Limitless Rot's 140+ mph Limitless GT build log

rotauq

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Hi guys,

I figured it's about time I share what I've done with my limitless up to now. Typically me, it's only the first iteration and many are likely to follow. Actually, i've had an earlier iteration, which was mostly destroyed when blew my first tyre. The limitless proved itself to be sturdy, cars broke into pieces at lower speeds.
The limitless deserves to beat my 224kmh personal record from my Hobao. And I've not hit 200kmh with this one yet. Some day it will...

The front:
front.JPG
  • Some led strips on the upper A-arms for visibility
  • GRP S3 wheels. The hoons weren't really holding up at high speeds.
  • Removed the original nose and replaced it with a 3D printed gopro session mount in the middle and some aero flaps in front of the wheels. Not so much for downforce, but a first attempt to influence the air resistance of the open wheel configuration.
Internals:
internals.JPG
  • TP power 4070CM - 2200KV motor with with dual heatsink fans
  • Mamba XLX with some cooling improvements (ducted 60x60 fan, thermal paste/pads and extra heatsinks to match the 60x60 fan) It's not properly mounted yet as i ordered some extreme delta fans (to replace the current ones on the motor and esc) and EC8 plugs to replace the XT90's.
  • I moved the centre brace bar a little to the side to make room for the esc and make it fit under the original body
  • The 4S Turnigy 5k graphene hardcase lipo's are rather big, but they give me less ripple and amp restriction than other batteries I've tried. I would rather not have them both on one side, but if i want to keep using the original body i don't see any alternative.
  • a DIY 7pack 2200uF low ESR 35V caps
  • The receiverbox is filled with cables, Flysky voltage/temp sensors, 12V ubec for fans and leds. I put the receiver(flysky IA4B) outside as i like accessibility (and i don't speedrun in the rain) and i was having some lesser range compared to what i am used to with other cars.
  • A cheapo DS3235SG servo rated for '35 kg', decent enough to keep a straight line.
  • Batterystrap/sideskirt holders have velcro to keep the body thight.
Underbody:
underbody.JPG
  • 3D printed sideskirts matching the rake (suspension all down they don't touch the ground) and a expansion bubble increasing the ground effect a little
  • Wheel case plates from thingiverse.(credit to <edit in later>). They should improve the ground effect and aero a little as well. I am still unsure if they really do.
Body:
body.JPG
  • I cut out the back of the body. Hot air needs to get out easily. And as i had to cut out i little in the front to fit the battery it's not fully enclosed and i definitely not want it to catch air and go airborne.
  • Removed the little piece at the wheel arc that scrapes when the tyres expand.
  • Cut down the rear wing to it's lowest line, still leaving a little downforce as i speedrun on mediocre lenght of road.
Please feel free point out my stupid choices :)
 
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jhunter

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The only stupid choice I see is you not giving me your car. Don't worry, there is still time to rectify your wrong!

Nice looking ride man. That gopro front end almost makes me want to buy a go pro... That's a really slick mount / front end you got going on there! kudos.
 

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Gopro officially stopped selling the session model. So this sleek model is hard to obtain. Caddx and some other brands do sell something similar.

I like to have a gopro during speedruns as it also gives me audio. Hearing the motor tells me more than the logs if the motor is still accelerating. At previous builds i had a lot of trouble getting a stable video image, everything was always shaking. But the limitless front/suspension seems to be very stable. So good even that at some point i would also like to try FPV speedrunning by attaching a digital DJI FPV system which i have for my drones. I would require someone to be present as a spotter to drive like that though.
 

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I keep having range issues and since my efforts on the receiver side don't seem to improve the situation my focus has shifted to the transmitter. I got many cheap/mediocre transmitters, even with more range (on paper), but I really like to keep using my FS-IT4S. I'm so used to it, it's like it's plugged directly in my brain. But it's no fun speedrunning when every launch is weak because of signal loss/delay. Something it never did before in the past...

Radical me ordered a 2W range extender to replace the regular antenna. Hooked up a switch, ledstip, 5V ubec and a 3S battery (FS-IT4S runs on 1S, which is too weak.
range extender.jpg
Now it's just ductaped, i will probably print some casing once i've tested it.
Right now my printer is now occupied with:

printing in proces.JPG
 

jondilly1974

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rotauq

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Here is a link to my booster build. You want to keep that booster cooled. I went with two high speed 30mm fans but that was overkill. I could have done a single 30mm regular fan.

I don't see the link, but i guess you did remove the heat shrink then? Did do a 15 min test to see if the ubec would hold up, at that point i could still touch it. I'll attach a 30mm fan and some heatsinks just to be sure. Thx for the tip.
 

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I don't see the link, but i guess you did remove the heat shrink then? Did do a 15 min test to see if the ubec would hold up, at that point i could still touch it. I'll attach a 30mm fan and some heatsinks just to be sure. Thx for the tip.
Oops I forgot to paste it.
 

