Bridge B/C

[sneepity]

As you guys probably already know, Division C is much harder than Division B this year due to the pass-through block being significantly larger and the width being wider than the loading block itself.

I briefly took a look at a simple scaled up version of my Division B design, but I wanted to try a tapered design to see how it would handle the design parameters.

I was able to build a 12.53g bridge that held 15.84kg for a competition score of 1596.2 ((15000 + 5000 bonus)/12.53).

While I doubt that score will win any major competitions, the goal was to give you guys a starting point to see how a tapered design works and could be built.

In my video, I not only show the testing and pretty cool failure in slow-mo, but also a bunch of steps along the way on exactly how I built it.

https://youtu.be/tvfrtC_1W58

I hope this helps some of the Division C students just starting out!

Marc

Hey Marc, it looks great! :) I had a quick question.. how did you cut the angles for the main four truss members when they're angled in and they touch the ground at an angle? Thanks so much.

Hi sneepity, thanks!! Do you mean how did I figure out the angle, or how did I actually cut it? For figuring out the angle, I just used scrap wood and did some trial and error. That seems like the best option.
For actually cutting it, I used a miter cutter which you can occasionally see in my videos. This is another extremely handy tool for this event. I only use the X-Acto knife for things like cutting thin/small balsa parts. For all basswood cutting and larger balsa, I always use the miter as it makes a much cleaner cut.

It looks like it's not available on Amazon anymore, but it does looks like you can get it directly from the manufacturer:

http://www.fourmostproducts.com/our-pro ... ion-tools/

Oh, I just re-read your question... the angle of the primary part of the beams touching the ground. Basically, I didn't bother worry about that at all. I probably would have made it worse by trying to sand them to the correct angle. Balsa is pretty strong such that it can be loaded along an edge like that and the load will still be distributed through the stick. Because the bridge is being loaded evenly across all 4 legs (theoretically), all those legs will get compressed in the same fashion. I just re-checked the bridge after testing, and all 4 legs at the bottom still had their angle, so they didn't even get compressed flat!

Hey! yeah I was asking exactly about that- if the angle for the 4 major members matters as much :) I actually am very paranoid about that and I use a protractor and everything to make sure it's perfect, lol. Thanks though- and rewatching your video I love the idea behind the jig. Is it okay if I make something similar? basically ive been struggling with getting the right width and making sure the sticks near the loading block sit right.
And I use the easy miter saw too! it's great- they give you such nice straight edges. I also use a exacto for the smaller sticks too. thanks a lot for your help I appreciate it!

[mklinger]

Hey! yeah I was asking exactly about that- if the angle for the 4 major members matters as much I actually am very paranoid about that and I use a protractor and everything to make sure it's perfect, lol. Thanks though- and rewatching your video I love the idea behind the jig. Is it okay if I make something similar? basically ive been struggling with getting the right width and making sure the sticks near the loading block sit right.
And I use the easy miter saw too! it's great- they give you such nice straight edges. I also use a exacto for the smaller sticks too. thanks a lot for your help I appreciate it!

Sounds good! Yeah, I would 100% recommend creating some sort of jig for assembly, even with more simple designs like the non-tapered ones. "Back in the day" when I first started coaching, we'd make the jigs out of wood, but now with 3D printers, it's so much easier and way more accurate! Blue painter's tape is also a nice option for taping things in place temporarily. It's strong, but not so sticky that it'll ruin the part when you take it off.

[sneepity]

Hey! yeah I was asking exactly about that- if the angle for the 4 major members matters as much :) I actually am very paranoid about that and I use a protractor and everything to make sure it's perfect, lol. Thanks though- and rewatching your video I love the idea behind the jig. Is it okay if I make something similar? basically ive been struggling with getting the right width and making sure the sticks near the loading block sit right.
And I use the easy miter saw too! it's great- they give you such nice straight edges. I also use a exacto for the smaller sticks too. thanks a lot for your help I appreciate it!

Sounds good! Yeah, I would 100% recommend creating some sort of jig for assembly, even with more simple designs like the non-tapered ones. "Back in the day" when I first started coaching, we'd make the jigs out of wood, but now with 3D printers, it's so much easier and way more accurate! Blue painter's tape is also a nice option for taping things in place temporarily. It's strong, but not so sticky that it'll ruin the part when you take it off.

mhm, im thinking of first making a jig out of wood then 3d print it. I used to use the painters tape and well it was not too stable to hold the trusses upright :/ and when I used too much it would peel some wood grain.

