I'm currently building a car with a simple 2-speed remote controlled synchronized transmission, shifted by a PF M-motor. Everything is fine except for one thing, the transmission driving ring is disengaging under the high torque due to the changeover catch going too far.

I have added parts blocking the catch from going too far, but when I stop sending current to the motor it spring back somewhat, disengaging the driving ring. Are there any ways of preventing this effect, or do I have to use a different motor altogether? Any help would be very much appreciated.

1 Answer 1


It sounds like the motor is "winding up" from drivetrain deflection. Lego parts "give" a little when stressed and therefore store some energy like a spring. This is likely being caused by the high speed of the PF "M" motor. A picture of the assembly would be very useful in determining a solution. Without seeing it, the simplest fix would be to gear down the motor. Given the fairly high speed of the "M" motor output, I would suggest doing this with a worm gear assembly. A worm gear will greatly reduce your output ratio to the driving ring while also giving you more precise control that is less susceptible to back-tracking from deflection wind-up.

Here is a photo demonstrating how a worm gear can greatly reduce output ratio in very little space:

enter image description here

Another potential solution, if you have the part, is to affix a linear actuator to the "M" motor. It would have the same benefits as the standard worm gear:

enter image description here

One last thing... Which version of part# 6538 are you using? 6538a & 6538b have ridges that allow the drive ring to "click' into place. If you're using part# 6538c then swapping it with one of the others may be all you need to fix your problem.

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A picture of your build would be appreciated to help reduce speculation. It would also be nice to see this just for the "neat-o" factor :)

  • 1
    I'll try the worm gear solution and see if it works, though it will increase the time needed to shift. I'm actually not using the old 2L driving ring, but the new 3L one with the 3L axle joiner with ridges. I'll see if I can post a video about it on YouTube. Commented Sep 2, 2019 at 13:46

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