Bellypan material and strength

Hey all! We’re incorporating a bellypan into our design for the first time this year (like so). Our robot is open-frame (U-shaped) so the bellypan will act as both support and an electronics platform.

Our prototype is currently using 1/4" laser-cut wood. We were going to use aluminum on the final robot, but as of now that isn’t looking like a possibility due to both funds and concerns about electrical conductivity.

There are points on the bellypan that take a lot of stress, all of which have to do with keeping it secured to the robot’s frame. Our catapult’s hard stop repeatedly pulls with immense force on the frame. This happened yesterday.

For the final bellypan, we were thinking either wood or something like polycarbonate, but given recent events… I’m not too sure about the outcome.

Which material gives the greatest strength? What do you use for your bellypan? Any advice is appreciated.

I’d use polycarbonate for both strength and electrical insulation properties. Are you limited to 1/4" thick? I’d also look at adding one strap of metal or aluminum angle to close the last side of the U.

We typically use 1/4" Baltic Birch plywood - very strong due to solid, consistent thickness ply…but with your demonstrated failure I would use 3/8 and reinforce it with some aluminum straps or angle if possible.

Is there no way to make your frame a complete perimeter? Then the belly pan only really needs to add stiffness.

If you are not pushing the edge of the weight limit - you can go as high as 3/4" Baltic Birch - and that would withstand a great deal of stress. Plus weight down low is a good place to have weight!

You may also want to think about where various elements are attached to the belly pan, especially the heavy battery. It looks like the battery is floating in the center of the pan’s unsupported edge and your straps that pull your catapult are attached to the very edge of the U. When loaded, the frame will flex. When suddenly unloaded, the frame will move the wood in one direction, but the battery’s inertia will resist. This will require the wood to accommodate the strain. The breakage will occur where the flexible wood is no longer allowed to flex – where it is bolted to the frame.

If you move the battery to another location within the U, that would help a lot. You may also consider mounting the battery to the frame instead of the pan. The rest of the electronics don’t have nearly the same mass, but the same principle applies. If you can reduce the flex by bracing the gap, that would help too.

Greg McKaskle

It’s amazing what you can do with some carbon fiber and Baltic birch plywood.
We have made many bell pans that are structural part of the robot. There are many video’s on the web of how to lay up some plywood. To deal with what you are doing would require more than a simple layer on each side.

For our electronics, we usually do a thin piece of wood, coated in Kevlar a few times. It’s very light and very strong. Wee also make our driver station out of it. It’s a great method if you have the resources to get it.

It appears the frame doesn’t go all the way around the belly pan? maybe a picture of the whole robot, or at least the design of the whole robot, will let us see what you’re doing. If the belly pan were only a belly pan, and not required to act as frame member, then the material you are using should be fine. But it doesn’t look like that’s the case, from this picture?

We are in the process of making our first WCD (Thank you 2791 for all the help).

I like the idea of using the 1/4 birch plywood for the belly pan and was just wondering how you go about attaching it to the frame. We are riveting the top together with gussets and I am not positive the best way to attach the plywood to the bottom. Can I just rivet the plywood to the tubing or is their another way? Thanks

When we run wood we rivet it every 1.5" or so with regular rivets and have been happy with the results.

Thank you for the fast response.

The past two seasons we have used 1/2" Baltic birch for our bellypan/electronics board. We haven’t relied on the wood for holding the frame together, but more to help add rigidity. We even used it 2012 to made custom 10" wheels for the drive train.

We typically only put holes in the wood if we are using them. From the photos, you may want to eliminate some of the unused holes near the high stress area. You may also want to look at distributing the load on your hard stop mount. Are you using a large fender washer? Could you make a quick ‘glorified’ washer that has an additional mounting hole about an 1" or so on either side of the main hole?

Another nice thing about wood is that it is very easy to use a router to recess things or “pocket” the wood.


Access to components can be increased by building a belly pan that can accept the components from the bottom. Talons make great parts that can be assembled from the bottom and wired through a cutout. Think of increasing your mounting area by using a layer. Access to your controls is as easy as flipping the robot on it’s side.

We typically use screws and nuts to hold the bellypan to the frame, then woodscrews to attach stuff to the bellypan. Something inside me (not sure what) likes having the ability to remove the whole piece if necessary - so I (personally) would not use rivets - but I can see the advantage of not having screws that can come loose. For an easy, but still removable method - self tapping sheet metal screws would work too.

We do rivets. I don’t think we’ve ever had to detach a belly pan, but if you do it’s just a few minutes of drilling out rivets. With our setup though we have the belt inside the tube so using lots of nuts inside the frame would be a little bit annoying.

If you want to use screws with a WCD belly pan (or really anywhere in / through square / rectangle tube) rivet nuts are probably the best option. We use rivet nuts almost everywhere except where we have to tap. I don’t know if this year’s robot even has a regular nut on it anywhere. Definitely worth the investment and less of a pain than PEM nuts.

Our team this year did our first custom drivetrain and for the bellypan we used garolite. It is strong and rigid yet bendable so it does not break, it is fairly cheap and easy to cut. Another plus is to doesn’t have conductivity like metal. I would highly recommend using it.

If you want an easily-removable belly pan and don’t want to deal with bolt heads, consider getting a thread setter, which is great for putting aluminum threads in polycarbonate and wood (as well as 1/8" metal, if you’re not happy with only 1/8" of thread.)

Actually just got out rivet nut tool and rivet nuts in today thank to Brian suggesting it to us when we came over for a visit. Learned so much that is going to make our lives 100 times easier next season.