# Speed Versus Capacity

Which will be more important in Sack Attack? Obviously carrying a large amount of sacks would require a robot with more torque than would a small capacity robot. I have seen numerous posts that vouch for speed. However, I am of the opinion that it is worth sacrificing some speed in exchange for immense capacity (and scoring/descoreing ability of course).

Here is my analysis (both theoretical robots use 4" mecanum wheels and n bar lifts):

Team 1 and team 2 each have an identical robot capable of holding a maximum of 10 sacks. Team 1 has a robot geared 1:1, while team 2 has a robot geared 1:1.6 for speed. The lift on robot 1 is geared 7:1 for torque, while the lift on robot 2 is geared 5:1. Obviously in this scenario robot 2 has a huge advantage because of superior speed and same, or close to same, capacity.

For the next tournament robot 1 makes modifications. Instead of working on speed, they vastly increase their robot’s capacity. The lift is now even slower (25:1 for torque) and it takes about five seconds to raise or lower all the way, but it also can now hold 50 sacks. During a match, robot 2 takes advantage of their speed by rushing to pick up 10 sacks, dump them in the trough, and repeat. Meanwhile, robot 1 camps the trough and descores sacks into their robot. During this time, team 1 has no need to raise or lower the lift, as the troughs are at a constant height. After robot 1 is full to capacity, it dumps all of the sacks it carries into a trough and then covers it.

If robot 2 was able to score 50 sacks (which were then descored by robot 1), now robot 1 has just scored 250 points (ignoring bonus sacks and high goal, because they are assumed to be even). Robot 2 now has to be able to score that much again before time runs out to compete with the score. Keep in mind that this would make pretty much every sack in the game scored. Additionally, after robot 2 scores a load of 10, robot 1’s alliance partner can come along and descore the sacks that were just scored for robot 2, giving robot 1 a clear advantage.

I hope that this argument has given you something to think about. Please respond with a counterargument if you would like.

Make your own decision based on your objectives for playing the game. There are positives and negatives for both and its just up to you to determine which is more important for you.

I think that there should be a balance. I haven’t competed in the last 2 years but last time I checked, robots that carried large loads could easily be countered with the right strategy and robots that carried small loads make fairly insignificant changes in the competition. The large load robots however, have always been really good in the skills challenges.

What I’d really want to know is if a chassis that is controlled by 4 393 motors is going to run smoothly with a robot that is holding a bunch of sacks. I haven’t properly tried the new 2 wire motors yet due to the fact that I haven’t been active for a very long time.

I would have to agree with you. It all depends on your teams strategy. If they want to be able to hold a lot of game objects, then torque is the way to go. If on the other hand they would like to be very fast, then they most likely want to have a fast chassis.

It all depends on what the team thinks is most important.

There are advantages and disadvantages to both methods. A fast robot can outmaneuver defensive robots, and score up quickly. However, a capacity robot can easily beat it by descoring everything the fast robot scores. The slow robot, however, is similar to the Super-Stacker from Gateway; the other alliance will do anything to block it, and it can’t outrun defensive robots. If the high-capacity robot is effectively blocked, then it does not score at all, and has wasted all of its time.

We think that there is an equilibrium in the middle, where a robot can have a medium-size capacity, but is not so slow that it can’t compete with the faster robots. A robot with a capacity of 20-25 and reasonable scoring/descoring ability should pair up well with almost any functional ally. If its ally is a quick robot, then the mid-range robot can act as the hoard-‘n’-dump partner. If the ally is a slow, 50-sack robot, then the mid-range robot can score in faster dumps and keep its opponent occupied.

Such versatility allows for many different strategies to be played, and a robot like this would be virtually unpredictable and best at adapting new strategies on the fly.

I agree with you on this, but 20-25 sacks 10-12 pounds which is a lot of weight for a robot. I think that will be the super stacker, and the medium capacity will be 10 sacks.

We asked about 4-bar arms in the forum, and someone ran some calculations stating that an 18-inch long 4-bar, with 2 393s on a 1:7 torque ratio, could lift over ten pounds without elastic. As for the drivetrain, we don’t think the extra weight will make to huge of a difference. Our Gateway drivetrain was similar to many others in our region (4 393s, 1:1), and despite our robot being about 15 lbs heavier than the others, the drive performed about the same.

