Rear wheel hub motors - do they free wheel?

[Stevew77]

Hi

I was contemplating buying a rear wheel hub motor and was hoping it may allow the rear wheel to freely rotate when the power is not applied?

I have a front wheel hub motor that slows the bike down when no power is applied, and for long distance riding unless your battery is large capacity, it makes for an un-enjoyable ride as the motor starts to act like a magnetic retarder on the wheel.

Are there any models of rear wheel hub motors that allow the rear wheel to just roll along without power, then you just apply power to engage the drive then pull back on the power once youre up to speed? If so what manufacturers might make these please?

Or do I need to go to a mid drive system to allow some form of clutch mechanism to be deployed to stop the motor from retarding the wheel rotation when power is not applied?

Any thoughts welcome.

 

[NCC1941]

Front motor vs. rear motor makes no difference for what you're asking.

You want a geared hub motor, instead of the direct drive hub motor you have now.

Geared hub motors (with the exception of the Grin GMAC) have the clutch mechanism you're looking for, to allow the motor to disengage from the wheel and not slow you down when it's not running. The feature tradeoff, in case it's something you care about, is that geared hub motors (besides the GMAC) can't do regenerative braking.

 

[JackFlorey]

Are there any models of rear wheel hub motors that allow the rear wheel to just roll along without power, then you just apply power to engage the drive then pull back on the power once youre up to speed? If so what manufacturers might make these please?

You are going to find, in general, two kinds of hub motors.

1) Direct drive. There is no gearing and no clutch. They are mechanically very simple (and rugged.) The "rotor" (center of the motor) is actually stationary, and the wheel (containing the magnets) rotates. There is almost no mechanical drag, and when below base speed (which is most of the tim) no regenerative drag. However there is some eddy current loss as the wheel spins. It is small but there. If your controller supports it, you can also get regen braking from these.

2) Geared. These generally have reduction gearing, allowing a much smaller motor, and a one way clutch. This means that when the motor is not spinning it is completely disconnected. Easier to pedal without power - but does not allow regen.

There are a few other varieties. The GMAC for example is a no-clutch geared motor; that does not have the clutch (so more reliable and regen braking) but results in a lot of drag without power. But those are the two basic types.

 

[2old]

One last aspect; the large, powerful motors are DD. AFAIK, motors like the MAC (and GMAC) are about as large as geared motors get.

 

[Stevew77]

Thanks everyone...very helpful

 

[Manbeer]

not to thread jack but NCC1941, your signature has me intrigued. ever going to post a build log on the ECR?

I have a thing for electrified Surly's

 

[neptronix]

BTW, geared hubs do have a variance between how well they roll.

Most larger geared hubs have excellent freewheeling. You will only feel their weight when riding.
Some smaller geared hubs that use dual reductions or sometimes even single reductions have a very minor but noticeable drag... ranging from a brake that is dragging 2% ( single reduction geared hubs of lower quality, Bafang G310 ) to about 10% at worst ( Xiongda YTW-06; the worst freewheeling geared hub ever designed ). Just depends on the motor design.

The ideal choice of geared motor depends entirely on your power needs.

 

[Carl18650]

BTW, geared hubs do have a variance between how well they roll.

Most larger geared hubs have excellent freewheeling. You will only feel their weight when riding.
Some smaller geared hubs that use dual reductions or sometimes even single reductions have a very minor but noticeable drag... ranging from a brake that is dragging 2% ( single reduction geared hubs of lower quality, Bafang G310 ) to about 10% at worst ( Xiongda YTW-06; the worst freewheeling geared hub ever designed ). Just depends on the motor design.

The ideal choice of geared motor depends entirely on your power needs.

I dare to say you are one of a few in the whole internet to say this, I was mislead to believe that all geared motors were drag free and the only difference was in their weight, I recently bougth the akm 75sx front hub motor and find out that it does not keep rotating by hand when pushed to the freewheel rotation side, I have not installed it yet on a wheel but I will like to ask about how hard was the xiongda to push by hand? The akm is fairly soft but it does not maintain any momentum,I saw the mahle x20 rotating by hand on a youtube video and believe my akm will behave similarly, I just hope I didn't bought another xiongda bad freewheeling design

 

[lnanek]

This thread is from years ago. Modern day, with a Grin controller, you can just use the virtual electronic freewheeling feature to pump enough juice into the motor to get it to act like it turns freely.

