NHRL 2022 Finals Prep – Bottom Feeder (and Apex Predator)

Bottom Feeder managed to qualify for NHRL’s December finals back in May, I won’t have any opportunities to compete between now and then, and after the end of August my free time for building will be severely limited. So that means I had about 3 months to figure out design upgrades for Bottom Feeder, get parts ordered/fabricated, and get everything built and tested. Plus I already had the new iteration of Apex Predator all built from the beginning of the year, so I’ve given it a few tweaks and flourishes and I plan to bring it along for some grudge matches.

Bottom Feeder Upgrades

12lb Bottom Feeder and 3lb minibot Remora, ready for the December NHRL finals with only 4 months to spare

The most obvious issue that Bottom Feeder faced in May was that its two losses stemmed from the disk being knocked off by vertical spinners. I had an aluminum gear for mounting the disks to, but it was damaged in the match against KillaJoule, so the rest of the tournament I was stuck using my backup UHMW gears which were not nearly so resistant to the disk screws pulling out. This time around I have aluminum gears, upgraded to larger fasteners, and have plenty of spares, so the disk should stay solidly connected this time around.

A less noticeable improvement is the shape of the gear teeth themselves. I had previously designed my gears using Solidworks’s gears toolbox. What I later discovered was that all the Solidworks toolbox can generate is basically placeholder gears with teeth that are actually fairly misshapen. This helps to explain why the gears on Bottom Feeder were noisier and had much more backlash than I expected, and both of those issues disappear after switching to proper involute tooth profiles.

Top – True involute gear teeth, no backlash, smooth power transmission. Bottom – Solidworks’s placeholder teeth, lots of backlash, noisy and inefficient.

While not as obvious as a disk that goes flying across the arena, the previous iteration was pretty rough on the motors driving the disk. All of the motors lost their retaining ring to hold the rotor in axially, which probably allowed them to float a bit on big hits. With the motors being located directly over the spinning disk, this may have contributed to how all of my motors were rendered inoperable by the end of the tournament with their bearings destroyed.

The first place I found to improve on this was to use better motors. The original PropDrive motors contained a small and a medium bearing for the rotor to spin with. The new Cobra motors have two medium and one large bearings, and thus should be able to take much higher forces. They also come with shaft collars, which should work much better for retaining the rotors than the dinky clips. They also have a few other design/manufacturing features that should make them more rugged, like having the magnets surrounded with epoxy from the factory (free battle hardening!), and having a more substantial interface between the rotor cap and the magnet ring. Hopefully these will last through the tournament.

The other thing I am doing to try to protect my motors is changing up the material I am using for the motor gears. The last iteration used UHMW for the gears since I could machine it myself and it is low friction. The unfortunate part is that I believe that it was still stiff enough to transfer a lot of the impact forces back into the motors and contribute to their failure. This time around I am trying out TPU gears, with the thought that their flexibility will help to smooth out the shock loads seen by the motors.

The upgraded disk setup for Bottom feeder, featuring an aluminum disk gear, larger fasteners, involute gear profiles, TPU motor gears, and a thicker disk.

In addition to the same disk design that I used in May, I also have a couple of other disk options I can use depending on what opponent I am facing. A longer option should be good against other horizontals to help out-reach them, and a thicker disk should be good against verticals to help the disk deflect less on weapon-to-weapon hits and prevent any resulting self damage.

The frame and drive systems held up great in May so there wasn’t much that need major improvements, but I made a few tweaks to improve how the drive axles are mounted. Previously the axles were just shoulder bolts threading directly into the side walls of the frame. This allowed the threads to be bent to where one of the wheels was rubbing enough to make driving straight nearly impossible, and with enough abuse one of the bolts eventually snapped off at the thinner neck of the threads. The new frame has the shoulder of the bolts go through the frame wall with a tight slip fit and thread into mounting points further inside the chassis. This should eliminate the bolt neck as a likely failure point and reduce the chance of tweaking the shafts into binding up the drive system.

