last year we used 70x70 steel chasis (20x40 box tube) for our robot, it was very durable but too heavy. next season, we are planning to switch our chasis to 75x75 cm chasis, which material would be more durable. For this design, 6063 aluminum weights 13kg with swerve and steel alloy weights 19kg. which material should we use and how can i make this design more weight and durability efficent
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First off, while I’m not sure what materials are available to you in Türkiye, I would point out that most of the teams that are using Aluminum box tube are using 6061 T6 Aluminum, as it’s generally stronger and easier to machine than 6063.
Beyond that, Aluminum box tube works fine, but it’s important to consider wall thickness. We use 0.1" (2.54mm) wall thickness on our 1"x2" (25.4mm x 50.8mm) box tube and it’s held up great, I wouldn’t recommend going thinner than that for a chassis, personally.
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You DO NOT want to use 6063 aluminum. 6061 aluminum is a substantially stronger alloy.
The other thing to consider is wall thickness. You want 3-4 mm, not 1-2 mm.
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did you had any problems with your chasis bending after a collision
While you don’t want to add unnecessary weight to the robot, extra weight down low (in the chassis) can be very helpful to keeping a low center of gravity. That being said, we typically build our chassis out of 0.125" thickness Al 6061 box tubing. It’s easier for us to machine. Some years (2023) we have added ended up adding extra weight to the chassis, and it would have been much easier to just make it out of something heavier to begin with. Other years (2024), we have had a big, wide, and overweight robot and it would have been a very bad decision to go with steel.
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do you have any advice about how can i change the structure to make this chasis more reliable for collisions with field elements like stage legs and other robots
Comparing our 2023 and 2024 swerve bots - 2023 we never bent anything even with bumper gaps exposing the frame to impacts, but 2024 with our under-the-bumper intake we bent the front frame rail several times. Having a belly pan sheet that spans the entire chassis front-to-back and side-to-side gave us a lot of rigidity in 2023.
We used McMaster 92985T52 because plastic sheet is easy for us to work with.
After an entire season and off-season of play (151 matches in total) there was some very minor bending in high-impact areas of the frame, but not to the degree that we would have needed to do anything about it. Our drive system is also one of the faster ones out there, so we had no shortage of high-speed collisions.
That said, it is important to note that our bumper mounting system also utilizes an 1/8" thick bent-steel plate (something like this), and we suspect that provides reinforcement for the center of the frame where it’s most vulnerable .
We also have a 1/16" thick aluminum belly pan riveted to the entire bottom-side of the robot which appears to help with stiffness as well.
If I was to build a robot without the steel brackets or aluminum belly pan, I’d probably use a thicker tubing (like 0.125" / 3.175mm)
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- I would define what “reliable for collision” first. This could be based on experience or just simply physics assumptions. Define this by something that has meaning (unit of measurement). For example, A bot chassis that protects the intake system when colliding with the wall travelling 20’/sec, etc. Once you have goal(s), then you can proceed with design that meets that goal. A modern car has bumper that is also a “crumple” zone. Our crumple zone is the plywood on the bumper.
- When I think about “robust” chassis under collision. I’m thinking energy absorption. Some of that energy is being absorbed by the bumper. So, perhaps better interface between bumper and chassis could be looked at.
- I wouldn’t make the robot “heavy” unless one of your goals is to not getting pushed around by other bots.
- There are ways to use different materials to increase the stiffness of the chassis. 6061 is pretty generic and everywhere. You could get better performance from 7075 in certain location. In fact, I would prefer to machine 7075 than 6061, if money is not an issue.
- Think about how you connect the frame members. The strength of the chassis depends on how good the joint connections are. If you know how to rivet well, great. Try to limits the amount of fasteners. The more fasteners you put in, the more you have to pay attention on assembling it correctly. I like welded frame, but this takes even more care and skills to do.
- Think about your CG. Even if you chassis is stiff and strong, high CG would create dynamic moment that could tip the bot under high impact collision.