Don’t forget to include your logic in your engineering notebooks, every judge I’ve talked to appreciates a team that can quickly and concisely explain important design decisions.

Why not both???

Definitely teams should do whatever they want, but the purpose of this thread is to evaluate what would happen in a match up between a high speed and high capacity robot.

Compromise
Having a balance between speed and capacity would not provide the best of both, it would simply land in between the two. From this hypothetical analysis (which might be completely false, considering we haven’t played the actual game yet) it seems like the more capacity a robot has, the better. Increasing capacity is worth the tradeoff in speed. Compromise would be an excellent option if speed is crucial for some reason which I am overlooking. If you can descore, it doesn’t matter that you get there a few seconds after your opponent.

Defense
The high capacity robot could likely be able to push robots that were attempting to play defense. The 1:1 drive combined with the weight of the robot and the sacks would give it excellent pushing power. Although, it might be blocked by a robot which travels under the trough and raises its arm to avoid being pushed back.

Both?
As for both, tradeoffs always have to be made. If you plan to carry 25 pounds of sacks in your robot, it will probably have to have a slower gear ratio than one carrying 5 pounds. The only way around this is a gear shift. We decided to abandon planetary gears because they don’t quite work the same way we thought that they did (not as much torque). You could have an effective piston powered gear shifter, but there are tradeoffs on that as well like the weight and space taken by the tanks. Also such a system adds complexity, which increases chance of something failing. We tried to build one of these, but it had a lot of problems. But a good, reliable, shifter might be the best option.

Yes, this stuff is great for an engineering notebook. Thank you for reminding me!

You can certainly have both capacity and speed if you carry one sack and go fast, or carry a dozen sacks and go kind of fast. The question is just how much of each can you get out of it? Maybe your team can build well enough to magically drive around 50 sacks at 2:1 on a 4 HS drive and lift it with a 1:7 arm reasonably fast.

Our teams drive is very simple, 2 X 269s on a simple cog system, it has a good ratio of speed and torque, so it goes semi fast but it can push alot of weight, we plushed 2 full stacks of balls/barrels at gateway (about 10 balls/barrels) … this moved very easily at some speed still

balls and barrels were alot easier to push than the sacks. team 357 had troubles pushing a stack of 5 sacks using a 4 motor drive. I dont think using a 2 motor drive will be enough this year…

i c what you mean, when our sacks arrive we will experiment, i might order another 2 X 269s then, but we are not planning on storing lots and lots, our limit is like 10 —> our driver has a really random idea … he wont even tell us, he says that it is a surprise, and he said it works on our robot at the mo … also we used metal weights to sympathies, it works … but sacks are obviously different to metal

you’ll notice in the vid your wheels are skidding

1. the sacks may have been lifting up your robot just enough
2. your chassis is light (not a full 25lbs robot)

that and their macanum wheels, i thik they look cool, and yes they can glide and stiff, but i prefer the traditional wheels … dont know how anybody else feels, also, does anyone know if it is easy to push a robot with macanum wheels ???

This has been discussed a lot, and there’s a video out there showing that omnis and mecanums have more traction than all the other wheels, making them optimal for pushing other robots.

Our 4-motor, 25lb Gateway robot can push at least 10 sacks across foam tiles with relative ease.

After participating in the REX scrimmage I feel that there is another thing that has to be taken into account other than just speed and capacity. This would be control. Many robots at the scrimmage were able to hold a lot of sacks, and many others were also able to move the sacks around amazingly fast. But neither of these attributes alone mad the robot good. The robot had to be built in such a way the the robot would be able to accurately control the sacks. Carrying 10 sacks mean’s nothing when over half of them will miss the target or get stuck. Similarly, being able to place the sacks in the trough quickly means nothing if the robot cant pick them up.This was why some teams emerged as better than others. The better teams were able to control the sacks and get them from point A to point B as efficiently and consistently as possible.

Well, having your robot work is a must. (By work, I mean pick up and score sacks without dropping them, ect…) At early meets, having a working robot will be able to put you among the top teams by itself. But, assuming the robots all work, will speed or capacity be more effective?

what you have just describes is not enough driver practice
remember, one last hour of driving practice the night before is better than one hour of “last second changes”