# Drive motor gear ratio

We’re currently at 1:1, but we’re hoping for 3:1 at least.

We’re holonomic, so that’s already a plus, but our biggest fear is one of those defensive bots with the huge walls. Gotta get our strafing fast enough.

Also for robot/programming challenge, speed = fairly necessary.

Suggestions? (will a 5:1 robot be able to move through a sea of balls?)

I’d die for the speed in 254 (not sure what letter it is) http://www.youtube.com/watch?v=YdK0uRSkYbg. What gear ratio is on it?

I am currently building a robot in which I gear the big gear to the medium gear. It’s not exactly 1:3, but I think it’s roughly 36:60, so 3:5. That’s not as fast as 1:3, but I think it maintains a decent amount of torque. And I think pushing power can be necessary especially when you come to the wall against another robot.

• Sunny

Once you get up to the speed that 254 is going at, it gets really hard to drive in a straight line. We also got really messed up when there where a lot of balls in our way.

One thing to keep in mind is that speed is not just a function of gear ratio. It also depends on the diameter of the wheels you use, the friction of your drive train and the weight of your robot. Study the information in the VEX Speed Charts to understand how fast you can go with certain combinations of gearing and wheels.

Last year, the fastest robots I personally watched were 723 (which I think was running 7:3 with 4" wheels), 575 and 418 with 3:1 with 2.75" wheels, and 417 with 16:5 with 2.75" wheels. All were fast and responsive BECAUSE they were very lightweight. The three Exothermic robots were right around 8 pounds, less than half the weight of the average VRC robot. Even at this weight, they used a drive train with six motors to get enough torque to turn well. The extra torque also gave them great acceleration, which is something you won’t see with a heavy robot geared up high.

Of the really fast robots from last year, I really liked how 575 moved because of its software and its very good driver. As Kendalls wrote, it takes great software to make a fast, maneuverable robot. Watch video of FTC matches to see really fast robots that are totally uncontrollable. Big motors do NOT make a good robot – only the right drivetrain with the right software and good drivers works well.

The robots listed above have theoretical top speeds of 3.6 to 4 feet per second. We experimented with some gearing at around 5 fps, but even with light robots they were too sluggish to rely on, and with the motors bogged down we were clearly sucking too much power and straining the motors.

Just remember it’s not just the gearing that determines how fast you can go, and how fast you can go in a useful, controllable way:

1. Gearing
2. Wheel diameter
3. Weight of the robot
4. Configuration of the wheels (a short wheelbase will improve your maneuverability requiring less power to turn the robot)
5. Number of drive motors
6. Skill and experience of the driver
7. The quality of the driving software.

I know I’ve posted this before 3-4 times, but it’s a great 30 seconds of what you can do with a fast robot, a good driver, and software that allows an otherwise twitchy robot to handle well. Watch the robot that starts on the far left corner of your screen, it starts with an empty mechanism and picks up three cubes from the other side of the field and then scores them in three different goals in 24 seconds. I also saw this robot pick up a cube from the floor and score it in the high goal with four seconds left in a match. http://www.youtube.com/watch?v=aIsQckNv9zM

(By the way, make your fast robots extra tough and reliable. Hitting the wall or another robot at 3.5fps is a great way to knock battery cables into resetting the robot. Ask me how I know. )

Actually, 723 used a 3:1 gear ratio as well, and that guy was a hacked driver.
He put omni wheels in the back and it was real exciting to watch him drive.

I think you pasted the wrong link.
Whats the link for the 30sec elevation bot?

3:1 on 4 inch wheels is 5.22ft sec.
We tried that once with standard 4wd software, and it tears up the motor gears quickly.

I’m guessing that all the above excellent advice is for 4wd, not holonomic.
Do you have any comments on gear ratios for holonomic drives?

• can be slower because they are more manueverable?
• can be geared faster because only 70% of power is forward direction?

Yes, it is for a “normal” drive train. I know 508 took a holo robot geared 5:3 with 4" wheels to Worlds last year. They got to the finals on Technology Division before one of their drive motors shredded its gearhead. I wonder if replacing your drive motors or at least the gearheads might be a good precaution before the elimination rounds at World Championships. One thing we have been playing with this year is building robots that are easier to repair. Little things like making sure all lock collars on axles are easily reachable from outside the robot and incorporating sensors into the design from the beginning instead of tacking them on afterwards. Just the little details that can shave minutes or seconds off maintenance between matches.

