Nexus 8 torque capability questionable ?

[lesdit]

I am considering a cyclone 1000w setup on a mountain bike that I ride trails with. I am very happy with an
internal hub transmission on a different non E bike I have, but I am worried about the use of a Nexus 8 on the new
bike. How rugged are they ? Are they all metal inside?
A human makes about 300 watts power max, and I want to know if the Nexus is OK with bursts 10X that
amount?

 

[MitchJi]

Hi,

I am considering a cyclone 1000w setup on a mountain bike that I ride trails with. I am very happy with an
internal hub transmission on a different non E bike I have, but I am worried about the use of a Nexus 8 on the new
bike. How rugged are they ? Are they all metal inside?
A human makes about 300 watts power max, and I want to know if the Nexus is OK with bursts 10X that
amount?

It might work on the pavement but probably not on the dirt.

 

[OneWayTraffic]

I think there's an MTB rated version of the Nexus. But I'd still be wary putting that much through it.

 

[JS Tyro]

Don't look at the wattage - look at the actual torque. Remember that you're comparing the motor's torque to a person (choose a weight) standing on a pedal (choose a crank arm weight and multiply). Work out the actual chain tension in both cases and compare.

A 200 pound (890 N) person on a 7" crank (175mm, on the long side) yields a 1400 pound inch (156Nm) torque. On a 28t chainring (max acceleration on a triple is the smallest ring - could be as small as... 20t?), that works out to 314lb (1397N) tension. For comparison, when playing with one of the bike motor simulators, the highest torque I saw was about 80 Nm or about half what a person could do. However, with a 14t cog on it (that's what Cyclone uses IIRC) then the chain tension works out to about the same (half the torque and half the teeth). If you can find the spec on the 1000W motor for what the actual torque is, you can fiddle with the numbers.

Don't forget that the Nexus would likely be somewhat conservatively designed - i.e. they probably figure on someone heavier than 200lb.

 

[lesdit]

OK, I see what you mean. The motor would not be as 'pulsed' as a human jumping on the pedal. I ordered a 1200
watt Cyclone kit, I don't recall seeing a torque rating. The motor has I think a 9:1 gear box reduction, and then
depending on the chain ring ratio, that would be the torque to the Nexus.

 

[MitchJi]

Hi,

Gary knows someone who has run Eteks (way more torque than any Cyclone) through Nexus 3 speeds and Nexus7 speeds without problems on the pavement. He told me he thought an 8 speed would be the same.

But on the dirt every time your rear wheel bounces up then hits the ground it will take a hit.

 

[John in CR]

JS Tyro,

You need to rework that math. That 7" crank is leveraging at the radius of the chainring, not the crankshaft (as far as the chain is concerned)...so maybe double, not x 7.

John

 

[JS Tyro]

I did - when I calculated the chain tension, I divided the torque by the chainring radius. A 28 tooth chainring is about 14 in circumference (1/2 in chain links). The radius is thus 14/pi = 4.46 in. Oops - that should be 2pi!! (I always do that!) So 14/2pi = 2.23in.

So: 1400 lb-in/4.46 in = 314 lb should be 1400/2.23 = 628 lb (2800N) tension on the chain.

The remaining comments are correct since I only compared relative sizes and torques - the math would work out the same.

Interestingly, dynamic loading can be twice that chain tension - over half a ton (1256lb).