Ideal weight for a robot cart

i decided to design a robot cart for my team since we were using our schools food cart as our robot cart, and well, by some calculation mistakes, my robot cart weighted around 75 kg. However im not quite sure if this is considered heavy for a robot cart like this


the rectangular tubes are 80x80 and 4032 aluminum
the whole cart is 1160x1000x760 without the upper part

What did you use solid profiles instead of hollow tubes? :laughing:

For real though, the answer is “as light as possible and no lighter”. If I had to give a number, I’d say for us that usually comes out around 30 kg.

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They were hollow, but i gave offset to wrong side while making the tubes. Still, they are pretty heavy.

You’re using 80mm square tubes? That’s excessive for anything like a robot cart, those are pretty much structural tubes for holding up serious weight (like, several hundred kg.) Make the cart out of more standard tube, something on the order of 50mm x 25mm or even 25mm square, with a wall thickness of 2mm or 3mm (the rough equivalent of SAE 2" x 1" tube or 1" x 1" tube with .062 or .125 wall thickness.) You overbuilt this thing so much that it’s no wonder that it’s heavy.

40" wide is way too big and wont fit through most doors.
Most carts are 24-30" max. I would lean to the shorter side like 600mm.

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+1 I would not rule out alternative materials like baltic birch plywood that can enable a solution that is more than strong enough and quite light weight.

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I recommend going back and defining the problem better. I don’t recommend focusing on a measure of the solution. Make sure you’ve defined the right problem.

Every team will have different resources, constraints, and operational uses.

As one person mentioned, 40” will be too wide for many doors. ADA codes require a 32” door. If you need to transport your robot to outreach events on the cart, then fitting in an ADA compliant 32” door needs to be considered.

Some teams have trailers with ramps and they can wheel their robot cart on and off the trailer. Other teams must load their robot, cart, and pit equipment under their bus. This requires a very different style cart.

Some venues have wide aisle ways and broad corners. Other venues have narrow doors and require zero radius turns. Some venues have a ramp between the pit area and the competition area where a too heavy cart could go out of control and injure someone.

Instead of defining a “right” cart weight, I recommend getting back into the problem definition and studying how the cart will be used and what its purpose is. Writing down a Concept of Operations (CONOPS) and developing User Stories or Use Cases will help you separate the problem from the solution.

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To literally answer the thread title, I want a cart that is exactly zero pounds. No more, no less.

However, if I can’t get exactly zero, cart with slight negative weight is far worse than a cart with slight positive weight.

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@erenorhun I have some recommendations. First, check out this post on the FRC blog, as it pertains to cart sizing: https://www.firstinspires.org/robotics/frc/blog/2023-event-rules-updates-for-2024

Second, I recommend first looking at several examples of what other teams are using for their robot carts. You’ll find that many don’t use a custom cart – they just get something off-the-shelf (e.g., we have this: https://www.homedepot.com/p/Everbilt-Structural-Foam-Adjustable-Hand-Trolley-690102/302883370 – weighs 9.3kg, made of plastic, folds up). This saves time: the scarcest resource in FRC. You might not have the same off-the-shelf options available, but I am certain there are some options.

Third, I generally agree with other folks in this thread, should you still choose to build your own: 50mm x 25mm x 3mm 6061 aluminum-alloy tube is roughly the right material for this job. Or wood – wood is also a good material for a robot cart.

One person should be able to lift the entire cart in a pinch. Doesn’t have to be easy, but if it’s over ~100 lbs, something is wrong. Especially if the isn’t storage for toolboxes and batteries and snacks and water bottles and the armoire.

There’s a JVN blog post (in the internet black hole now) featuring a hilarious exchange as they slowly realize I’m using the robot like a kick scooter instead of getting the cart.

Maybe don’t go that far, but consider integrating a shopping cart handle into your robots.

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Something that hasn’t been brought up yet: small carts are likely to have field access this year (unless they’re unsafe). I’d have to find the blog post for exact size limits. But a small light cart that can hold a robot may be worth looking into.

My cart requirements, no particular order:
-single person to move
-hold varying 150lb robots over the years
-carry some selection of tools and at least one spare battery
-easy to transport, either in one piece or by quick disassembly

Optional: include dedicated spot for driver station (becomes mandatory if driver station is unwieldy)
Optional: be under field fit size.

I went exactly where @gerthworm did. The ideal robot cart would be made out of that infinitely rigid massless stuff in all the high school and freshman physics textbook problems. Really ideally, it should instantly fold up into something small enough to put in your pocket, like Jane Jetson’s “car”. Just big enough so you don’t lose it.

More seriously, treat this like any engineering challenge (like your robot, hopefully!). Define and prioritize your requirements. Examples might include (in rough order from near certain to far less so):

  • Carry the Robot
  • Be small enough to get on the field with the new rules, or at least small enough to fit through doors and into the pit with the robot on it.
  • Carry the driver station
  • Carry a spare battery (or two)
  • Carry a (hopefully small) set of important tools
  • Be budget friendly (includes both money and time/energy).
  • Carry paperwork (e.g. checklists, match schedules, strategy sheets)
  • Be easy to “tuck away” fieldside and in the pits, or to double as a workbench or some other function.
  • Display team colors theme, logos, sponsors, etc.

Once you have your requirements and priorities in order, come up with a few basic designs. Pick the one that best meets your prioritized requirements, go forth, do more detailed design, build (or buy, sometimes a COTS item is the best answer), and iterate as needed.

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You can check these two posts to get inspiration for your robot cart design. They both have the CAD of their robot cart publicly:

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our robot cart is super heavy to move simply because we have 2 giant wagon tires in the rear that rarely get inflated, so it feels like you are moving a boulder up a hill all the time during a comp
this is just me and team bias but I’d maybe consider making the bottom a flatter surface and adding a jack to raise and lower the robot platform, it may reduce weight depending on how you build both sides

The weight of our last made robot cart from 2023 is 22kg and is made of 30x50x2mm aluminum tube and 10mm plywood.
As mentioned earlier, I would advise first look at your requirements and wishes. And I think it makes life a lot easier if it’s less wide. You can always take a look at our blog post and if you have any questions, I am there to answer them.

We’ve used a pneumatic lifting cart for quite a few years now. It’s small enough to easily fit through a doorway and can easily hold our robot. The lifting capability is really nice and allows the people moving the robot to not need to raise the robot too high. We can then lift the entire platform and work on the robot.

The only problem is it weighs about 90 pounds. This hasn’t actually been an issue, besides some slight inconveniences and not being able to ship it to Houston.

Size matters much more than weight.

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