[WIP] Small Scale 4x4 Off-roader with Brushless Motor

[gyenesvi]

Hi! This thread is meant to document my first build using a custom brushless motor. If you follow the "Brushless Motors in Lego" thread you may have seen the motor design, but I give a quick summary here as well. Apart from the brushless drive motor, I am using a GeekServo for steering, and control them with an RC pistol transmitter/receiver.

So lately I started experimenting with 3d printing lego compatible mounts for brushless motors. I bought a small Injora Purple Viper motor with a corresponding ESC to control it; they come in a package for about 50 Eur, that includes a 11T gear and mounting screws as well. The idea was to make a motor that fits into a 3x3x6 space including a two stage planetary reduction, which is taken out from a 3rd party PF M motor (costs about 3 Eur on AliExpress). Apart from these parts, only the 3d printed housing is needed. The result looks like this, more info in this thread.

 

The key to this design is that it is possible to run a driveshaft right below the motor, furthermore, there's even space for heavy duty CV joints, as the motor itself is narrower:

There are two obvious gearing options, 16:16 and 20:12. I checked Injora's transmissions that are applied with these motors, and they also come in two options, the "normal" provides a 1:13 down-gearing, and the "underdrive" version provide a 1:24 ratio. Since the two stage planetary reduction from the M motor is about a 1:24 down-gearing, applying a 16:16 on the output results in exactly the "underdrive" ratio, while applying the 20:12 up-gearing on the output results in a 1:14.5 total down-gearing, which is roughly the same as the "normal" ratio with the Injora transmission, so there's a pretty good speed correspondence here with RC parts. By the way, these Injora parts are designed to be a drop in replacement for TRX4M Traxxas models.

So this was the starting point for my chassis, and I wanted to see first how small a 4x4 off-roader I can build, aiming for 56mm tires and two live axles. I also happened to have some soft and sticky Injora tires too that I wanted to try for a couple of years now..

For the front axle, I have been wanting to test this tricky wheel mount technique I have seen used by some builders around here, making it possible to use heavy duty CV joints at this small scale (9-wide axle between the pivots), combined with my linkage based steering techniques I used before:

This is a fairly slim and dense axle for what it achieves, quite happy with it, however it misses two upper links, and the panhard rod also proved to be a bit far away, so it became a bit wobbly under the power of the servo, so I had to use other tricks to make the axle stable..

For the rear axle, my aim was to make it as flat as possible to be able to run beams right above it without obscuring the trunk space. It is an often used negative caster (torque tube suspension) setup with only one CV joint and two links and a panhard rod for stabilization. The springs are also mounted on the back to be able to lower them and so that they don't protrude to the trunk.

A chassis that holds all these components together came out quite slim as well, I'm really happy that it actually looks somewhat like real RC chassis platforms!

The servo is mounted fairly low at the front, and there's just enough clearance for the drivetrain even when the axle is articulated. Another key element is that the servo is in between the springs, and right behind the front axle's cross beam, and it actually stabilizes the axle, not allowing it to move too much sideways or tilt forward/backward (just enough to let it articulate freely). These limiters together with the panhard rod make the front axle stable enough so that the servo does not pull it sideways when steering, resulting in a really good steering response and a quite high steering angle!

As you can see, there's plenty of space left for building internals / body and also for placing a battery and ESC/receiver to the trunk. For now, I have mounted these items with tacks onto the chassis for testing, it looks like this:

 

As for performance, I don't yet have a video, but I am pretty amazed by this little thing. Speed control is just very smooth at startup, and with those RC tires, it can climb steep ledges with such confidence that I never thought possible with lego parts. First I tried the 16:16 gearing, but I though it does have enough torque for the faster one, so now I swapped it to 20:16, which has the advantage that the gear below sticks out less, and it does not get caught up so much from the bottom. Also, this way the top speed is very good, like something with a buggy motor (or two), but with much better speed control (and steering). Will make a test video soon, just need to practise filming while driving.. :)

 

And the next step will be:

• building a bodywork; I'm thinking of a small Jeep, like a CJ or a Suzuki Samurai, but ideas are welcome
• printing some casing for the battery / ESC / receiver to enhance their placement / cable management (in the trunk)

Edited 19 hours ago by gyenesvi

[Appie]

That's nice little chassis and I am sure it's a blast with these third party motors 

One thing for the front axle you might want to consider is to add a 9L liftarm at the bottom of the steering liftarm or even better at the bottom of the "hubs" (though would probably collide with the gears and would cost ground clearance). The wheels get a little bit of negative camber with the current construction when sat on the ground. You can leave it as it is since it is nothing major, but if it annoys you, that would help it a little (not completely).

[gyenesvi]

26 minutes ago, Appie said:

That's nice little chassis and I am sure it's a blast with these third party motors 

Thanks!

26 minutes ago, Appie said:

One thing for the front axle you might want to consider is to add a 9L liftarm at the bottom of the steering liftarm or even better at the bottom of the "hubs" (though would probably collide with the gears and would cost ground clearance). The wheels get a little bit of negative camber with the current construction when sat on the ground. You can leave it as it is since it is nothing major, but if it annoys you, that would help it a little (not completely).

Sure there's a little camber, but it's really nothing major, much better than I expected, and the goal here is definitely to maximize ground clearance, so I don't plan to add anything to the bottom.

[Krxlion]

This is becoming a very neat, small off-roader. One thing that I want to mention, is that it is much better for climbing purposes to have a battery on the front. I know that it is often very hard to do, especially in this scale. I don't say you should remodel it completely, but for a future projects, you might consider flipping the motor, so it's more on a rear side, then battery in the center (or as much to the front as you can do). Recently, I was building your Toyota Hilux chassis, but powered with BL setup and moved the motor to the back, so I can put a battery on the front and improve climbing performance.