Many common questions asked by new builders are about how to choose the correct motor, or belt drive ratio or any number of other aspects of a spinning weapon. In this post I’d like to summarize information I have shared throughout a bunch of different YouTube videos on my channel over the years in one place, so all the relevant info is right here!

Right off the bat, let me say that if you hate math, or any of the info below seems confusing or overwhelming, the simplest thing to do is use one of my off the shelf Hubmotors and a handy blade size/tip speed chart like this to determine the correct combination of battery voltage, weapon RPM and weapon size for you. If you welcome a challenge however and want to learn something, let's move on!

 

Resources

First off I built a couple webpage calculators to help you understand and play with the variables for designing a robot. First is a 4 part calculator that follows the multi stage approach outlined here. This calculator has a section for Hubmotor weapons and bite calculator which lets you pick presets for the options I sell, or enter manually for 3rd party motors. Second, is a dynamic calculator that allows you to change variables on the fly to show in real time the effects of higher KV or higher belt reductions, or just picking a different battery voltage. 

I will try to avoid going too deep into the rabbit hole in this article, but if you want a true deep dive, watch my Spinner Design videos here:
Part 1: Math and Theory

 Combat Robot Spinner Design Tutorial (Part 1): Theory and Math

Part 2: Motor/pulley selection, and real-world examples 

Combat Robot Spinner Design Tutorial (Part 2): Motors, Pulleys, Applications and More

Part 3: CAD tutorial for flat asymmetric weapons 

Asymmetric Weapon CAD Design in Fusion 360! Combat Robot Spinner Design (Part 3)

Part 4: CAD tutorial for beater bars 

Beater Bar Design + CNC Quoting! Combat Robot Spinner Design (Part 4)

You can also reference this handy calculator to get more info!

And the AskAaron calculator is great as well. 

1: Weapon Diameter

If you don't know what a diameter is, go back to middle school! When designing a combat robot with a spinner, the design process often involves picking a spinner diameter near the start and designing your robot around it. For horizontal spinners it's possible to switch to a slightly smaller or larger weapon but always near the same. On Mini Mulcher and Subtraction for instance I have blades ranging from 7 up to 9 inches in diameter, and Subtraction even ran a 10” one once! but for vertical spinners, it’s nearly always just one fixed diameter. “What is a good diameter for a weapon?” Well, take a look at what other people are doing. For a 1lb bot, a vertical spinner might be only 2-3 inches in diameter, while for a 3lb bot a horizontal spinner could get to be 7, 8, or 9 inches as shown below. This choice comes down to personal preference, combined with the practicalities of what the smallest or largest size you can fit would be.

2: Tip Speeds

Tip speed is a pretty simple concept and is commonly brought up on Battlebots and NHRL. Tip speed is simply how fast the tip of your weapon tooth moves (in linear speed) and it's calculated by a simple formula.

[Revolutions per minute] * pi * [Blade Diameter] = Tip Speed. If you used diameter in inches, this will be inches/min, which can then be converted to any unit of choice. [Pi * diameter] is the circumference of a circle so this is just the distance the tooth travels per revolution times revolutions per minute.

When you are looking to compete at a high level tip speeds on vertical spinners start getting crazy high, as when two collide often the one with the highest tip speed wins the engagement. However, designing your robot to have a crazy high tip speed like 400mph comes with a lot of tradeoffs.

• Higher tip speeds mean that your weapon motor needs to work harder to get to speed and will take a lot longer to get up to max speed
• The air resistance on a spinning blade increases by nearly speed^3 which means a 20% increase in speed means your motor needs to draw over 70% more power to keep it spinning! 
• This drains your battery super fast at best or blows up your motor or ESC at worst
• You start losing bite (essentially how far into your opponent your tooth actually can hit) the faster you spin. Bite depends on the RPM of the weapon, how many teeth it has, and how fast you can drive into your opponent. Higher RPM minimizes bite, while faster driving increases it. Going from 1 tooth to 2 cuts bite in half!
• The lowest tooth count is 1, which is why we see so many asymmetric single-tooth blade designs. 

