Okay, thanks for reply. To anyone interested, after looking at this video:https://www.youtube.com/watch?v=H9_Llzcz2cY
how I understand it is that, when we assume that input's rotation speed (our cadence) and torque (we're tapping along) is constant, then ratio is in inverse proportion to torque on the output, so
- on the underdrive: the smaller the ratio, the bigger the torque on the output
- on the overdrive: the bigger the the ratio, the smaller the torque on the output
However, talking about gearbox, every time it has to convert rotational speeds of input and output, it's under torque (except for the 1:1 in single planetary gearhubs, like 3-speeds), but, judging from the above, the torque is bigger on underdrive. Given the small size and great proportional differences of input torque (when we stand on pedals or start on high gear), IGHs' gears are subject to greater torque, than properly used car's transmission.
Back to the topic xD - that justifies using EP additives in grease for IGH (apart from what Brucey wrote about great differences in rotational speed of bicycle bearings compared to what greases are normally engineered for).
Another question - what would you say about greases for (coaster and roller) brakes? It's written everywhere that you should use high-temperature grease for coaster brake hubs, but I never understood, if it means the whole hub, especially the left side bearings, or just the brake shoe... Also how high temperatures should it withstand? Bearing greases' labelled "high temperature" normally go up to 150 celsius, which is not much higher, than standard bearing greases I checked. In my city, there was old bicycle mechanic, one of few, who would deal with IGHs, and he told me, he uses standard, cheap graphite grease just on the brake shoe.
My tactics with coaster brake hubs is to use normal, NLGI 2 grease for bearings (more of it on the left side), semi-fluid one for gears, and high temperature, super thick MoS2 paste (meant for brake pistons and backside of brake shoes to prevent squealing). My intuition tells me, that:
- bigger amount of grease on the left side bearing will compensate for its melting under high temperature.
- MoS2 paste will prevent it from seizing, while, being thick, stay on the brake shoe and not migrate to other parts of the hub.
To be honest though, in 99% cases you could probably use anything, as coaster brake hubs are mostly used for super leasure cycling and won't see that much of this heat
When it comes to rollerbrake, MoS2 paste is so thick, it definitely induces drag. Since Shimano grease is so expensive, but also out of curiosity and experience with their white Nexus grease (which turns out to be inferior to widely available competitors), I was trying to find an alternative. What rollerbrake grease seems to be, is MoS2/grafite (it's black), low viscosity (NLGI 0?) grease. Similar things I managed to find, were meant for 150 celsius, or were, at least, NLGI 2 thickness. However, a copper grease seems to match the criteria - it's both supper high temperature (actually used around brakes in automotive) and I managed to find low viscosity versions of it. Do you think it's a good candidate? Or maybe it's wiser to try to dilute one of those ceramic/moly grease (if yes, than with what? gear oil?).