Re: Elevated Bridge B/C
Posted: May 5th, 2010, 4:57 am
From BalsaMan (abbreviated from page 71): "JimY, a couple comments:
"When I say precise building, I'm talking about two aspects. The first is the joints – right amount of glue- just enough, and no gaps- a precise fit. The second is geometrical precision and symmetry. We have all seen seriously imprecise bridges, such as one side visibly lower than the other, leg span on one side a cm larger than on the other, leg lengths different enough that it teeter-totters on two legs. They tip over or break at low loads. "
Yes, I agree. I consider this incredibly important, but issues with both are easily remedied with a truss pattern that is carefully checked before the first piece of wood is taped down to it. So, when we make truss patterns, after the node points are marked, we check several diagonals that span across the bridge (such as from node 2 to 9, etc). My rule of thumb is that each length should match up with its mirror image by not more than 1 mm. This step is completed in September or October, and those patterns are used for the entire season.
The second thing we do is work on getting our main compression joints to fit together with no gaps on the entire surface of the joint. This is actually easier and easier to do as you use flatter and flatter pieces of wood. So, square is not the way to go. The compression pieces later have a perpendicular piece added to make them L shaped. This gives higher strength in compression than square pieces of the same total cross sectional area. Two reasons to not go square.
Regarding the wood, since the bass used is much more consistent on a percentage basis than balsa, we don't go to the point of weighing sticks. For the small amount of balsa used, first, I try to purchase sheets that appear to have consistent density throughout the sheet. For that, I just hold it up to the light in the store. If it looks like a zebra, I leave it at the store. Then I weigh the sheets purchased and mark them. So each strip cut from a sheet will have somewhat limited variability. I don't weigh the strips cut from these balsa sheets either. I did bass stick weighing when boom was a C event a couple years ago, but just about drove myself nuts. We earned a 7th and a 5th at nats in the event those two seasons, but weighing the bass for that event in the end didn't matter that much because we didn't do enough building to know what density we actually needed. I'm not saying that weighing isn't useful, but rather that we don't do it.
Somebody else asked what wood dimensions we use. All I can say is that except for the smallest pieces, nothing is square. Flat gives the advantages mentioned here, plus greater glue surface area, which I mentioned a page or so back, for things like lap joints and gussets.
I hope this helps the teams that are competing at nats.
"When I say precise building, I'm talking about two aspects. The first is the joints – right amount of glue- just enough, and no gaps- a precise fit. The second is geometrical precision and symmetry. We have all seen seriously imprecise bridges, such as one side visibly lower than the other, leg span on one side a cm larger than on the other, leg lengths different enough that it teeter-totters on two legs. They tip over or break at low loads. "
Yes, I agree. I consider this incredibly important, but issues with both are easily remedied with a truss pattern that is carefully checked before the first piece of wood is taped down to it. So, when we make truss patterns, after the node points are marked, we check several diagonals that span across the bridge (such as from node 2 to 9, etc). My rule of thumb is that each length should match up with its mirror image by not more than 1 mm. This step is completed in September or October, and those patterns are used for the entire season.
The second thing we do is work on getting our main compression joints to fit together with no gaps on the entire surface of the joint. This is actually easier and easier to do as you use flatter and flatter pieces of wood. So, square is not the way to go. The compression pieces later have a perpendicular piece added to make them L shaped. This gives higher strength in compression than square pieces of the same total cross sectional area. Two reasons to not go square.
Regarding the wood, since the bass used is much more consistent on a percentage basis than balsa, we don't go to the point of weighing sticks. For the small amount of balsa used, first, I try to purchase sheets that appear to have consistent density throughout the sheet. For that, I just hold it up to the light in the store. If it looks like a zebra, I leave it at the store. Then I weigh the sheets purchased and mark them. So each strip cut from a sheet will have somewhat limited variability. I don't weigh the strips cut from these balsa sheets either. I did bass stick weighing when boom was a C event a couple years ago, but just about drove myself nuts. We earned a 7th and a 5th at nats in the event those two seasons, but weighing the bass for that event in the end didn't matter that much because we didn't do enough building to know what density we actually needed. I'm not saying that weighing isn't useful, but rather that we don't do it.
Somebody else asked what wood dimensions we use. All I can say is that except for the smallest pieces, nothing is square. Flat gives the advantages mentioned here, plus greater glue surface area, which I mentioned a page or so back, for things like lap joints and gussets.
I hope this helps the teams that are competing at nats.