These are the real thing when it comes to super heavy duty tubes. Super heavy-duty wall thickness to help prevent snake bites when you are admiring the scenery and don't see that pesky rock in the middle on the track.
Place these ultra heavy-duty tubes in and minimise the risk of that unwanted flatty.
300X 21 TR4 - 80/100 - 90/90X21
Check out this video on how to change a tyre, very useful: