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The bicycle I am thinking to buy has a stepless shifting hub that is officially rated for 75 NM typical and 90 NM maximal torque. The electric engine of the bike can provide 75 NM at the maximal setting of assistance. Me, only 15 NM? I am not sure.

I need a bicycle to commute and there are some rather steep hills on the way. Hence it will be heavily used and I want it to last. There is an option to select a more usual 10 or 12 speed transmission with chainrings. In terms of durability, would it be the better option? How much momentum can by handled by the chainring-based transmission? I was not able to find the official maximal ratings for them.

If to be precise, my specific case is Ibex eAvantgarde Neo that has an option for Enviolo TR. The technical manual says 75/90 NM for the total weight of 180 kg. And the engine is Bosch Performance CX Cruise, 25km/h.

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    The hub may be rated for electric assist rather than total load. The torque load can be changed by altering the sprocket size anyway. Derailleur drivetrains suffer increased wear in electric assist applications too. What is the specific hub and motor combination?
    – Noise
    Aug 21, 2021 at 22:10
  • It is a good point that there is still a chain gear before the hub and the rear chainwheel is obviously smaller.
    – nightrider
    Aug 22, 2021 at 9:14
  • I also wonder whether the rating is, in fact, the allowed motor torque value only. Aug 23, 2021 at 6:22

2 Answers 2

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Torque at the hub is reduced by the gear ratio. Take an example: a 55t ring with 22t sprocket gives a 0.4 torque multiplier.

this translates the 90Nm motor torque to 36Nm at the hub. Not much problem!

You are buying a mass market bike and the components will be correctly chosen. The Nuvinci hubs are heavy but strong, i don't see you'll have a problem. Rotalis uses this combination for some of their e-bike holidays.

In response to @Vladamir_F:

I selected the example ring sizes based on what is commonly used with a Gates belt system, as the Questioner's proposed purchase has. It is equivalent to around 69inches on 40mm 700c tyres, quite a common choice for singlespeeding and certain hub gear combinations.

Regarding other gear combinations, a smaller chainring with a larger sprocket of course increases the torque at the hub using the same maths. However, Internal Gear Hubs have a minimum sprocket/chainwheel gain specified by the majority of manufacturers for this reason, so as not to damage the internals.

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  • However, the easiest gears on the bike could be the opposite, e.g. on an MTB, 28 at the front and 46 in the back! A 55t front chainring is even more than the classical road 53. Aug 23, 2021 at 6:19
  • @VladimirF see updates. Thankyou.
    – Noise
    Aug 23, 2021 at 9:30
  • Thanks, I hope I understand it now. Aug 23, 2021 at 9:52
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Me, only 15 NM? I am not sure.

I am sure about my torque.

If I stand on the forwardmost pedal, it's 110 kg weight. If I pull up from the rear pedal with 25 kg weight (clipless pedals) and push down more on the front pedal, it's 50 kg additional weight. If I also pull up from the handlebar with 30 kg load, it's 30 kg additional weight. This all adds up to 190 kg weight, or 1864 newtons. This at a 0.17 meter long crank is 317 newton meters.

I don't know what the hub torque means. If it means torque at the cranks, it's game over even without electric assist. Only some lightweight occasional rider who rides 1 km to the nearest shop on non-hilly terrain might find it durable.

If the hub torque means torque at the hub, then do note there's about 1:2 torque reduction gearing at the chain. So after the chain drive, 317 newton meters of load would be only 158.5 newton meters. Still it's more than the 90 newton meters so I suspect this hub is made for lightweight occasional riders.

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