The Parts That Outlive the Robot

"A man who could afford fifty dollars had a pair of boots that'd still be keeping his feet dry in ten years' time, while the poor man who could only afford cheap boots would have spent a hundred dollars on boots in the same time and would still have wet feet." — Terry Pratchett, the Sam Vimes "Boots" theory

Your shopping list is really two lists mixed together.

The first is stuff that gets used up. Wheels get shredded, weapons shatter, frame parts crack and get reprinted, and motors eventually wear out.

The second is your workshop: the radio, the charger and the tools. None of that belongs to any one robot. It belongs to your bench, and it'll still be there when the robot sitting on it is the fourth one you've built.

If you don't separate the two, your budget goes wrong in both directions. People spend big on parts that get wrecked in one match, and then go cheap on the workshop stuff and end up buying it three times.

The radio is the clearest example

The radio transmitter (the controller in your hands) will probably be the longest-lasting thing you buy. It outlives every robot and every weight class you move up through. It's also the only thing connecting your hands to a spinning weapon.

The simplest move is to buy a popular budget radio and stop worrying about it. You don't need the expensive one. You do want one that lots of people use, with spare receivers easy to find, room to save settings for several robots, and the ability to fine-tune each control, because those adjustments end up being free performance later on.

The one limit on a budget radio, found the hard way

Read five guides and four of them will recommend the FlySky FS-i6. It costs around fifty-eight dollars and it's the right first radio.

Then a kid asks a good question. When the robot lands upside down, all the controls are backwards, so can't you just flip a switch on the controller to make them normal again?

Unfortunately, the FS-i6 can't do that at the flick of a switch.

There are three features in its menus that look like they should work:

• Reverse
• Dual Rate
• Mixes

Reverse does exactly what you want. It really does flip the control, and the robot really does drive correctly afterward. The problem is where it lives. It's buried in a menu you have to click through, and you can't assign it to a switch, so it's no help while your robot is upside down and the clock is running.

Dual Rate is the tempting one, because it's the only one of the three you can put on a switch. It changes how much the stick affects a control. But that's all it does. It can turn your steering down, all the way to zero if you want, but it can never go past zero into reverse, so it can't flip anything.

Mixes are the interesting one, because they're the most important feature on the radio and they also fail in the most interesting way.

A robot with two driven wheels needs two separate motor speeds, but your stick gives you forward-and-back and left-and-right, which is a different thing. The mixer translates between them. It reads one direction as how fast to go and the other as how hard to turn, and then does simple math. The left wheel gets throttle plus steering, and the right wheel gets throttle minus steering.

Push straight forward and both wheels get the same speed, so the robot goes straight. Push right and the left wheel speeds up while the right one slows or reverses, so the robot turns. That's called one-stick tank drive, and it's how nearly every combat robot is driven.

A mix can only add, though. The radio's own manual shows it on screen as Ch1 <= Ch2+Ch1, which is a sum.

So if you feed a switch into a mix, you don't flip anything. You just add a fixed amount, which shifts everything to one side and pulls your steering off center. The robot then drifts to one side all the time, which is worse than the backwards controls you were trying to fix. Adding two positive numbers can't give you a negative, and a negative is what flipping the controls needs.

That's a limit of the radio's built-in software, and the popular custom software doesn't fix it either. The well-known ten-channel mod adds channels and sensor readouts but never touches the mixer.

That's what people mean when they say the cheap radio can be the expensive choice. The radio doesn't break, but a year later it can't do something you want.

So what should you do? Buy it anyway.

The limit is real, but you won't hit it for a while, and in your first season you're better off practicing driving upside down than reaching for a switch. Good drivers can all drive with the controls flipped, and that skill still works when there's no switch, when you forget, or when your thumb misses.

Just know where the limit is before you spend more money. If you decide you really want the switch, the cheap fix is a small add-on board rather than a new radio.

The FingerTech tinyMixer costs about sixteen dollars and sits between the receiver and the speed controllers. It does the mixing itself, and it has a dedicated input for flipping the controls that you can connect to a spare switch. It also has a proper failsafe (it shuts things down if the radio signal drops), which is what makes it legal at every competition, and cheaper mixers leave that out.

The other option replaces more of the robot at once. The Malenki-Nano is a receiver and three speed controllers on a single 2.3 gram board with control flipping built in, and the Malenki-HV is the same idea built for higher voltage and more current. Either one replaces your separate receiver and drive speed controller, which usually makes the robot lighter, not heavier.

A radio that can flip controls on its own costs a hundred dollars and up. If you go that route, make sure it has a multi-protocol module, or it won't talk to the receivers you already own.

The charger is where people go wrong

This one catches people because the cheap charger looks like the same product for less money.

