From what I gather, direct drive motors (DDM) are capable of bigger power. I suppose it's because their heavier nature allows them to withstand higher voltages and currents. They are also very quiet but they generate some drag due to the direct rotation of the rotor when the wheel wants to spins freely.

Geared motors allow the wheel to spin freely but are less powerful and noisy.

Which leaves me with no option. I don't want a direct drive motor which has all the advantages except that by choosing it, I'm adding quite a big drag. I heard someone say about his own bike: "no, you don't want to pedal this bike".

Because of the constant drag, pedalling would no longer contribute much to the range and speed and so IMO I would really classify such upgrade as basically making myself a moped, not an electric bicycle.

The problem with geared motors is the noise: I don't want everyone I pass by to notice that my bike gives out an unusual noise (like a swarm of bees) and eventually everyone would know that it's because of the motors. My goal is to create a something that only by a close look you would know.

I came up with couple of solutions which are very impractical, unless someone might help me make them a reality.

Solution 1: I want two somewhat weak direct drive motors, hoping that the drag would not be annoying as much if I decide to leave the battery at home for some short commute. They might be designed for 250W~24V, but say I would want to plug a much more powerful battery (like 48V) which would instantly give me much more power.

Problem with solution 1: I couldn't find any DD motors which on its own would have so few watts. I would like two of such, front & rear; 36 holes, disk-brakes.

Solution 2: Because I believe that the lack of freewheel on DD is just a poor design, I myself dared to sketch the design of the basic idea:

Problem with solution 2: I don't know how to get someone manufacture this for me.

enter image description here

If it were possible to have it built like this, I would wait and I'm sure I wouldn't be the only buyer.

  • Are you designing a pedal bicycle with motor-assist? Or an electric motorbike. Laws and Limits vary by location, but often there's a wattage or speed limit. A 250W motor at 24 W when run on 48V will increase the effective wattage assuming the circuit survives. – Criggie Jul 22 '18 at 6:24
  • Removed most tags as we tend to be tag parsimonious on this SE. – RoboKaren Jul 23 '18 at 1:10
  • I thought even “direct drive” motors usually have their own free wheel, thus should contribute minimal drag? – Michael Jul 23 '18 at 10:57
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    P.s. geared hubs are often more torquey than direct drive hubs, because they can be better geared for low speeds. They only lose out at higher speeds but newer dual-gear geared hubs effectively erase this difference at the expense of more mechanical complexity. – RoboKaren Jul 26 '18 at 3:53
  • @RoboKaren definitely +1 for double speed hubmotors, interesting option! I don't know but I suppose when switched to the higher speed setting, they might not be as much noisy. – Daniel Katz Jul 30 '18 at 23:23

You could get two low-cost, low-power direct drives and put them on the front and rear wheels. Plenty of people do this with high power hubs to get an all-wheel drive mountain bike.

But you don’t gain much with two low-power hubs and you get increased complexity and weight and resistance. Your own custom hub motor design with a large bearing race with a one-way ratchet would be impossibly heavy as well as fragile. One bump and your wheel is toast.

If you want silence combined with efficiency then it sounds like what you really want is a bottom bracket motor. They’re absolutely silent when coasting (your rear wheel will click just as with any bike), and there is no coasting resistance. Because they use your existing derailleur drivechain, they’re highly efficient both powered and pedaled. They’re not as stealthy but you can’t have everything.

P.s. if you want absolute stealth and have the engineering chops, the bottom bracket motors that are built into the seat tube and are used for “motor doping” of bike races. But one of those will easily cost you $1000 if not $5,000 or more in custom machining.

  • Doesn’t a bottom bracket motor cause a tiny amount of drag when pedaling (while the motor is turned off)? – Michael Jul 23 '18 at 10:55
  • They do but it’s the resistance of a one-way bearing and not that of the motor windings of a direct drive. – RoboKaren Jul 23 '18 at 12:16

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