How Much Motor Do You Actually Need

"Measure what is measurable, and make measurable what is not so." — attributed to Galileo Galilei

You've found the robot you want to build, and the description says something like "use N20 gearmotors and a 5010 brushless." You go to order them, and the drive motor is out of stock. Or it's in stock in three versions, 11:1, 22:1 and 50:1, and the description never said which one. Now you're stuck guessing, and the description won't tell you why it picked that motor in the first place. Maybe the person who wrote it worked it out. Maybe they just used whatever they had on their first build.

Most of the time you can copy the recommendation and the robot will work fine. This piece is for the times you can't, and it comes down to one idea. A motor is never the only thing that limits a robot. Something else gives up first, and once you know what that is, picking the motor gets a lot easier.

For the motors that drive the wheels, the thing that gives up first is the tires.

For the weapon, it's the weapon itself.

So they get worked out in two different ways, and we'll walk through them one at a time.

Your tires give up before your motor does

Picture pushing a stalled car in sneakers on a wet driveway. At some point you're not pushing any harder, your feet are just sliding. A robot works the same way. It can only push as hard as its wheels can grip the floor. Past that point the wheels spin and the robot goes nowhere, no matter how strong the motors are.

That changes the question. You're not looking for the strongest motor you can find. You're looking for a motor that's a little stronger than your tires can use, and anything past that is extra weight you're carrying around for nothing.

The good news is that how hard your tires can push is something you can actually estimate, and it only takes a few numbers.

How hard can your tires push?

Two things decide it. How much weight is sitting on each wheel, and how grippy the tire is on the floor. A heavier robot presses its wheels down harder, so they grip more. A softer, stickier tire grips more too.

Engineers put a number on grippiness called the coefficient of friction. Don't let the name put you off. It's just a score for how much a tire grips compared to how hard it's being pressed down.

Team Run Amok's Ask Aaron, a Q&A archive they've kept up for decades and one of the few properly rigorous resources in this hobby, uses 0.8 for rubber on a painted floor. That's a fair starting guess for foam wheels on an arena floor, and it's where the method in this section comes from.

So the most a wheel can push is its weight times that grip score.

So let's put real numbers on it. A one-pound robot weighs about 454 grams. Most of that weight, call it 400 grams, is sitting on the two wheels that drive it, so that's about 200 grams on each wheel. Times the 0.8 grip score, and each wheel can push with about 160 grams before it starts spinning out.

Okay, 160 grams. That's about what a baseball weighs, so it doesn't sound like a lot. Both wheels together get you to about 320 grams, which is still less than the robot itself weighs.

Now picture who you're pushing against. The other robot weighs a pound too, and its tires are gripping the same floor, so you'd need about 360 grams just to slide it. That's why two of these robots meeting head on mostly just shove each other and go nowhere.

A bigger motor won't break that tie.

Better tires will, and so will a wedge that gets under the other robot and lifts its wheels so they can't grip.

So keep 160 grams per wheel in your head. Next we'll work out how strong the motor has to be to push that hard.

Turning that into motor strength

A motor's strength is called torque, which is just how hard it twists. The wheel is what turns that twist into a push on the floor, and a bigger wheel needs more twist to push just as hard, the same way a heavy bag is harder to hold out at arm's length than close to your chest.

So the twist each wheel needs is the push times the wheel's radius. Our wheel is 1.5 inches across, which makes the radius about 1.9 centimeters. 160 grams of push on a 1.9 centimeter radius works out to about 0.3 kilogram-centimeters of torque. That unit sounds odd, but it just means "this much weight, held out this far from the center."

Now look at what a typical drive motor actually gives you.

The usual choice at this size is a 16mm brushed gearmotor with a 22:1 gearbox. Brushed is the simpler, older kind of motor that most small drive motors still use. The 22:1 means the motor spins 22 times for every one turn of the wheel. That trade gives up speed and gets you pushing strength in return.

The datasheets list torque at stall, meaning how hard the motor twists when something is holding it completely still. For this motor it's around 0.84 kilogram-centimeters on 6 volts. A one-pound robot usually runs a 3S battery, which is three cells at 11.1 volts, and stall torque goes up roughly with voltage, so on 3S it's closer to 1.55.

You need about 0.3. You've got about 1.55. That's five times what it takes to spin the wheels. The usual rule of thumb is to give yourself 1.5 to 2 times what you need, for margin, so this motor is already way past it.

That's the whole drive question in one line. Torque isn't what's holding you back, so a stronger drive motor won't make the robot push any harder. The tires will just slip sooner.

What to do with all that extra strength

You've got three good options, and none of them is buying a bigger motor.

You can trade some of it for speed. The same motor with an 11:1 gearbox instead of 22:1 spins the wheel twice as fast with about half the strength. That's still about 2.5 times what the tires can use, which is inside the rule of thumb, and the robot gets noticeably quicker. Speed is something you can actually use in a match. Strength the tires can't put down isn't.

