Elevated Bridge B/C

[lllazar]

O wow....that rule was removed? Kinda sad....but then again i guess i using anything bigger than 1/4 by 1/4 is just a waste of extra weight.

Can anyone answer my questions on bass wood?

[jazzy009]

Basswood is stronger, but if you make an entire bridge out of it usually it's too heavy to be really competitive. If you've got free time you may want to try a bridge of bass and one of balsa, maybe even mix them up. But I like balsa, it's very light and holds a good amount.

[nejanimb]

A piece that was 1/4 by 1/4 would probably be too big to be useful, but our design from last year had a couple pieces that were 1/4 wide in one dimension. I was very glad that the restriction on size was removed, because last year, after buying pieces that were that 1/4 width, we actually went and sanded down the whole thing again and checked it with a vernier calipers to be sure that the place we bought it from hadn't left it slightly wider than 1/4 (in some spots, it was slightly over, in fact). That was a huge pain. This year, one of my new design ideas actually incorporates a few pieces that're 3/8 in one dimension, and it seems to be working as I'd hoped. I loved this part of the rules change. It was an unnecessary restriction, I thought, because it limited design possibilities and required further rules-checking, and going over that limit could only be done in small parts to be effective.

The debate over Balsa and Bass is not such an easy one to answer. If you read more of this thread and much of last year's, you'll find information on this topic and the thoughts of many people. JimY, who won B division nationals last year and has impressive credentials otherwise too, swears by Bass. I and many others on the other hand, are fans of balsa. Granted, I haven't tested it as extensively as I'd like to and I'm hoping to try some Basswood designs this year, but there have been very good bridges made out of both. Some even use mixes. The top 10 at nationals last year included several all-balsa bridges and several bass bridges (though I think most or all of them included some balsa pieces), and both types broke the 2,000 mark under last year's specifications. The main idea is that while some would argue vehemently on either side, it is not the most important factor in making good bridges, because you can do it with either.

[blue cobra]

Another advantage to bass no one seemed to mention is its density. The density of bass is significantly more consistent than that of balsa. While balsa can range from 6# to I think nearly 20#, bass stays around the same density, which I think is about 23#, but I'm not sure. This can save you time and wood because you don't have to search for the piece of the right density-if you need a lighter, weaker piece, go smaller.

Anyway, I just tested my bridge. I made it entirely of bass, except for balsa for the lateral bracing. It did much better than my other bridges, holding about 12 liters of water. Assuming this weighs about 12,000 grams, my bridge weighed 13.48 grams, giving it an efficiency of about 890. I used the safety tower (why is it called that, by the way?) and that definitely helped. Interestingly, one side was completely undamaged. Is there a way I can get both sides more perfectly parallel without building a jig for every bridge? Also, the "tension" members that were under the lightest load-going from the joint where all the tension members meet to the bottom of the legs- were beginning to buckle, indicating that the compression member I was worried about did affect them. I think I may replace the strips with 1/8 square.

[sewforlife]

Though the density bit is true, sometimes (or most of the time) different parts of our bridges need different densities, depending on how light we want that part to be.

[nejanimb]

Agreed - I don't personally like the idea that in order to get the weight down when I don't need a piece that strong, I have to also make it smaller. I like that, with balsa, I can make decisions about mass and cross-section independently of each other.

[dragonfly]

Anyway, I just tested my bridge. I made it entirely of bass, except for balsa for the lateral bracing. It did much better than my other bridges, holding about 12 liters of water. Assuming this weighs about 12,000 grams, my bridge weighed 13.48 grams, giving it an efficiency of about 890. I used the safety tower (why is it called that, by the way?) and that definitely helped. Interestingly, one side was completely undamaged. Is there a way I can get both sides more perfectly parallel without building a jig for every bridge? Also, the "tension" members that were under the lightest load-going from the joint where all the tension members meet to the bottom of the legs- were beginning to buckle, indicating that the compression member I was worried about did affect them. I think I may replace the strips with 1/8 square.

1. A safety tower is called that because it keeps the loading block from wrecking your bridge, thereby it can sometimes help discern what went wrong with your bridge better.
2. About keeping your bridge parallel... We actually don't use a jig (though it might be nice), since last year we meant to make one but in the mean time made a make-shift set up with counter-top samples Funnily enough, we've been using them ever since, and have yet to have a problem with uneven connecting or sides. Two things to keep tabs on though: Make sure your template is perfectly symmetrical (i'm assuming it's meant to be that way) before and after building, since that might lead to uneven weight distribution, and also make sure to sand everything so that your bridge sits evenly on the table before testing.

[icyfire]

How do you usually come up with designs? Also when you are designing a bridge what part of the bridge do you start with?

[sewforlife]

How do you usually come up with designs? Also when you are designing a bridge what part of the bridge do you start with?

I usually pull designs from my experience with last year's. I think last year's came from basic stuff. Like, warren trusses, and just applying that, and experimenting.
When I design a bridge, (I hope you mean like, when you draw it out on paper, cuz that's what I'm talking 'bout) I usually design the sides, because I don't use a 3-D jig, I build the sides and then erect it later

[AlphaTauri]

We just tested our first bridge yesterday and got an efficency of about 430. The bridge weighed about 20g and held approx 8.2 kilos before it broke. For those of you who are more experienced than me, does the winning bridge at a state comp usually hold all 15kg, or not?