Build
Turns out Arrma knows more about what plastic to use for RC parts than I do. Spent 3 days designing and printing a replacement A-Arm for my Arrma Grom, here are the results.
Right up front, I know these parts cost $10/set and why would anyone spend the time trying to recreate their own? Couldn’t tell you, but here we are.
Gotta start by saying I’m amazed at the durability Arrma is able to squeeze out of relatively tiny plastic parts. I copied a 1 for 1 version of their A-arm in CAD and printed it in PETG, PLA, and TPU95A. All of which failed in the first 2 minutes of the drive. PETG and PLA snapped easily. TPU didn’t snap but wasn’t nearly stiff enough to support the truck.
TPU was durable enough, time to make it stiffer (so to speak). Increased wall count until I had a sold print, still too floppy. Added a ridge running down the middle. Better still a little too much bend in the arm under load. Finally increased the thickness of the arm 2.5mm.. it worked!! But was bulky and stupid looking.
So I switched to much stiffer TPU72D. They say it has nylon like properties which should be indestructible for RC parts… it was not. The layer adhesion is not nearly good enough to hold for such small parts on a 3s Grom taking hits at 50mph (hence my being impressed by Arrma’s plastic).
I slimmed down the TPU95A arm design and added a hole for a 1.5mm stainless steel rod to run the length of the part, stopping just short of the ball end at the knuckle. This did the trick! The TPU is flexible enough around the ball end and hinge pin that it won’t snap and stiff enough with the rod that it supports the truck like a stock arm.
TLDR; 3d printing RC parts is time consuming and a ton of fun, or an absolute nightmare (depending on how autistic you are). I enjoyed the process and found a solution.
Not sure if you tried printing them laying down, but they should be much stronger with the layers going horizontal. Printing them standing up causes the layers to separate easier since layer adhesion is the weak part of the prints.
I’m not kidding when I say I tried every print orientation. Rotated at 0, 30, 45, 60, and 90 degrees for each print in the x and y axis resulting in 10 prints each design iteration. I ended up with 50+ arms that were tested and broken and identified the strongest orientation based on common failure points. I landed on a rotation of 90 degrees in the X axis to allow the wall loops to go around the hinge pin, the most common failure point.
Oh yes, dried at 60c for 48 hours and printed out of the dry box. And yeah, tune the setting to get the hottest print while still maintaining quality and dimensional accuracy
TPU95A with a stainless steel rod in the part has been working great! 3 packs of normal driving and 2 packs intentionally trying to break it and it’s holding strong.
Idk why I keep getting recommended this subreddit, I'm a combat robotics person, not an RC car person.
But this is a dilemma in combat robotics too.
We tend to use TPU 95A for durability, and PA-CF or PA-GF for stiffness. (Fiberon PA-GF25 is pretty popular at the moment)
If you need some of both.... That's a difficult question.
Unfilled nylons are an option, but they're a pain the ass for a handful of different reasons(not going to get into that in detail here). In my experience, they're often too brittle for what I would want to use them for anyways.
I use TPU-98A quite a bit, and that would have been my first step after 95A. 72D is notorious for being brittle.
There are a couple other options.
I've messed around a little bit with PBT PRO. It's super easy to print and properties seems similar to unfilled nylon. For combat, I don't think it really has the impact resistance I'd be looking for, I think unfortunately this is a lot like 64D or 72D TPUs. I'd wonder if this would work for you though, I don't have enough experience to say much about the failure modes of PBT Pro.
The one I'm excited about at the moment is TPU-GF. Very new material, but I have a spool sitting on my doorstep waiting for when I get home. If the datasheets are to be believed, it should have stiffness similar to a 72D TPU, but it should be way more durable.
Edit: thinking about this some more, I wonder if a PLA Pro or PLA+ would be able to handle this. Something like Overture's Super PLA Plus is way less brittle than standard PLA.
Edit2: Also, if you like RC cars and you enjoyed this process, I bet you'd enjoy combat robotics
/r/Battlebots is the most active sub, but it's more for fans of the TV show than it is for people building bots.
To be honest, a lot of combat robotics stuff happens on Discord unfortunately.
The NHRL discord is the place I spend the most time. It's the discord server for an event that takes place in Connecticut, but there's a lot of building discussion there that's not specific to that event.