You can spend it on grip. Softer or wider tires raise that grip score, and raising it is the only way a robot really pushes harder. That's why builders obsess over tire material, and it's why a lighter robot with grippier tires can shove a heavier one around.

Or you can leave it alone. A big margin means the motors run cool, and cool motors last through a tournament. That's a perfectly good choice, as long as you're making it on purpose and not because you assumed more torque was always better.

One thing to keep in mind about these numbers. They assume both wheels are flat on the floor with the robot's full weight on them. When your robot gets lifted onto someone's wedge, or tipped, or some of its weight is resting on a guard, there's less weight on the wheels, so less grip. That margin is what carries you through those moments.

Check how much electricity the motors pull

The number that actually ends matches is usually current, meaning how much electricity the motors draw, measured in amps. It's easy to forget.

A motor pulls the most current at stall, and stalling happens a lot more than the word makes it sound. Every time your robot is shoving a wall or another robot and the wheels aren't turning, the motors are stalled or close to it. That's a big part of most matches.

The Silver Spark, the 16mm 22:1 motor in our build, pulls about 1.3 amps at stall on 6 volts, which is roughly 2.4 amps on a 3S battery. Two of them stalled together is about 5 amps. That fits comfortably on a small dual speed controller, the ESC, which is the little board that takes signals from your receiver and feeds power to the motors.

So add up the stall current of every drive motor and check that your ESC is rated for more than that. If it isn't, the ESC protects itself by shutting the motors off. A lot of small ones turn back on once they've cooled down, but until then your robot just sits there with dead wheels, and it'll happen right when you're pushing hardest. One without that protection can just burn out, and then you're done for the match.

Your weapon gives up before its motor does

The weapon is a different problem. Grip has nothing to do with it. What limits a weapon is how hard it can hit before it breaks itself, and for a 3D printed weapon that's not as hard as you'd hope.

The number that matters here is tip speed, which is how fast the outer edge of the weapon is moving. Think of a baseball bat. The handle barely moves, but the end of the bat is flying, and the end is what does the damage. A spinning weapon works the same way.

Just 'Cuz Robotics' spinner design guide is the clearest explanation of this around. They suggest aiming for 200 to 250 miles per hour at the tip, with about 300 as a practical ceiling. The ceiling is there for a reason. Doubling the tip speed from 200 to 400 takes roughly eight times the power, and all of that comes back into your weapon every time it hits something.

Picking the weapon motor

Brushless motors are rated in KV. That's how many times per minute the motor spins for every volt you give it, with nothing attached. Once a weapon is on it'll spin a bit slower, but KV is close enough to size with.

So the motor's speed is roughly its KV times your battery voltage. The weapon motor in this course is 1100KV. On a 3S battery that's 1100 times 11.1, or about 12,000 spins per minute.

How fast the tip moves depends on how big the weapon is. With a 5 inch weapon, those 12,000 spins put the tip at about 180 miles per hour. With a 4 inch weapon, it's about 145. If you want to check those yourself, the spinner guide has the formula, and it's the motor's spins per minute times the weapon diameter in inches, divided by 336.

Both of those are under the 200 to 250 the guide suggests, and for a printed weapon that's the side you want to be on. Plastic can't take what aluminum can. We landed under the usual range on purpose, which is the same reasoning behind limiting the weapon throttle in the weapon module.

When you're shopping for a weapon motor, work it backward. Decide how big your weapon is. Pick a tip speed you think your plastic can survive. Work out how fast the motor needs to spin to get there, then divide by your battery voltage, and that's the KV you're looking for.

Two quick checks before you order

Give the weapon's ESC some headroom. Pick one rated about 20% above the most current the motor will pull. A bigger ESC costs a few dollars more, and one that cuts out mid-match costs you the match.

Keep the weapon to about a quarter to a third of the robot's total weight. Much less and the weapon won't do much. Much more and the drive starts to suffer, and a robot that drives badly loses to one that hits softly.

Most people skip all of this, and that's fine

Most builders at this size never do any of this math. They copy a combination that's known to work, which is a completely reasonable way to get a first robot to an event, and it's what this course recommends in Module 2, The Decisions Before the Build.

The math earns its place in two moments.

The first is the one this piece started with, when the motor in the description is gone and you need to know what will stand in for it.

The second is when something isn't working and you need to figure out whether you picked the wrong part or built it wrong.

Both will happen to you eventually, and it helps to know the answer before you're standing at the checkout trying to guess.

Go deeper

• Ask Aaron, Team Run Amok is the most technically serious free resource in combat robotics. Decades of questions answered properly, with the math shown. The gear ratio page is the source for the drive method above.
• Just 'Cuz Robotics on spinner design covers tip speed, KV, ESC sizing and belt reductions in one readable page.
• The RioBotz Combat Robot Tutorial is a full engineering textbook on combat robot design from a Brazilian university team. It goes far deeper than a first build needs, and it's the right thing to reach for once a first build stops being enough.
• Duke's combat robotics wiki walks through an antweight's electronics from end to end and is the closest thing to a plain, part-by-part introduction.