Direct Drive Gearless Hub Motor Drag Technical Q

Hello,

I know that direct drive gearless hub motors have some sort of drag, and is often used for regenerative braking. I'm looking at the Crystalyte NSM 13 motor specifically, from Canada's Grin Technology at ebike.ca. Apparently it's out of stock, but if they do restock, I am interested in this motor.

I looked at their motor simulator on ebike.ca, and it seems like at higher speeds, the torque drops and even goes negative! Does this mean that at very high speeds the motor will slow you down?

Additionally, does the motor have drag, if for example, my battery runs low and as a result a very low voltage is run to the motor, or I need to reduce the thottle or pedal assist to maintain battery levels and I try to pedal faster than its unloaded speed at this reduced power? Again, from the motor simulator, it seems there is always some residual drag force if the motor isn't powerful enough for said speed.

Lastly, does running a higher voltage minimize this negative torque at higher speeds, while keeping the throttle lower to maintain the same output power? I modeled both 36V and 48V battery packs on the NSM 13 motor and simulator indicates that motor drag kicks in later as the torque line is translated to the right (increased torque at all levels for a given speed).

Motivation
I'm looking to run a low power build, as I want just a 'little' boost but keeping the bike as light as possible, as after trying out some e bikes, I don't really need 500+ Watt motors for what I'd like to ride

DDs will always have drag. if you want a light, occasional assist motor, and not having regen is okay.. you will want a geared motor. Geared motors are also much lighter when you're in the <1000W range anyway.

If you see negative torque, i really wonder what your settings are on the simulator ( did you point the bike downhill? )
It's easy to get the wrong idea of a motor's performance if you don't understand how to use the simulator. A screenshot of the parameters you set would be helpful.

Welcome to the forum.

Yes, direct drive motors do have a little drag. It's often called Motor Cogging, due to the way it feels at low RPMs. For a 500 watt motor, the drag is about like running a treaded tire instead of a slick on a normal bike. You'll be able to tell you aren't as fast, but it won't seriously effect your ability to pedal.

Since you are familure with the Ebikes.Ca simulator, Juston once posted here that you can use it to calculate the drag of a hub motor. Set the throttle slider on 3-4% (it doesn't work on zero), then use your mouse to slide the speed bar on the graph to your cruising speed. In the left colume under the graph is a field called Mtr Power. For that Clyte motor, it will read -15w at 20mph. that means it has -15 watts of drag at 20mph. Consider that at 20mph, you're dropping around 10 watts just in losses of the chain, you can see the drag from that motor isn't much.

The motors do have some mass, though. changing speeds to accelerate or brake, you'll notice the wheel is harder to speed up, and harder to slow down, a little. The mass causes a little bit of a flywheel effect.

Regen only happens when a circuit is closed by the controller, allowing power to flow back to the battery. the resistance during regen is about like hitting the brake hard.

There is an effect where if you pedal a bike with a low speed motor faster than it's theoretical top speed for the voltage present in the controller, the eddy currents caused by the motor spinning will be reversed and can cause the FETs to activate, effectively shorting the windings. It happens suddenly and slows you down hard until your speed drops below the threshold.
This effect is rare, and not every controller reacts this way. I've only experienced pedaling it on a Clyte 4012 (Very low rpm motor )with an analog controller when running at 36 volts. Coasting down a steep hill at high speeds can cause it as well, but it's very rare.


Since you say you want just a low power boost, might I suggest looking at geared hubs? They weigh less, have no drag when not in use, and deliver more torque that similar specked direct drive motors.

I see, thanks for your input

I've been looking all over the ebikes.ca website as they have a lot of well written information that's easy to navigate.

I decided that since I want a top speed of 25 mph, direct drive motors are the most suitable, additionally since I'll be putting them on a brompton it sort of limits my motor choices anyway. I'm not sure what other motors are technically suitable for the brompton but ebikes.ca has one that seems to work well the crystalyte SAW20.

