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The humble direct-drive electric bike hub motor has been powering e-bikes for many years and is now a favourite of DIY electric bike enthusiasts. Its success is down to a simple and durable design. The motor fitted to the mountain bike below is a typical 1000w direct-drive rear-wheel electric bike conversion kit.
The direct drive hub motor is the simplest form of electric bike propulsion: The outer shell of the hub is an integral part of the motor and has a big ring of powerful magnets fixed to it.
When the motor runs, it drives the wheel directly (that’s where the name comes from). Put simply this means that the wheel is a motor with the shaft fixed in place so that the body of the motor (the outer hub shell, and thus your wheel) spins instead of the shaft.
It is a simple design, but comes at a cost – the motor needs to be quite big and heavy to produce enough power. A smaller motor spinning slowly wouldn’t produce enough torque, and the speed you want your wheel to turn is relatively slow, so the motor needs to be as big as possible to produce torque at low speeds.
There are several pros and cons to direct-drive motors:
- Very cheap
- Reliable – no moving internal gears or parts to wear out
- Can be fitted to most bicycles
- Powerful – direct drive-hub motors can handle a ridiculous amount of power!
- Runs almost silently
- Very heavy
- Generally 500w and above – illegal for road use in the UK, Europe and Australia
- Low torque – compared to geared hub motors and mid-drives
- Not very efficient – You will get significantly less battery range
- Looks like you have a big electric motor in your wheel – not very inconspicuous!
- It is very common for spokes to break or need tightening – this is due, in part to the weight of the motor.
- The drag of the motor can make pedalling harder (especially with the motor switched off)
Front or Rear?
Personally, I think a powerful front hub motor is a bad idea, regardless of the legality of the motor. If your bike has disc brakes, it is highly likely that there will be not enough space between the brake calliper and the hub casing. Another thing to consider is having an extra 4-5kg in the front wheel is definitely going to have a negative impact on handling.
A rear-mounted hub motor is best, but there will still be compromises to consider when fitting: If your bike has a cassette-type gear block, then it will not fit, as all the direct-drive hub motors I have come across, only have fitted for a screw-on Shimano type gear freewheel. This will mean that you may lose a couple of gears on the back.
Another issue can be the brake calliper may rub on the hub casing, and also the rear derailleur may do the same when the lowest gear is selected – this can be mitigated by fine-adjustment of the derailleur’s limit screw.
Are they worth fitting?
This really depends on what you are going to be using your bike for. The most important thing to consider (for UK, European and Australian riders) is the power output.
Due to their size and weight, most direct-drive hub motors are usually rated to 500w and above. This poses a legal problem in a lot of countries. Federal e-bike law in the USA states a maximum power output of 750w and assisted speed of 20mph – the bike has to have functioning pedals.
Direct-drive hub motors are certainly very reliable, and are capable of lasting for years. The big downside is they are ungainly and inefficient.
UK, European and Australian laws are 250w continuous power output with assisted speed limited to 15.5mph (25km/h). This puts most direct-drive hub motors firmly in the ‘illegal’ category in these countries.
To put things into perspective, a typical cheap 1000w electric bike conversion kit with a good quality 48v battery fitted will produce roughly 35-45nm of torque. This is about the same amount of torque produced by a good quality 36v 250w geared hub motor, on the other hand, a decent 250w mid-drive system like the Bafang or Tongsheng will produce closer to 60nm of torque.
Because of their durability and simplicity, large direct-drive hub motors can handle very high power outputs – because of this, they are very popular in electric bike racing (on closed circuits).
I don’t feel that they are particularly suited to mountain bikes, due to the extra weight in the rear wheel, and the lack of torque.
Cheap DIY twist-and-go electric commuter bike – a great idea, but unfortunately totally illegal in some countries.
The Direct-drive electric bike hub motor is certainly very reliable and capable of lasting for years. The big downside is they are ungainly and inefficient.
Despite the 250w legal limit in Europe, an increasingly large number of riders are using 500w, 1000w or even 1500w converted e-bikes on the road.
The demand (in the UK at least) seems to be for more powerful electric bikes, a lot of people are unsatisfied with the law as it stands and are willing to take the risk. My opinion is this: Make an informed decision, and if you do decide to go down the illegal route, be prepared to accept the potential consequences.
If you take risks and ride at high speed, using only a throttle, you will draw attention to yourself. Ride sensibly and only use the power when you really need it. Alternatively, buy a kit that has a power restriction plug in the controller box.
I personally think that in time, big hub motors will be confined to the history books. For the moment though, they offer the most bang for your buck, and for that reason, they will continue to be popular with DIY e-bike builders.