Weight Issues

Hello again,

We are a rookie team and need some advice

Just one question:

My team’s last plywood prototype before our final design weighs 50-60 lbs. During testing, it was moving pretty fast and was able to climb well (with four 12’ pneumatic wheels). Should we try to stay at this weight, or should we try to build a robot somewhere closer to 100 lbs? Our main strategy will be, in order:

  1. Herd small balls
  2. Uncap and move mobile goals
  3. Climb to the center and block other robots from hanging

Our final machine will be built of four aluminum 2’’ X 4’’ channels with a sheet of aluminum (1/8’’ or 1/4’’ thick) for mounting motor, electronics, etc. http://members.cox.net/rambot/Documents/chassis%20concept%20rev4.pdf We should easily be less than 100 lbs because we are most likely not going to build any arms or other mechanisms on top of our drive train other than a plow. My question is: What are the advantages and disadvantages of both a light and a heavy robot? Should we add weight by using a thicker mounting sheet or would a lighter robot be better?

Once again thanks


It seems to me that a lighter robot might get pushed around more.
Then again, I’m probably still thinking in terms of last year’s game, when being pushed around was a big issue.
With your goals, you should be a happy medium between strong and fast. Closer to 100 lbs seems like a better option for you, to me, anyways.

So, to sum up, you have a ~80 pound chasis and drive train. Why stop at 100 pounds?

What is your gearing? Have you really tried herding balls? Do you have some sort of way to latch onto 2 goals and move them?

I recomend that you take a long hard look at your ‘plow’. From what I’ve heard/experienced, ‘plows’ are not terribly effective. You may want to put a lot of time into making yours as capable as possible. At the vary least, it will give you drive expereince.

You may consider adding additional motors to your drive train. Since you have no arms, you may find you self playing an agressive game against the other bot’s. Many 'bots are skimping on multimotor drive trains this year, you may be able to bully them around. This could be espically useful if you plan on dominating the top platform.

In short, you have time, weight, and not much to lose. Why not add some new components? At the vary least, practice, practice practice with what ever you do have.

Oh, and make spares, since you have the time.

-Andy A.

dont forget that friction between the wheels and carpet is directly proportional to wieght, so you may get pushed around if the other robot is strong enough. also in a shoving match where both robots have the torque, the one with more tire-carpet friction always wins, so consider if you can either make your wheels more grippy or add wieght to make sure your robot can hold its own. as was said before, this year might not be a pushing game, but if you think about it, you might still get in a shoveing match to keep opponents from getting to put the big ball on, or get their balls to the hopper, or get to the bar. I think shoving will be playing a very central role.

Our gearing is 3:1 using the two drill motors tied to two wheels apiece set in low speed.

We have been doing some tests with our plow. We have put rollers on our plow that help to prevent our robot from riding up over balls and getting jammed. We have two ~20’’ arms (with latex tubing as a safety shock absorber allowing full retraction when rammed into things), giving us ~50’’ of reach. Our strategy will be to run into as many balls as possible and push/shoot them into our goal (controlling them is too hard).

We are setting aside the last week for testing and will go to a regional pre competition event before shipping.

How do our designs look?

I am sure they are not terribly original but we hope it produces a sturdy and simple machine. Any thoughts?

Best regards

not to be the naysayer, but I think you should consider doing exactly the opposite.

with a simple robot, the best course of action would be to make sure that you are as fast and manueverable as you can possibly be. this will aid you immensely in herding the small balls, and in general the fastest robot should win. yes, as such a lightweight, you probably will not win a shoving match, but why would you ever need to be in one in the first place? there is no reason for it in this game.

consider throwing an extra set of motors on your drive train. as you probably don’t want to have to design an entire gearbox for them, just put them in at as high a reduction as you can get them and scale the voltage down to bring them to the same RPMs. you don’t get as much power that way, but it is a lot easier and better than nothing.

