[WIP] TRX-4 1:10 Lego version

[Krxlion]

Hello everyone, this presentation of a MOC will be a little different from usual ones.

Genesis
I started this project, right after I finished building the Lego 42177 G500. In the meantime, I learned about an RC car named TRX-4, which was on paper what I was always aiming for with my model - locking differentials, two-speed gearbox, 1:10 scale. I wanted to create a car that has no problem going uphill on a 1st gear, and on 2nd offers decent cruising experience (8-10km/h top speed).

I powered it like I usually have with a non-standard motor, this time C2830 brushless motor, and familiar to my readers housing, with planetary reduction (~5:1 geardown).

In the middle of designing my model, I took inspiration of the @Zerobricks G500 modification regarding locking rear axle - I modified it, as I was trying to use the same motor for locking rear and front (using pneumatic system) axle. To lock front differentials, I needed to aim for solid axle, and not double-wishbone suspension like with regular Lego G500 version. I took mine from SUV Badlands MOC, and tweaked it a little, so it would offer space under the hood for all the electronics, specially designed in 3d lego-suited housing.

My car look from the outside like there is nothing that actually powers it, which give my a warm pat on a shoulder.

 

 

Features
As this model was supposed to be a better version of my other - SUV Badlander, features are very similar

• Sturdy drivetrain with 2-speed gearbox
• Remotely lockable both front and rear axles
• 1:10 scale
• Utility vehicle - it must be able to climb step hills and also provide decent speed on plain terrain
• Precise low speed control with good amount of torque
• Detailed body of Land Rover Defender body, like TRX-4 RC car, to solidify authenticity of the model. I also wanted to have openable doors, driver, seats, etc.
• Using slightly better C2830 1300kv BL motor (with 3d-printed lego-compatible housing - including geardown) and other RC components to power it (AM32 ESC, and 3s battery)
• Moving center of mass to front and lowering it down, to prevent flipping over during climbs.
• Working front and rear lights
• 2.5 kilograms of weight

Suspension
The rear axle is more or less the same as in Lego G500. I have modified it, so it has the option of locking differential. The front on the other hand is something I am really proud of, multi-link suspension with positive caster and also locking differential option. By designing the front axle that way, I was able to put all electronic, including the battery just above it, on the front, which was very helpful for rock climbing. The downside of front axle is that it cannot use G500 wheels (due to steering rack system), but on the other hand it is very narrow, and using different wheels is actually beneficial here, as I have achieved the same stud wide axles both on front and rear.

 

Locking mechanism system
As you can see from the image above of the rear axle, I have also included some details of the locking differential mechanism. Green geek-servo offers continuous rotation in both directions. To lock the rear differential, there is a series of gears that help navigate to correct location of the shaft locking mechanism. Then, using a combination of 8t and 24t clutch gears, I can flip on and off switch directly above the driving ring. It was kinda tricky to design this, but in the end it works harmlessly. To avoid piling up the amount of geek-servos used in the model, I wanted to have the same green geek-servo to be used for locking front, this time using pneumatic. I did some research and someone years back designed this tricky solution using pneumatic valve (although old one)with worm gear and 8t gear. After fair amount of tests i did it, and I was very happy how it turned out. Though, the front sometimes need more time to get locked.

Gearbox and driveline
The gearbox was something I was testing for a while. I don't know who actually come up with the initial idea, but the design of it is not 100% mine. 1st gear offers not only more torque, but thanks to the combination of gears, it also locks the central differential, this makes the car actually control remotely locking three differentials. 2nd gear offers more rpm in the system, and opens the central differential to narrow down the stress in the driveline. As you will see in the second image, this amount of motors, multiple systems is kind of enormous, but every system on its own worked as it should.

Bodywork
I was aiming straight from the start for the old Land Rover Defender 100/110 look. The same one I believe was used in TRX-4 RC version. Bold, big, massive, you could feel power from it. There is nothing much to say here, I was just trying to also have possibility to install lights, which are just converted version of RC lights to Lego Technic system, and keep the look as close as it was possible for me to the original version.

 

 

 

Few notes before ending words
The model is using non-standard electronics and some custom parts. I know that everyone is not happy with that, but the origin behind each individual mechanism is pure Lego Technic stuff we all love and share. I have also lubricated like I mentioned above all needed parts, and used some custom ones, like small M4x4 snail screws, to hold pin towball 6628 parts. Of what I am not happy about is I cut two 6L axles to 5.2L, I tried using 5.5, but they were too long. I used them for front axle driveshafts to each wheel. On paper, it was not needed, but due to the design it was much better to have it this way (the 5L were constantly disconnected, to outer push). It's a pity, but like I said, I want to be crystal clear how I have made this model. If you want to know more, just ask here, I am willing to share are the tricks and tips I did here.

The rest of the story
The past 2 months were mostly calm, waiting for firmware upgrade of my AM32 ESC, but this wasn't delivered, and I ended with the old one. The new one would actually don't matter in the end, because at the very end I started to realize I hit a wall. Driveline wasn't working as it should be. I made sure that I reinforce frame, gearbox, everything, but 2.5kg of mass made me realize that plastic elements of driveline are too vulnerable for this model. I do not drive my models like a maniac, I even start with 50% of brushless power. The motor was suited, so it will deliver approximately 3.000 rpm at full throttle, but I was using it at 50%, so 1.500 initial rpm. I started to break things. First few rounds were my issues, which were later solved, but after so many disassembles back and forth of the model, which were taking a lot of time, I began to give up. I lubricate all moving parts, and learned that some shouldn't be lubricated (like driving ring part, where teeth meet teeth of a selected gear). Yesterday, I broke a reinforced CV joint, and Differential clips - I have never broke those before. This situation made me choose to share my journey now with you guys.

