Finally putting together a build/competition diary for my most successful fc robot - Caustic

Phoenix · 29 July 2026 14:06

This bot starts its life as another of my bots’ ends - MITE ants IV…

I had 3 entries into this competition:

A shuffling hammersaw - irrelevant to the story, so will not be discussed, just know it sucked

A 4WD grabber named Farrago, which is pictured below - My first bot where I had tried gears, featuring the use of idler gears to drive front wheels without 4 motors and an electronics layout I quite liked. At the event, I discovered my newly reprinted pinion gear had a flaw, with poor wall generation in the slicer leading to it failing under any amount of force, which led to my amazing idea to just cut off the entire grabber assembly and run it as a pusher. This led to me realising I had a super effective drive base with 35g of spare weight to modify, although I wasn’t sure what I would build. Eventually, Farrago was knocked out due to a combination of my poor driving and my opponent simply having better ground game.

And finally, my first somewhat functional spinner, Uppies, also pictured below. It ran 1000rpm drive, PLA+ and steel forks and a TPU unibody frame, with the 24g mild steel (emphasis on the mild) disc and 1806 motor both mounted to the PLA+ top plate, with a thin channel on the back to route wires and connect the top plate, which somehow was never hit, along with the exposed weapon wires in this area. Uppies managed to advance to the quarterfinals before losing to Ripper remix, once again due to my poor driving and my opponents superior build quality. Even with the positive W/L ratio, I had been unhappy with both the drive and ground game of Uppies for a while by this point, so it was scrapped, with another bot to replace it. I had found a use for Farrago’s drivebase - A gear based 4WD vert

And thus the first CAD for Caustic was born, a branch of farrago, with thinner wheelguards, 4 TPU forks, which would be sprung to the floor by their own flexibility, and the same weapon setup as Uppies, a mount attached to the PLA+ top plate

I had also added a flared front wedge, which didn’t really do much other than look cool and give it a more distinct form factor than the normal square 4WD vert. The electronics layout from farrago was kept, with the brushless ESC to be stacked above the malenki nano.

After printing all components of this design out, only 2 changes were made before competing in January GROCS:

The forks: Following printing, I realised being so thin, they would not mount properly without splitting, so I beefed them up to twice the thickness and removed the barbs, which I realised would only get in the way. To make weight for this change, I removed half of them, but kept their bolt holes, allowing for different positions to match my opponent

The drive: I discovered that due to the flex in the frame, the wheels had a upward camber, which meant the wheels did not contact the ground, due to the gears getting in the way. I solved this by switching to a belt setup of the same wheel size from another bot. This change is not reflected in the CAD, as this was done last minute using pre-existing spares

This is where this build log ends for now, just before the debut event, as I am out of writing time, but I will likely finish this later tonight. Until then, thanks for reading!

TheVigilante · 29 July 2026 14:45

YOU!!! Youuuuuuuu…

Don’t think I haven’t forgotten what your monster did to Sunday Drive. It was nothing short of spectacular. I await the day we finally cross paths in the arena again. If there’s any good bot to test my girl’s defenses against, it’s yours!!!

EDIT: Spoilered because the fight footage hasn’t actually been posted yet.

Phoenix · 16 August 2026 16:27

So, by tonight, I definitely meant *checks notes* 18 days?!?!

After posting the first entry into this topic, I went along and just straight up forgot I made it, so uhhhhhhh

Update later today (“surely I won’t forget this time” he says, hubris-ly)

Phoenix · 16 August 2026 20:39

So January GROCS..

After very nearly not arriving in time due to a delayed train making me miss a connection (damn you Avanti!), I found some pit space near a couple other builders, the most relevant of which to this event recap being the builder of Vanquish, a very large 2WD vert.

After putting in the battery and fighting to squish down all the electronics into the chassis, only somewhat helped by a small amount of hot glue holding stuff in place, I went over to the tech check, only to be 4g overweight! From there I had two choices:

Be smart, and accept my weapon was too big, and swap to the 16g “penguin” disc from team croc, saving 8g
Say “screw it” and swap to a smaller 120mAh 15C battery from my 250mAh 25C battery, which saves the exact amount of weight needed, while reducing my number of fights per charge from 2 to 1 with no other adverse effects, because surely the max discharge current of the battery is irrelevant (definitely not foreshadowing)

As you could have guessed, I went for the option which gave me the bigger weapon, like a fool

Let’s get on to the fights!