you will want to keep the wheels you have, they are perfect for what you are doing. the only other thing i would be sure of is that there is at least one front of your robot that does not have exposed wheels. remember, wheels made to climb up on the platform will just as easily climb up on another robot and flip you over in a heartbeat.

in the end, it boils down to this. a heavier robot is better at pushing matches, but turns slower and therfore requires more power in order to be as manueverable. as a team not utilizing a four-motor gearbox, you probably wouldn’t be able to win a mushing match even if you max out your weight allotment. therefore, the other choice, and the more realistic one from what you’ve said about your bot, is to make a robot that is as light as possible. it will not win a pushing match, but it will easily be able to simply drive right around anyone trying to block you - speed alone may very well win you the game.

what ever you do, just be sure not to bite off more than you can swallow. if you can make a simple robot that is fast and manueverable, you will still be better off than at least 1/2 the teams out there. the only component that i would suggest adding on to is your plow - we’ve experimented with them, and found that in their most basic form they are extremely innefective. you could perhaps try to make a modification to the plow that corrals the balls inside it, so that when you turn/stop/reverse or do anything other than go in a straight line, the balls come with you? its up to you.

what modifications have teams done to make an effective plow?

Short story:
At 100 lbs with those wheels, you’ll need to be a little slower than that if you don’t want to trip your breakers on the drill motors in pushing matches…

Long story:

Drill in Low Speed:
Free Speed: 450
Stall Torque: 320 in-lbs
Stall Current: 127 Amps
Breaker Current: 40 Amps
Torque at stall current: (40 / 127) (320 in-lbs) = 103.3 in lbs

100 lbs / 4 wheels = 25 lbs / wheel
25 lbs * 1.2 COF = 30 lbs

12.5 in diameter wheel = 6.25" radius

30 * 6.25 = 187.5 in-lbs * 2 wheels / motor = 375 in lbs of torque needed.

Minumum Ratio Needed: 375 in-lbs / 103.3 in lbs = 3.63

The ratio of 4:1 that FIRST gives you in the kit is a good choice, even though you have less weight. Try running your robot against a wall on carpet- if the circuit breakers trip, you should considering reducing the gear ratio in your drive train.

I hope this helps! :slight_smile:

Good luck everyone.


well, there’s lots of stuff you could do. you have, for example rollers on your robot to herd the balls. if you put another roller on the end of a large square that flips down from an initial standing position, you could trap the balls that you have in front of you at that point inside a corral of sorts. you probably wouldn’t want to make it any bigger than, say, enough for 3 balls, but its better than nothing. something like that would let you actually grab balls and move around with them, instead of just hitting them.

Really? Which motors and reduction included in the kit would work best for this addition? Is it worth the trouble if we hope to stay light?

hmm. lets see, if i remember correctly one of our neighboring teams (173) threw the chips onto their drive train (which used the drills) at the last second to help them turn better. I’m not sure exactly what reduction they used, though I’m pretty sure that they ran the chips at in and around 30% voltage. that being said, their high speed was about 12 ft/sec - since you probably won’t be going quite that fast, you would need a bigger reduction then they had in order to get the same efficiency (remember, 30% voltage is not the same as 30% power. If you want details on how they did it i suggest you send a pm over to Joel J. the period is part of the name, not the sentence)

you would have to buy some different sprockets as the ones in the kit probably won’t work. i do remember seeingthis robot which has the chips tied into the drills. you could send an email over to whoever posted that pic if you needed more info on how they did that.

its always worth it to give yourself more power. believe it or not, it actually takes less battery power if you have more motors. battery power drains the fastest when you are completely stalling out a motor. if you have more motors, you have more power, and are therefore much more likely to simply spin your wheels instead of stalling. you will turn faster, go faster, trip breakers less, and all in all be more manueverable and versatile if you throw in another set of motors. the the 2 or 3 pounds that you pick up by adding them is paid back tenfold by the increase in power. remeber, lightness only matters because acceleration = force/mass. if you increase the force a lot and increase the mass a little, you’re still better off than where you started.

