Why is weight so important?

This will be our fourth build season, but I’ve only been utilizing chief delphi and other resources over the last several months. I’m curious about something that seems to come up frequently in technical discussions: weight. Why are there so many individuals and/or teams concerned with having the minimum weight on their robot and parts (some recent threads on wheels that weigh less than a 10th of a pound would be a case in point)? Of course, I understand the need to remain under the limits (our bot had to be drilled at the last minute to make the weight requirements last year), but really, once you’re under a certain maximum, what’s the point? Is it just something to add to the challenge from an engineering perspective? Is it bragging rights? Are their special awards/categories that I’m unaware of in my ignorance of FIRST culture? :o I appreciate any enlightenment you are willing to bestow.

Team’s sometimes like to keep their weight to as light as they can get, while still being structurally sound, in order to give them leeway when adding on components and other parts at regionals. There’s also the case of weight distribution on a certain type of robot. Teams spread their weight evenly or lopsided in order to maintain CG and while building their robots they keep this in mind.

The vast majority of FRC robots are right at or very close to max weight. Any weight that you can shave off one part (gearbox, chassis, etc.) is weight that can be used to add functionality or additional strength to the parts of the robot that you feel need it more.

Its alot like Backpacking, if it takes cutting off the handle of a toothbrush to make it work, then so be it!!!

It can also be a weight / power trade off.

Eg to get a robot moving at the same speed at different weights takes different amounts of motor power. By making the robot lighter you may be able to use lower power ( cheaper ) motors.

Also if the motor doesn’t have to generate as much force ( torque ) to move the robots then you use less electrical energy from the battery. So the battery charge lasts longer or you can use that energy somewhere else like a stronger / faster motor for the arm.

Materials is also another area where you can save money.
To keep it simple I’ll use an example of castors.

Say you have 100lb robot on 4 castors. So say you decided you need 25lb castors ( 4 * 25 = 100lb) to spread the weight ( we’re ignoring a lot of things to keep this simple ) and they cost $10 each = $40. But you lighten the robot to 80lbs now you could use 20lb castors at $8 each = $32 a saving of $8. Not much of saving but every bit counts.

Also a lighter robot means you can ship more spares / tools in your crate.

I know this is an over simplification but I hope it gives you the idea.

It’s a matter of balancing everything out.
Like you wouldn’t try and tow a caravan over the Rockies with a British Mini car. In the same way you wouldn’t use a Kenworth truck to tow one of those luggage trailers you see towed behind mini vans.

There are many reasons to keep the weight of structural components as low as possible.

First of all is the aesthetic argument that a strong, lightweight piece is simply more elegant and better thought out than a needlessly heavy piece. This isn’t the most convincing argument, but in a contest that focuses on engineering design I believe it is relevant.

On a more practical basis, by building structural components light, you can have more of them… this means that you can get your robot to do more things. For instance if you can shave a couple pounds off your frame, then you can perhaps add an extra motor or manipulator component.

Designing with a focus on weight also prevents last-minute robot diets… I’ve seen teams have to shed as much as 30 pounds on a Thursday at competition, but in our experience it mostly comes at the end of build where we need to re-inforce or replace one system or component, but don’t have room in the weight limit. So we break out the hole saws and start lightening the robot elsewhere. Now we *try *to design the parts lightweight right from the beginning as it is much easier to shed weight at the design stage than at the “oh sh…” stage.

Building light also allows for changes to components at (or between) competitions… or repairs that aren’t as light as the original part. At GTR one year a major component broke and we fabricated a heavier replacement part. Going in to elims we weighed 120.0 lbs. Had we not taken the time to replace the steel machine screws holding the electrical components to our robot with nylon screws two months earlier during build we wouldn’t have been able to compete on Saturday afternoon.

Finally, even if you can build absolutely everything you want in to your robot, come out ridiculously under the weight limit, with lots of room for upgrades and repairs, you can improve the performance of your robot by building lighter still. It gives you the choice of either having lower mass for higher accelleration (F=ma applies here too) or adding ballast to lower your centre of gravity and/or improve traction and handling.


This long and the short of it is that the lighter your robot is, the more stuff (or the more complex stuff) you can put on it. While saving, say 1/2 pound isnt much, doing it three or four times in a few different places can make a huge difference. Most importantly, 1/2 pound may mean nothing if your robot weighs 115 pounds, but makes a huge difference if it is already at 120.

As stated before, a lighter robot we be able to accelerate faster than a heavier robot, which can be very important in certian circumstances (rabbit robots this year is a perfect example).

I’m somewhat fond of saying, if your robot dosen’t weigh 119.99 pounds, you forgot something.

