Rookie Team - How much should we worry about weight shaving?

Like the others, plan for a simple robot (that means cutting scope - functions, mechanisms, etc. Adam Heard has great talk on this). You should not put yourself in a position where you’re “obsessing” about weight savings as a rookie.

However I DO recommend you at least track your weight as you build the robot, and maybe even go so far as to make a weight estimate before you build it. This will help your team both refine the skills and processes needed for future seasons, and allow you to identify if things aren’t going to plan as early as possible and course-correct if it looks like you’re on track to be too heavy.

Like Connor stated, don’t make inspection the first time you weight your robot. You

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Our rookie year the robot was 30 lbs or so overweight, weighed a few days before it had to be crated up. So yeah, you need to pay attention to weight, and asking about it now is an excellent indication that you are aware of the issue.

But you can control robot weight many ways, and there a bunch of old threads on CD about how to do so. Planning ahead is the biggest one…

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As a fellow rookie during the Bush administration, I’d venture between a greater understanding of materials now and the added protections inside the frame perimeter it’s a lot easier to keep it lightweight now. :slight_smile: (Our last two robots have gone first round or captain at regionals without cracking 100 pounds at inspection. Pretty sure the 2020 one was under 90. And that was with the heavier 6-Mini-CIM setup.)

The best way to avoid needing weight savings:

  1. Analyze the game and determine bang for buck. Yes, the Simbots and Cheesy Poofs of the world will do it all. Will you? Probably not year one, and that’s cool. In 2019, we saved a ton of weight by not going for the top level of the rocket at all.
  2. Picking materials wisely. For an astounding number of situations these days, 1/16" wall tubing is plenty. (I’m not saying all situations, but an astounding number of them.) And my favorite example is comparing that to a heavily-swiss-cheesed 1/8" wall tube:
    Both 18", both 6061 aluminum. The one on the right is 0.246 pounds lighter with no weight relief.
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I wouldn’t worry too much about it at this point. As others have mentioned a rookie team, particularly this year, should focus on mastering one thing and then consider adding other functionality once that is operating acceptably.

I’ve been a RI and LRI at many events over the years, 4-6 per year since the start of the PNW district. Many of those years I was also the FSM for our state, so I have worked closely with many rookies, especially at events. I’d say that the average rookie I’ve seen is 80-90lbs and that is w/o them putting much if any consideration into how much the robot weighs.

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True, but having a hole pattern can have other advantages like making adjusting stuff simpler. For example, I think we’ve tried three different mounting locations for our intake and it’s as simple as unbolt, slide, rebolt. Not to mention we’re now on our second intake, climber tower, and winch. Maybe there are teams that get everything perfect on the first go, but we’re not there (yet).

My advice is to make sure you set your part materials correctly in your CAD program and you should have a pretty darn accurate estimate of your robot’s weight (and CG) before you even make your first part.

Premature optimization is a well-understood “anti-pattern” in (Software) Engineering — measure and understand first, then you will know where to focus your time & effort so you will get the most bang for your buck. So if your complete CAD says “91 lbs”, is worrying about weight the best place to spend your time now? Quite probably not.

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We’ve discussed this at length over the summer. Mounting patterns and weight relief patterns are two very different things. While you do remove some weight with a mounting hole, nobody in their right mind is doing weight relief with a #11 drill bit. Heck, nobody in their wrong mind either.

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It’s a great time to develop a weight tracking system.
I make a spread sheet and then weigh every component.
Then I weigh assemblies and track them.
You will always know where you are weight wise and it also makes creating the BOM so much easier (if we still need such a thing.)

One data point: In my team’s rookie season, we really built the robot to be sturdy and reliable. And the first time it was weighed, it came in at 165 pounds. Without the battery or bumpers.

Yikes!

The lesson here is that you don’t need to obsess about weight, but do pay attention to it. Create a basic weight budget and pay attention. Like 45 pounds for the drivetrain & frame, 25 pounds for the thingamajig system, and so on, and weigh things as you go. If the thingamajig comes in at 28 pounds, OK, but if it’s 50, then you need to rethink it.