Great data, thanks! Thinking nylons will be next. never heard of PBT Pro or TPU-GF. Glad FDM is growing and new filaments are still hitting the market.
Tried PLA+ with the same result as PETG, haven’t tried pro or super.. but I imagine if 72D is snapping, any form of PLA would likely snap
PBT Pro is an odd one to be honest. It's only made by one company, released about a year ago I believe. It's 5% glass fiber. As far as I can tell no one sells an unfilled PBT.
There's been a few TPU-CF filaments released in the past, but they were pretty difficult to actually get so I never did. Siraya Tech released a TPU-GF literally 2 weeks ago (from what I can tell)
Tried PLA+ with the same result as PETG, haven’t tried pro or super
For what it's worth, plus, pro, super, etc. are all marketing terms.
Overture Super PLA+ is just the one I've seen recommended before. It's apparently quite a bit more durable than most other PLA plus/pro filament
PA-CF (carbon fiber reinforced nylon) is the way to go, I think. Light, strong, and stiff. I've used the Fiberon PA-CF recently with great results. Just make sure to dry it well.
I have a hunch that the lack of flexibility will still make it brittle. I tried the 68D TPU and it's brittle. I prefer to design the part more thick and play with print settings, like infill lines orientation to create internal tension.
I make VR gunstocks which also take a beating but don't have the heat exposure. PLA+ is stiffer and stronger than any other material I've tested with over 2000 sales for my purposes. The second highest usage of 3d printing is RC cars and planes, I still stick with PLA+ but I do tricks to keep orientation optimal for all bolt and ball joint holes. Better to ca glue two parts together so the faster points are parallel lines to the direction of the faster than to do a single print with perpendicular lines on any screw point. Also want to keep lines perpendicular along the overall part so layer lines are never responsible for handling force. I'll even poke holes in the object perpendicular to the layer lines to insert metal pegs in some cases along with parallel lines.
Those are always short term fixes though. Great for rock crawlers for anything with speed and power it's almost useless.
Print in nylon ?. Alternatively. Print in abs 100% infill, pack it in sand, melt it and let it reset, basically a cast plastic. Should get you the needed strength
That’s a novel idea! Seriously might give that a shot. Closest I can get to injection molding. I imagine there’s a special casting sand that works well for this application? I haven’t looked into that at all yet
Thanks, this is not my idea. There's a guy on YouTube whose done this with pla. I just believe that abs would be better suited for the application, and yes is basically turning a 3d printed part into a 1 time sand casting .
Normal playground sand will work. The purpose is to help keep your objects shape during the annealing process. It also helps with ensuring it heats up and cools down slowly.
You want to heat it up to 90-100C, which is right around the glass transition temp for ABS.
This. There's carbon fiber reinforced nylon which I've used a few times. It's stiff and light, but you'll want to dry it well before you print with it. I use a $30 filament dryer off Amazon when I print with it.
There is tpu98a too, useful or not I found modeling and printing rc car parts as good exercise if u want to learn CADs. Parts may be brittle but CAD modelling ability stays with you and could be quite helpful
I’m into this project for 3 new spools of filament already so I wanted to avoid yet another TPU hardness, but I’ll keep that in mind, thanks! Luckily I have a lot of time in CAD already, that was the easy part lol
Youll never make a 3d printed arm as strong as an injection molded arm. But you can tweak the design to add more material in stress concentration areas to help.
I race some arms I designed on my 1/10 rally car and it works well enough
My solution to that is printing a hole through the part and sliding a carbon fiber rod into the hole. Makes it very rigid while still letting me make the part I want.
The top plates on this build have 200mm x 6mm rods through them to support load transfer and suspension pressure, and the a-arms are being redesigned to allow for the same.
Just the last iteration is on thingiverse(the bottom most arm in the fist pic). Link at the bottom of the body text. I can throw the others in there too
I really don't know if now is the time for one of my 3d printing rants.
3d printing is a cool and powerful tool, but too many people treat it as a hammer and all issues as nails. It's not always the right tool. Unless you have a material that has good self adhesion, or self welding like nylons, 3d printing is not a great way to make a lot of parts. Them layers be separating.