Fight one: Caustic vs Tongue Twister

An SVRN kit, Tongue Twister uses the flipper variant of the design, which meant I was expecting a difficult fight if I am unable to get around to rear, which is the main weakpoint of SVRN bots, being the only non TPU area

(Why did youtube remove the ability to create clips, this is annoying)

Turns out Tongue Twister was far faster than expected, which combined with my lack of experience driving a this version of the bot leads to a successful boxrush from my opponent, although I succeed in escaping from their forks soon after, only for their extreme speed to get the better of them, leading to an accidental driveout, a lucky win for me, even if a win was possible

Fight two: Caustic vs Thunderbolt

A big wheeler! I had fought Thunderbolt in the past, so was very confident for this fight, even with my inexperience driving the bot, which was slowly reducing. No real strategy other than driving at the opponent and not the pit

After one or two big hits and a good push, Thunderbolt went over the (relatively) short side wall, as expected. By this point I had got a lot more familiar with how it drove, and was mostly satisfied with the pushing power

Fight three: Caustic vs Grey Fox 2

I have fought Grey Fox several times in the past, and (If I am correct), at this point we were even, so I knew I was in for a driving challenge, especially since Grey Fox runs a lot of TPU, making large amounts of damage unlikely. A boxrush was likely.

Everyone remember how I swapped to a smaller battery?. Turns out with the lower C rating, the current provided to the weapon motor was not sufficient for self righting, leading to a quick loss

To remedy this, I decided to take the first option of the two from before, and swap into a smaller weapon but return the battery to its original size, which solved the problem

Fight four: Caustic vs No Big Dill

No Big Dill, a flipper which has been very successful in the past, with a skilled driver, this fight was looking to be a difficult one. No Big Dill also runs a lot of TPU, including the side forks and side armour, although, the back is all PLA of some sort, so that was my target.

Here I was really feeling the drive not being the fastest, as I was not getting much bite and outmanoeuvring my opponent was difficult, however, at the time I simply put it up to my bad driving and poor fork geometry (I was driving decently well and the forks are quite good)

Quarterfinal: Caustic vs Angler Merkel

Oh dear. A smothering control robot which I had lost to 2 times already at previous events with other robots, Angler Merkel was looking to be my hardest fight yet, with the only glimmer of hope being the all PLA+ (or was it PLA-ST? Eh, close enough.) rear end and side armour, a common theme between a lot of robots I was facing. Of course, I assumed I wouldn’t be able to get around to there, so I just hoped I could get a single hit

Well it is what it is. After two good hits, my weapon snowed, Angler Merkel’s grabber clamped down on the weapon motor, stopping me from rapidly throttling my weapon to attempt to escape the grab, then a swift pitting ensued. And thus my record of Angler Merkel still stands at 0:3 across all my bots. Maybe one day.

Now, being out of the competition does not mean that is the final fight! Remember when I mentioned I was sat near the builder of vanquish? Well, I had been discussing how I had not fought a spinner with any of my own spinners in a main competition in actual months, and was hungry for some stress testing of my bots, to which we ended up with a whiteboard planned, leading to:

Whiteboard fight: Caustic vs Vanquish

Being a whiteboard, I had no strategy, I simply planned on throwing my bot at my opponent until one of us broke

Yeah I broke first, and badly. The first hit occurred as Vanquish caught my weapon guard, cut right through it, took a bite out of my weapon motor, and with that bite, caused my weapon support to shear right along the layer lines, while also flipping the bot. After enough hits, I decided to tap out, as my drive had began to fail, and I would rather not have to replace any more motors.

Overall, 3:2 a good opening event for the bot, showing good potential with good ground game and a durable everything (but the weapon)

Next event: ORCS 11!

Dylan_Furniss · 16 August 2026 22:24

Bigger weapon better is never a bad strategy, despite my bot being quite anemic at the time the Vanquish whiteboard was still pretty impressive, was no small hit that ripped your upright off