven't done is test to failure. Harder to do when you don't have a custom-built dynamic load cell We will try that more for next season so we have a better idea how much of a safety factor we have built in. It's possible his bridge was just barely hanging on and a second test did it in and if we had tested more to failure we would have a better idea of that model's limits.

It's definitely useful to test to failure for a given design/build if you can so you can understand the weak points and the general factor of safety (overdesign) you are dealing with. The thing that really helps is high speed video. I wish I had had the Chronos while coaching, but that is a recent acquisition. Most phones these days do a decent job of high speed video, so try to do that while testing to failure to see what breaks first. Sometimes is really hard to tell or even impossible to tell what failed first from just the debris after the fact.

If you only have 15kg of sand, don't be afraid of using something like 5 to 10kg of steel weights in the bucket first. Before we had access to an auto-loader of any kind, for pre-load testing we'd often just use steel weights as it was much quicker and less messy than dealing with sand.

If the device can hold close to 15kg, it will be no problem at all to place 5-10kg of weight in the bucket (gently) before adding sand to get the final result.

Marc

Great advice. We are fortunate enough to have an auto-loader and I think we have 50 lbs of sand but the 5 or 10 kg weights are great ideas to speed things up.

[mnoga]

This is from my experience. The same thing happened to me as the previously held full weight bridge broke at 14.2 kg when it was inside the Silica box.
I guess we need some moisture at least. Less moisture makes it brittle. Also, pre-testing to full weight weakens the structure.

My experience is that just using silica packs to dry out your structure by ~2-2.5% will not negatively effect the strength. Baking down 5-6% is a different story and is more risky.

I think what you ran into was more of pre-testing too close to its eventual maximum load. It's been my experience that you can only safely pre-test to 75-80% of the eventual maximum load. If you get closer to 90-95%, you risk doing internal damage. One way to try and predict this is to listen very carefully during the loading to see if you hear any "creaking or cracking". That is usually a bad sign.

FWIW, we would never pre-test just once for that exact reason. To really be safe, it's best to pre-test in steps up to a maximum and do it multiple times. Over 6 years and dozens of competitions we never had a device break at less weight than our pre-test.

The problem/challenge of course, is that with the 15kg bonus, to actually guarantee you can hold 15, you need to design to more like 16.5 or 17kg. By the very definition, that is overdesigned. So, every team has to make the choice of risking early failure or guaranteeing a better score, but maybe not a winning score. When there is such a big bonus for holding 15kg, I like to think of it as something like this: 50% chance of 1st or 2nd place with a 50% chance of 10th+ place (for early failure). OR a 90% chance of 3-5th place by properly pre-testing to 15kg with a slightly overdesigned device.

That is also why sometimes you see might higher scores in Invitationals during the year than in important competitions because you can hurt the team a lot more if you go for 1st place and come in 10th.

As for CA getting "old", we used an 18 month old boomi that placed 3rd in Div C Nationals in 2021 and I have re-tested about 10 devices that were all over a year old and they all held more than they originally tested, so I don't subscribe to that theory.

Marc

Just another quick follow-up. This is how I would approach trying to win with a device that was optimally designed for 15kg.

1. Have a known design and build that has been tested to failure and it's exactly how you want. Say it breaks at 15.3kg. Ideally, the team can reproduce this and has done so multiple times in practice.

2. Rebuild multiple devices for a new competition

3. Put them in a dry-box and then pre-test them all to 12kg 3 times in row. If more than one survives, take the lightest one (they should all be within 0.1-0.2g of each other).

4. Cross your fingers that it will hold 15kg during competition. This is basically the best you can safely do.

The idea is that you are using your previous builds and experience to give you confidence that your device will hold 15kg again like it's done in practice. The team needs to build A LOT of devices to gain this kind of experience. Think on the order of 10-20 devices per student over the course of the season.


If you want to be more conservative, do the same steps above with a build designed for 18kg and pre-test something like 2x to 12kg and 2x to 15kg or design to ~17kg and 2x to 12kg and 1x to 15kg and small fingers crossed (this is what we'd typically do).

FWIW, that is why the 5kg bonus for holding 15kg makes the event MUCH more challenging. They usually wait until the second year of a device to add that scoring tweak and I'm surprised they had it this year instead of waiting for next year.

