For my setup under full continuous power, my calculations suggest about ~60Nm of torque is being placed on the 11T sprocket teeth, ranging to ~185Nm of torque on my 34T sprocket teeth. So even though this might suggest that the larger cluster sprockets would wear first, the fact is i spend most of the time in higher gear (smaller rear sprocket) and this combined with less teeth to handle the load means they wear the fastest - just like most non-electric on road bicycles, but on a faster timescale.
Drivetrain theory suggests much if not most of this wear is due to chain stretch to which sprocket teeth wear rapidly to match. For this reason i monitor my chain stretch with a linear measuring chain checker tool (Park Tool CC-2) and replace my chain at the 0.75% mark to give my cluster longer life. This seems to work out about every ~500km . Three chains cost about the same as one cluster so it is cheaper for me to follow this approach, although it is still annoyingly expensive.
To increase the lifespan of your sprockets i suggest you also monitor and replace your chain quite regularly if you have not been doing so. I dont think that shifting under load will directly wear your sprockets any faster, rather it will stretch your chain faster and increase the chance of bent links, which in turn will wear your sprockets faster. Also, keep in mind that the angled leading teeth of the sprockets you mention is apparent to some extent on BRAND NEW sprockets. This teeth form profile is an attribute of modern Hyperglide cassettes and is used to aid smooth shifting. From what i have noticed, It seems to be more apparent on the lower tooth count sprockets within the cassette cluster. Make sure your not confusing this feature as part of normal wear.
As you said IGH is ideal as it allows the use of beefy 1/8" BMX chain with non-tapered link pins which extends chain life enormously with a dead straight chainline.
however.... few IGHs can take the torque of electric drive. The Rohloff is well suited due to its off-road/Tandem rating, 526% gear range, however they themselves cost as much as a decent bike. There is a small community of MTB riders who have reported success in using the Alfine 8spd in an overgeared configuration for offroad use (a lower chainring:rear sprocket ratio than Shimano recommends = higher torque on the epicyclic gears), so that in itself is placing torque loadings on the hub approaching that of electric power when using manufacturer redcommended chain gearing for on-road use.
The SRAM i-motion9, and Sturmey Archer 8 speeds are not much good. Apart from having clunky shifting and being noisy in some gears they have reports of high failure rates from human power alone.
Although strong and beefy, the Nuvinci IMHO is just too heavy, let alone on a rear sprung offroader.
For my new Non-frock project bike I myself are hanging out for the Alfine 11 speed hub with 409% range. The proven pedigree of the Alfine 8spd being overloaded combined with rumours that the new version may well be spec'd for XC riding means it is shaping up to be a good option for electric assist. Greater gear range, oil bath lubrication and helical gears are a promising improvements over the Alfine 8spd, but still it'll be About half the price of a Rohloff..... i'll still take the plunge though.
I have a cunning plan
to combine motor power and pedal power at the hub's input despite the limited room for two sprockets. No room for even one freewheel though, so freewheel cranks and and a freewheel jackshaft are still required just like a chainring driven system
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8)
my data is all real test.
