I have used a Sturmey Archer AW, three speed hub for years. I could build one in a trash bag if asked. Except for loading the pawl springs, I could get one back together from a pile of parts without looking.
This might seem like a lie to anybody reading this that doesn't know exactly what's in that little can shaped "Black Box", but if you have ever built one twice correctly, you can do it again and most likely without looking. Its funny to me that most mechanics that know how to assemble an AW, don't know how they work. True! Ask.
I like to explain how the hub works by telling a story about a fishing reel. High gear (3rd) is like your legs powering the hand crank and the bail is the wheel.
Low gear,(1st) is like your legs powering the bail and the wheel is the hand crank.
Second gear is direct drive.To explain any more would require a hub in hand to show how the clutch is pulled and pushed to engage the planet cage or disengage the high gear pawls.
Both the three speed hub and the fishing reel are what's known as a "Planetary Gear" or "Epicycle Gearing."
Here is the magic and a story. As long as the "Sun Pinion" on the axle doesn't break, almost anything else can and allow you to keep riding in 2nd gear.
On my last day of my tour in Totnes, Devon, Uk, I spent the day riding around going back to GPS points to set up and take photos.
1937 Raleigh Golden Arrow
After 3 stops, I needed to ride down town, about 200', and ride up on the other side of the valley for 3 more photo shoots.
Riding through town I felt and heard a large crack. After that a click, click, click. Not Good.
I needed to take the photos at 250 feet above town, ride back and then climb 265' up to the B&B to pack the bike for the trip back to Boston.
Other then the click, the bike worked fine in all three gears. I stayed seated in the saddle and did the climb expecting the hub to give way.
Returning back to Boston, I dismantled the hub and found a broken Planet gear.
As seen in "Raleigh Project One" the AW hub will work for years missing one of these little gears. In this case, the broken chip was never found.
An extra note to this story, I have had 3 Sturmey Archer AW hubs "Explode" while on tour. Another story for another day.
1937 Raleigh Golden Arrow with broken hub