250 watt hub motor E-bike; Is it Enough?

August 8, 2012
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250 watt motors are the most used motor on commercially available electric bikes. They are popular in Europe (and Asia) where in many countries there is a 250 watt power limit on E-bikes. Because millions of these 250 watt motors have been made for the European and Asian market, the cost to buy them from the factories in China is low, but at the same time it is a motor that is proven to be reliable and efficient.

The wholesale price from China for a geared hub motor is around $60 before shipping. In comparison, a larger 600 watt geared hub motor such as the BMC will cost $250 wholesale from China. An ultra-affordable 250 watt motor makes it possible for an E-bike manufacturer to sell a decent (but low powered) electric bike, and still make a nice profit.

The 250 watt hub motor comes very close to being a  perfect piece of E-bike technology, cool looking, lightweight, reliable, and cheap. It could have been the very best way to make a bicycle into an electric bike. However….the only caveat is the simple question…is 250 watts enough? If 250 watts is enough for you, then this might be the ideal power plant for your E-bike.

Small is good

When it comes to computers, phones and hub motors, small and lightweight is good.  As electric bike technology has improved over the years, engineers have figured out how to make a smaller and lighter hub motor. The 250 watt hub motor is an evolution of hub motor technology, and is in fact a beautiful piece of E-bike porn which  made #4 on  our top 10 list of E-bike Porn. It would in fact be a perfect electric bike solution if it was possible to pump more power through it reliably. (Check out some examples of beautifully built mountain e-bikes utilizing 250 watt hub motors.)

Quietness and Ride Quality

250 watt hub motors make very little noise…just a barely audible hum. Because they are almost silent they are like a magic carpet ride. Because they are so small (and not powerful) the power will come on smoothly.

What does a 250 watt hub motor consist of?

Most 250 watt motors are geared hub motors, meaning they have a planetary gear reduction so that the motor spins at an optimal RPM. This makes the motor more efficient than a simpler direct drive hub motor. At 250 watts, the planetary gears means a more torquey and more efficient motor. In this motor pictured below, you can see the planetary gears which allow the motor to spin up to 5 times faster than the wheel. Since electric motors love fast RPMs (more efficient, more torque, and more reliable) the planetary gear allows the use of a smaller and lighter motor, with the same power output of a larger motor.

Power to weight, the geared hub motor is much better than a direct drive hub motor (read our story comparing geared and direct drive hub motors).  Although a few companies make a 250 watt direct drive motor (such as Bionx’s 250 watt motor, the same motor that goes on the Smart Bike), we strongly suggest that if you buy a bike with this size of motor that you opt for the geared hub motor which is lighter, smaller and more efficient than its direct drive counterpart.  Like most hub motors of today, most of the 250 watt motors currently on the marketplace use highly efficient brushless motors.

 

the inside of a Bafang 250 watt motor (8fun)

 

Advantages of 250 watt motor

  • Lightweight (6 pounds) THe biggest advantage of these 250 watt motors is they add very little weight to your bike. 6 pounds is the same as a Kryptonite U-Lock. To compare, a Crystalyte hub motor tips the scale at 15 pounds. Weight in the wheel that you can really feel. Even a pound less makes a big difference in the feel of the bike.
  • Stealth looking. You can’t beat a 250 watt hub motor for stealthiness. Even when installed on the front hub they are hard to identify. They look almost the size of a standard wheel hub. In the rear wheel they hide perfetly behind the rear sprocket and disc brake.
  • Free wheeling. Geared hub motor free wheel just like a regular bike (but no possibility of regenerative braking)
  • Easily fits on front or rear hub. Because 250 watt motors are so small, they can easily be mounted in the front or rear wheel. Because they put out so little power, you don’t have to worry as much about this puny motor snapping drop outs like you do on the more powerful set ups.
  • More range. 250 watts is an extremeley efficient wattage to operate at, and forces the rider to provide input. Therefore 250 watt bikes will get a long way per battery charge.
  • Super reliable. Because 250 watt motors have been mass produced and tested on a mass market, they are super reliable. And should give you years of trouble free use. If one breaks no problem, they are cheap to replace.
  • Cheap purchase price

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when is 250 watts enough?

Lets say you do all your riding on a paved flat beach trail. And in beach tradition you like to cruise while sipping your virgin piña colada with plenty of your own pedal input, so you are getting exercise…a 250 watt motor might just be your cup of tea. Lets say you ride on hilly terrain but just want a boost to smooth those hills out, and you don’t mind pedaling hard up these hills and providing plenty of your own pedal input…250 watts might be all you need.

Also the less you weigh, the more likely a 250 watt motor will fit you. If you are thinking of giving your 90 pound 10 year old an electric bike for Christmas, 250 watts is plenty of power. For a 300 pound rider, you really need to look at 600 watt motors and beyond.

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A 250 watt motor and hills

definitely more than 250 watts….try 2000 watts on a mid driven m55

 

where you will feel the deficiency of a 250 watt motor is when traversing hills, especially off road. Hub motors are generally inefficient when climbing because they are usually geared for flats. When you hit a hill, that 250 watt motor is really going to have to work to pull you up, and if you your part and vigorously pedal-assist it, you can easily burn that little motor out. Generally since efficiency is so important when limited to 250 watts, if you have a hilly commute it is better to with a 250 watt mid drive which are extremely efficient and great hill climbers despite their minimal power output. (you can climb in your bike’s granny gears).

250 watt motors are less reliable than larger motors

Because 250 watt motors are so small they tend to be more fragile than the bigger and heavier higher wattage hub motors.  Also most people ride 250 watt motors at full throttle all the time, where as with larger motors you can ride them at half throttle which is easier on the motor.

