Originally Posted by
Bonzo Banana
The normal granny gear arrangement is the chain running between the smallest chainring at the front and the largest cog of the cogset at the back which is super easy to turn enables you go up hills at low speeds. Also on many cassettes and freewheels the largest rear cog can be aluminium or lower grade metal but the smallest cog will be highest grade or heat treated metal possibly a decent steel alloy. Wouldn't that indicate the level of strain and wear?
Admittedly I'm not really factoring how internal hubs work because I'm not too knowledgeable with regard planetary gears but assume it is similar situation to a derailleur based gear system.
Besides isn't it how Shimano configures nexus/alfine hubs if you have a smaller rear wheel (20") you can have a larger front chainring but if you have a larger rear wheel (700c) you need a smaller front chainring?
When you said
"The normal granny gear arrangement is the chain running between the smallest chainring at the front and the largest cog of the cogset at the back which is super easy to turn enables you go up hills at low speeds.", that is correct. But, keep in mind you need high torque to go up hills. I have spun out (lost traction) when climbing steep hills because my rear wheel torque was high enough to spin on the ground surface.
When a cassette manufacturer uses a softer metal for a larger sprocket, perhaps they do not expect that sprocket to be used as often? Or perhaps they wanted to make it lighter? But if you are using the smallest chainring on a bike you are maximizing the chain tension and that would put more torque on the sprocket than if you were on a larger chainring.
I can't comment on specifics on Shimano IGH hubs, the only IGH hubs I have are a Rohloff, Sram Dual Drive and a vintage Sturmey Archer 3 speed.
The point with a IGH is that if you put too much torque on the sprocket, it could damage the inner gearing components. That is why Rohloff sets minimum ratios for the chainring to sprocket ratio. For my weight, that ratio that Rohloff sets is 1.9, but I use a range of 2.25 to 2.75 depending on where I am and how much weight I plan to have on the bike, it is a touring bike.
The reason you use a larger chainring on a folding bike with small wheels is because if you don't you spin out.