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  1. #1
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    How much power will an Alfine handle?

    I am sure I will be bashed for doing this, but I am looking to build an electric bike that uses the Alfine 8-speed hub. Does anyone know what kind of power these hubs can take? I was hoping for 4 kw. Thanks -David

  2. #2
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    Long answer

    Power isn't the only number you should be looking at. Torque is going to be the most important number. Internally geared hubs typically have a minimum gear ratio to limit the torque on the hub. That minimum gear is typically picked based on the max power that humans can output (or more to be safe).

    This is totally speculation, but I'd guess that they're probably designing the hub to take around 1500 watts being applied at slow rpms (standing on the pedals) and therefore fairly high torque values. If you up the rpms (as I assume you will), you can probably get away with higher power numbers.

    That leads to the next problem: the hubs aren't designed to take any higher rpms than a cyclist can produce. That's not to say that they won't, but you may be the first to try it.

    All that being said, it's unlikely that you'll take off and have the hub explode. Many people on this forum are pushing the limits of what these hubs are made to do and are getting away with it for some period of time. Some people never have any problems. If you do end up running 4kw through a Nexus, I wouldn't expect it to work forever.

    Have you looked at the Nuvinci CVT hub? Staton motors has been using these hubs with gasoline engines for a year or two now, so one would assume that they're handling the stress.

    I hope I've been some help. Good luck with your project!

  3. #3
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    Eddicycle,

    Thanks for your response. I think I will skip the Alfine in favor of the cassette for now. Once it wears out, I might try the Alfine or its replacement at the time. Thanks again, -David.

  4. #4
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    Quote Originally Posted by damcard
    Eddicycle,

    Thanks for your response. I think I will skip the Alfine in favor of the cassette for now. Once it wears out, I might try the Alfine or its replacement at the time. Thanks again, -David.
    I would love to see you try this, I've been thinking the same thing for some time, but not with anything near 4 kw. What motor and batteries do you plan to use?
    JW

  5. #5
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    Have you looked at the Nuvinci CVT hub? Staton motors has been using these hubs with gasoline engines for a year or two now, so one would assume that they're handling the stress.
    I have a fair bit of experience with the NuVinci. I have built two motor assisted dual-suspension MTBs using the Nuvinci, with a Staton reduction gearbox, and a Robin 33cc 4-stroke engine, rated at 1.6HP, though it is nearer to 1.2HP in the real world. You won't damage the NuVinci with human power alone, or with the Robin engine alone. However, my mate and I use the bikes for serious off road use on inclines that require human and engine power together,and we have both damaged our hubs by applying excessive torque.

    I spoke to the NuVinci engineers about the torque limitation for these hubs. In the case of the NuVinci CVT, the clamping force on the drive balls (required to stop the balls slipping) is proportional to the input torque. If the input torque is excessive, the clamping force becomes so great that the surface of the balls becomes permanently flattened. If this happens, the drive becomes really "rough" due to the flat spots on the balls - sound painful, doesn't it! This is different to a geared hub, where a large torque will either do no harm, or else cause catastrophic damage due to tooth or shaft breakage.

    NuVinci gives a maximum torque specification for these hubs - look on their website and find the specs. Note that the maximum torque rating is referred to the INPUT shaft, not the output, because the clamping mechanism is driven from the input.

    It should be a simple matter to calculate whether your emotor will exceed the NuVinci input torque rating. Which motor are you thinking of using?

    Rohloff also give a max torque spec for their geared hub, which from memory is similar to the NuVinci. However, I suspect that in the real world the Rohloff can handle more torque. I say this because a number of very fit people have successfully used the Rohloff on off-road tandems, and from my experience I am fairly sure the NuVinci would not handle the power output of two fit people.

    I'm not knocking the NuVinci though. It definitely performs to spec, and with a little restraint and common sense I have not damaged my second hub, with the combined power of myself and the petrol engine.

    I am very interested in hearing people's experience with all types of geared hubs, in torque-abusive applications such as tandems or motor assist. For me, I would prefer the lighter weight and higher efficiency of a geared hub, compared to the NuVinci, if a geared hub was available that could handle my torque requirement.

    Colin

  6. #6
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    NuVinci CVT is not rated for tandems, but...



    ...what fun would there be if we always followed the rules.

    However:


    technically it's not a tandem...

    BTW: Rohloffs are rated for tandem use, even for a fit team.

  7. #7
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    Quote Originally Posted by itsdoable
    NuVinci CVT is not rated for tandems, but...



    ...what fun would there be if we always followed the rules.

    However:


    technically it's not a tandem...

    BTW: Rohloffs are rated for tandem use, even for a fit team.

    OK, um, that kicks ass.

  8. #8
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    Quote Originally Posted by damcard
    I am sure I will be bashed for doing this, but I am looking to build an electric bike that uses the Alfine 8-speed hub. Does anyone know what kind of power these hubs can take? I was hoping for 4 kw. Thanks -David
    It's torque that kills gearboxs (which is what an Alfine hub is essentially) not power.

    Bear in mind that a strong cyclist is capable of producing as much torque through their legs as a small to medium sized car engine (yeah seriously!), but the cyclist only delivers it at say a maximum of 100rpm. Most car engines rev to 6000rpm or more, hence they produce a LOT more power. Without going too far into the maths, take something like a Toyota Yaris with a 1 litre engine, which produces circa 90 Newton Metres of torque (easily within the realms of possibilty for a strong cyclist to produce), but circa 48KW of power at approaching 6000rpm.

    As power is a product of Torque x RPM, you can see how the car engine produces its power... Via high revs! By comparison the cyclist would producing circa 800 Watts ((48KW/6000RPM)*100rpm) at 100rpm.

    Anyway, to cut a long story short... What RPM does the electric motor in the bike produce 4KW at? If you can find out the RPM, you can work out the torque of the motor. I'd hazard a guess, but I reckon the 4KW electric motor doesn't produce as much torque as an average cyclist, it will produce it's high power figure via high RPM like a car engine. Personally, I'd be inclined to give an Alfine hub a go on an electric bike! If you do, let us know how it goes...

  9. #9
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    Quote Originally Posted by itsdoable



    ...what fun would there be if we always followed the rules.
    Ok, I've been absent from the forums for a while, so what happened here? Got a link to another thread? How'd you jump from the Speedhub to the NuVinci? How's the shifting action on that thing? Lots of twisting? More pics?
    speedub.nate
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