My personal recommendation is to always design for a max tip speed of at most 300mph, with an ideal range around 200-250mph, as you can always run your weapon slower to increase bite, but your motor won't have to work super hard to spin up to these more reasonable speeds. You can pick other tip speeds if you want but note that it’s pointless to design for much higher than 350-400mph as air resistance gets so intense, it’s just not physically possible to fit enough battery and motor power into a robot to achieve this. Going from 200mph to 400mph requires ~8 times the power! 

3: Weapon RPM, Voltage and Ideal kV

Now that we have the weapon diameter and tip speed, we can figure out your ideal weapon RPM, and start figuring out what motors would be suitable! To start with we are going to figure out the correct kV of the motor to use for a given weapon tip speed and diameter.

I already went over calculating tip speed from RPM and diameter, but let's do it backward! 

To get RPM from diameter and mph, we can use a simple formula:

[Tip Speed in mph] * 336.14 / [Diameter in inches] = Spinner RPM 

336.14 is a conversion factor I got from 63360 in/mile / (pi * 60 min/hr)

Let's say our weapon diameter is 4 inches, and tip speed goal is 250mph, then we get an ideal weapon RPM of around 21000 RPM. If instead, we want to use an 8-inch weapon, we find the ideal RPM is half that at 10500 RPM. 

Let's take both those numbers to do a bit more math to figure out what motor kv might work for us. The kV of a brushless motor is simple the voltage constant, for every volt this is 1RPM that the motor can spin at its fastest. So at 12V, a 100kV motor spins at 1200RPM. For antweight and beetleweight bots, typical weapon motors have a kV value between 1000kv and 3000kv as that's what is widely available in the size range we care about. 

So what kV do you need? Well to know that you also need a battery voltage. The ‘nominal’ voltage of a lipo battery is the voltage at around 50% state of charge and is just 3.7 times the cell count. For a 2S lipo this is 7.2V, for 3S, 11.1V, for 4S, 14.8V, for 5S, 18.5V, and for 6S, 22.2V. Most beetleweight bots use either 3S or 4S, while most antweights run 2S or 3S, so let's stick with 3S for this example.

For the 4 inch weapon at 250mph as described above we need 21000RPM, so that will be 21000/11.1V = 1891kv for our 4-inch blade. For the 8-inch weapon, we would ideally have half that or 946kV for our 8-inch blade.

If instead, we decide to run on 4S voltage, we find the ideal kV to be 1420kv and 710kv for 4 inch and 8 inch respectively. 

4A: Weapon Motors - Choose a Hubmotor!

From here we have two options. We can either decide to use a direct drive weapon setup, where the spinner speed is 1:1 coupled to the motor, or we can use a belt drive setup that allows us to use different size pulleys (typically larger on the weapon, and smaller on the motor, so the weapon spins slower than the motor). Belt setups are simple in principal but there are a lot more elements to consider.

The main advantages of a belt drive setup are isolating shock from your motor, and allowing a higher kV motor than direct drive 1:1 setups. The shock isolation is important, since when the weapon hits something the forces can often shatter magnets or snap motor shafts! In the past, this was almost a necessity. Thankfully I have developed the first-ever battle-hardened motors which are designed to mount a weapon directly and survive these immense shock loads. Still, you aren’t forced to use my products and this is a general guide, so I'll cover both options from here on. 

Using a hubmotor weapon motor such as my RDY-5536 or RDY-5022, you can just attach the blade to the motor directly. However, the tradeoff is you are stuck with the kV options available so you might not get the exact tip speed you wanted, or maybe you’ll want to change the blade diameter a bit to get the tip speed you're going for. My beetle-sized RDY-5536 motor is available as of writing in 900kV and 1200kV, while the RDY-5022 is available in 1700kv and 1100kv. I also offer a more traditional style, but still ultra durable JCR-4935 motor in 1000kV which is built specifically for single side supported horizontal spinners, be it undercutters or overhead spinners like Subtraction. 

Let’s say you really want to stick to an 8-inch diameter for a big beetleweight horizontal spinner. Well, my 900kV 5536 hubmotor will be a great option, and with a 3S lipo you get a bit less than 250mph, (237mph to be exact) or on 4S you can get quite a bit more (317 mph). 

How about an antweight that is using a 4inch blade? Well, my 1700kV RDY-5022 will allow you to run a 4 inch blade on 3S at about 225mph, which is still plenty fast. My MakerBattle champion bot Sonic ran a 1700kV weapon motor on 3S for just 170mph tip speed and still tore other bots apart! Remember that 250mph really is just a baseline and not a rule. 

To help pick a hubmotor, check out my Hubmotor Calculator here: 4 part calculator. I also have a handy spreadsheet to find tip speeds for my hubmotor options at different blade diameters and voltages.  

4B: Weapon Motors - Belt Drive System!

Okay so now the more complicated selection - belt and pulley setups!

For the sake of keeping the math to a minimum let’s assume we are building a beetleweight with that same 8-inch blade diameter, sticking to that 250mph target tip speed, but this time we really want to use 4S. We need to pick a motor and pulley ratio combo that works for this.

We already found out that a 1:1 Ratio setup with a 720kV motor gets us there, but there aren’t any motors that low kV around that are lightweight and small enough. Generally speaking, you get more power at the highest kv available for a given size motor - as long as it can handle the chosen voltage! So for the best power to weight we probably want a much higher kv. 

For Division V3 I used a quite large Cobra 2814 1850kV motor on 4S, and with a 10 tooth pulley on the motor and a 30 tooth 3D printed pulley on the weapon, I got a 3:1 reduction or belt ratio. This meant the motor spun 3 times for every 1 turn of the weapon. In that case, my motor RPM was 27380RPM at 14.8V, and the weapon maxed out at 9126RPM, which for a 7.5” disk meant a tip speed of ~204mph. You can use the same tip speed formula as above, but simply multiply the ‘ideal kv’ by your pulley ratio to get the new motor ratio. You can get pretty much any ratio between 1.000 and 2.00 or 3.000 depending on how small of a pulley you will fit on the motor and how big you can fit on the weapon. 

Returning to our 8-inch blade on 4S at 250mph example, say we decide we can fit at most a 2.2:1 ratio in our design. We need a motor that is between 720kV and 720 * 2.2 = 1584kV, which is definitely doable. There are a couple high quality BadAss 23 series motors like the 2315 1480kv that fit the bill! For a budget option, we could try a Turbo 2830 1300kV which is a similar size. Remember that we can calculate the exact ratio needed by just dividing the motor KV and the ideal KV. So for the 1300kv option, we need 1300/720 = 1.8:1 ratio, and for the 1480kV we need 2.06:1. Now you just need to find a pulley you like for the motor and the correct relative tooth count on the weapon. Let’s say we are using the same 10T XL pulley that Division uses, well we can find an 18-tooth or 20-tooth design for the weapon. If we find that an 18 tooth pulley is too small to fit a bearing for a dead shaft weapon setup, we can always make the motor and weapon pulleys both larger too. Maybe we use a 15 tooth on the motor and 15 * 1.8 = 27 tooth on weapon. 

 

The mechanical design and construction are outside the scope of this article but below are some really useful tips for belt setups. 

Through many iterations of my 3lb combat robot Division, I learned a lot of things that help with belt-driven robot weapon design. 

