So I know that the general consensus for rubber banding a DR4B is that a triangle is the best shape. However, I have a few questions about some more technical aspects on the choices that teams make when rubber banding their lifts.
For one, why do most teams only rubber band the bottom part of the DR4B? I’ve been rubber banding both the top and bottom four bars and it works just fine, so is there a particular reason that the bottom is usually the only part rubber banded?
Also, why do most teams do the rubber banding all the way around the triangle shape? Most do it like this:
with the rubber bands going all around the triangle, which doesn’t exactly make sense to me because the distance between the two top points never changes, so that tension never changes, only the tension of the two sides of the formed triangle.
Wouldn’t you get more tension out of the rubber bands this way?
This way has the rubber band starting around the top, curving around the bottom, and back up to the top, maximizing tension.
I have no idea why, but our lift doesn’t go perfectly vertical if we put rubber bands on the top 4-bar. It is farther forward when the lift is down and farther back when the lift is up. For some reason, it works out okay when rubber bands are only on the bottom.
You actually struck the exact reason on accident. The purpose of triangular rubber bands is not to have greater tension, it’s to have more uniform tension. Rubber bands are cheap enough that maximizing the tension gained from each one is not necessary, but having uniform tension at all lift heights (or more uniform tension, at least) is extremely useful.
This triangular method is more than just about providing tension. The main intention is to achieve uniform tension throughout the lifting and and retracting.
There are lots of discussions abt this during sack Attack.
Interesting. Just curious, how does the triangle achieve more uniform tension if the top part never changes in tension and the sides do change? Whereas the bottom picture has both sides changing in tension, so wouldn’t they be closer together to each other in tension?
Unless I’m misinterpreting what “uniform” is supposed to mean in this scenario. Regardless, thanks for the info
Uniform is supposed to mean that both sides have an equal amount of tension running through the lift.
This wan’t answered clearly, so I’ll answer it slightly more clearly.
Certain teams actually do rubber band the top part. Because of the way ost teams (Including us) built their dr4b, the top rubber band will actually try to stop the bar and pivot point from moving upwards and downwards uniformly.
For a little simplicity in the explanation, draw a line from the middle of the long side that runs along a single bar over to the one point where it connects to the other bar. Your question then boils down to one of geometry. A direct, 2-point connection would be the short leg of the right triangles you just made. Try playing around with the geometry. You’ll see the change in the length of the hypotenuse is always noticeably less than the change in the length of the short leg. The change in length is what is important since the bands will at least nearly obey Hooke’s law.
You have to adjust the standoffs according to your robot until you get a nice curve on the graph. I believe the creator has made an updated version so you could check that out too.