Marc

As a follow on, since I've mentored balsa/bass structures for a number of years, I believe the best strategy to lock in the 5kg bonus is to test your device to approximately 15.2kg listening for any funny sounds (creaks). In our experience, if your structure holds 15.2kg with no sounds, you can rerun it again with high probability that it will hold together again past 15kg. We've never had an actual failure using this methodology. In year one of boomilever, before COVID, we ran the same boom build across several competitions with no failures.

[Tall]

My experience is that just using silica packs to dry out your structure by ~2-2.5% will not negatively effect the strength. Baking down 5-6% is a different story and is more risky.

I think what you ran into was more of pre-testing too close to its eventual maximum load. It's been my experience that you can only safely pre-test to 75-80% of the eventual maximum load. If you get closer to 90-95%, you risk doing internal damage. One way to try and predict this is to listen very carefully during the loading to see if you hear any "creaking or cracking". That is usually a bad sign.

FWIW, we would never pre-test just once for that exact reason. To really be safe, it's best to pre-test in steps up to a maximum and do it multiple times. Over 6 years and dozens of competitions we never had a device break at less weight than our pre-test.

The problem/challenge of course, is that with the 15kg bonus, to actually guarantee you can hold 15, you need to design to more like 16.5 or 17kg. By the very definition, that is overdesigned. So, every team has to make the choice of risking early failure or guaranteeing a better score, but maybe not a winning score. When there is such a big bonus for holding 15kg, I like to think of it as something like this: 50% chance of 1st or 2nd place with a 50% chance of 10th+ place (for early failure). OR a 90% chance of 3-5th place by properly pre-testing to 15kg with a slightly overdesigned device.

That is also why sometimes you see might higher scores in Invitationals during the year than in important competitions because you can hurt the team a lot more if you go for 1st place and come in 10th.

As for CA getting "old", we used an 18 month old boomi that placed 3rd in Div C Nationals in 2021 and I have re-tested about 10 devices that were all over a year old and they all held more than they originally tested, so I don't subscribe to that theory.

Marc

Just another quick follow-up. This is how I would approach trying to win with a device that was optimally designed for 15kg.

1. Have a known design and build that has been tested to failure and it's exactly how you want. Say it breaks at 15.3kg. Ideally, the team can reproduce this and has done so multiple times in practice.

2. Rebuild multiple devices for a new competition

3. Put them in a dry-box and then pre-test them all to 12kg 3 times in row. If more than one survives, take the lightest one (they should all be within 0.1-0.2g of each other).

4. Cross your fingers that it will hold 15kg during competition. This is basically the best you can safely do.

The idea is that you are using your previous builds and experience to give you confidence that your device will hold 15kg again like it's done in practice. The team needs to build A LOT of devices to gain this kind of experience. Think on the order of 10-20 devices per student over the course of the season.


If you want to be more conservative, do the same steps above with a build designed for 18kg and pre-test something like 2x to 12kg and 2x to 15kg or design to ~17kg and 2x to 12kg and 1x to 15kg and small fingers crossed (this is what we'd typically do).

FWIW, that is why the 5kg bonus for holding 15kg makes the event MUCH more challenging. They usually wait until the second year of a device to add that scoring tweak and I'm surprised they had it this year instead of waiting for next year.

Marc

As a follow on, since I've mentored balsa/bass structures for a number of years, I believe the best strategy to lock in the 5kg bonus is to test your device to approximately 15.2kg listening for any funny sounds (creaks). In our experience, if your structure holds 15.2kg with no sounds, you can rerun it again with high probability that it will hold together again past 15kg. We've never had an actual failure using this methodology. In year one of boomilever, before COVID, we ran the same boom build across several competitions with no failures.

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Since this year's Nationals is going to be a satellite competition, do you guys think that teams using auto-loader have an advantage over manual loading teams?

[mklinger]

Since this year's Nationals is going to be a satellite competition, do you guys think that teams using auto-loader have an advantage over manual loading teams?

Yes, I would say it's always an advantage to use an auto-loader over manual loading. You want to minimize the time at high loading conditions, so start out slow for maybe 5-10 sec and then go full speed with the auto-loader. If you made it to Nationals, definitely try and track down a fast auto-loader to use if possible.

