Rim width: The rim width of a typical road bike would be 20mm or less, and the width of a typical mountain bike rim would be 24-28mm. The rim of a fat bike is two to three times wider than that, with different models’ rims at 65, 88 and 100mm.
Tire size: The wider rim makes the tire wider and taller than a tire with a smaller rim, while creating a tire space of a significantly greater volume.
Tire pressure: Because the much larger volume already creates a greater cushion of air than that of a smaller tire, fat tires require far less tire pressure. A typical mountain bike might be filled at 28 to 35 pounds per square inch; a fat bike is best when filled to just 5 or 6 psi.
Where fat bikes excel: On sand or snow, where a narrower tire might sink in and the fat-bike tire glides along the top.
Why that is: The greater surface space makes all the difference. Try pushing one finger down onto beach sand; see how it goes right in? Now take your entire hand, palm open, and push it onto the sand. It stays on the top. For that same reason, the fatter the tire, the easier it will glide along the surface of sand and snow, instead of sink down into it.
How it feels: Like the smoothest ride you’ve ever had on a mountain bike.
— Scott Sandsberry