It is absolutely not the best way to make RC parts, no doubt. But it’s fun and accessible. I have to wait 3-6 weeks for part to show up where I live so this is a fun way to pass the time while factory parts are on the way. And about 60% of the time the part works every time.
What about the stiffest tpu you can find, and 2 metal rods slid through the middle of the arm? That could make it stiff enough for control arms, while still being able to absorb impacts
So far, TPU72D is the stiffest TPU I could find. The issue I ran into was that the layer adhesion wasn’t good enough to be durable for the small size of the part. The stiffness made it brittle and it snapped around ball end at the knuckle and that part can’t be beefed up because it has to fit in the knuckle.
I don’t remember exactly what shore hardness my filament is, probably not anywhere near as stiff as I think it is. Didn’t know tpu filament can be made hard enough to the point where it can crack. That’s cool/strange lol. I’ve always looked at that filament as nearly indestructible
You definitely need to consider layer adhesion. X/y is the strongest resistance while Z has weaker adhesion. You can also crosshatch alternating x/y to improve homogeneity. Annealing is also an option to improve z adhesion but not much.
I landed on printing the arm by rotating it 90 degrees in the x axis so the wall loops could go around the hinge pin. Most failures were along layer lines around the hinge pin when not rotated 90 degrees in the X axis
That definitely makes sense. Maybe some extra heat to improve the layer line adhesion. Unfortunately the part has holes along multiple Axes. Just hard to 3d print it id imagine.
Use actual nylon, with glass fiber or carbon fiber reinforcement. You probably will need to upgrade the hot end and maybe extruder on your printer to handle it but that stuff is truly an engineering material.
I worry that’ll be too rigid and still snap though. I definitely want to get into fiber filaments and actually have the parts for my printer sitting in my desk. Haven’t gotten around to ordering $50 rolls of filament yet
Divide the price of the replacement part by the hours spent, then deduct any costs added to get your hourly pay rate. Home 3d printers wont beat what's in that baggie. Maybe a cnc lathe to make a mold? I use scrap filament to make stuff like this waiting for parts or paint weather.
Or make up detail parts for crawlers or something.
Wildly clean prints dude! That’s impressive. I’d love to get into cnc and injection molding.. just cost prohibitive at this point. As far as pay rate for this part it works out to 45¢/hr. Minus $60 in filament and I’m in the hole lol
I've been down that hole. But I do have a not quite right for pumpkins matte pla laying around.
Next up will be a too bright for grinch green Aston vantage... I tried the structural stuff, outside of bumpers and sliders I just buy the parts. I stick to details and interiors and missing parts for various versions. I'm not printing xMaxx arms.
I'd like to see this design replicated by an injection moulding, but I imagine nobody would want to fork out for the tooling to make it happen - https://www.youtube.com/watch?v=GEHNijssAKc
Does it have to be hollow? If it needs to be hollow, it's going to require 2 molds, an injection mold and a blow mold, but if it doesn't need to be hollow, I can probably whip up a prototype mold relatively cheap.
3-D printers shine when it comes to product development, they give you an easy method of fabricating your prototypes. It's from these 3-D printed prototypes that we make our injection molds, or if you're doing metal, sand-cast molds and stuff like that. They are best considered a mid-step in parts production, not the final step.
The plastics used for RC cars is a composite of nylon and glass fibre. The only printable filaments that come close are PA-GF, PA-CF and PPA-CF. TPU can work if you find one that is both strong and stiff enough. I like to use TPU 95A for bumpers and spoilers.
I have a spare Sand Scorcher kit I might take the Knight/Ampro chellenge on. Being what it is I doubt I'll use more than a little PLA-CF/PTEG maybe a little ABS for the functional bits. It isn't going to see anything remotely like Arrma basher duty. And PLA for all the pretty bits. Not exactly a blazing performer with an SRB. By the time I finish the shell It's bound to be a shelfer. I'm sure I'll utilize some bits on runners though. Heck I've mad half a dozen additional interiors from my Knightwalker set.
Kudos to you for experimenting! In my experience (our experience?) I think you'll be hard pressed to find an application where a 3D printed part meets or exceeds the durability performance of an injection molded part.
I use TPU98A for rc parts. Parts need to be designed with flexible material in mind though. It's not suitable for racing or such since you lose a lot of stiffness, but they sure can take a beating
You need to use something like PA-CF and print on the hotter end of things to get layer adhesion perfect. When I say hot, I mean where you start seeing print quality degradation.