The frame and drive setup of Bottom Feeder.

Minibot – Remora

Remora – 3lb minibot for Bottom Feeder

At the May NHRL event my father pointed out that many other teams were taking advantage of NHRL’s unique multibot weight bonus for minibots which could actually have a decent impact on the outcome of a match, and that he would be willing to drive one for me. Since he has exactly zero experience with combat robotics besides watching me at a few competitions, I decided to keep it simple and give him a 4WD wedge bot that he can use to get under my opponents and let me line up for a better hit.

I wanted something in the vein of the D2 kits, but with a drive train that (hopefully) has a better chance of surviving going into the ring with my 12lb opponents. The frame is designed to be the same build style as I have for Bottom Feeder with interlocking aluminum walls, CF base plates, and a bit of UHMW armor, but brought down to the 3lb scale. Building this first actually allowed me to use this build as a testbed for some of the frame tweaks for Bottom Feeder. I don’t have the best history with making titanium wedges myself, so just throw an off-the-shelf D2 wedge on the front and I’ve hopefully got a bot that can take a hit from a 12lb opponent (or quite possibly my own 12lb bot) and keep running.

The frame and drive system of Remora.

The drive train is also like a shrunk down version of the 12lb bot, with brushless outrunner motors through planetary gearboxes (Fingertech Mega Sparks), XL belts to drive the wheels, bolts through the frame walls for the axles, and BaneBots compliant wheels. In my testing at 4S without the wedge in place it has enough drive power to wheelie itself into cartwheels from a standing start if I hit the throttle, but I can make adjustments in my transmitter to try to keep the wheels on the ground, and I hope that means it has enough power to wedge itself under a 12lb opponent.

Another fun fact, three generations of my family will be involved with getting this minibot to the finals: I Designed it, my daughter helped build it, and my father will be the one to drive it.

Apex Predator Upgrades

Despite some people having strong opinions to the contrary, Apex Predator has a few key differences from Omnivore, namely that it does not have the capability to run as an undercutter and as a result has more weight available for heavier and more substantial armor. The vertical spinner configuration of Omnivore from May is much the same as Apex Predator’s setup, so there were a few takeaways that I have implemented before I bring it along for grudge matches.

The new tooth plates for Omnivore worked pretty well and held up much better than the standard flat head screw teeth, so I carried the concept back over to Apex Predator, but this time with the aesthetics tweaked to look more like fangs to better fit the theme. The 2L belts worked well to drive the bar, but there were no off-the-shelf options for the motor pulley, and the limited selection of belt sizes meant I had to use a fairly large pulley on the bar, which I think might have contributed to Shifty being able to snipe the belt/pulley on his first hit. Consequently I am trying out an XL belt with a smaller pulley, but I am printing it from TPU to let it have some little bit of give since it can’t slip like the V-belts. Finally, I’m trying to speed up the bar a bit to help me continue to come out on top in weapon-to-weapon hits.


Bottom Feeder is basically ready for the finals, I just have to solder up some spare electronics, tune my transmitter settings, and put it on a shelf to wait for the day to come. I’m a little sad that I can’t formally compete with Apex Predator in the tournament, but hopefully bringing it along for grudge matches will give me an opportunity to test out some of these new tweaks before bringing it back another year. I also have Portable Apocalypse and Omnivore sitting on the shelf in the state they were at the end of the November/May tournaments. They are a little worse for wear, but mostly just need a fresh belt/wheel/receiver to be combat ready again, so they may get chucked in a box and dragged along for grudge matches as well just so I can make the most out of my trip to Connecticut.

In thinking back on my build history, I have built a reasonably successful series of 2WD undercutters ranging from 150g up to 12lbs before transitioning to 4WD for what I feel have been my best undercutters to date in Bottom Feeder and Omnivore (hard to argue with a place in the finals and a record setting knockout). I also have a successful 3lb 4WD vert design in Apex Predator, and I now have a bit of experience in the 12lb weight class, so maybe next I will take a crack at a 12lb vert design.


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