After traversing some distance, many small particles (1-3mm across) got locked into the tyre. This grime built up over time. The strength of the wheels are actually in the rivets that hold together the titanium strips of metal. Some of the abrasive soil and pebbles were grinding in between the strips and ran a risk of rivets 'popping apart.' This could cause an equivalent of a blow-out.
Some pictures show the Lunar Rover with no tracks seen on the surface. There are five answers to this 'problem.' They are all listed in the revised book (4th October 2018) but here is one for free...
On one or two occasions, the astronauts halted the vehicle, lifted each wheel and extracted the material as best they can just by tapping them gently with a hammer. Then they carried the vehicle a few feet away from all the loose debris, put it back on the deck (away from the existing tracks) then continued with the EVA after taking a picture or two.
On earth the Rover weighed 209kg, on the moon with just 17% of earth's gravity, it weighs just 35.5kg. Moving the vehicle around by hand is no big deal for two healthy astronauts.
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