I did test ride the electric version of the motor and the limiter was definitely noticeable, even when going up a small hill. It was a little strange as I would have a lot of momentum and suddenly it felt like the brakes were literally being applied as I rode up the hill. Also I've looked up the 'gear inches' on my bike and found that riding at 100 gear inches on the flat was fine for short bursts for me and that translates 20+ mph.

The only issue I've been running into while doing research the past week was the choice of batteries, as I am looking for a small 300Wh battery, but with a voltage high enough that I don't run into the back emf issue if I happen to sprint close to the unloaded speed of the motor. At 36V, I'm a little worried that I may run into the motor speed limit, and I feel more comfortable if I could find a 48V battery with around 300 Wh, but it seems like those are not that common. The next best option that seems like a good vendor was the 52V lunacycle box battery, but they seem to be sold out on their website.

The small DD in a 16 inch wheel is probably the best motor choice you could imagine.
Get a programmable controller that doesn't have stupid limits or weird hitting the brakes type behavior when you hit 20.00001mph. Ainnobodygottimefodat 8)

25mph on a 16 inch wheeled bicycle with no suspension is probably going to be the most uncomfortable bone rattling experience on the planet. 25mph is where i start thinking 'gosh, front suspension would be nice, at the least' on a bicycle with 26 inch wheels.

For batteries, i really like em3ev even if ( currently ) the process of ordering and shipping will be very slow, due to us being in peak ebike building season. Maybe the 'preditor' is up your alley, though it's a bit on the large side.

https://em3ev.com/shop/preditor-l-47v-8-4ah-11-8ah-frame-battery/

Yeah I know that it might be uncomfortable, but like with a car, we don't necessarily drive at the maximum possible speed (which is usually well over 100mph for modern cars), but we'd like to know that we have the freedom to do so. I expect the the same with ebikes, I'm going to look around to see what I can do

Yes, when you exceed the motor's free speed and its back EMF exceeds battery voltage, it applies a braking force that clamps your maximum speed. Back when I used a power analyzer that I could watch while riding, I could see the system voltage rise past the battery voltage and start to apply passive regenerative braking (though because it happened at 28mph, it didn't feel like braking). Whether all controllers allow current to flow back into the battery when the motor develops higher voltage, I don't know.

This phenomenon is separate from eddy current drag, which will occur even if the motor is unplugged.

25mph on a Brompton will be a lot more unpleasant than you understand before you try it. My current e-bike has a 20 inch front wheel and goes 20mph, and it regularly makes me cuss when I encounter road flaws I didn't see coming. That's on a heavy, stable, long wheelbase bike with a 700c rear wheel that carries most of the weight. In those same situations, a Brompton will punch you twice, harder, and then likely fall down at speed. Keep in mind that 25mph represents 56% more kinetic energy than 20mph.

If your local street surfaces are flawless enough for 25mph on a Brompton to be feasible, I think it will be more benefit than harm if a direct drive motor limits you to 30mph when you coast downhill. So don't worry too much about that. For you, the real drawback to a direct drive hub will be the noticeable drag it causes when pedaling without motor power. It isn't insurmountable, but it's discouraging.

I see, I am currently looking at the brompton kit on ebikes.ca

Right now it boils down to whether or not I want to get the phaserunner vs the CA4820. While the CA4820 is great, I like having the best equipment I can get. The main benefits of the phaserunner seems to be it has a better signal, but I'm not really sure if I understand the differences between a field oriented/vector control vs a sinewave. It seems like sinvewave is already great, but maybe field oriented is even better?

Additionally it has more programmable features, which seems to give it flexibility if I want to put it on another bike with a different setup.

it also has field weakening which might be useful if I need another 10% in my top speed without changing the battery, and it seems to allow higher phase and battery currents and voltages, which can help me get the most out of my motor both torque and regen.

