Re: To deadweight or not
 

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.

Re: To deadweight or not
 

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?

Re: To deadweight or not
 

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.

Re: To deadweight or not
 

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.

Re: To deadweight or not
 

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.

Re: To deadweight or not
 

Momentum typically makes for better defense than raw pushing power.

Re: To deadweight or not
 

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.

Re: To deadweight or not
 

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.

Re: To deadweight or not
 

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.