Belly Pan Lightening Holes

So I’m having a discussion with one of my former teammates on why a hexagonal pattern for a belly pan is a bad idea and that we should just go with a traditional diamond pattern. So why not go to CD and get all you guys(or girls) to pitch in on why a diamond pattern is superior to a hexagonal pattern. His biggest “claim” is that a hexagonal pattern is easier for “mounting hardware”. He just doesn’t seem to listen to me, so maybe he’ll listen to CD. Perhaps not, but it’s worth a shot.

We did a hexagonal pattern because we wanted to use it as a design for our entire robot. It didn’t work out as well as we hoped, but the belly pan worked just fine.

Hexagonal as in a bunch of small triangles or actual hexagonal holes?

As in just hexagonal holes

Proper diamond has the same # of diamonds wide as tall, which leaves a strip of metal directly connecting corner to corner. This “string” is very strong (at resisting parallelograming).

Arguably a pocketed bellypan isn’t what you should really focus on though. Unless you have a reasonable excess of resources, there are better ways to allocate them.

The diamond pattern contains straight lines running corner to corner, which act in pure tension/compression diagonally across the bellypan to add torsional stiffness to the frame. The hexagonal pattern’s members doing this job are a bit zig-zaggy.

Realistically though, this is a nit-picky design criteria. Any bellypan is going to significantly increase torsional rigidity, and I’d say that this choice can be safely made purely based on actual weight lost, aesthetics, etc. There are much more important things to worry about on a chassis design.

That can depend. Our sponsor, for example, provided us with a belly pan for free, and actually wanted to show it off to potential customers because they loved it. If you needed to spend money or your own time to make one though, I would just use a wood bellypan.

The amount of cut time (and material) going into a single fancy bellypan could provide gearbox plates, gussets, etc. for many systems.

So if an appreciable amount of your total sponsor and in house resources is used for the bellypan, it should instead be used on other aspects of the robot.

If getting the bellypan cut doesn’t rob resources that could be better used elsewhere, that’s a different story (few teams actually fall into this category, but many think they do).

The first question you should be asking yourself is what the purpose of the belly pan is. Depending on how you construct your chassis, the belly pan my play more or less of a role in the structure*.* A well-designed belly pan can greatly improve the structural efficiency of the chassis and allow you to get the same stiffness with a much lighter design, but it’s not always that integral.

The structural purpose of a belly pan, as Adam alluded to, is that it resists “parallelogramming”. That is the tendency of a rectangular frame to become a parallelogram under certain load cases, usually when a force is applied at the corners or in shear. Without a belly pan, the corner attachment is the only thing that resists the parallelogram deformation. Gussets can help, but only so far. The belly pan is effectively one big gusset.

With that in mind, the structural efficiency of the belly pan is related to how well it can transmit the internal loads. If all you cared about was stiffening the frame against parallelogramming, you could achieve a similar effect by tensioning cables across the diagonals. When the frame parallelograms, the diagonals increase/decrease in length depending on which direction it’s going. The cables would resist that.

The belly pan can do more (like provide electronics mounting), but the key point is that the load that’s being resisted is along the diagonals of the frame. No matter what you do, that’s the load path. The diamond works with that.

That corner to corner strip equivalent to your “cable”. It’s actually a metal strip, so both diagonals contribute something because it can take a limited amount of compression as well. You have several strips (it is a pan after all) so the load is distributed among a much larger area. It’s very efficient for the job that you want it to do.

If you put hexagons in there instead, you lose that direct load path. Now the load can’t travel in a straight line corner to corner, it has to zig-zag around the pattern. Ultimately, that greatly reduces the structural capability and stiffness in this application. If you take a hard hit and part of your frame flexes, the hexagonal pattern is much more likely to show signs of buckling/crippling than the diamond. Even a diamond pattern that’s not the same aspect ratio as the frame is still better than the hexagon. It doesn’t have the same direct corner-to-corner strip, but it still has straight load paths all the way through the material.

In FRC applications it’s rarely that critical unless you’re designing on the very edge of the structural capability, but diamond is still objectively “better” than hexagonal structurally, which usually translates to lighter weight, since it can handle the same amount of abuse with less material.

If you are looking for rigidity and easily mounting parts onto a belly pan, I would suggest using perforated lexan since the material won’t break under normal circumstances, and all of the holes in the lexan make mounting components easy and can help with clean wiring.

Diamond strip is better. I’m not going to get into why because plenty of people already explained it. However, I will add that making bellypans yourself is, in effect, a massive waste of time. If your robot actually needs the bellypan as a vital structural component, odds are you did something wrong. Build the frame right, and it won’t matter. Personally, I’d get the hexagonal pattern because it looks cool and it’s easier to mount electrical components to. However, if it were an option, I wouldn’t do a diamond or hex pattern bellypan at all. I’d suggest just getting some perforated aluminum sheet. It’s cheap, it’s fast, it’s light.

I have to disagree with the statements above. Bellypans can make a structurally rigid frame that is lighter than one without.
However, I will agree that any resources poured into some complex bellypan design are likely wasted, when a wooden bellypan can do the same job effectively for less weight.
5254 has this perforated lexan sheet they use for mounting electronics, because it already has a standardized hole pattern and they have a ton of it.

Between diamond and hex- for the most part it doesn’t matter on the FRC scale if everything else is designed well, but diamond tends to be the better option structurally.