With all due respect… there’s a lot of things here that aren’t quite correct.

More motors will use more power, which will drain your battery faster if all your motors are stalling out.

When your motors aren’t at stall, assuming that you’re running them optimally pulling equal current at 12 volts… you’ll be pulling either (for example) 30 amps from 2 motors or 15 amps from 4 motors… since power is a function of current and voltage, there will be a more or less constant power pull. I could argue that due to efficiency, more motors will always require more power draw… but that’s besides the point. More motors will mean draining your battery at about the same rate when you’re not stalling your motors, and when you’re in a pushing match, they’ll drain faster- but it’s usually well worth it since the matches are so short, and the batteries can be recharged!

It should be noted that the Chips weigh in at around 3 lbs each… chain is a fraction of a pound per foot, and sprockets weigh a fraction of a pound each. You also need to mount the motors, including fasteners, brackets, wire etc… the weight addition of two motors to a drive train will most likely be closer to 8-10 lbs than 2 or 3.

However, I do agree with Jake that more motors is a good idea if you’ve got weight. I’d strongly suggest that you look into it! I’ve believe it’s well worth it if you have the weight to spare. You can ask me for help on combining motors if you’d like, or I know that JVN would love to show you his cool white paper that could help you do it too. :slight_smile:

Good luck!


I believe it. We ran our QuardaMax G2 off a power supply with just the Chias and it drew like 3 amps no load and then we ran it with both the Chias and the drills and it ran at like 2 amps no load. And then squeeze a tube of grease in there and you’ll harldy be pulling any current at all :smiley: j/k Anyway, I’m sure the more motors - less current relationship depends on the motors involved and is only true up until a point.

For a low weight robot, the cool thing about that is that you can pile on all the extra junk you want later (lights, horns, hampster wheels, random blocks of lead, etc.) because you’ll have the extra weight to do it.

Our rookie year, our robot was like 80 lbs and we added a 20 lb bumper. Solid iron bar. Worked very well. All we had was a simple drill motor drivetrain and one arm. We still view it as a very effective robot that was well designed and played the game well.

I would say that you should look at your weights again. I think that your numbers sounds low for accomplishing all those goals. But not knowing what systems you use, I have no idea.

I think thatits important to use your weight limit as much as possible for good systems. If you are still under, make huge push bars. If you ever play us, you will need them. J/K

1/4 inch Steel Diamond-Plate is perrrty.

if you realy intend on going to the center to stop other robots from hanging you ned to be able to push them off because a robot with a climbing mechanism will way at least 90-100lbs so you need to be able to counter that.
Last year, in our rookie year, we were at 81lbs, and last year pushing was a big thing… which we failed at… so this year we’re aiming at 100 pounds… nice round figure… should be good

… and it is once again proven that Jake isn’t nearly as smart as he thinks he is.

while I don’t think it would be as heavy as 10 pounds, Matt is correct when he says that it will be heavier then my original assertion.

whatever you do, don’t try to outdo yourself (i.e. this would not be a good time to attempt to build a technocat transmission). it is more important that the things you do have on the robot work well than it is that they do everything.

if you are able to drive up to a wall, and spin your tires on a carpet, then adding more motors to your drivetrain wont give you anything but more complexity and greater probability that something will fail or break

the more parts you add, the lower your machines reliability.

once you are able to spin your wheels, more horsepower will only let you spin them faster - and the coeffecient of kinetic friction has nothing to do with how fast the tires are spinning - once they start to spin the force they produce is the same, no matter how fast they spin.

to the original post?- all that matters is that your under the 130lbs. like some 1 said before… y stop at 100?

Its gonna go up the stairs no matter what the weight is (lets be practical here :)). Just be sure that your center of gravity is low. That way when you do climb up the stairs the robot wont tip over.

Just a FYI, our robot is 90 lbs without an arm and it gets up the stair just fine.

Also when you use metal you frame will probably be .2 times heavier so no matter what you do it will be heavier.