I recall that some early DD hubmotors had skewed stator teeth faces. I was told that the slight overlap reduces the felt clogging. However, it is my understanding that the angled stator teeth also reduce the potential power that the motor is capable of (for a given weight and size)

Justin has posted that experiments with spacing out the permanent magnets in a variety if distances has provided a variety of results, when it comes to the balance between low cogging and best possible power potential...

niwrad said:

I looked at their motor simulator on ebike.ca, and it seems like at higher speeds, the torque drops and even goes negative! Does this mean that at very high speeds the motor will slow you down?

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If a motor exceeds the RPM at which it would spin at the voltage it's being run at, then it will begin generating a voltage higher than that being input.

This means that with a typical controller, it will be higher than the battery voltage, and the FET bridges become rectifiers and begin charging the battery.

This then means it begins adding drag to the wheel, because it's no longer a motor, it's a generator.

This process generally only happens when going down a hill at a high speed, high enough to get that motor RPM faster than it could go as a motor with the system on the bike. So, not a problem unless your needs include coasting down hills at a faster speed than you can power down them.

niwrad said:

I did test ride the electric version of the motor and the limiter was definitely noticeable, even when going up a small hill. It was a little strange as I would have a lot of momentum and suddenly it felt like the brakes were literally being applied as I rode up the hill.

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The "limiter" you're seeing there is just not having enough power to climb the hill at the speed you had been going. So the bike slows down.

If you're seeing braking happen while going *up* a hill when not going beyond whatever speed limit the system might have, if any, then something else is going on, and isn't any kind of normal limiting or anything to do with a DD hubmotor vs any other kind of motor.

Yes, if you coast down a hill fast enough, at some speed your motor will slow you down, if its a DD.

I have occasionally seen on my CA, a few watts flowing back into the battery on bikes with no regen, but only at very high speed on very steep hills. Don't really feel much difference, but by then the motor alone is definitely limiting speed. With the stuff I have used, you generally can't coast down a hill faster than 35 mph ish. With a regular bike, or geared motor bike, you could hit 60 mph on this hill easy. The lower the rpm of a DD motor, the lower will be the max coasting speed on a given grade. This is part of why I built one bike with a slow rpm motor. It was nice when coasting down a large hill, to not need to put on brakes at all, except to actually stop.

When you hit a limiter going up hill, that sounds like a speed limiter in the controller, or the CA, or whatever, that actually cuts power at a set speed. With a CA, that limiter is adjustable.

Yeah, the turnkey bike I test rode seems to have a 25 km/hr or 15.5 mph speed limit built in. I'm trying to avoid any motors that might have an intrinsic speed limit as I sometimes sprint to 25 mph. From what I've read it seems that geared motors typically have freewheels built in as they are already going through the steps of having gears already while direct drive motors do not have freewheels. At first glance it seems like geared motors are the way to go, but I couldn't find any geared motors that would fit the narrow fork on a brompton, and direct drive also gives the benefit of regen which could mean I don't need to change my brake setup as often. Also with direct drive, I can up the voltage without much harm to the motor if I need the extra speed while it seems geared motors suffer from overheating when run at higher speeds much more so than direct drive motors

niwrad said:

Yeah, the turnkey bike I test rode seems to have a 25 km/hr or 15.5 mph speed limit built in.

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This would be common to have a built in limit on any OEM bike, because many countries/etc have laws restricting what can be manufactured and sold as an electric bicycle (beyond which they're either some vehicle class, or not road-legal at all). These laws may not apply to DIY bikes, which may have different restrictions based on local laws instead.

However--most ebikes with limits don't actively brake when they reach it, because the law usually simply specifies that no assist (motor) can be provided beyond the limit, so it just cuts power to the motor, it doesn't actively prevent the bike from going past that speed.

If a manufacturer chooses to actively limit speed by causing the motor to brake past the limit, that would be strange, and could actually make the bike unusable for a lot of it's potential customers. So I don't think they'd do that. Not on purpose, anyway.