I do think that for Clean Sweep holonomic and slide drive robots don’t have to be as fast because they are more maneuverable. I’ve seen a few 6WD robots which will are very maneuvarable if they drop the center wheels a little (575 used this at Gladstone). For a big robot without a slide drive, 575 turns really well, and their center wheel is only dropped about an 1/8". It’s still not nearly as maneuverable as 10q, though, which is a true holonomic robot with a 1:1 drive with 4" wheels.

I see lots of teams leave potential scoring opportunities laying on the ground because their robots are too slow. I cannot emphasize enough that every team should print out the VEX speed charts (there are now three) and study them carefully when building a robot. Using four drive motors, a heavy robot should stay in the green, lightweight robots should stay in the yellow, and I don’t know who should be in the red. I know a 4-motor robot weighing less than 8 pounds couldn’t turn very well in the red (it wasn’t a holo bot though).

All of you except Sunny have the gear ratios backwards. The ratio goes like this: the number on the left of the : means for every x amount of turns of the pinion gear, the spur gear will turn x amount of times (the number on the right of the :.) So you probably mean 1:3. 3:1 is pretty slow and torquey.

Thank You for adding this it was beginning to bug me. We need a sticky with this info in it.

First, not all VEX drive trains have spur gears (or pinion gears for that matter). Some use chains.

Second, if you want to get really picky, you wouldn’t write “1:3” you would write “.333:1.” You are using a non-standard format to clarify your meaning. This is why I frequently write things like 84:36 rather than the more-correct 2.33:1. When I use the number of teeth of the actual gears I am also letting the reader know which VEX parts they can use to achieve the same result.

Third, I don’t think anyone was confused by what was being written, and I personally don’t mind if someone writes that their drivetrain gearing is 84:36 instead of 0.428571:1. I understand what was written, and I believe the non-standard description is, in this case, easier to understand.

Fourth, you are absolutely right that it would be more in line with mechanical engineering practice to use the format “driving mech:driven mech” as a ratio of to 1, but I might still slip from time to time. I hope you all understand.

I respectfully disagree with your statement of “.333:1 instead of 1:3”. .333 is not exact, you can never get a true 1/3 with decimals, so 1:3 would be correct. Due to gear slop, this would be quite adequate for Vex, however from a precise standpoint, such as a maker of microchips, you need everything to be exact. You might as well just save the extra breath and get a more precise measurement. Also, I believe everyone should be using this same method for writing gear ratios, as making a 3:1 ratio on a robot and 1:3 is a HUGE difference. Please, everyone use (x turns of motor) : (x turns of shaft). It just saves a ton of trouble.

Agreed, I was very confused of what he was trying to say…

After considering most of the competition-winning robots, my impression is that worlds is going to be full of holonomic robots, including defensive robots.

Any estimate about the range of lightweight? Judging from the fact that 8 lbs is about half the average weight, i’m guessing 12lbs and under?

I guess weight and consequently speed is a drawback to a robot of high capacity.

that is true
if you had a huge dumper robot, it would be hard to drive fast and have the torque to drive the heavy load

fast drive trains are recommended for lightweight robots that win for efficiency rather than quantity in this years competition
just my 2 cents

btw if you were at the gladstone 12/12 competition, you would have seen the 2 verginea teams that had a huge dump, but a (even thought its a holonomic) slow drive train
just the different strategies used this year

12 pounds is a little heavy to be considered light, by my standards. If you want great speed, shoot for 8 pounds and under, above that I would think would be slower andmore sluggish. My 3 (three) cents.

By my standards, anything under 10-12 lbs is a lightweight, zippy-esque bots are featherweights (8-10 lbs). medium weight bots are what our current bot is at around 12-14lbs. Heavy weight I’d start at around 15 lbs which is what our bot last year was at worlds. Since 721 has predominantly used 4" wheels, my personal experience is that 1: 1.7 is about as far as you can push a reasonably heavy bot (15lbs). At that ratio our driver had to be extremely careful to avoid nasty clutch problems.

Ah, this is quite handy. 60 tooth on the motor and 36 on the wheel? As far as I can tell, our robot will be 15 lbs with our dumping mechanism full.

I don’t know how many robots will be featherweights, considering some level of capacity seems to be necessary for scoring efficiency (maybe except for catapults). I guess it’ll be a matter of trial and error to identify a capacity that’s both efficient and fast.