If you absolutely have to hand load, go as quickly as possible, especially at the end. Good luck!!

[Tall]

Since this year's Nationals is going to be a satellite competition, do you guys think that teams using auto-loader have an advantage over manual loading teams?

Yes, I would say it's always an advantage to use an auto-loader over manual loading. You want to minimize the time at high loading conditions, so start out slow for maybe 5-10 sec and then go full speed with the auto-loader. If you made it to Nationals, definitely try and track down a fast auto-loader to use if possible.

If you absolutely have to hand load, go as quickly as possible, especially at the end. Good luck!!

Makes sense. Thanks Marc.

[random-person]

Hey, I'm really short on time(States is 5 days from now(well actually 4 depending on how you count)) and i have three bridges that i've pretested once, they all hold everything. (one is bass, the other two are all balsa.) since we're on spring break right now, I only have one more chance to test these before going to competition, what would be my best option? I think i can cut off a couple more tenths of a gram from my current bridges but i'm too scared to bring in an untested bridge(or one that's been tested once) Any advice would be greatly appreciated. I'm sorta new to forums so sorry if i've messed up and asked a question that's already been answered...

i just realized i sorta rambled so simplified version of the word blob above:
i have two bridges that score decently high that i've tested once, and i think i can improve my score with a bridge i'm building right now, but that one's untested. i only have one more chance to test my bridge, what's my best option at this point? Bring the lighter of the two that have been tested once, or bring the new one that has been tested no times? (after the one chance i get for testing the first two will have been tested twice and the new one will be tested once.)

(this is my first year doing any balsa events so i'm kinda paranoid and terrified right now)

[jgrischow1]

Hey, I'm really short on time(States is 5 days from now(well actually 4 depending on how you count)) and i have three bridges that i've pretested once, they all hold everything. (one is bass, the other two are all balsa.) since we're on spring break right now, I only have one more chance to test these before going to competition, what would be my best option? I think i can cut off a couple more tenths of a gram from my current bridges but i'm too scared to bring in an untested bridge(or one that's been tested once) Any advice would be greatly appreciated. I'm sorta new to forums so sorry if i've messed up and asked a question that's already been answered...

i just realized i sorta rambled so simplified version of the word blob above:
i have two bridges that score decently high that i've tested once, and i think i can improve my score with a bridge i'm building right now, but that one's untested. i only have one more chance to test my bridge, what's my best option at this point? Bring the lighter of the two that have been tested once, or bring the new one that has been tested no times? (after the one chance i get for testing the first two will have been tested twice and the new one will be tested once.)

(this is my first year doing any balsa events so i'm kinda paranoid and terrified right now)

IMHO, considering the 5,000 g bonus, I would not bring an un-tested bridge. If you have already pre-tested a bridge once, it should be fine.

[mnoga]

Hey, I'm really short on time(States is 5 days from now(well actually 4 depending on how you count)) and i have three bridges that i've pretested once, they all hold everything. (one is bass, the other two are all balsa.) since we're on spring break right now, I only have one more chance to test these before going to competition, what would be my best option? I think i can cut off a couple more tenths of a gram from my current bridges but i'm too scared to bring in an untested bridge(or one that's been tested once) Any advice would be greatly appreciated. I'm sorta new to forums so sorry if i've messed up and asked a question that's already been answered...

i just realized i sorta rambled so simplified version of the word blob above:
i have two bridges that score decently high that i've tested once, and i think i can improve my score with a bridge i'm building right now, but that one's untested. i only have one more chance to test my bridge, what's my best option at this point? Bring the lighter of the two that have been tested once, or bring the new one that has been tested no times? (after the one chance i get for testing the first two will have been tested twice and the new one will be tested once.)

(this is my first year doing any balsa events so i'm kinda paranoid and terrified right now)

IMHO, considering the 5,000 g bonus, I would not bring an un-tested bridge. If you have already pre-tested a bridge once, it should be fine.

And, I would add, make sure your bridge holds greater than 15.1 Kg, which gives you a safety margin and assurance that you can clear the 15 Kg bonus threshold.

[random-person]

Thank you so much! tested twice to 15.5kg and I feel much better. Again, thank you, I really appreciate the help :)