Drive Train Speeds

not sure if anyone would know the answer to this question, but we are planning on having a really fast robot for sack attack. If we are planning on having a drive that runs at 4.07 fps (7:3 ratio). How many motors would be recommended for a drive that goes at this speed, for example how many motors would we need for this robot to run without stalling. We were planning on using 6 393 motors in the high torque mode, but we are not sure if there is anyway to calculate the amount of stall torque we need to drive our robot around.
Simple Specs: robot weight ~13 pounds, planning on holding 14 sacks ~ 7 pounds max.

6 393s with that weight will definitely be fine.

yeah, i would also say 6 is fine
just make sure you keep your chassis light

I have never seen a robot that runs above 2:1 speed not stall, if you plan to strafe, there is no way you can use such a high ratio as one wheel will be powered by a single motor when strafing, even if you aren’t strafing, I think you will notice a big performance drop during continuous operation, eventually ending in it burning out. It will be down to your testing if it lasts long enough.

Just a word of warning (although you probably already know), if you are going to attempt a gear ratio that fast, you HAVE to either use clutches (even on 2 wire motors)(I don’t recommend using clutches, just because I’ve never used them myself) or make it so that in your drive code, the motor powers change at a slow rate, so the power on your motors is “ramped” up and down when you start/stop driving. This will prevent you destroying the gears in your drive motors. (You probably already knew that, but thought I’d give warning just in case, because stripped drive motors aren’t fun ;))

The motors may also need ice packs to prevent overheating.

With such a speed ratio, train your drivers not to drive into anything, and avoid pushing matches at all costs! We avoid speed ratios for just these reasons.

The formula is: Torque = Wheel Radius * Robot Weight * Coefficient of Friction * Gear ratio (for speed) / % of stall torque
Wheel radius is 2 in
Robot weight is a max of 20 lbs
I’m not sure what the COF will be but I heard with omni wheels it will be about .7
Gear ratio is 7/3 for speed
% stall torque is so you don’t run the robot at the stall torque so you have some room to push without tripping immediately a good number is .7
280/3 = 93 1/3 in/lbs

A 393 torque’s stall torque is 13.5 in/lbs. 6 are 13.5*6 = 81.

One thing to consider was that I was assuming that you run at max weight the whole time and that you agree running your motors at full speed when you are at 20 lbs causing you to be at 70% stall torque.
So if 70% stall torque is 93 1/3, 100% would be 65 1/3. (65 1/3)/81 = 81% stall torque at full weight with 6 393 torques. I would not recons trying to run at 80% but since that is at max weight a simple solution is just slow you bot down when it has a lot of sacks.

I’m not the best at explaining stuff so if you have a question feel free to ask.

i made a robot that was six wheel drive with this ratio. it used 8 269 motors (4 on each side) and had no problem stalling even when bashing into walls and pushing around full trash cans (approx. 10 lbs). I also put a 10 lb weight on it and it could handle it without a problem. I dont know how 269s compare to 393s but i hope this helps.

I’m just going to leave this here…

this tutorial helped so much. thank you sir

That test doesn’t take into account that one this years winners was on 1:2 for strength :stuck_out_tongue: or that strafing puts more load on the motors, if you want to try a fast ratio, you just need to test every ratio in the book until you find one that gives you the ideal balance of pushing/speed ratio, which will change depending how the game is played in your area. Things like that massive equation look phenomenal in a design notebook, but there are just too many factors in vex to completely model it.

I personally prefer drive trains that are designed for torque, sadly i think this years game is going to be a lot different than last years game(Gateway/Sackattack). I think it is going to be about being a the right place at the right time, and a torque drive might be to slow. There are also some other reasons on why we are going with a fast drive, but you guys can see why in a reveal sometime. There are a lot of reasons on why we have decided this but if we see that in scrimmages we are getting pwned by torque drives, we will have to reconsider our decision. :slight_smile:

are you talking about 2W?
they had 1:5 ratio for strength o.o

I sort of thought this video would be forgotten about pretty quickly but I keep seeing it, so I think I’d better point out that it isn’t a great way to calculate an optimum gear ratio. Hopefully this doesn’t come across as too harsh.

There are a lot of errors in this tutorial. They are:

    • The biggest one: it assumes that the optimal gear ratio is the critical gear ratio where the robot becomes able to spin its wheels on floor tiles. This is a bad assumption as this is not a useful thing to be able to do for most robots. There are big advantages to higher gear ratios which this model doesn’t account for. Any robot that can spin its wheels when pushing things is going to be a very slow robot.
    • (tan(30))(1.8 kg)(9.8 ms^-2) = 10.2 N, not -113. Your calculator was in radians.
    • It mixes up metric and imperial units (without incorporating conversion constants).
    • it confuses a 1:12 ratio for speed with a 12:1 ratio for torque (but at this point in the video the numbers themselves are meaningless because of the above two points).
    • Where did 40% come from? It’s not an error per se, it’s just arbitrary and shouldn’t be treated like it has all that much authority behind it. It won’t always be the best value.

And a little nitpick:
-11.94 ≈ |12| is wrong. It should have been |-11.94| ≈ 12, but you probably knew that.

The process is sound, but it’s not working out a particularly useful quantity and the implementation shown in the video contains errors.

I might make a more nicely worded version of this post and put it on the video itself to discourage people from applying the model beyond its limitations as the title implies they should :/.

when I made this, I was in a hurry to get it in for the online challenge for last year. I know I mixed up the ratio. It should have been l -11.94 l and not just -11.94. I do not work as well on camera expecially since I had to remember how to do it right then. I have a PDF document that explains it alot better and is much more clear.

even with the use of 6 motors you will still run into overheating issues those issues killed us at worlds so run them at 90% power or less with those speeds

Having a high ratio, then running it at a low speed is pretty pointless :smiley: unless you are tripping breakers, you should probably gear them down to 1.6:1 for speed and run them at full power (this is a well tested value, there is a reason most NZ bot used it last year) The fact that you have 2 more motors on what most teams had last year should just be used to make sure you don’t burn out from pushing/climbing over sacks.

Actually, we found that a drivetrain running at 90% power runs at almost exactly the same speed as a drivetrain running at full power, but with less current draw.

Was this on a very lightweight robot (eg. squarebot)? because a lightweight robot takes less torque to move at the same speed as a heavy robot, so reducing the power to the robot would make much less of a difference than if it were the weight of a full competition robot.

No, this was our 25lb Gateway robot.