Strange SCX10 type drivetrain phenomenon!
I have an SCX10 esque vehicle, think all metal (Aliexpress) Chinesium, which is properly shimmed and buttery smooth. However, today I have received a 55T motor and I've been busy messing.
I noticed, after removing the spur gear and the attached clutch/friction pad, that when I rotated the opposing alloy friction disc it was buttery smooth but in the other direction there was resistance. By resistance I don't mean friction but more like magnetic or gravitational, if you get my meaning.
I was using the pad of my finger on the edge of the friction plate and it was not a problem in the one direction but slipped in the other, regardless of pressure.
I'll say again, everything is shimmed to perfection but what comes to mind is the differentials and the direction the gears rotate.
Is there some sort of mechanical phenomenon that produces this? What is also strange, is that if I lift the vehicle off of its tyres the buttery smooth prefection is the same in both directions of drivetrain rotation.
Good job going through a chinese truck and getting it shimmed to perfection and buttery smooth. It sounds to me more like the gears need time for their mesh to wear in. Probably run a couple packs through it and recheck.
Also might want to scuff up the friction surfaces on your slipper clutch while its apart.
Or being chinesieum, it could have an off spec bearing trying to act more like a One-Way beaing.
I've just experimented with the orientation of the gearbox/motor as a process of elimination. I've been running it with the gearbox/motor flipped 180 to counter torque twist(spur gear to the front) so I reverted it to its original, factory setup, with the spur to the rear.
The effect is the same in all ways but in the opposite direction!
The gearbox, on its own, was slick and equal in all directions be it spun from the input shaft or output. Therefore, the only thing left are the axles which have not changed orientation.
I also hand tested the axles/diffs in each direction, via both the drives shafts and wheels, and they too are perfect and free, needing only the slightest effort to move and keep moving.
How can this effect be explained if it is now in the oppposite direction?
I just tried the gearbox connecting each axle individually and then followed by flipping the gearbox and repeating.
The effect is the same on both individual axles and with the flip of the gearbox the effect reverses.
This is known thing when the gear are worn.
It will go smooth in one direction, but feel notchy in the other direction.
This can be in both gearbox and axles.
Usually, bad pot metal gear wears down much faster than quality steel or aluminium gears.
Shimming won't help if the gears are a little worn.
And when you looked at the pinion on the motor, does the teeths look the same on both sides of the pointy end of the teeths?
If the teeths "leans" over to one side, or the teets are thinner one side, it means it's worn and should be replaced.
Also make sure you are using the correct pinion, you have both metric and inch pitch, and also MOD.
Also some wear may be picked up when the drivetrain have resistance, i.e on the ground, table etc, but not holding in the air.
This is a sure sign of either worn gears or low quality production or the quality of material used.
Take off the pinion or remove the motor and push the vehicle back and forth.
Does this make it feel smootly or notchy?
Does it have more resistance in one direction?
If it is, that is surely a sign of worn gears.
I highly recomend to change to hardned gears, from example Axial.
Hope this helps.
Edit: You said turning the gearbox, made the effect reverse, and that is down to the gearbox.
So the gearbox is the problem.
Either pinion, or the gears inside.
Hot Racing among others makes good gears.
I can't say for sure which gears are best as it's about 10 years since I drove a vehicle with a SCX10 transmission, and it had the orginal plastic gears and it held up fine for my use (2s on 55t motor).
These days I have only one SCX10 based rig running a hybrid of HPI and RC4WD transmissions.
And that's the problem with chinesium products, some may work, some may work for a while, but other things are utter crap.
You never know what you get.
But if you switch to Axial or other known brand gears, it should work ok as long as the tolerances in the gearbox are the same as Axial SCX10 which it's based on.
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