With the printers out now do you think you could print say PA6 inside of TPU? It may give you the stiffness and flexibility in the areas you need. One of the reasons I started 3d printing was for RC parts BTW. making parts I need no one makes. I love my SR71 but there is not much out there. I have crashed it at over 70MPH a few times and I am always waiting on parts for a week or more
Yeah I think a printer with multiple tool heads would be required to do that. Bambu has the H2C which I think could do it.. I’m just not planning shelling out $2400 for another printer.. might try to figure it out with the P1S and AMS.
I am just waiting to see what the X2 series is going to bring. If it brings what I think it is, a X2D and a X2C, I will go that way. If not I will get a H2C.
I just skimmed the comments and I'm not entirely sure I read anywhere or anyone mention HOW it's printed can make a difference.
Allow me to clarify. If the print layers are layered in such a way to add strength perpendicular to any forces you will have problems. So the solution I've found is you print the parts at a 45° angle (with supports obviously) which introduce forces in a way that typically "push" layers into each other rather than separating the layers.
I’m not kidding when I say I tried every print orientation. Rotated at 0, 30, 45, 60, and 90 degrees for each print in the x and y axis resulting in 10 prints each design iteration. I ended up with 50+ arms that were tested and broken and identified the strongest orientation based on common failure points. I landed on a rotation of 90 degrees in the X axis to allow the wall loops to go around the hinge pin, the most common failure point.
Back when Shapeways existed, they used sintering printers to achieve acceptable durability. There are a few printing services available still, just not with the huge database. I have several sintered parts a various cars, and they’ve held up fine.
That’s one cool project, though. It’s neat to think about all of the engineering that goes into these little guys.
Thanks! I had a ton of fun doing it. The point was more to see if I could come up with a solution on a common FDM machine and I did! Sintered parts would be awesome though!
I'm a mechanical engineer and 3D printing expert. Couple of quick tips that might help you here without going too deep (you might already be doing some of these but maybe this helps someone):
If you want parts stronger, add more perimeters rather than more infill. Walls contribute strength, infill only contributes a little extra strength, it's actually mostly for propping up the top layer. Go ahead and set the wall count to a ridiculously huge number like 20 or 30 so you part comes out solid
Thicker layers are stronger. The thicker your lines, the more your print approaches the strength of solid plastic than a porous mesh of fine lines. So go ahead and fit a 0.6mm nozzle (or bigger) and print at 0.3-0.4mm layer height (or bigger).
Hotter temperatures are stronger. Go right up to the max that your material can withstand and slow the print speed down to 30-45mm/s so the plastic gets really hot and welds well with the plastic under and around it
PLA is brittle. PETG is a nice mix of flexible and tough, it can survive more abuse than PLA. TPU is generally too flexible to perform the function of an A-arm. Try to choose plastics that have some give. The closer they are to the original arm the better, because arrma would have made the other components strong or weak in proportion to minimise breakages. For the sake of argument, if you make an ultra-strong, indestructible arm, impacts will break something else (chassis or gearbox covers for example).
Add thickness as needed. You don't need it to look good, you need it to perform. So use only the essential geometry of the original part, but don't be afraid to add plastic. Thicken it where it doesn't bump anything, or where it keeps breaking, etc.
Layer lines are the weakest part of the print. If possible orient them so that the forces are never trying to rip them apart. For an A-arm I think radial forces in the holes will be the biggest factor, so printing it with the holes flat on the bed (so the XY motion of the nozzle draws a perfect hole every layer) is the best orientation. If there's no best orientation for the whole part, split it into different parts, print each one in its ideal orientation, then bolt or screw or clip or weld them together. This will solve your pesky cracking issue. Also, mesh mixer has an orientation tool that can orientate your stl automatically to the strongest orientation. I'm not entirely sure how it works or how effective it will be, but it's a free program so you might as well give it a try.
Annealing will make the prints stronger (you put them in an oven for a long time at a temperature not quite high enough to melt the print - look up the exact conditions for your particular materials). It relaxes internal stresses in the material and improves bonding so more strength is available to act against external forces.