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Another little update:


update1.JPG
  • I redid my wiring, 8mm bulletplugs everywhere.
  • Swapped most bearings with ceramic hybrids. Feels way smoother. It's 'most' because Arrma really locktighted the hell of some screws :(
  • Moved my batteries to the sideskirts (original body wont fit anymore) I don't want to 3d print the whole car as for weight and strength. Maybe i will print a mold then make it epoxy/fiber. I've done that before.
  • Got my hands on some ridiculous delta fan's (40x40x28) witch run on a separate 3s battery, so i can cool between runs and don't strain the ESC. They are noisy though. I've added some scoops to test the effect on aero/cooling, obviously it looks silly them being a-symmetrical.
scoops.JPG


Unfortunately speedruns are not going so well atm, during some earlier test(with different fan's) one of the motor wires in the esc let go and it's a nightmare to solder it back, thus every time i pull more than 200A it just let's go again. Seems like i need to upgrade my soldering station :(
 

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So it seems my castle XLX has seen it's best days. I managed to resolder the motor wire with the help of a heatgun next to my 48W solderstation. I wrapped all but the connector pad with lots of aluminum foil to prevent the heatgun from burning the fet's. I managed to have the solder properly flow on the pad and make the cable stick to the pad regardless of the load. Yet, it still randomly shuts down, according the datalog it's a mix of "motor start failure" or "excessive load detected". I am quite sure these shutdowns started occured when the cable came lose, i just didnt bother to look at the logs as the lose cable was likely the cause.
As motor the motor could play a role in this, i inspected it. And also did a testrun with a castle 1717, in which it acted up the same way. I am afraid it's a goner. Tough ESC, it should have died way earlier, like any other brand would. Mysteriously perfect timed with the release of it successor the XLX2. So i ordered one at castle, but it seems they launched their new flagship product with low stock.

Whiles I am waiting for them to get stock and ship it overseas, I will put my focus on 3D printing:

donkervoort.JPG
donkervoort2.JPG
donkervoort3.JPG

The bracebar really helps with making a modular design where i can easily swap out roof parts and keep everything nicely accessible.
These prototyping ~3mm PLA parts just snap on to the bar.
- The bonnet curbed in line with the upper A arm, gives it some kind of Donkervoort D8 look. It does fly of on high acceleration.
- The F1 inspired sharkfin/helo coolingduct rearcover does look nice, even tough it a very bad print. I haven't tested it on high speed yet due to the esc problem, but i guess the added cooling doesn't compensate the extra drag

Considerations:
- The nose is a tough one. I have the gopro nose, but i want to change it for a smaller DJI fpv camera and cover/smoothen the shocks&arms.
- My battery tray's will move more outwards/forwards, in line with the front wheel arches.
- The rear, i don't know yet. Covering the side and wheels perhaps. I am tended to remove the whole wing construction, but i like the f1 look of it.
 

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Some more updates: (or just things gone wrong)

- As my XLX was acting buggy I built Flier 300A from my boat in the limitless, but it stopped working suddenly too. I replaced the wiring on the XLX and it seems to work again for now. So I was like, than i should give my 5694 SSS motor from the boat a run too. Yet 1000Kv on 8S doesn't give high rpms and i lack the 8mm M1 gears to increase the gear ratio. So I returned back to my good old 4070CM.

- Replaced the center dogbones by gpm ones that looked thicker. Result was clear, they are weaker then the original:

gpm.JPG

- Drawing in fusion360 takes time and i got into this hobby to not spend all my time behind a computer. I will continue with my 3D printed designs but at a lower pace. So i decided to go for a GT body. Ordered the jaguar XRS from delta plastics as everyone else seems to fancy the Toyota. Also ordered a bunch of new paint. After paying the import taxes today, I realized this shopping spree cost me 200€. It looks nice though, can't wait for the paint to arrive.

limitless GT.JPG
 

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While my first layer of paint is drying, I printed a fitting dynamic spoiler. I adopted the concept from my old 1/8 speedrunner.

I like the initial downforce and have the drag coefficient go down as I throttle up. And the airbrake effect is a nice extra :)

Capture.JPG
 

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So a little update might be in place. I changed so much since my last post. Let's say bad weather has it's upside.

top.JPG

backside.JPG Motor.JPG top with cover.JPG back with cover.JPG cam spoiler with cover.JPG front with cover.JPG

Summing up all changes makes me feel like buy to win. Here is just a few: XLX2, new batteries, new motor, real a carbon fiber splitter.
I am aware my fpv tower might cost me some topspeed, but it will give me much better range as my eyesight is not that awesome. I actually didn't see any depth up to 10 months and i am still struggling at distance.
 

