Originally Posted by
wphamilton
I am a little skeptical of 2% more drag of IGH. I think it may be true in some case but not in general, because if I'm not mistaken they generally have planetary gearing which have greater losses than that due to having multiple stages......
You're right to be skeptical of the "precise" 2% figure. However it's still a reasonable working rule of thumb. All bicycle drive train systems have 2 sources of inefficiency. All have some fixed parasitic drag from things like lube viscosity and unloaded movement, and there's load related drag which increases with load. The fixed losses factor more at low power, while the load related losses become more (relatively) significant at higher power levels. So any efficiency comparison will depend on the power level tested.
IGH hubs are tricky to rate for efficiency because it varies greatly by gear selection. Simple hubs, ie. 3s and 5s, use a single planetary stage and are most efficient in middle gear which is direct drive to the shell, so there's only the fixed parasitic drag. They're also pretty efficient in the higher gears because the loads tend to be at the lower end of the range. OTOH they can be more inefficient in low where the loads are higher. Muti-stage planetary systems are more inefficient because each stage adds it's own drag.
Derailleur systems have their own problems due to sprocket sizes, chain misalignment and power lost to the idler cage pulleys and lower loop tension. So any comparison can only be a rule of thumb guideline and as such the added 2% estimate is reasonable, but should not be taken as gospel.
In any case, efficiency is only one factor in the choice, with needed gear range, weather/riding considerations, purpose, weight, etc being others.
IMO- IGH shines for utility where the needed gear range is narrower, but is a poor choice when gear range, weight, and efficiency become more important. If anyone making IGH hubs were to ask me, I'd suggest focusing on simpler lighter 3 and 5s hubs of varying ranges, especially narrower ones to offer urban riders closer spaced gear choices for relatively flat terrain. They shouldn't chase the wide range multi-step market where they'll always be at a disadvantage to derailleurs.
FWIW- I own both derailleur and IGH, along with fixed wheel bikes (including a 2s fixed) and my choice depends on the application. What I'm still looking for is narrower 3s (or 5s) IGH suited to gently rolling (and varying wind) riding for utility and commuting. Sadly the IGH makers still don't realize (or feel they cannot market) that closer gearing steps are more important to serious riders than wider range. If I were to design a hub it would be a 5s single stage with the 3 middle ratios as closely stepped as practical, and the two outer steps being wider. (anyone out there listening?)