To deadweight or not
 

Our robot is nearing completion, and although I am only approximating, I think our final weight will be no greater than 90 pounds. Even if I am off by 10 or 15 pounds, my question still stands: Should we add some deadweight to the robot to get up to 120 pounds?

We will absolutely be be doing our own testing soon to determine what is optimal for us, but I am curious to know what the community's opinions on this issue are. I think that this year will see many more robots that are underweight than in previous years, so I think that this could be a good topic of discussion.

In my opinion, this will be the year where many matches will be decided by pushing contests. Even with the wide open field, I don't see top speeds of greater than 12 fps gaining you anything. In addition, the argument of "we're going to be so fast/maneuverable that we don't need pushing power" does not hold as much weight (haha) this year since few robots will be exclusively offensive. Therefore, I would be very much in support of my team adding weight right up to 119.9 lbs.

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We're currently planning to ballast our mecum drive so the weight will be evenly distributed among the wheels.

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Speed kills. So many teams bragged about their defense last year but none of them stopped our light fast robot.
If I get there first I win no matter how well you can push me around. Keep pushing we'll be gone.

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Keep in mind you don't have to be able to win a pushing contest to defend successfully. You do, however, have to be where you want to be when you want to be there. With how open this year's field is, you don't have the luxury of field elements helping you block the other bot.

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I liked to think of your robot last year, in the nicest way possible, as the squirrel on caffeine. It was so fast, I was impressed your drivers were able to control it. Was there any software used to help with controlling at high speeds?

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This. Maneuverability far trumps "pushing power" in my mind. You'll be surprised by how fast a robot can jump to speed at 90lbs versus at 120, and what a difference it'll make in gameplay.

Keep in mind, if you think defense is just a contest of who can push more, you're doing it wrong.

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I would add a little ballast if necessary to get the drive train to run smoother/straighter. Otherwise any added weight should be functional, whether it be added mechanisms for defense that you can easily remove, or LEDs that are aesthetic and/or sends signals to you alliance partner. (Ex. making your ground pickup light up when ready to recieve the ball)
Granted you can justify a bit of frame reinforcement for the same reason you can justify adding more weight in the first place.

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If you're offensive, even if you "win" a pushing match, you still lose. Time is everything in FRC. Anybody can get three Assists and score the ball, but if it takes 2:20 to do that you won't win many matches.

Making sure you can't easily be pinned or pushed sideways is, however, a very worthwhile endeavor.

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A lot of things depend on what drivetrain you have... can you provide details?

A few things to think about:

Your acceleration rate with, and without, added mass.

Are you going to make your robot much more likely to stall its drive motors if you get into a pushing match? I would MUCH rather spin wheels on carpet than stall 4-6 CIMs. Handy reference chart here.

There are many different styles of defense, including just 'being in the way and making them push you sideways to get by' and what I like to call 'kinetic defense' wherein you simply repeatedly ram the other robot, throwing them off course and disrupting whatever task they're doing. Serious pushing force isn't always required for effective defense.

Making the weight easily removable so that you can be a 'heavyweight' for matches were you expect to play a lot of defense, and a 'lightweight' where you expect to play mostly offense might be the best of both worlds.

You could invest that weight into simple mechanisms to add functionality. Brakes, for instance. More durable and stickier wheels, a passive catching structure or something else to easily get the ball from the in-bounder, or a goal-tending mechanism come to mind.

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Correct me if I'm wrong, but wouldn't you have to get re-inspected every time you changed the weight of the robot, even if it is pre-planned?

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I think you should add some weight to your base if at all possible for two reasons.

1. This year robots will be taking A LOT of abuse and any structural help you can provide is key and

2. You want to keep your center of gravity as low as possible.

And to play devils advocate, speed has been great in past years when all you had to do was get to a safe zone as quick as you could and score from there. This year there is no such safe zone anywhere on the field so yes you might be able to maneuver around a heavier bot, but unless you can score on the fly then the second a pushing bot catches up to you they can slam you and mess up your shot. So having a heavier bot or at least a really solid one with a low center of gravity will be more beneficial than a quick bot this year for the case of scoring in the high goal/catching the ball.

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Nope that was all Nick. Best driver I ever had.

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We've got the same issue. Does not deadweighting really improve the speed of the bot by that much?

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See [R5]

And more importantly, [T8]

So, presuming you got inspected with all the weights attached, and all possible mechanisms attached/present, you could remove any of the weights at any time with no issues.

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Ok cool thanks:)

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Another successful type of defense if you have tank drive would be to turn side ways and as long as you weigh enough, you won't be pushed side way for the most part. To be successful at this it is also important to have good acceleration.

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I think that the dynamics of play this year will be very different compared to previous years.

In previous years, defense and offense were generally mutually exclusive, and often the former was a position relegated for rookies and/or teams with non-functioning mechanisms. This year, many robots will do both, and the best robots will do both very well.

In previous years, the defensive robots generally blocked the "best" opponent robot. Take last year as an example, the best cyclers might get 5-6 runs in a match. If a solid defensive robot takes away two of those, that is (2 cycles) * (4 discs/cycle) *(3 points/disc) * (75% accuracy) = 18 points worth of defense. Since the average robot scored <18 points last year, it would not have made sense for the above defender to totally shut down a below average team instead of taking 2 cycles away from the top team.

But this year, the positions are reversed, it makes more sense to shut down a below average robot than to merely slow down a top level robot. If this robot has the ball while you are playing shut-down defense on them, the entire alliance is in deep trouble.

