Trying to figure out which tires/tubes I can use
50-559 means 50mm wide tire, for a rim that measures &quot;ERTO 559mm&quot; in diameter. This is a very common size.
The &quot;inch&quot; equivalent is a tire for a 26&quot; wheel, with a width of about 2 inches. Any &quot;26 inch&quot; tire that's about 2 inches (or 50mm) wide will fit your bike, such a 26x1.95. The difference between 1.85, 1.95 and 2 widths is not large.
You could potentially get a wider tire, like 60mm / 26x2.4 inch, and it will fit your rim, but you would have to check if the wider tire will rub on your frame or fenders. Wider tires will ride better, but be heavier.
You could also get a narrower tire like 35mm / 26x1.5, and if should fit too, but the current trend is towards wider tires. Narrow tires will ride worse but be lighter.
Tire pressure that you should use is relative and will depend on your weight and the tire size. The number printed on the tire is the maximum tire pressure not to exceed. Usually, you will run less pressure than the max. On my cargo bike I run 26x2.5 tires with a max of 60psi. I actually run 35-40psi.
Tubes will work for a range of tires and you don't need to be exact. Practically any 26 inch tube between 1.5 and 2.5 inches will work for that whole range of tire widths. So you can use a 26x1.95 tube for a 26x2.25 tire, for example. The important thing is the correct wheel size (26&quot;) and a width that is reasonably close to rhe tire width.