1. For antweights, S3M or HTD 3mm belts work fine for weapons, like you can get from fingertech, but for beetles, I strongly recommend XL belts if you are in the USA or HTD 5mm internationally. There are also V Belts which require proper tension to work well, and the small motor pulleys often need to be custom-made, but since they slip on impact they can give the best results.  
2. Buy aluminum pulleys with either set screw or clamping hubs for your motor. Find these on Ebay, Amazon, McMaster Carr, BBMan, Polybelt, Servocity, Misumi, tons of other places. See my robot part sourcing post for more! 
3. For the weapon, usually you can get away with 3D printing these rather than expensive and heavy metal ones. Getting 3D CAD files for existing pulleys from Fingertech, Misumi, or other places and modifying them is a great option. 
4. Use filaments like Nylon or Super PLA for S3M pulleys, the teeth are too small for TPU. You may be able to use TPU for the XL/HTD5 size but rigid filament works better. If you can't print these yourself check out my 3D printing service! 
5. You need an exact center-to-center distance between the motor and weapon pulleys. Calculating this is easy using several online tools. I like the BBMan calculator, there is also this super fancy tool for FRC teams, and Fingertech has a calculator as well. 
6. Don’t only support your weapon or motor with TPU, you need to have a rigid structure to maintain belt tension, especially for V belts! Larger-toothed timing belts with bigger teeth are more forgiving. 
7. If you don’t need the maximum torque on your weapon, you can use a timing belt with a smooth pulley on the weapon to add slip with a much cheaper and easier-to-fabricate design than V belts. 
8. Don’t forget you need to assemble and fix the bot - leave room for the belt and think about how many things need to be removed to replace it!
9. For some designs, you might be able to get rid of the belt entirely and use gears to drive a weapon, but this requires putting your weapon motor super close to the weapon and will transfer a lot of shock into the motor when the weapon hits things. I did this successfully on my antweight Mini Mulcher.

Motor Selection and Sizing

Now when it comes to how big and powerful of a motor you need, things get tricky. The brushless motors we tend to use are build for drones and meant to spin propellers that weigh 20 grams and not 200 gram steel flywheels. They will work fine for this in most cases but you should be aware that the power and current ratings are often inconsistent. The maximum continuous current for a motor gives a baseline reference for how ‘powerful’ it is, but this won’t be a perfect comparison method. Many motors have propeller charts, which bcan give a more reasonable expectation for the power draw the motor can sustain, but not a lot about what happens when spinning up your weapon. 

The best advice I can give you is - take drone motor ratings with a grain of salt. Consider that if two motors are saying they have 30A max current, and are both 1200kV, but one motor weighs twice what the other one does, one of these might be a 30A continuous rating and the other might be a 30A for 10 seconds peak rating. Most often motors with the same kV, size, and similar weight will perform very similarly regardless of the numbers written on the product page. 

It’s worth playing around with a few values in this combat robot spinner calculator to get a feel for things. 

The AskAaron calculator has an Ri input value for the motor winding resistance which can provide more accurate data for spinups. I provide this value for my weapon motors, but most manufacturers don’t. It is possible to measure this on a motor yourself using a bench power supply. Set the supply to 1 volt, and connect it to two brushless motor phases. You will get a current of, say, 3 amps. Then Ri = 1V/3A = 0.333 ohms or 333 milliOhms. 

5: ESC Sizing and Choices

For pretty much any modern brushless motor weapon in 2024, we probably want an ESC running the new AM32 firmware. Just ‘Cuz Robotics currently offers 35A and 70A ESC options which work great for almost any ant and beetle respectively. If you want to choose the minimum size ESC for a given motor, this can be a little complex. Like I said in section 2, tip speeds increasing drive up the weapon motor power requirements, and thus current requirements. ESC manufacturers also do not have a standardized rating system for the ESCs themselves, so a 35A ESC from two different bands could be vastly different sizes and weight, and in reality, the smaller one might die at only 20 amps sustained. The good news is that spinning up a weapon is a relatively short few seconds at full power, followed by a much lower sustained load. Picking what has worked for others with a similar motor and weapon is the safest option, but when in doubt, choose an ESC that has at least 20% higher current rating than your motor’s “max current” at the chosen voltage, for a safe rule of thumb. 

