Modular RC 4speed 24/12 gearbox

[Sophraves]

Hi everyone,

This is the drive module of a modular RC car I’m working on (uses 2 of these).
The intentionally compact design pushes LEGO tolerances, so I’d appreciate some experienced eyes on the gearbox — also to help spot weaknesses or improvements.

You can download the Stud.io file from my BrickLink Gallery.

What it is:

• Pure LEGO (no modifications)
• 4 speeds: 1x / 2x / 4x / 8x (no z8 gears used);
• Compact layout;
• Motorized shifting with indexing;
• Cam-actuated Idler gear;
• Driven by two motors;
• Designed for RC use, not display-only.

Things I’m aware of:

• long axles are used instead of bushes to reduce friction, at the cost of less lateral support;
• Several gear clearances are very tight;
• Gear pair linking in the frames is not yet optimal;
• Axles are not fully constrained; this longitudinal play could cause friction issues;
• The full model is currently digital; I’m actively checking whether physical assembly is realistic.

Specific feedback I’m looking for:

• Better ways to connect or brace the internal structure to the main support beams;
• Experimental knowledge about axle length vs bush friction in high load applications;
• Where would flex become an issue and how would you improve frame rigidity;
• Any red flags regarding friction, wear, or gear alignment in real RC use;
• Would you build this as-is? If not: what would you change, and why? Reliability matters!

I look forward to honest, well-founded criticism - this gearbox is meant to be used, not to look nice.

I will update this thread as soon as the missing parts arrive for physical testing — can’t wait to see it in action!

 

Many thanks in advance!
Sophraves

Edited 17 hours ago by Sophraves

[vascolp]

Looks cool. 

If it is pure lego, will you use a PU hub to control? PU motors have position control, you could connect the motor directly to the orange gear selector, no need to use the knob gear.

[Sophraves]

3 minutes ago, vascolp said:

Looks cool. 

If it is pure lego, will you use a PU hub to control? PU motors have position control, you could connect the motor directly to the orange gear selector, no need to use the knob gear.

Hey vascolp,

Ty for your reaction!

The control setup is conceptual, but it requires the idler gear to perform a full spin and hold position.
This is more compact than a 4:1 ratio with gears, which might be better anyway to link the indexing to the idler.

[2GodBDGlory]

I always like to see gearboxes in development! This is a novel gear setup--interesting!

I do wonder about how much gearing up you're seeing, though. Having a full 8x speed increase is a LOT, and I wouldn't be surprised if the motors stall very easily just trying to manage the friction of the gearbox while running at 1/8 of its torque. I also don't understand what the function of the cam-activated idler is, but I do worry about that 12T-12T meshing there. Those gears can kind of mesh in that spacing, but there's very high friction involved.

I also agree with vascolp that connecting the shifting motor directly would make more sense and cut out a lot of complexity, though I'm not sure how that would interface with that idler mechanism (since I don't understand the purpose of the idler mechanism).

Thanks for sharing!

[Sophraves]

1 hour ago, 2GodBDGlory said:

I always like to see gearboxes in development! This is a novel gear setup--interesting!

I do wonder about how much gearing up you're seeing, though. Having a full 8x speed increase is a LOT, and I wouldn't be surprised if the motors stall very easily just trying to manage the friction of the gearbox while running at 1/8 of its torque. I also don't understand what the function of the cam-activated idler is, but I do worry about that 12T-12T meshing there. Those gears can kind of mesh in that spacing, but there's very high friction involved.

I also agree with vascolp that connecting the shifting motor directly would make more sense and cut out a lot of complexity, though I'm not sure how that would interface with that idler mechanism (since I don't understand the purpose of the idler mechanism).

Thanks for sharing!

Hey 2GodBDGlory

Thanks for the thoughtful feedback — these are exactly the points I’m trying to validate.

An 8× speed increase is indeed aggressive. In practice, shifting into 4th would only happen near maximum speed in 3rd.

The cam-activated idler disengages the drivetrain during shifting. The drive motors are briefly unloaded and the wheels decoupled, so no back-driven forces return into the gearbox and there’s no risk of two clutch elements engaging simultaneously under load. The idler is only engaged when a gear is fully selected. I agree this adds complexity, and it’s one of the first subsystems I’d remove if the drawbacks outweigh the benefits in testing. The timing, positioning, system response (rubber band actuation), and supporting software all need proper evaluation.
You’re absolutely right about the 12T–12T mesh: while it works geometrically, friction and wear are real concerns, especially for RC use. This is one of the areas where physical testing will be decisive.

On the input side, the two drive motors are currently coupled via a 20/12 ratio, but this can also be changed to 16/16 or 12/20 without major redesign. I’m keeping this flexible for post-build tuning once real-world losses are clearer.

Finally, directly coupling the shifting motor is a fair suggestion. I opted for mechanical indexing and locking to ensure defined gear positions under vibration and load — especially since the idler mechanism itself benefits from precise indexing — but this choice is also open to revision if it proves unnecessary.

Thanks again for the critical look — it highlights exactly the weak spots I want to test next.

[gyenesvi]

I agree with others that the switching motor could be directly connected, it's pointless to complicate things in the RC case.

My philosophy about RC gearboxes is that more than 2 speeds are pointless, and often impractical :) Though I mainly build off-roaders where a high and a low gear makes sense, but anything in between does not really. So I assume this is more for the sake of doing it, than the actual practical utility of having more speeds.

Also agree with others that 8x speed range is way too much. The lego motors won't really be able to handle it. If it was me, 3x is the most I would try, something like 1x, 1.66x, 2x, 3x (or 2.77x) for the 4 speeds. Lego motors are weak enough for a single speed for a larger car, not to mention a range of speeds and the friction of the gearbox.

As for the 12T-12T diagonal connection, just try it manually with 2 gears on an L beam - you'll see immediately that it does not work, way too much friction.

I guess you could start out with testing a 2-speed gearbox and having a complete body of the car to see the weight, and find out which speed range works, and then you'd see how you could fit more gears in between the two extremes. What's the scale of the car you are building?