Why normal load matters
Drive force acts at the contact patch. If a powered module carries little vertical load, it can spin even when motor and gearbox torque are ample. Frame twist or ramp transitions can shift load away from the drive wheel.
Why a friction coefficient is not enough
Real surfaces are contaminated, worn and irregular. Tread temperature and contact pressure change during operation. Use a conservative model for preliminary work, then test the actual wheel–floor pair.
System response
Mechanical compliance can help maintain contact, while current limiting and slip-aware control can reduce abrupt wheelspin. Both must be evaluated with steering and braking behavior.