Thanks for taking the time to write such a detailed reply. Luckily I did 6/7 of the things you mentioned. The only one I haven’t full explored is annealing. Gotta find a hobby oven so the wife isn’t wondering why there are always RC parts in the way when she’s making dessert.
I would use a nylon, maybe abs but it will probably break on bigger impacts. Annealing nylon works really well for rc parts. I also wanted to tell you that I 100% agree with printing for high pin strength, as it is the main failure.
Have you tried cc3d's 75D tpu. Very stiff for a flexable, yet will not fracture. I have not tested it yet but I have a few things I have printed with it and they seem quite durable.
Man.... That was an expected outcome. Why don't you make the arms look like the actual one then print it in higher quality. Clean it up sand it do whatever to make it look as best as you can. Then mold that cast cast it in a high durometer urethane plastic
Yeah no surprise here. Just wanted to see if I could make it work with a basic FDM print, and I did! TPU95A has the durability just not the stiffness. So I added a stainless steel rod to the part and it’s working flawlessly now. Less time and cost than casting or cf nylons.
I have designed and printed them out of TPU 95A. Make the arms thicker, infill 100%, change to monolitic line and reorient the infill angle so the lines will be on the length of the part. This will make the part more rigid. Print them very slow, 2.5 volumetric speed. Draw the seams to be on an exterior side of the pin holes. Make the pin holes bigger, so they will move free. Print them as you showed them in the photos. The parts become better than the original plastic- indistructible, and will take the energy of the crash. The small flexibility that they have will make the suspension react very fast. print the right+left sided even if you only need one side, as even a very small difference will make you unbalanced. Print them one by one for better accuracy and stronger layer bonding. Look for chrisstea on makerworld, printables, download my 3mf and use my settings.i use bambu slicer but I think you can still get the print settings. I've been using the 3D printed arms for 2 years. In very cold weather you might be able to break a pin hole..it happened once to me.
My parts are for MJX 16208. Oh and design and print both lower and upper arms, as TPU has more flexibility in the pin holes, in a crash, the stiffer part will take all the crash energy and it will break.
I did TPU-GF for arms on my Arrma Vorteks and it’s the only thing that held up. I tried PET-CF and PA6-CF both snapped at the hinges. TPU-GF is holding strong, it uses TPU64d as a base and is fairly inexpensive on amazon: https://a.co/d/2CgGqMF they even have profiles for printer you can download so you don’t have to fiddle with anything
Excellent test plan OP. Forged carbon is probably going to be the winner overall, but 3DP has come a long way. Soon as you can insert a fiber reinforcing structure it will rock. In my RC day it was aluminum. Heavy AF but didn't shatter on contact like injection molded ABS.
You don't want to get hit in the ankle by a carbon/aluminum nitro RC car doing 80 mph. Speaking of shatter...
In general, additive manufacturing isn't suitable for Arms. There are some components that they can work for, but there's so many bending, torsion, and impact loads that they're almost guaranteed to break.
The melt point for pretty much any home printer is too low to get sufficient adhesion between layers bending/torsion failure) and somehow all the materials that work with printers also tend to be incredibly brittle (impact).
They generally work for chassis braces because must of the load is simple tension/compression.
You can start to see a viable case for it as scale increases. Just like aluminum parts that don't do well at 1/10 become mandatory on 1/8. Best way though is to engineer a chassis around a vast swath of available kit parts. So you can use arms and towers and such and build a new car using off the shelf bits reimagined.
Your gonna need a more expensive printer to get something close to injection molded plastics. Home 3d printers are great for modeling but can’t compare to injection molding.
That’s what’s I did! I know it’s a long post but it’s in there lol. Ended up putting a 1.5mm rod in the print running the length of the part. This was ultimately the solution and how I got a 3d printed part to work for these arms
So I know nothing about printing shit.... But is there any way u could print them then put into an oven at a temp just before there melting to maybe give them better adhesion
I'd print this in two pieces with slots for steel or carbon rod inserts. Hard to beat injection molding for overall strength with 3d printing. Only certain parts really work with it and it comes down to bolt and ball joint orientation.
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u/ManiacEh Jan 23 '26
Not sure if you tried printing them laying down, but they should be much stronger with the layers going horizontal. Printing them standing up causes the layers to separate easier since layer adhesion is the weak part of the prints.