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  1. #1
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    Motor choice and battery for uphill riding

    Hi,

    I'm building a heavy duty electric bike to help me pull stuff upp a long hill. No need for it to go very fast ~15km/h is fine, but it needs to be strong so that it can work for a long time with load and uphill. Weight of the total vehicle + rider is ~130 kg and the slope angle is max ~20%. Would be great if the engine could work continiouselly for at least 2-3 hours.

    Any idea if is possible? Can I use a "standards" e-bike engine? What power do I need (500W or more?). Which batteries would be suitable? Do I need to "gear down" a standard e-bike engine to fit this purpose? Any idea of typical weight of a motor + battery pack for this application?

    Thank you very much in advance for input.

    Best regards

    Carl

  2. #2
    Senior Member CigTech's Avatar
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    I think the Mid-drive system would be best for you with Lipo4 battery. Check this out http://www.gngebike.com/450wbrushless.htm See the Mid-Drive has more low- end power for hills, and with a 20% grade hill you'll need a lot of power. The only problem I see is the run time up long hills. The battery drains fast with a heavy load up hills. But if you battery has a lot of amp hours then it is possible.

    The bike, batteries, motor wouls weight about 70 lbs. See I would go with at least 2 Lipo4 48v 20 amp batteries to give you about 2 hours run time on hills or 4 hours on flats.

    Hopes this helps
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    +1 on Cigtech's mid-drive/LiFpo4 advice.

    Another battery chemistry type worth consideration would be LiPo. This type of battery can offer significantly higher "C" ratings that other battery types often struggle to match. Note: LiPo batteries do require that you exercise caution while charging, storing, and use (always follow the manufacturer's instructions regardless of battery type).

    Here's a link that explains battery "C" ratings:

    http://www.commonsenserc.com/page.ph...explained.html

    You probably already know about this, but I thought I'd mention it anyway.

    Btw, welcome to the Bike Forums Carl.

  4. #4
    Transportation Cyclist turbo1889's Avatar
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    I absolutely agree that a mid-drive or crank/drive set-up that allows you to gear down is the way you need to go. Beyond the commercial offerings you could also consider doing a build using a hub-motor as a mid-drive/crank-drive motor like the stoker-monkey set-up or similar.

    I don't normally make this recommendation and usually recommend LiFePO4 batteries specifically packs made out of repairable/replaceable prismatic cells with screw top terminals but considering how much weight your going to be trying to haul anyway you might be best off just overbuilding for double the cargo weight and using some big heavy deep cycle SLA units. I'm thinking a cargo trike or quad build for that kind of set-up.

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    Hi,

    Thank you for your replys and tips! Much appreciated. I guess it's not possible to gear down a hub motor to have a lower speed at optimum rpm? I like the idea of a hub motor since from what I read it's more reliable with less wear than a Mid drive motor since all parts are encapsuled. Just for me to understand, what is the problem with a 500W motor and a Lipo4 battery? would the motor burn or what's the key issue? If the slope would be less steep, say 10% would it work then?

    Thank you!

    Carl

  6. #6
    put our Heads Together cerewa's Avatar
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    10% grade is not really what hub motors are designed for. Normally, hub motors aren't very efficient when climbing hills that steep. My hub motor would function fine if used continuously at 10% grade (or at least it would when the weather isn't hot) but it partly depends on the weight the hub motor is carrying (including bike, batteries, person, and cargo).

    Note, "geared hub motors" may work for what you are doing, but if the internal gears are plastic (as many of them are) you are likely to melt the gears.
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    I have 2 electric bicycle conversions: one being a direct-drive rear hub motor and the other a (rear) geared unit. I use a single 36v 10Ah for both. The DD bike = ~ 720 watts (36v x 20 amp controller) and the geared machine = ~ 450 watts (36v x ~ 13 amps).

    Both of these bikes work well for how I use them (14-18mph top speeds on nearly flat terrain with an over-all weight of ~ 275 lbs.). However, neither of these "run of the mill" ebikes would be suitable for your needs imo. I say this because the DD bike would probably over-heat the hub and/or battery while the geared machine would most likely "melt" the plastic internal gears.

    A high performance BMC or MAC geared hub (@ 48-52v) might work out for you. However, these hubs are much more expensive than your "run of the mill" geared units. Both of these geared hub motors are pretty heavy as well.

    Imo, the mid-drive option would be a much better setup for your needs (due, in part, to the selectable gearing).

  8. #8
    Transportation Cyclist turbo1889's Avatar
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    Quote Originally Posted by carlww View Post
    . . . I guess it's not possible to gear down a hub motor to have a lower speed at optimum rpm? I like the idea of a hub motor since from what I read it's more reliable with less wear than a Mid drive motor since all parts are encapsuled. Just for me to understand, what is the problem with a 500W motor and a Lipo4 battery? would the motor burn or what's the key issue? . . .

    Okay, long story short, electric motors unlike internal combustion engines have a fairly flat torque curve. Generally it can be said that for an electric motor torque is a reasonably stable constant, the exact winding and mechanical configuration of the motor has some effect on that of course, usually though the torque slightly decreases with increased RPM. Unfortionatly, this has resulted in a whole lot of people unnecessarily abusing electric motors by not gearing them down appropriately either mechanically or electrically (it is possible to wire a motor with multiple winding configurations that you can switch between electrically that are nearly as effective as using mechanical gears) and just dumping more power into them to pull heavy loads at lower RPMs then they were designed to operate at.