[mklinger]

Hey! yeah I was asking exactly about that- if the angle for the 4 major members matters as much I actually am very paranoid about that and I use a protractor and everything to make sure it's perfect, lol. Thanks though- and rewatching your video I love the idea behind the jig. Is it okay if I make something similar? basically ive been struggling with getting the right width and making sure the sticks near the loading block sit right.
And I use the easy miter saw too! it's great- they give you such nice straight edges. I also use a exacto for the smaller sticks too. thanks a lot for your help I appreciate it!

Sounds good! Yeah, I would 100% recommend creating some sort of jig for assembly, even with more simple designs like the non-tapered ones. "Back in the day" when I first started coaching, we'd make the jigs out of wood, but now with 3D printers, it's so much easier and way more accurate! Blue painter's tape is also a nice option for taping things in place temporarily. It's strong, but not so sticky that it'll ruin the part when you take it off.

mhm, im thinking of first making a jig out of wood then 3d print it. I used to use the painters tape and well it was not too stable to hold the trusses upright :/ and when I used too much it would peel some wood grain.

There is nothing wrong with making a jig out of wood if you can get it the way you want. I would recommend some sort of hard wood for it to be as square as possible. The big benefit of 3D printing is the sub-millimeter accuracy. It's really hard to cut wood super accurately without a CNC machine or some router/template setup which is a hassle.

With any tape, you have to be careful, but the blue stuff is much better to use than normal masking tape (unfortunately, it's also much more expensive).

[sneepity]

Sounds good! Yeah, I would 100% recommend creating some sort of jig for assembly, even with more simple designs like the non-tapered ones. "Back in the day" when I first started coaching, we'd make the jigs out of wood, but now with 3D printers, it's so much easier and way more accurate! Blue painter's tape is also a nice option for taping things in place temporarily. It's strong, but not so sticky that it'll ruin the part when you take it off.

mhm, im thinking of first making a jig out of wood then 3d print it. I used to use the painters tape and well it was not too stable to hold the trusses upright :/ and when I used too much it would peel some wood grain.

There is nothing wrong with making a jig out of wood if you can get it the way you want. I would recommend some sort of hard wood for it to be as square as possible. The big benefit of 3D printing is the sub-millimeter accuracy. It's really hard to cut wood super accurately without a CNC machine or some router/template setup which is a hassle.

With any tape, you have to be careful, but the blue stuff is much better to use than normal masking tape (unfortunately, it's also much more expensive).

exactly! And yeah tape freaks me out. Thanks for your input!

[mklinger]

Just in case anyone is trying to duplicate my Division C design I posted earlier, someone just correctly pointed out to me that I missed a very important rule that the test supports that the bridge sit on are exactly 6" (15.24 cm) wide. That means my bridge as shown (190 mm wide) is too wide and wouldn't work.

I think the general design is fundamentally good, but it definitely needs to be reworked to be less than 6" wide. I'll work on a new version and see what I can come up with.

[JonB]

Just in case anyone is trying to duplicate my Division C design I posted earlier, someone just correctly pointed out to me that I missed a very important rule that the test supports that the bridge sit on are exactly 6" (15.24 cm) wide. That means my bridge as shown (190 mm wide) is too wide and wouldn't work.

I think the general design is fundamentally good, but it definitely needs to be reworked to be less than 6" wide. I'll work on a new version and see what I can come up with.

We missed that too- I would say this year's bridge requirements are some of the most challenging we have seen for bridges. At least, in my opinion. But, that's what makes it fun!

[mklinger]

This is my first attempt at a 140mm wide modified design that will properly fit on the 6" testing block specified in the rules.

The bridge weighed 11.71g and held an incredible 23.68kg for an actual efficiency of 2021.9 and a competition score of 1707.9 ((15000+5000 bonus)/11.71)

https://youtu.be/RlAgtdSUD_k

The slightly narrower design turned out to be much better while still providing enough lateral stability to work like the previous design.

I'm very happy with this result and it shows that 2000+ competition scores for Div C will be very doable.

Marc

[Tall]

Impressive design Marc. Thank you.

[mklinger]

Quick informal survey, would people have any interest in me creating a long-form video where I record myself building my latest Div C bridge design and talking through the build process? I could show the glue techniques I'm using as well as more of the general process rather than just the final result.

Thanks!

Marc