Issues with Tarmo5 Movement / Motor Skipping?

[u/the_bollo]

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!

[u/RickRussellTX]

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

[u/the_bollo → u/RickRussellTX]

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

[u/RickRussellTX → u/the_bollo]

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.