I'm trying to avoid any motors that might have an intrinsic speed limit as I sometimes sprint to 25 mph.

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All motors have an intrinsic speed limit for the wheel size used vs the voltage the system is using, vs the load on the system.

Geared clutched hubmotors, or any middrive that has a freewheel between the motor and wheel, arent' going to cause any braking beyond that speed, but they do still have the limit.

Simply increasing the system voltage to something higher than what the motor generates at the fastest speed you'll be going at will increase it's limit beyond what you'd ever need.

Also with direct drive, I can up the voltage without much harm to the motor if I need the extra speed while it seems geared motors suffer from overheating when run at higher speeds much more so than direct drive motors

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No, they don't overheat from higher speeds.

They overheat from higher loads than they are designed for.

That load *can* be from higher speed, if the speed needs more power than the motor can handle...but the same is true of any other motor.

That said, using a higher voltage is often done to get higher torque for quicker launches from a stop, or other needs, and *that* can break the gears or clutch (freewheel) on a geared hub, if the torque is beyond what they can handle.

I see. I think the motor cutting off is enough to feel like a brake. I do want an electric bike and DYI way allows me to set the limit to American standards of 28 mph with assist for less money. Right now I’m considering getting the kit from Grin and upgrade the motor controller to the phase runner so I can have the most flexibility using their LiGo as thy can be used in 36V or 72V and are airplane portable which is definitely something I’ll use as I travel between CA and the Northeast. It seems like they only have one motor available for Brompton but they did mention they are beta testing a geared motor.

I thought about it and I think the direct drive is already proven and it’s more suitable for top speed rather than low end torque and has regen I’ll just get their direct drive
Motor instead of waiting for them to finish beta testing their geared hub.

The main benefit of the heated hub in my view is that it’ll be both smaller and lighter, how much so may change my choice

There is a lot of good info above...I am just attempting to add to it.

Not sure where you are with your project but the GMAC geared hub motor has some advantages compared to a Direct Drive motor...lighter, more torque, and it has regen. It actually a Geared Direct Drive Hub. Disadvantage is that if you push it past its limits, any geared hub motor can not reject the excess heat very well because of the air gap between the stator and the housing.

Direct Drive motors have an air gap too...it is just a LOT smaller.

You can bridge the gap and thermally connect the stator to the housing with both the Geared Hub Motor and a Direct Drive motor it just takes a lot less fluid to do it in a Direct Drive motor. It will help cool both motors a lot.

All motor types...Mid Drive, Geared Hub Motor, Direct Drive Hub Motor have advantages and disadvantages. You just have to figure out which one works the best for the way you are going to use it.

Riding relatively slow off road with high torque demands...I would choose a Mid Drive like the Bafang BBSHD.

Riding off road and on paved surfaces with some hills...I would choose a Geared Hub Motor like the MAC.

Riding on paved surfaces and/or if you want to go fast...I would choose a Direct Drive Hub Motor and select the size and winding according to how fast you want to go so it is hard to give an exact recommendation.

Bullfrog said:

There is a lot of good info above...I am just attempting to add to it.

Not sure where you are with your project but the GMAC geared hub motor has some advantages compared to a Direct Drive motor...lighter, more torque, and it has regen. It actually a Geared Direct Drive Hub. Disadvantage is that if you push it past its limits, any geared hub motor can not reject the excess heat very well because of the air gap between the stator and the housing.

Direct Drive motors have an air gap too...it is just a LOT smaller.

You can bridge the gap and thermally connect the stator to the housing with both the Geared Hub Motor and a Direct Drive motor it just takes a lot less fluid to do it in a Direct Drive motor. It will help cool both motors a lot.

All motor types...Mid Drive, Geared Hub Motor, Direct Drive Hub Motor have advantages and disadvantages. You just have to figure out which one works the best for the way you are going to use it.

