Any freewheeling hub motors?

[IdeaGuy]

Rolling resistance in my brushless front wheel mounted Ateoma hub motor seems pretty high. After a lot of searching I found at Renaissance Bicycle company a Crystalyte Motor FAQ that covered the topic (text below). What I am wondering is this common to all brushless hub motors? It seems odd that we have freewheel hubs to allow the rear wheel to turn forward freely when the tire is moving faster then the sprocket cassette, but for non regenerative hub motors it seems like you just get a braking effect unless you have your power turned on. Are there ways of fixing this with the Ateoma or other brushless hub motors? Are there any manufactures that make freewheeling hub motors?

Part of the reason that I am asking is because I recently ran out of battery power about 3 miles from home. I live in a very hilly part of southern California, so it was about 2.5 miles uphill pedaling for me. This is the same stretch where I have seen several experienced riders have to stop to catch their breath. Adding a lot of weight for the hub and the battery/controller etc made it a tough ride, though I'm happy to say I was able to pedal the whole way back, with my wife laughing at me for not charging up my battery! :? Anyway, it just seems like part of the challenge was having to overcome the inertia / drag force in the hub, which seems like a poor design.


" I just got my motor but can barely turn the axle there is so much resistance, is this a defective unit?

No, with any direct-drive motor there is significant drag force required to turn the hub due to the strong interaction between the magnets and the stator. With the Crystalyte 400 series motors, this is about 0.4 N-m, while the torque to turn the axle of the 500 series is closer to 0.7 N-m. That is more torque than most people are able to apply just with their fingers turning the axle. However, if you mount the wheel on the bike and spin it, you will see that the wheel can still turn more or less freely. If you give it a whirl it should turn 3 or 4 revolutions before coming to a stop. The drag force caused by these hubs is comparable in magnitude to the rolling resistance of the tires on a regular mountain bike."

 

[amberwolf]

Most of the geared hub motors have freewheels (or rather, roller clutches) in them.

In theory, you could turn any hubmotor into a freewheeling motor if you can bore out a larger hole in the stator, press in an appropriate one-way bearing, freewheel, or roller clutch assembly, and press the axle into that, if you could find a way to get the phase wire power and hall signals across the now-rotating surface.

Why it's not done is probably a few things:

• --It's not that easy to get the power from the axle across the freewheel into the stator, because now the stator has to be able to rotate but the wires can't twist around so they can't be directly attached to it. You'd have to have a brush ring that can handle high currents, for at least three wires (phases) and possibly five more (hall signals, power and ground) if it's not a sensorless system
  --That brush ring could corrode, arc-pit, wear, etc. Reliability goes down.
  --The freewheel could break if the torque load ever exceeds it's ratings. It will wear out over time, too. If the freewheel breaks, the motor does nothing for you even if you still have power. Reliability goes down.
  --Depending on the type of freewheel, it may add noise to the system.
  --It adds to the complexity both of design and of construction, and adds to the materials/parts count and costs, not just directly but by the added complexity, storage and handling costs of the extra parts, etc.
  --Few people care if it will freewheel when they buy it, only after they have a situation like yours. So it's not a selling point, since you do not want to sell them a solution to a problem you don't want them to think about like that. You'd really want to sell them a solution like a bigger battery, or lithium vs lead/etc. So it's hard to market.

If you could make a large enough diameter freewheel, you could solve the connection problems by not putting it in the stator, but instead putting it outside the magnet ring. Then it would allow the magnets to stay stationary with the stator when not energized, and only engage the wheel to push it when the controller runs the motor forward.

But that is a lot of extra material and weight, and added cost that again probably wouldn't be desirable to most buyers.

 

[rkosiorek]

no it is not a poor design. it is part of the design. it is the price that is paid for a simple mechanically efficient direct drive system.

the idea behind the hub motor is simple. the rim and tire are part of the motor and driven directly by the magnetic forces. this keeps the design very simple. no mechanical losses, no gears and clutches to get in the way. it also makes regen designs possible. and short of letting it rust up solid not much will stop it. but it will not freewheel without some drag. it was never designed for that.

if you wanted feewheeling you should have bought a different design to begin with. BMC/PUMA and Ezee are both freewheeling and geared hub designs. more complex, gears to wear out, roller bearings to break, much higher maintenance, shorter lifespan.

in reality it is not a poor or flawed designed, but instead a poor or flawed understanding of what a direct drive hub motor is.

rick

 

[IdeaGuy]

Thank you both for your insights here. On my next motor purchase I'll look at the designs that you mentioned since I don't have the tools to do a quality job of changing over the engine to freewheeling status. In the mean time I think I'll just keep my battery fully charged.

 

[John in CR]

Thank you both for your insights here. On my next motor purchase I'll look at the designs that you mentioned since I don't have the tools to do a quality job of changing over the engine to freewheeling status. In the mean time I think I'll just keep my battery fully charged.

The easy and cheap answer is add more battery capacity.