It will be fascinating to see high-level robots with great drivers play defense. It will also be interesting (and somewhat sad) to see inexperienced drivers get defended by 1 or 2 good robots. Driver practice is (dare I say it) more important this year than in any other year, both for offense and defense.

This is why I am supporting pushing power this year. Everything we thought we knew about offense and defense from previous years is out the window for this year. Why? Because we have only ever seen above average teams being defended, and we have rarely seen teams like 254 and 1114 play defense.

Re: To deadweight or not
 

Just like everything else in FRC, it all comes down to what are you doing with your robot.

If you want to be in the Goalie Zone, blocking shots, then yeah, I'd deadweight, partially to keep your Center of Gravity low when the blocking mechanism is extended, and partially to make your robot harder to push if the opposing alliance sends another bot in to push you out of the way, like defending full court shooters last year.

Now, if you're strictly offense, or if mobility is key to your robot, then you really don't need to. The only advantage to deadweight if you're only offense is to make it harder to push you out of the way if you have to stop when shooting/passing. But seeing as most teams will need to stop to pass/catch/shoot, it might be wise to deadweight.

I'm definitely going to enjoy seeing what everyone else is doing, as well as watching the week one events, to see what works and what doesn't. Maybe you'll find an answer watching one of those events... Who knows?

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I'll ask again: are you traction-limited at 90lbs?

At full weight (~150lbs with battery and bumpers), 100% of weight on driven KoP wheels, and a 4-CIM single-speed drive, you will be traction-limited if you're geared for 7-9ft/s or less. If you're geared for a faster top speed and you try to push anyone with more traction than you, you'll stall your drive motors and likely start popping breakers. These numbers shift around if you use different wheels, add or subtract drive motors, and change the robot's weight.

IMO it will be very important to have a robust drivetrain this year, and every year. I don't want my drivers worried about pushing too hard and popping breakers. I want a traction-limited drivetrain. Then the drivers can get a lot more aggressive on defense without the risk of partly (or totally) disabling the drivetrain during a match from blown breakers.

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We are currently geared for approximately 8.5 fps with all weight on driven wheels that have CoF=1.07 (default wide AM14U kit chassis). According to your handy chart, it would seem that we would be traction limited with this current setup, and we will be borderline if we add more weight.

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Well, if you aren't going to gain any more pushing force by adding more weight (since motors will start to stall) I would say skip adding ballast and enjoy the improved acceleration and longer battery life associated with a lighter robot.

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Momentum typically makes for better defense than raw pushing power.

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If you're geared so slowly (under 9 FPS), you have little to lose and a lot to gain by adding weight. You'll push and resist pushing better with more weight.

Less weight is helpful for acceleration but a 4 CIM 9 FPS drive does not need the help.

Re: To deadweight or not
 

Hi, Question.

I'm not too versed in terms of the dynamics of traction-limitations and rates of accelerations vs weight.

Our team is shifting between 6 ft/s and 12 ft/s. We have a 6 WD with a 1/8" drop and 6 AndyMark 4" performance wheels w/ roughtop nitrile tread.

We're probably gonna clock in around 80 or so lbs.

So my question is: What kind of weight should we be looking to add?
We want a good compromise between accelerating quickly in high gear and still being able to push well in low gear.

With our current weight, I assume we won't get all of our torque out in low gear, but am not sure as to how much weight we should add before the decrease in acceleration is more significant than the increase in pushing force.

I guess short way to say it is: at what point will added mass no longer add to pushing ability? I'm assuming this is what's referred to as traction-limitation.

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Whoops, got it backwards. We are currently NOT traction limited, and we will be borderline if we add more weight. We would certainly not add any unnecessary weight if we were traction limited.

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Wait... that doesn't make sense.

Traction limited means that your drive wheels spin before drive motors stall. Adding weight pushes you out of the traction limited regime and into the torque/current limited regime.

If you run the robot up against a wall, do the wheels spin or do the motors stall?

Re: To deadweight or not
 

A drive is considered to be traction limited when, at competition weight, will spin it's drive wheels at some point before drawing stall current. This means that if you were to hypothetically push against a wall at full power, you would spin wheels against the wall rather than drawing stall current and popping your breakers.

As far as how much ballast to add, JVN's design calculator is awesome for this sort of stuff. Plug in your gear ratio, wheel CoF, etc, etc and then it does the math for you. Without knowing your exact gear ratios, it's a bit hard to give exact numbers, but assuming you're geared somewhere around 10:1 in high gear (right around 12fps floor speed with a 6" wheel) and your tread is good for 1.2-1.3 CoF, you're good to about 130-140lbs in total robot weight before you'd start stalling your motors in high gear.

With that being said, having a traction limited high gear is a bit unnecessary, since you can always shift down into low gear for better current management and/or motor loading.

If anything, the question of how much ballast to add before the additional pushing power starts to be overshadowed by the loss of acceleration is a very subjective one. There are a few acceleration calculators in CD media, but if I were in your shoes, I'd get some sort of dense and/or easy to work with ballast (steel sheets of a known size are nice) and add weight incrementally and do some testing. Sometimes testing like this will help to ground the calculations in reality since the math may produce results that aren't always easy to 'see'.

Re: To deadweight or not
 

Whoops, got it right the first time. We currently ARE traction limited, and we will be borderline if we add more weight. If we run the robot into a wall, the wheels spin out from underneath us.

I need to understand my terminology better.