# Heavy Bumpers

I’m imagining someone arguing with the robot inspector…

“What do you mean we can’t inject concrete into the pool noodles?”

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I’m not sure we’ve seen that before, but we have seen teams stick steel bars/pipes in the middle of the tubes before…

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According to Google, standard plywood has a density of 680 kg/m^3. If your bumpers are 3/4" plywood, 5" tall, and wrap around the whole robot perimeter (120"), that’s 11 lbs. Aluminum 1x1x1/8" angle weighs ~0.3 lbs/ft, so covering the whole perimeter weighs about 3 lbs. There’s about 15" of fabric wrapped around the circumference of bumper profile, and the 1000 Cordura fabric that AndyMark sells weighs 11 oz/yd^2, so that’s about another pound in fabric.

In total, that’s ~15 lbs, not including any other mounting hardware (bolts, nuts, wood screws, etc). That’s about the maximum reasonable weight for a set of bumpers without actively adding weight to them though. They can weigh a lot less if you have a smaller robot, a lighter mounting system, or don’t have full perimeter bumpers.

Heavy bumpers will lower your center of gravity, which can be helpful if you have a robot prone to tipping. And they’ll add you your total robot weight, increasing your maximum pushing force. But on the other hand, heavy bumpers increase your robot mass, meaning it takes more force to accelerate the robot. In general, heavy robots are slower and drain the battery faster. So in general heavy bumpers are good if you want to design a robot that’s better in pushing matches; if you want to have good acceleration and quick cycle times, you want to cut mass out of every part of the robot including the bumpers.

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Last year we used lightweight aluminum brackets on the front and sides, and a big chunk of steel L on the back, to not only get us up to weight, but to put as much of that weight as possible in the back of the robot to counteract our cube mechanism when we went tall.

Bumper brackets are great for shifting CG a little bit.

You most certainly cannot put sheet metal on the inside of the bumper. However, as mentioned in the other replies, if said sheet metal is part of your bumper backing because it’s integral to your particular mounting system, more power to you.

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Cool. Thanks for the clarification.

To reduce the center of gravity, we fill our bumpers with concrete instead of pool noodle. Gives ramming defense bots quite a surprise.

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As others have said: don’t do anything illegal like filling your pool noodles with weights. Instead you can take advantage of how your bumper weight includes the bumper half of the “bumper mounting system”, and just build an extremely heavy and rigid “bumper mounting system”. Eg thick steel angle instead of thin aluminum.

The other tip would be to use steel L-channel for the permitted bumper fabric clamping material. This isn’t always allowed to be steel but that was permitted this year.

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We used baltic birch plywood this year. It is a little denser and much higher quality then typical plywood. It definitely helped increase the robustness and quality of our bumpers.

For my team we used ply wood and added steel brackets to increase the wight, this helped us many times when we got close to tipping over.

it took 118’s reveal video this year for me to figure out we’ve been making bumpers to an antiquated rule set.

its clear to me now that you can create a metal interface or support to the bumpers that defines your robot frame perimeter, but is completely detachable from the frame for easier bumper removal than trying to find a way to create latches, pins, etc. to the bumper wood itself.

it also eliminates the need for the popular design of a welded frame rail, typically 3/4" or 1" square tubing parallel to the drive rail. the detachable bumper support/frame perimeter serves this purpose.

and yes, of course, that metal frame perimeter must be counted toward robot inspection weight.

Oh so that’s why we hit you and we break… makes sense now.

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I had to go look at it myself after reading it. Here’s a capture from the video, when they have their bumpers off:

Based on that image, I believe they have a full frame perimeter identified, with appropriate backing for the bumper. A shot from a different portion of the video shows nothing that I would call a removable support on the open side where their wheels are - in fact, I believe the white tabs are part of their mounting system, which would place them right at the point where their supports are.

The cold anodized “frame” that goes along the top of the bumper is clamping the fabric to the wood, not creating a frame perimeter or interface.

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there are moments where they drive bumperless and i realized my ignorance of the bumper ruleset for the last 4 or 5 years. i always assumed there had to be a continuous backing that defined the frame perimeter, in order to do so you need a rail over the wheels (see 254 drivebase). you can also make out how they use a bracket that remains on the bumper and frame.

maybe im wrong that its counted in with robot inspection. but i’m really just admitting to my ignorance of the latest bumper rules.

@Tyson - if you look at the first screen capture I posted, you can see white frame pieces that stick out between the wheels. Those standoffs provide the bumper backing required by the rules. I assume they are set up so the space between them is <=8", and the space between them and the front/back rails is <=8". Those white pieces, along with the front/back rails, constitute the frame perimeter. Any other parts you noticed on the bumpers are considered part of the bumpers, and are weighed with the bumpers.

Yeah, I agree. What 118 actually did is not really my point.

I think you could design a detachable frame and bumper support brackets as part of the actual robot frame perimeter, with the intention during an event it mostly stays with the bumper.

How metal bumper frames and supports get inspected is potentially a grey area but really, I’m not a rules nerd and dont care to discuss it or argue any side. and dont mean to point out 118, just recalling the moment I realized the frame perimeter rules have changed since I started, and quite a few years ago without me realizing it.

Our construction method:

Full frame bumpers, 1 per side unless intake cutouts etc. are needed, in which case there are likely 2 shorter bumpers on a side. Each bumper solid piece of 5" height 5/8" thick plywood, held in place by what McMaster-Carr calls “steel adhesive-mount studs” screwed into the front of the plywood (https://www.mcmaster.com/#standard-threaded-rods/=513c227bb7784ab0bc040997f33cd641jvfre7pa). The studs are fixed with wood screws to the front face of the plywood and go through a hole drilled into the plywood and a corresponding hole in the robot frame, wing nuts in the back hold everything in place. Pool noodles (solid if available) are hot-glued to the plywood to try and make them straight. If making this kind of bumper, ensure you calculate in the plywood thickness plus the thickness of your robot frame plus at least 1/2" for threading the wing nut and general extra tolerance when ordering your studs.

The pool noodle/plywood assembly is wrapped with printed and plasticized Cordura or similar, which is wrapped as tightly as possible and stapled to the plywood in the back. Final step is aluminum angle (3/16 I think?) top and bottom, screwed into the back of the plywood with countersunk wood screws at about 4" spacing. Upper angle is equivalent leg length, lower angle is higher than it is deep to help keep CoG low.

Not sure exactly what they weigh, probably very close to the max weight. They’re very durable and stay in place. Drawbacks for sure - not quick-change and the pit crew are not fans of changing bumpers for red/blue alliance swaps as it involves threading wing-nuts in hard-to-reach places.

R31-F and figure 10-5 show that you can use metal between the noodles and the plywood to reinforce corners. There is no stated limitation about size other than the 1 inch limit for hard parts in the cross section.

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