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Unfortunately youtube encoding ruins the video quality, but this is video recorded from my dji glasses. I wasn't driving FPV as i do require a spotter to do so and there was none available today. Image is really stable and definitely driveable this way :)
 

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Time for an other little update

First of all, my TP 4070CM-2200KV died. It had been acting up during braking for a while. Then it started making a clacking sound. I opened it up and a big hole appeared, I guess that's no good o_O No bad feelings towards TP though, this is a 3 year old motor and has seen it's fair share of abuse.

Hole in my TP.JPG
Fakecastle.JPG
I temporarily put in my 'castle' 1717 motor. Once bought it on AliExpress and it's clearly fake. Only 1100kv of it's intended 1580kv, yet somehow it just runs so smooth and efficient. So it will get the chance to show it's capability on the XLX2 with 34P/27S gearing on 8S. I don't expect miracles, but i am still waiting on some other gears before i can try out some more promising motors i have at hand. For now it's good enough to test my handbrake setup:

frontside brake.JPG

I swapped the arrma cup's on the spool for mugen's as they can hold the brakedisc. It required some trickery to place it in the front as i have my motor mounting reversed. I really hope it will hold :D
For now i disabled reverse and mixed it with my throttle. On 0 throttle it already engages some, so it will function like drag brake. I prefer not to have it lock up on high speed. When i do some testing tomorrow i will also try to have it engage from 90% braking and throw in some ABS.

My FPV tower has been removed as i made some changes to my active spoiler. This design is more sturdy, ripped the idea i used for the tower. I will add some new design FPV tower once i actually improved my PB. Top speed has priority...
spoiler update.JPG
 

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Ha, this handbrake is insane!

On the log below it was obviously locking up, but from 200km/h(125mph) to 0 in 4 seconds seriously reduces my braking distance. Going nice straight and no visible flatspots on the tires (6C outside) This really gives me that extra confidence i need. Often i brake way to early.

braketest.JPG

I checked some random other logs and didnt find any where i did it in less than 6.5s on motorbraking (+spoiler airbrake) The ABS settings on ch3 don't seem to work (yet), it should have been visible at the rpm line...
 

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It's been a month since my last update.

Initially much didn't happen. Work, cold, dark early and plenty of rain kept me of the streets. What did happen is:

  • Replaced my GRP's for foams from SP tyres. It made me relearn my trigger finger. Foams have no grip at all o_O

  • My new Flysky noble NB4 arrived. On first install my XLX2 stopped working. Hard to tell the cause, i won't point any finger to neither flysky or castle. Made a castle ticket and surprised their support with esc behavior which according to them, they never seen before. Send it back for rma.Back to the noble NB4. Next to build quali, the new AFHD3 protocol reacts ridiculously good. Response time combined with the accuracy of the wheel and trigger make it so much better than i expected. Except for the range on the original receiver(s). I've changed it to a FTr8B dual antenna receiver and now i get my first low single warnings at 230m (750ft) which is acceptable.

  • I did some testing on new cappack designs before the XLX2 died. In the end i disassembled one and used the caps to replace all on my old XLX. It did stop acting buggy, but ripple is now of the charts, even with a fat cap pack. I figure these reading are faulty as my replacements (even on the rails) are different specs. My careful testing showed promise, so I did do a more serious speedrun. My gps battery was down, so no formal records. But man, it ran smooth as never before. Then i crashed it at top speed... Car flipped forward, body flew way up in the sky, all 3dprinted parts broke and pieces everywhere... but no car to be found. After a long search i finally found it submerged in a ditch 40m further. CRASH 1.JPG crash 2.JPG
I was able to find all critical parts back, except for the my XLX PCB (including the datalog) Next day i went looking for it and found it again. Damage report: 2 broken wheels, servo, fan and all PLA pieces. Examining the datalog: ripple galore.JPG Ripple is obviously absurd high, but the rpm and current line are nearly perfect, hitting plateau before crashing. Loving the Noble NB4 trigger. Arguably the moment of liftoff might have caused the rpm bump in the end. (In my memory i was already off throttle before i crashed) According to my really accurate excel sheet 51042rpm would be 243kmh(151mph). Before the bump it was ~48000rpm which makes it 229kmh (142mph). Both are above my 'official' PB of 224kmh(139mph) so as far as i am concerned, the Arrma limitless is now my fastest RC car and feel confident it will soon be validated by gps.​

  • I've rebuilt the car. All 3D print in bright yellow, so it's easier to find/trace flown off parts after a crash ;) I did add carbon fiber trays to make it more sturdy. This is what 'minimalistic with 3D printing' means in my book:
yellow internals.JPG

My body did got a little bruised, so i am gonna strengthen it with epoxy/glasfiber. I've added these carbon fins and am still in doubt how to finish the spoiler. Current weather conditions are real bad, so driving without downforce is asking for disasters.

spoiler work in progress.JPG
 
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