Riding relatively slow off road with high torque demands...I would choose a Mid Drive like the Bafang BBSHD.

Riding off road and on paved surfaces with some hills...I would choose a Geared Hub Motor like the MAC.

Riding on paved surfaces and/or if you want to go fast...I would choose a Direct Drive Hub Motor and select the size and winding according to how fast you want to go so it is hard to give an exact recommendation.

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I've decided to stick with my current motor the crystalyte saw 20D, as weight was important to me and it was the lightest one at the time. The next lighter is the MSOFP16 but it doesn't have regenerative braking, and I feel that it's sort of a game changer when you are in city traffic like around the NYC area.

Should be good. Neptronix makes a good point: 16 inch wheel. Geared motors don't give you multiple gears, they just allow the motor to operate at a faster speed than the wheel, which (I'm sure you've already figured this out) is what you're already getting with a 16 inch wheel.

The lady of the house has been thinking about going electric with her Brompton. I haven't really looked into it. The only concern I'd really have is that its real virtue is that it's this marvelously compact, well made thing that pops open to a bicycle - and the motor and battery might add so much weight that it takes the fun out of it. Or worse, gets in the way of the fold, but I suppose they have that figured out.

donn said:

Should be good. Neptronix makes a good point: 16 inch wheel. Geared motors don't give you multiple gears, they just allow the motor to operate at a faster speed than the wheel, which (I'm sure you've already figured this out) is what you're already getting with a 16 inch wheel.

The lady of the house has been thinking about going electric with her Brompton. I haven't really looked into it. The only concern I'd really have is that its real virtue is that it's this marvelously compact, well made thing that pops open to a bicycle - and the motor and battery might add so much weight that it takes the fun out of it. Or worse, gets in the way of the fold, but I suppose they have that figured out.

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yeah, the biggest issue is the weight. The geared motor while less ideal, is nonetheless lighter then the DD.
However, I guess the weight difference between the two is about 4-6 pounds, so I guess it's not necessarily a deal breaker

Chalo said:

Yes, when you exceed the motor's free speed and its back EMF exceeds battery voltage, it applies a braking force that clamps your maximum speed. Back when I used a power analyzer that I could watch while riding, I could see the system voltage rise past the battery voltage and start to apply passive regenerative braking (though because it happened at 28mph, it didn't feel like braking). Whether all controllers allow current to flow back into the battery when the motor develops higher voltage, I don't know.

This phenomenon is separate from eddy current drag, which will occur even if the motor is unplugged.

25mph on a Brompton will be a lot more unpleasant than you understand before you try it. My current e-bike has a 20 inch front wheel and goes 20mph, and it regularly makes me cuss when I encounter road flaws I didn't see coming. That's on a heavy, stable, long wheelbase bike with a 700c rear wheel that carries most of the weight. In those same situations, a Brompton will punch you twice, harder, and then likely fall down at speed. Keep in mind that 25mph represents 56% more kinetic energy than 20mph.

If your local street surfaces are flawless enough for 25mph on a Brompton to be feasible, I think it will be more benefit than harm if a direct drive motor limits you to 30mph when you coast downhill. So don't worry too much about that. For you, the real drawback to a direct drive hub will be the noticeable drag it causes when pedaling without motor power. It isn't insurmountable, but it's discouraging.

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Hi,
My gmac 10 makes a very loud noise when coasting down hills. Can it be related to this topic? What can I do?

zinaambiorix32 said:

Hi,
My gmac 10 makes a very loud noise when coasting down hills. Can it be related to this topic? What can I do?

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Without knowing what the noise is like, I can only guess it's a result of GMAC not having an overrunning clutch like most geared hub motors. The sides of the gear teeth that bear on each other in that scenario probably aren't as run in and smoothed as the sides that transmit forward power, and they're spinning faster than they do in other conditions. So it may be a result of that hub's "advantage".