My back wheels do the same thing. There is no metal axle that connects both back wheels like the axle design does for the front wheels, but I'm sure LEGO designed it that way.
So, imagine the train's motor being in an "ON" state just a rolling. Now enters a kid, just being a kid. Grabs the thing, and starts a rolling in whichever direction kiddie wants to roll. Not a caring that the motor is doing it's own thing too. How does LEGO protect the device from the free rolling madness of a child?
So, I think this slippage is a built in feature to kid proof protect the motor from being over torqued. Some type of reverse rocket booster dampening system. Analogous maybe to a circuit breaker which, is an automatic device for stopping the flow of current in an electric circuit as a safety measure.
(01) Battery check.
You said it no longer can pull extra weight, and that it stalls even on flat normal gravity leveled curves? Huh? So, before we go too deep, I would recommend first replacing the batteries with new ones. Then, retest it.
(02) Balancing our time vs. the money justification check.
Tearing down the housing to get inner access to remove excess hair and debris from this unit is fairly easy, and just requires buying a T9 Star Torque from the local hardware store and maybe 15 mins of our time.
Even though this thing is LEGO engineered like a tank, we can still imagining extensive damage, let's say a smoosh over by car, where we'd need to replace the circuit board, motor, or destroyed plastic gears.
Then, maybe it might be time to do a price check and consider purchasing a new/used unit versus the time to invest and produce a 3D printed replacement.
(03) Axle wheel hub tear down (setup only).
I'm not taking mine apart, becasue I haven't justified a reason that works out for me, but this is my observational approach if anyone else wants to venture further into the unknown.
So, let's look at the wheel design and note the similarities/differences between both wheel hubs.
Break down wheel hub anatomy consists of at least three pieces.
- Inside hub piece.
- In between trans-clear rubber tire tread.
- Outside hub piece.
- Unknown non-visible pieces. Possible notched out axle bar.
The inside hub pieces and in between trans-clear tire tread all look to be the same molds, where the outside hub pieces differ in aesthetic design, but function in the same way when interlocking with the inside hub piece.
Here's where things get dicey. There seems to be six interlocking tabs that need to be simultaneously unhitched to separate the pieces without permanent damage. Unless you happen to be a six clawed mutant named Wolverine, this will be a challenge.
Six interlocking tabs release damage free setup and approach:
- Hot glue one end to work desk for stability.
- Insert six flat head precision screwdrivers.
- Wiggle until it's free.
Six interlocking tabs release egg breaking, omelette making approach:
Goal: 3D print a replacement, so permanent damage doesn't matter.
- Using a Dremel rotary tool, carefully drill out 4 of the 6 interlocking tabs ONLY. Don't go deep. We can 3D print plastic, but not the clear-trans rubber treading.
- Insert two flat head precision screwdrivers to disengage the locking function of the 2 remaining tabs.
- Using a digital caliper measure out the dimensions of the working 2 remaining tabs.
- 3D print replacement piece.
- Consider using some Loc-Tite glue adhesive when reassembling the