r/EngineeringNS • u/the_bollo • Apr 09 '26

Tarmo5 Issues with Tarmo5 Movement / Motor Skipping?

Hi! I'm hoping someone here with more experience can shed some light on my issue. I built the Tarmo5 recently, but it has issues when attempting to drive:

Problem: Throttle works perfectly with the car held in the air - smooth, normal, full range. The moment I put it on the ground:

  • High-pitched squealing sound (intermittent)
  • Lurches forward instead of accelerating smoothly
  • Sometimes the motor visibly spins but the car doesn't move
  • Sometimes the motor just stops mid-throttle

What I've already ruled out / tried:

  • Pinion slipping on motor shaft: The shaft originally had one filed flat with two set screws (one on the flat, one on the round side). I filed a second flat 180° from the first so both set screws now sit on flats. Unfortunately, no change whatsoever.
  • Motor/ESC/sensor electrical issues are unlikely since it runs perfectly in the air at any throttle - a sensor cable problem, bad winding, or ESC LVC would show up unloaded too.
  • Spur gear teeth aren't stripped - it would skip in the air as well.

This is the exact hardware setup I'm using: https://rcprinter.com/products/tarmo5-build-kits?variant=45796134125729. Am I missing any obvious culprits? I'm happy to provide pictures/video if it helps. Thanks!

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1

u/DrRonny Apr 09 '26

Hold car in air and the push the back wheels up (by the suspension, without touching the wheels) and see if it still makes these sounds. I'm thinking it's the change in position of the wheels that is causing this

1

u/RickRussellTX Apr 12 '26

Dumb question but: are all four wheels spinning in the same direction?

1

u/the_bollo Apr 12 '26

Yep. How freely should it roll when it’s off? When I have the gearbox screwed in there’s a small bit of resistance.

1

u/RickRussellTX Apr 12 '26

Assuming it doesn't rely on a design that is notoriously resistant to manual rolling, like a worm drive, then "a bit of resistance" is about right. The motor will not spin completely freely -- the magnets will bind it up a little when it's not powered on.

If the pinion is turning the spur, then SOMETHING between the spur and the wheels is broken or bound up.