It isn't, though. Cheap chargers often charge at one fixed current, and that's only safe for batteries above a certain size. One popular budget charger puts out a fixed 800mA, and the seller's own page says not to use it on anything smaller than a 400mAh battery. Batteries in small combat robots are usually 300mAh, so that charger would be pushing yours at more than 2C, on a battery size the seller specifically warned against.

C is a multiplier, not a fixed amount. It's based on the battery's own capacity. On a 300mAh battery, 1C is 300mA and fills it in about an hour. 2C is 600mA, fills it in half the time, and runs hotter while doing it. Unless the battery maker says otherwise, assume it wants to be charged at 1C.

⚠️ The big number on the battery label is the discharge rating, not the charge rating. Ours say 65C, which is how hard the battery can be drained while the robot is running. It tells you nothing about how fast you can refill it. Mixing those two up is a common mistake, and an expensive one.

Charging faster than the battery is rated for makes heat. Heat swells the battery and shortens its life, and in the worst case it starts a fire, which is why the fire-resistant bag is on the workshop list. A fixed-current charger takes that choice away from you on the one part of the robot that can actually burn your house down.

So get an adjustable charger and set the current to match the battery, which for a 300mAh battery means 0.3A. A charger that handles one to six cells will also cover whatever class you move to later, so it's squarely a workshop purchase.

Check what's in the box. Chargers often sell the power supply separately, and the adapter that fits your battery's plug separately again. Ending up with a charger you can't plug into the wall or into your battery is a common and very annoying way to lose a weekend.

Tools you actually need

This is a short list, and each item on it prevents a specific problem.

A scale that reads to the gram. Weight limits are checked when you arrive at the event, and an overweight robot doesn't get to fight. There's no substitute for weighing at home.

A soldering iron with temperature control. Every electrical connection on the robot depends on it, and bad solder joints fail when they get shaken, which is all your robot ever does. This isn't the place to save thirty dollars.

We had a Pinecil, decided its tip was too fat for small robot wiring, and bought about eight replacement tips. That was sixty dollars, and we returned all of it, because the original tip was fine. The problem is almost never the tip. It's the temperature, keeping the tip clean and coated with solder, and holding the joint still while it cools.

Blue thread locker. Loctite 242 is the common one, and any hardware store has something similar. It's a liquid that keeps screws from shaking loose. Some motors specifically call for it on their gearbox screws in combat, because without it the gearbox can come apart from the motor mid-match. ⛔ Don't buy the red kind. Red is permanent, and you'll lose the part the first time you need to take it apart.

The hex key for your power switch. It sounds like a small thing, but this is the one that leaves people stuck. Combat power switches and tiny set screws often need an inch-sized key that nothing else on the robot uses. For us that was a 3/32" and a 0.050". Look up yours while you're ordering, not the night before.

Buy three of each. They cost about a dollar, they're small enough to lose in a carpet, and you can't turn the robot off without one, which makes it a safety-inspection item and not just a convenience. Keep one in the pit box and never take it out for anything else.

Digital calipers. They're about twenty dollars, and you'll reach for them on nearly every part.

Where to spend, in one table

The idea behind it is simple. If a part's job is to take damage, buy it cheap and buy spares. If its job is to prevent damage or measure something, buy it once and buy it properly.

Hubs and bearings are in the buy-well column for a reason that surprises people. A hub carries all the force from the drive motor through one tiny set screw, and in a plastic hub that screw strips out and leaves the robot stuck in place. It looks like a part you'd wear out, but it behaves like workshop gear.

How the numbers really work out

If you sort a first-season order into those two columns, about two thirds of it ends up on the workshop side: the radio, the charger, the safety gear, and the bulk supply of wire, screws and bearings that every build draws from. Almost none of that gets bought again next year.

That's the fair way to read the price tag. The first robot looks expensive because you're really buying a workshop. The second one costs about a third as much, and that's usually when this stops feeling like a big purchase and starts feeling like a hobby.

Where to buy it

On a first build, the combat robot shops are worth using even when a general hobby site is cheaper. Their catalogs are built around parts that work together, which saves you from the fit problems that Module 6 is all about.

A handful of small shops sell most of what a small combat robot needs, and they overlap a lot: Palm Beach Bots, FingerTech, Repeat Robotics, Just 'Cuz Robotics and ItGresa, plus The Robot Marketplace for general parts.

None of them is the one right answer. Check the same part at two or three before you order, because prices on identical items really do vary, and so does shipping. awesome-robot-combat keeps a longer list of suppliers, including ones in the UK and Australia.

The bulk supplies are the exception. Wire, heat shrink, screws, standoffs, bearings and bushings are standard parts with standard names, and buying them in bulk packs from a general supplier is cheaper and makes more sense. Module 6 covers how to tell which parts are which, and getting that right is most of the difference between a sensible first order and an expensive one.

Put this to work. Before you place a first order, split your list into the two columns from the table. Then confirm the charger lets you set the current, and look up the hex key size for your power switch.