A 250 watt motor is not more efficient than a larger hub motor

Lets compare a 250 watt motor with a 500 watt motor. If you limit the 500 watt motor to 250 watts you will not see any difference in efficiency. This means a 500 watt motor can serve as both a 250 watt bike and a 500 watt bike depending on your needs and wants. Most electric bikes have some kind of efficiency mode where power is limited, so that you can switch to half power with the flick of a switch.

250 watts forces you to be a miser

When the maximum wattage you can burn is 250 watts you will naturally be super efficient, since at 250 watts you will rarely find youself in situation where you have too much power. For most situations, 250 watts is “just enough” to travel at a moderate pace. Without the temptation of having extra power on tap, you will always use a maximum 250 watts….turning you into an electric power miser whether you want to be or not. It’s hard to get in trouble when riding with only 250 watts on tap.

 

World’s smallest e-bike, read about it here.

 

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250 watt E-bike allows a smaller battery pack

 

A 250 watt motor allows manufacturers to offer a smaller battery pack and get away with it. A great example of this is the Faraday electric bike which as you can see uses a 250 watt geared hub motor in the front, hidden behind the front disc brake. This bike has a meager 110 watt-hour battery.  Can you see where it is hidden? The battery pack is actually built into the frame tubes of the bike. If you put a 600 watt hour motor on it you would only get a pathetic 10 minutes of wide open throttle before the battery died. With a 250 watt motor, the Faraday is able to get an acceptable 25 minutes of wide open throttle before the battery dies.

250 watt bike gets you into shape
250 watt bikes means less cheating and more exercising. Many times when you have plenty of power on tap you end up being lazy and using more of that power than you really need. A 250 watt just give you enough power to help you up the hills, speed you up on the flats and keeps you totally honest.

250 watt bike in the USA?

The real problem with a 250 watt powered bike in the USA is that you are allowed to ride up to 750 watts legally in the USA…why settle for a bike that was built to adhere to European standards? You can always have a switch on your throttle which limits your bike to 250 watts and be just as efficient as the smaller hub motor…why not have the option for 500 watts or 750 watts when you want it? 250 watts is not enough to give you serious help up a hill, where 750 watts is. Even on the flats, you will be hardpressed to get to 20-MPH on a 250 watt bike without pedaling hard.

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DIY 250 watt conversion

This custom built Power in Motion bike has AWD 250 watt motor in both front and rear…super sweet.

 

250 watt bikes are super easy and cheap to build. You don’t need to stress as much  about torque arms etc,  because 250 watts is not strong enough to snap most drop outs. You don’t need as big as a battery, even a 250 watt hour battery will keep you moving at full throttle for an hour.

Types of 250 watt motors

There are 3 main manufacturers of 250 watt hub motors at this moment. Bionx, Bafang, and Cute. To read about these 3 motors read our  story here.

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250 watt motors and E-bike manufacturers

 

Many major manufacturers are opting to use the 250 watt motor for upcoming bikes that otherwise would be pretty exciting. The list of bikes included the Smart Electric Bike, the Specialized Turbo, the Faraday electric bike, and the Grace E-bike. These manufacturers have decided that 250 watts is enough.

So is 250 watts enough?

I really believe that 250 watts is not enough power for most consumers in the USA. It is refreshing that  electric bike manufactures in the USA such as Currie, Pedego and Stromer are offering 500+ watt bikes in their product lines. You might think a 250 watt hub motor is enough, but once you test ride both a 250 watt bike and a 500 watt bike together, it is hard to settle for the measly 250 watt hub motor. However, if you really are the type who will never need more than 250 watts, its hard to beat the stealthiness of a 250 watt hub motor bike.

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Hot rodding  250 watt motors…to 1000 watts

Check out the rest of this builders beautiful 250 watt bike builds here.

 

Some hotrodders, attracted to the small size and low weight of the 250 watt motor, but not satisfied with its puny power output, have over volted them and run as much as 1000 watts to them (read our hot rod primer). If they could do this reliably, this could be the E-bike motor solution we all have been waiting for. However cooling is a real issue with these small motors and its easy to strip the nylon planetary gears. Once hotrodded, these motors become  fragile and must be run with care and common sense when it comes to throttle input.

Therefore they are not very popular platform for modifying.  However check out in the above bike how the tiny 250 watt motor fits nicely into the mountain bike frame that a full size hub motor could not dream of fitting.  For another great example of a mid mounted 250 watt motor check out this forum thread.

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2WD?

Most of these motors get 250W by using 36V (which we agree with, 24V is a waste of time), and they then limit the power with the controller capped at 8A (36V X 7A = 250W). Here’s a tip, you can traise the amps to 10A to get 360W, and I recommend adding a temp sensor to keep track of how hot the motor is getting. In that situation, you can raise the amps to 12A (36V X 12A = 420W), and as long as you don’t take it onto a long and steep uphill, it should only get warm. But…why not use TWO of them?

These motors are small, light, stealthy, and cheap. If you already have one, why not just add a second one, instead of selling it and buying a bigger hub? Get a second 250W hub of the same make and model, along with a second controller that is the same as the first. You only need one throttle and one battery.

Plus…if you add temp sensors to keep track of the internal heat (keep it under 93C/200F), you could raise the volts to 48V and run both of them at 10A…for a combined 960W of 2WD! If you put a pair of saddle-bags over the rear cargo rack, nobody will even know that you have a second motor. Especially since these small motors fit well between a rear brake disc on the left and the wheel sprocket on the right.

To get an idea of what a single unit of these little motors can do when hotrodded, check out this video:

 

 

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Written by Eric, August 2012

Eric has been involved in the electric bike industry since 2002 when he started a 6000 square foot brick and mortar Electric Bike store in downtown San Francisco. He is a true believer that small electric vehicles can change the way we operate and the way we think.


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