For me, I design all moving parts to be as light as possible, and all fixed parts to try and hit the sweet spot between strength and weight. Lighter moving parts = less force required to get them moving = faster acceleration. Speed = win.

Because the new control system weights as much as a fully-loaded M1 Abrams Tank.

Oh my… so does that mean we such do tank track drive setup to squish the red fishy? Hmm.

You’re some kind of a math genius, Craig. :wink:

In the several responses to your question, no one has yet directly answered some of your questions

No - It is the result, not the source of the challenges.

Sometimes - For some people this topic does become their harmless obsession … Other folks find other ways to improve designs.

No; but all other things being equal, lighter is usually better (and can be pointed out when the obvious awards are being sought).


I view weight as a limited resource on the robot, you can only have so much of it.

If you could buy drive gearboxes that worked great, were reliable, and fit your application perfectly for $100, why pay $200 for something that does the same thing? I feel the same way about weight, if you can cut weight WITHOUT killing reliability (which should be obvious, you should be making parts strong enough regardless), I see no reason not to.

Being lightweight in component design allows you many benefits; you can fit more on a robot as it all weighs less, you can keep the robot weight light for better driving & decreased battery use, and you can make up that weight with ballast real, real low on your robot to put the CG exactly where you want it.

I’ve seen a recent trend on CD that many people feel that removing weight through machining operations is unnecessary, and even a foolish waste of resources. It is a worthwhile operation, and doesn’t even need to be a CNC’d accurate operation, much of the cosmetic pocketing you see on gearboxes with fancy curves and such can be approximate with a manual mill (if you know integrals, imagine you’re approximating the pocket with a bunch of rectangles X units wide. X being an endmill small enough for a decent amount of material to be removed, but large enough as to not take forever). You can even draw the pattern you’d like, and drill or dremel material out; just don’t take more material out than you designed in the drawing.

Before trying to machine weight out, assuming your fabrication resources are limited, you should consider thinner and lighter materials or just try to make parts smaller overall.

As mentioned, weight is an issue with the new control system being about 3 times heavier than the old one. Generally speaking, teams have a hard time staying in the legal weight limit with a bare minimum running much less. Last year I had one or two teams that weighed in at less than 90 lbs. with the bumpers. I had one team weigh in at 143 lbs. due in part to a faulty scale at their shop. Note to self…test the scale using last year’s robot with the battery out to be sure.

Weight is definitely very important. There are different ways of dealing with weight, though. If you start out with a realistic weight budget for different parts of the robot, and revise the design as needed to meet that budget, then you might be able to get away without having to do any “weight removal” machining at all. We did this last year, our robot was pretty big and capable and weighed 10 lbs under the limit.

There are some folks who seem to get carried away with making everything as light as possible, and they probably have a great future in the aerospace industry. Others don’t really worry too much about spending any extra time making things light, and they probably have a great future in the consumer goods industry.

I fully concur! If anyone ever calculated their final power consumption under load (sans pushing and shoving), they’d probably find that 5 lbs of weight can easily translate into several extra watts of available instantaneous power. We all know what more power does: a more responsive drive train, faster manipulator movement, and more room for error when you’re lifting a game piece.

For those of us who do offseason design, mainly drive trains are what you see. If we can shave 20% of the weight from a 35-45 drive train concept, we inherently add watts of power to be used in acceleration of the drive train. Another benefit of extra power is that drivers can simultaneously move more things at once, such as the drive train, and say 2 degrees of freedom on your manipulator.

Weight isn’t to be blindly removed as a trophy piece, it should be carefully considered as to what it would affect. Sure, having a light-weight robot is very good most of the time, but under specific circumstances, it can be iffy. Last year, our robot weighed in at 99.1 pounds (without battery and bumpers). However, this was a 13-foot max height forklift, and it’s center of gravity was actually fairly high with the lift lowered. In a practice match, 118 ran into us when we were putting the ball on the overpass and we went down, and on competition day in our second match, 118 just barely pulled us over when they got hooked on our arm early in the match. Then we added 4 steel bars about an inch from the ground and ended up at 119.1, dead on 20 pounds. Now they’d probably be the ones flopped over.

Weight is a structural property, and just like any other structural property, it can be used to help or hurt the robot.

But also consider if we’d had exactly 21 pounds of bars. To put that in perspective, that’s a medium Red Bull away from being disqualified until another 20 minutes of work could be done, by which time we could have completely missed a match. Doing careful consideration on the generalized weight and weight distribution of your robot can really save you sometimes.

Note to all rookies:
Teams never give up trying and find all kinds of ways to improvise. However, unstated here is the need to reinspect before continuing with competition. Please involve your inspectors when making changes in the heat of competition. They can save you a lot of work and heartache.