 

[offGridDownUnder]

Any thoughts welcome.

Grin is devoted to hub motors, and they also are strongly in favor of regeneration for braking. They sell motors that freewheel and motors that do not - and they offer a service of welding the clutch on some motors that freewheel so they regenerate instead.

This is a foundation to mention they also thoroughly characterize the motors they sell on dynamometers, including wind tunnel tests for cooling on some (not all - it takes real time to do this).

And they offer an option in their controllers to bleed a small constant current to their motors to overcome the 'cogging' (drag) from the magnets when turning. A motor with this turned on freewheels - and you can adjust it so the motor 'just barely' freewheels.

And with the exacting characterization they do, they also assert that in general, the small amount of power recovered from the regeneration - without any special riding style to maximise it - is greater than the power consumed by the 'electric freewheel' effect.

You can have your cake and eat it - regenerative braking means greatly reduced braking wear and greatly increased brake lifespan, while also having the benefit of freewheeling. Your full brakes will be there when you need them - but with their All-Axle on two bicycles I've converted, and with their GMAC, I can bring my machine to a full halt only by the regeneration - I tune my brake adjustments so there is a clear space between when the switches trip (engaging regeneration) and the mechanical brakes grab.

There is also a facility that changes the throttle action when the brake switches trip to be a variable regeneration/braking input so I can ease or clamp on the electric braking.

 

[amberwolf]

I dare to say you are one of a few in the whole internet to say this, I was mislead to believe that all geared motors were drag free and the only difference was in their weight, I recently bougth the akm 75sx front hub motor and find out that it does not keep rotating by hand when pushed to the freewheel rotation side, I have not installed it yet on a wheel but I will like to ask about how hard was the xiongda to push by hand? The akm is fairly soft but it does not maintain any momentum,I saw the mahle x20 rotating by hand on a youtube video and believe my akm will behave similarly, I just hope I didn't bought another xiongda bad freewheeling design

The clutch design in a geared hubmotor is not the same as a typical freewheel; it is usually a roller-ramp clutch, so that it still has friction in this clutch that's greater than that of the typical 3-pawled freewheel.

Additionally, you still have the gearing friction between the motor casing and that clutch. The more stages of gearing the higher this will be. The more grease and/or the thicker this grease is, the worse that can be as well.

So none of the hubmotors with these types of clutches are going to roll freely the same way a bicycle wheel does.

However, it's usually not very noticeable (if at all) while actually riding a bike equipped with such a motor, vs the drag you can have from a DD hubmotor (depending on that motor's design); also the drag from the DD motor will increase noticeably with speed, whereas the drag from the geared-hub's clutch doesn't normally.


There are also some geared hubmotors that do not have a freewheeling clutch of any form inside, like the GMAC and others, and so they actually can have higher drag than the DD hubmotor since their internal motor drag is multiplied by their gearing ratio (just like their motive power is).


I don't know of any of them that use a pawl-type freewheel between the gearing and the hub casing, which would then roll as freely as a bicycle wheel.

I don't even know of any that use the roller-ramp clutch between the gearing and casing; which would would roll more easily than the typical design that puts the clutch between motor and gearing.

 

[neptronix]

I dare to say you are one of a few in the whole internet to say this, I was mislead to believe that all geared motors were drag free and the only difference was in their weight, I recently bougth the akm 75sx front hub motor and find out that it does not keep rotating by hand when pushed to the freewheel rotation side, I have not installed it yet on a wheel but I will like to ask about how hard was the xiongda to push by hand? The akm is fairly soft but it does not maintain any momentum,I saw the mahle x20 rotating by hand on a youtube video and believe my akm will behave similarly, I just hope I didn't bought another xiongda bad freewheeling design

The Xiongda was pretty bad, almost like it didn't have a clutch. The motor also died quickly. The replacement motor wasn't much better.

A well designed motor will have no perceptible drag on the road.

 

[neptronix]

In owning maybe 7 different geared motors, i find that the higher quality the motor, the less drag.
MACs ( without Grin's clutch lock modification, anyway ) just spin forever in the air, compared to some cheap 250w hubs. In practice, this small amount of drag doesn't matter too much even if present, because you'll find yourself on the power more often than not..