Looking at your design above, one plane has more rigidity (left and right) compared to front and back. Tying the front and back to the cross member frames would increase the rigidity in that plane. Or adding the cross section of that front and back frame by bolting on another 1X2.
You could also replace that front and back cross member with 7075, while maintaining the weight. Just because it is steel, it doesn’t mean it is “stronger” than Aluminum. Different grade of steel has different mechanical properties.
- Make high quality bumpers. This is your first line of defense, and with the new bumper rules there will be options to absorb impact even better than in the past.
- Avoid long, unsupported lengths of 2x1 on the perimeter. The biggest problem last year with bent frames was that under-the-bumper intake designs often required long unsupported beams. I think your design looks pretty good in this regard!
- A nice belly pan will increase the overall strength and rigidity. Lots of material options here, but consider it a part of your structure. A polycarb bellypan will give you space to mount electronics, but it won’t improve your chassis strength like aluminum (or steel) would.
- You could use 40x40 instead of 20x40. I don’t think this is necessary, but it would add a lot of strength in the proper direction.
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its gonna depend on your robots weight and how you want to tune your center of mass, in the past we have used both (i.e. mostly aluminum with a rear steel tube) to shift weight backwards to offset the extension of our arm
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I’m not the mechanics guy. But our chassis frame had a severe bent after colliding with another robot, too severe that we had to quit in playoffs. It’s currently a 20x40 box tube with 2mm thickness, how can we make it stronger?
I think this says it all. More wall thickness on the chassis tubes.
Those of us who have been around since before bumpers wince when someone uses anything thinner than 1/8" (3mm) in the drivetrain. With the bumpers helping, you might get away with thinner… but with the increased damages seen, 3mm should be the bare minimum.
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@Mark_Wasserman can say much more on this:
There’s a good team I know of that builds chassis using thin wall steel tube. I think it is similar to this. Weighs about .336 lb per linear foot. I don’t know if the same material is commonly sold in metric sizes (e.g., 20x20 with 0.9 wall) but I would be surprised if it isn’t.
They reinforce and weld the frame joints.
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I will echo what others have said, weight down low is not necessarily a bad thing, most of the time it is highly desirable.
As far as things I did not see explicitly stated on my skim of the thread: if the game comes out and it is a complex/ busy field with little room for high speed collisions (e.g. 2018, 2019) then this is all a bit of a moot point and most materials will perform just fine, even thin walled aluminum.
As others have pointed out, more wall thickness (at least 3mm) is always a good way to start, and as I pointed out in my first post, make sure you’re using 6061 (or better) Aluminum instead of 6063 as there’s a significant strength difference.
Additionally, consider how your chassis frame is supported. Placing other framing elements behind your box tube and attaching them can work wonders to reinforce the outer structure, even if it’s thinner-wall than the outer tubing itself. Here’s our 2023 chassis as an example:
Granted, this is a bit of an extreme example, many teams would probably consider this “overkill” (the two long tubes running down the center were also used for various component and mechanism mounting, in fairness), but this chassis was rock-solid. The two smaller tubes on the left and right were thin-wall and only there for structural support and were pocketed to reduce the weight. Everything was riveted with gussets on the top and to the belly pan on the bottom.
Lastly, while not perfect, I have also seen teams use 3D printed inserts to fill the space in the middle of box tubing in a pinch. When done correctly it can help resist impact bending. This isn’t a solution in it of itself, but it can help a bit and the weight penalty is minimal.
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Thank you so much for answering! I’ll pin our cad lead @yzhhl to see this.
Wood is also a practical anti-crush material for teams with access to table saw, planer, joiner — the usual wood shop equipment. Sadly, HS wood shop has become much less usual than it once was.
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I’ve used 3d prints, plywood and polycarbonate blocks, all three worked well assuming a high infill on the prints.
We exterminated with HDPE. Strong lighter than steel. https://youtube.com/shorts/GHSv99yzYWM?si=WTuMzWZboggHX2Q-