    If you have a 1,000-watt direct drive hub-motor that is designed to run at 350-RPM (about 27-mph with a 26” wheel) and you instead run it up a hill at 7-mph pushing the full wattage into the motor over half of the power you are putting into the motor isn’t being used to produce mechanical motion to propel the vehicle forward but is instead being used to heat up the motor. So unless you are using the motor to cook your lunch to sit down and have a picnic at the top of the hill its wasted energy and abuse on the motor.


    Yes, it is entirely possible to gear down a hub-motor. You take the hub-motor out of the rim and mount it up in the frame instead of inside the wheel and put a chain between the hub-motor and the rear wheel with a bigger sprocket on the rear wheel and a smaller sprocket on the hub-motor.


    I have a bike where I did exactly that where I used a 250watt 24V cheap geared front hub motor and rigged it up to the right of the rear wheel to drive the big 34-tooth granny gear on my rear 7-cog freewheel off of a 16-tooth drive sprocket on the hub-motor attached to the six disk brake mount bolts on the left side of the motor hub so it was a 2.125/1 gear down ratio on a second chain running up to the hub-motor mounted in a home built rack on the right side of the rear wheel inside the right side pannier bag with its back side cut out so it was just a cover over the motor, controller, and small 24V battery pack all together in a tight package under the pannier used as a cover and then just used only five of the seven rear gears for the main pedal drive chain (not using the #1 granny gear on the rear because it has the second chain on it for the hill helper motor or the #2 gear, its just a clearance spacer between the two chains). In that case I was setting up my gearing such that the motor would be in its peak power output at a little less then half the normal speed so its peak power output would be at the speed I'm climbing a hill at rather then at full speed on the flat which is how hub-motors are usually set-up.


    It works fine for my needs as a hill helper motor that is "stealth" and very few people ever notice the second chain going up into the right side rear pannier and I still have the left side pannier empty for carrying cargo. Actually balances better with a little cargo in the left bag to equal out the weight of the hill helper motor and battery pack under the right side pannier.


    Obviously, for pulling heavy cargo you would need a more powerful and heavy duty motor then that and/or gear it down even more then I did.


    So, Yes, you can certainly gear down a hub-motor and it is a good plan just realize that you can’t gear it down inside the wheel itself but instead need to mount the hub-motor without a rim around it somewhere in your frame or such with a good chain line to the wheel and drive the wheel off of the motor through a gear down chain drive. Custom brackets fabrication and such hands on ingenuity is usually required.

  9. #9
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    Then there are the geared hub motors. These hubs have reduction gears similar to a drill motor. My new bike has a 500watt 48v bafang geared hub and it pulls 350lbs (rider bike and cargo) up hills well. It's a new bike though so I don't know how it will last yet. I can say that its far more powerful climbing than my direct drive hub.

  10. #10
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    I also support the center drive application for your purpose.

    Another area to consider is wheel size.

    Your application is all about torque.

    Using any of the systems, or combinations of systems, with a
    smaller wheel size would increase the torque available to the
    system.

    Using a system designed for larger wheels on a 24" or ideally 20"
    platform will accomplish this.

    Going the other way ( small wheel design built into a larger rim),
    the speed would increase but torque would be diminished.

  11. #11
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    +1 on wheel size...that's a big factor in gearing.

    What about two medium or larger ( Bafang BPM) sized geared hub motors? One front and one rear, for balance - yet the bike would still be freewheeling. The geared hub motors are quite torquey - two of them would be lots of torque at slower speeds, without stressing them. Plus you have redundancy and balance. One or two controllers and one throttle? There are dual motor controllers out there (Crystalyte), but two cheaper controllers would work. With a big Lithium iron pack or two you'd have a good hill climber. The simplicity and reliability and quiet of the hub motors is excellent, although through the gears type driving is more torquey - but there can be a lot of issues with chain wear, chainlines, and NOISE. And much harder to build. For example the GNG kit - has not yet proven to be very reliable at higher than average power. Same with cyclone - not very reliable - and issues with moisture.
    Perhaps one of the bigger geared hubs like the MAC from emissions-free.com (British guy in China).

    I'd check the simulator at ebikes.ca for torque stats on the various motors available - and you can always use two of them.

  12. #12
    Senior Member chas58's Avatar
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    Correct, wheel size is a way to gear down a hub motor. Putting a 200rpm motor (designed for a 26 wheel) on a 20 wheel (but still full size) bike will get you a lot more torque where you need it, but of course is going to limit your top speed to about 20km/hr. That might meet your needs.

  13. #13
    coprolite fietsbob's Avatar
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    IDK how much of it* is patented.. hub motor, out of a wheel, driving the left side of a tandem crossover crank,
    allows the rest of a mountain bike drivetrain to be used to apply the power..


    but, to make room for the motor, the bike needs to be a longtail, like an Xtracycle, or a Big Dummy,
    or built from scratch.. , for the purpose..

    * Aka 'stoke monkey'..

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