This all hits me very badly, because I was working on that model for so long, and yet I need to abandon it. All those functionalities, which you don't usually see in every MOC, are tossed away, because of simple weight. This leaves me with an open question to you guys. Have you got into a situation like this, how do you handle this, and what would be your advice to me? Should I forget about some of those functions? Try to minimize exterior look, thus reducing weight, maybe change wheels to smaller diameter? I was even considering trying to build metal driveline, but not all parts comes in metal.

 

Now that I finish the writing, I remember I have tested a modification of G500, which had the same gearbox, driveline (except front axle was original), it only got rear-locking system, and... smaller wheels, less weight(2kg). It got me thinking that maybe aiming for 85-90mm tires in 1:10 scale off-road models is the key (although I don't know if you can tell it 1:10 after switching wheels), combining it with smaller weight 1.5-2kg, maybe it could work.
On top of that, I just weighted wheels from this model and original wheels from G500 82vs52 grams less in G500. Here is actually old footage of it performing a decent climb. I have also tested it on a field, no issues.

Thanks for reading, if you have any questions or would like to have any other pictures taken I can do this (I have not yet disassembled the model) and have a wonderful day, take care.

Edited 4 hours ago by Krxlion

[CK28]

Looks great! not a big fan of the cutting work but well you gotta do what you gotta do.. 

What is the reason behind the diff lock being operated by a motor on one end and a pneumatic switch on the other? I can imagine it will be hard to sync both axles with this setup and to keep everything remote you also need one extra motor to deliver air pressure.

[Krxlion]

Thank you for the comment @CK28.

The reason is quite simple. I couldn't find a way to navigate yet another axle to the front, so it would operate a switch for locking front differential. Switching both axles to be operated by pneumatic system on the other hand require two 5L cylinders, which I don't have.

It was not that hard to synchronize them, it's just that the pneumatic system usually takes up to 5 seconds to start working. One motor is switching valve, pumping the air and also through selection of gears, switch driving gear in the rear.

The concept of having one motor switch and pump at the same time is explained below, as the idea was there, I just needed to adapt it to my model.

 

[2GodBDGlory]

Oh, that's disappointing that you couldn't get the drivetrain to hold up in the end! That is understandable, though. You're working with a lot of power, weight, and drivetrain complexity. Is there any way to software-limit your motors? Maybe with greater limitations you could at least have a functional model, even if it's not as fast as hoped?

[Krxlion]

11 hours ago, 2GodBDGlory said:

Oh, that's disappointing that you couldn't get the drivetrain to hold up in the end! That is understandable, though. You're working with a lot of power, weight, and drivetrain complexity. Is there any way to software-limit your motors? Maybe with greater limitations you could at least have a functional model, even if it's not as fast as hoped?

Thank you for the comment 2GodBDGlory. I don't think that the issue lies in motor, as I was driving it really carefully. I would understand if things start to break after 50% of possible power (as I described in the topic, I was driving with power cut to 50% only).  On the other hand, maybe the motor gives too much torque, but then better option would be to lower the weight and/or use less heavy wheels.

I didn't give up on this model completely yet. I will get back to the "drawing board" and see what possible solutions I can use. I am currently aiming at reducing weight, shorten main driveshaft (bring the wheels closer together), and use smaller wheels (from G500). We will see where it will lead me to. ;)

Edited 4 hours ago by Krxlion

[Krzychups]

Really nice RC car, I like the whole look of it. I appreciate that you have built a chassis with a 2-speed gearbox and how you have dealed with locking diffs, even if they don't work properly. I have also tried, but the gearbox haven't worked well, there have been a lot of friction. Regarding the diff locking system, I've tried with small SG90 servos, but it hasn't worked as I've wanted. Finally, I don't have these functions. It's better for me to drive without the thought in the back of my mind that something might not work and break down.

The weight of your vehicle is similar to real trial RCs (RC rock crawlers). In extreme cases, they can weigh up to 4 kilograms. I wonder how these little springs can lift the whole chassis. Your front and rear axles are massive. I don't say that they're bad, but how about @HorcikDesigns's 3D printed live axles parts?

And this is where we reach the limits of LEGO bricks and 3D-printed parts. Not everything can be done with them. But on the other hand, this starts to be a departure from LEGO bricks and a bit of improvisation. At this point it's better to go into normal RCs, I think.

22 hours ago, Krxlion said:

waiting for firmware upgrade of my AM32 ESC, but this wasn't delivered, and I ended with the old one

What do you mean by this?

22 hours ago, Krxlion said:

small M4x4 snail screws, to hold pin towball 6628 parts

Could you show us how you have done it? I generally usually use M3 screws.

I wouldn't put a lot of metal parts, because it will stress on other parts and potencially break.

Could you show us how the chassis looks wihout the bodywork?

I wish we could meet someday and drive our models together :D

Edited 1 hour ago by Krzychups