As you get more experience, you’ll find that CAD software can estimate assembly weights fairly well. The goal is for almost everyone on the team to be proficient in CAD, so things are designed and built in software before any metal is cut.

All that being said: Metal is stronger than it looks, and the robot doesn’t need to be armored and bullet-proof. 1/8" aluminum is too thick for almost every application.

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That’s because they should be using a #9 drill bit :slight_smile:

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I did this FEA around 10 years ago. I’m going to keep posting it until everyone understands how ineffective lightening holes are in tubing.

Each tube is the same linear density, one is 1/16in wall, the other lightened 1/8in. The lightening pattern destroys the strength of the tube. In the last ~10 years no one has ever demonstrated a lightening pattern in 1/8in wall tube that didn’t adversely affect strength while also reaching the same linear density as 1/16in wall tube.

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As you said not that important this year, but it will be when in-person competitions resume! I give a presentation at some local training events each year about the top 10 ways teams fail inspection and how to fix them. the top reason is ALWAYS weight. Competition is not the time to shave weight - it can be extremely time consuming, not to mention difficult! Think about the difference in building with thinner material from the start, versus drilling 100 holes to try to lose weight. Saving weight as you build, when you have pieces before they go on the robot, is the best time. Weighing the robot, and robot components, frequently is key to having an understanding throughout the season and being able to estimate where you’ll end up.

By far the best tip you can get is to buy a scale! My team got this one a few years ago and built a robot-sized platform for it. It works great, you can zero out the platform weight and having the readout on a cord lets you actually read it while the robot is sitting there.

This is a great year for learning with this sort of thing, as the results are less important than in other years - if you try to save some weight and discover you’re still over weight, you learned a lesson without having to go crazy trying to fix it at an event!

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I’d be interested to see that with a triangular “truss” pattern rather than circles.

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If you model something, or provide dimensions, I can run it quickly.

Obviously I’m no expert on this, but my guess would be that in order to get a triangle windowed tube of comparable strength to the circular windowed one you would end up with more weight. I think the triangles would create more intense stress concentrations which means you would need to window less aggressively to make up for it. I’d definitely also be interested to see the simulation tho.

Also, you can’t really drill triangles. Maybe some creative filing could do it, but for a team without cnc capabilities, it’s not really a viable option.
Triangles are actually going to be better, imo, strength to weight wise, because of how they can be packaged.

Can’t emphasize this enough. For the money, this is way better than a bathroom scale because of the remote reader. It’s also a good idea to get “calibrated weights”. Get 2, 45lb. barbell weights and find someone with a calibrated scale and record their weight. You can then calibrate your scale (know how far they’re off and do some math, mx+b right?). This is only critical if you’re one of those teams whose robot somehow always ends up at 119.8lbs.

There’s another issue and that is the official scales at the competition aren’t as calibrated as one might think. They do their best and their scale is the judge and jury. To avoid conflict the goal should be to stay 5lbs. under max so scale tolerances don’t bite you.

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I’d be more convinced if you actually tested this (in a UTM or whatever). FEA can be useful, but it has some limitations that aren’t always obvious. For example, anyone who’s ever bent a piece of square tubing knows that the most common failure mode is the side walls (in the plane of the force) fold up accordion-style. This is called “columnar buckling”, and is a lot more common than “real” compressive failure. With respect to columnar buckling, the 1/8" wall is going to be MUCH stronger than the 1/16" wall. However, most FEA software (I don’t know about Solidworks specifically) looks at things in the perfect world of CAD; since there’s no lateral load on the side walls, there’s no way for the buckling to “start”. Therefore, for the compression forces on the side walls, the FEA software is probably using the compressive strength of the aluminum, when in reality the walls will buckle at much lower stress.

What’s a good rule of thumb for how high your CoG is before it’s problematic? I could have sworn I heard something a long time ago about teams trying to make sure their CoG was under their bumper height, but for tall robots, that’s really difficult to hit. Is there a general rule teams typically go by here?

Our rule is to draw an equilateral triangle with the base of the triangle going between the points where your outer wheels contact the ground (ignore your drop). Your CG should be in the triangle if at all possible.

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one of my favorite teams, lightening their robot at their first competition. 1717 went on to do great things…

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