NEVER use an ESC at a higher voltage than it’s rated for. A motor usually can take 4S if it's designed for 3S, it will just run hotter, since its just fancy copper coils and magnets. But some components on ESCs will immediately smoke and die if given too high of a voltage. You might even find you need to add a 500uF to 1000uF 35V+ capacitor to your ESC if running 5S or 6S as voltage spikes still can cause issues at times. Solder this between the two power leads, mindful that the ground and positive are correct.  

For my beetle 5536 and 4935 motors, I would recommend my Pariah 70A ESC for 3S up to 5S or 6S. For the antweight 5022, my 35A ESC is plenty to run up to 4S.

6: Bite - What’s it Mean? 

When you are designing a spinner blade from scratch you will want to consider how much bite you will realistically get. Bite is how far into an opponent your tooth will go at a given RPM. Bite is determined by your weapon RPM and drive speed, so if your robot spins super fast but moves super slow, you will get very little bite and just grind away at the opposing bot. If instead, your weapon spins relatively slow and/or your bot can move super fast to engage, you can hit opponents super hard into the ceiling or across the box with ease! You’ll need to do some drive train calculations to know the forward speed of your bot before you can know how much bite to plan for, but once you do, keep in mind that the bite calculated is the absolute maximum you will get when the weapon spins it’s fastest, and if it spins slower, you will get even more. I like to make the stick out of my weapon teeth about 1.5X the max bite so that even on slower engagements I can still hit just with the teeth. Be careful if designing an asymmetric weapon or disk so that the counterweight cannot get easily hit!

Hits with very little bite make pretty sparks, but they don't send your opponent flying or tear pieces of armor off.

7: How heavy should my weapon be, and what’s all this about Kinetic Energy? 

How big and heavy a weapon is comes down to personal design choices more than anything. Almost anybody in the sport of robot combat who does well at competitions will tell you that the number one priority to make your robot good is a super durable and reliable drive system, with the weapon coming secondary to that. It’s better to make a robust drive base and add a weapon with the weight you have left, vs the other way around! That said, generally speaking, people tend to dedicate maybe 25-35% of their robot weight to the weapon system, including motor, pulleys, belts, blade, bearings, shaft, and in some cases the ESC as well.

What is KE or Kinetic Energy then? KE is a measure of the amount of total energy stored in the spinning blade. This is a factor affected by but not directly related to its diameter and weight, but it’s extremely dependent on the exact shape. The formula for this is 1/2 * I * w^2, where w is the RPM and I is the MOI or Moment Of Inertia of the weapon about its rotational axis.
Calculating MOI by hand is a fool’s errand. It's something I was forced to do for simple examples in Mechanical Engineering classes, but your CAD software calculates this in an instant for any shape imaginable. MOI is basically the average position of mass from the axis, m*r^2. The r or radius term is squared, so if you have two disks, one thick and R=1, and one thin and R=2, the larger diameter disk of the same weight has about double the MOI. Also note for the kinetic energy formula, again the speed term is squared, so smaller diameter weapons spinning really fast can store a ton of energy without being too heavy. 

Some people will obsess over their weapon design trying to maximize the MOI per weight, but personally, I feel like KE isn’t that important. If you are making a really small and lightweight weapon, it might help to calculate this as you tweak the design to optimize for hitting a bit harder without eating too far into your weight budget. But beyond a certain point, you end up making shapes that are ineffective - either you have too much weight in the ‘ring’ of a blade, and super thin spokes that just bend, or your tooth is too small to be effective, or your counterweight is too close to the max diameter of the tooth... There are many pitfalls to be aware of. 

Hope this helps!

If there is any pertinent info you think is missing here, feel free to shoot me a message with my Contact Us form or just ask if you have any further questions.