E-bike mid-drive motors hardly outperforms hub motors

[avandalen]

I wrote an article where you can read that a mid-drive motor hardly outperforms a hub motor.

https://www.avdweb.nl/solar-bike/hub-motor/mid-drive-vs-hub

In practice there is no significant difference in performance. You can clearly experience this yourself during test drives with both kind of motors, I did this myself too. Because I wanted to know the cause, I researched this subject extensively. The conclusion is that the advantage of mid-drive motors is only noticeable in extreme situations, on steep slopes while the cyclist pedals only a little or not.


I would like to know how others think about this.

 

[dogman dan]

Well, a motor is a motor, and has its performance limits. What I mean is x amount of copper and magnets, in general, delivers y amount of thrust.

However, huge differences occur depending on the gearing of a mid drive, or size of a hub wheel, combined with load in the form of grade, weight, and headwind.

It depends on what you are trying to do. But if the basic goal is 30 mph on relatively flat terrain, with the weight of the whole vehicle including the rider under 300 pounds, and the motor is rated 500w, there would be little real performance advantages to mid drive vs hub motor in 26" wheel. Both perform outstandingly under those limitations.

Change that to 400 pounds and 15% grade, and now your mid drive in a very low gear does the job, slowly, but without melting the motor.

Or make the motor big enough to zip up that hill. Of course, guys selling stuff make dumb claims about motor performance all the time, like calling a low rpm hub motor high torque. So those dumb claims are out there, big time. A 500w rated mid drive does not have more power than a 500w hub motor. It just has the ability to change its gearing while a hub motor is locked into the wheel size it got built into.

 

[spinningmagnets]

For a given performance level on flat ground:

Mid drive is potentially lighter.

A hubmotor is easier to install, and direct drive hubs have more power potential in high-power applications. Also, a hubmotor kit of similar power is typically more affordable.

'Xxxxxxxxxxxxxxxxxx

Where a mid drive is worthwhile:

Power limited countries, such as those with a 200W-500W power limit.

Steep uphills, because a mid drive can downshift gears to trade top-speed for wheel torque, in order to keep the motor RPM's in the efficient zone.

 

[AHicks]

Any thoughts regarding mid drive gearing requirements, or available power, need to be qualified by the size of the motor. Clearly a 250w Euro motor is going to be in a completely different class than something like a 1600w Bafang.

Same story when talking "hub drives". You need to break that "hub drive" terminology down further, much further. Comparisons without regard to size, weight, or power output of a high voltage DD or a 500w geared hub, which are both "hub drives", are not going to be worth much.

 

[Mclewis1]

For a given performance level on flat ground:

Mid drive is potentially lighter. ...

How? If a mid drive contains some additional gearing (vs. a geared hub motor), and additional material in the housing for mounting plus the weight of the non driven hub which gets replaced in a hub motor setup. The outer shell of a hub motor is a bit more robust (for mounting spokes). There is a benefit from a built in controller but it appears to be very small, mainly made from not having an external housing and some wiring. There is also the difference in mid drive specific frames but this also appears to be a wash vs. the mounting material of a traditional bottom bracket.

With all of that is there really any appreciable difference in weight if we compare apples to apples (same wattage, total system weight)?

 

[donn]

I wonder if anyone has any comments on the article, beyond just the subject statement? He recites the theoretical explanation of the virtues of a crank drive in the article. What I'd like to know is whether the analysis is accurate. The terms he's reporting are a mystery to me - η, Pin, Ploss. I can guess Pin is power input, but ... maybe it's too early in the morning, but anyway I'm sure some of you guys are familiar with this kind of analysis.

Mid drive is potentially lighter.

• It is much heavier in weight.
• It gives serious resistance when you ride without assistance.
• It is many times more expensive, €750 instead of €100 and repairs are not done, you have to buy a new one.

I was surprised to read the weight and resistance comments, but I've never really considered a crank drive.

Power limited countries, such as those with a 200W-500W power limit.

He seems to have used a "250W" motor for his simulation. I suppose a key factor in how the comparison works out, is the hub motor gearing. I don't know how much commonly available geared hubs vary on that parameter.

 

[spinningmagnets]

The BBS02 is very common. It has been verified to work well with 52V x 20A = 1,040W

For a "fair" comparison, we would have to use a 1,000W rated hubmotor. Which hubmotor would you like to use for a comparison?

That being said, if we restrict ourselves to 250W, that a fairly important distinction, and the TSDZ2 is even lighter, though it has been successfully run at 750W.

There are mid drives that are even smaller, if you like.

 

[spinningmagnets]

The author has done a good job, but the article limits itself to 250W systems. This is understandable if the audience is from countries with a 250W power limit.

 
"...The conclusion is that the advantage of mid-drive motors is only noticeable in extreme situations, on steep slopes while the cyclist pedals only a little or not..."

I can agree with the conclusion.
 

 

[Tommm]

A motor is a motor. With given power, other than a few % efficiency difference it will do the same thing (assuming it is sized correctly).

I am not sure what the point is?

Now if you put them in a bike, they sure handle very differently off road though.

 

[MadRhino]

Mid drives are best at both ends of performance scale, but disadvantaged in the middle.

I mean, if you are building a low power bike, a mid drive does let you build it much lighter and still give it good torque. And, if you want to build a motorcycle, only mid drives can give it competitive performance. But in the middle, if you want to build a fast and powerful ebike, the numerous advantages of a hub motor are making a mid drive a poor choice in terms of cost, simplicity, reliability, robustness, top speed, and noise.

 

[donn]

I mean, if you are building a low power bike, a mid drive does let you build it much lighter and still give it good torque.

Would you consider his "250W" motor to be "low power"? He uses the same motor for both, in the simulation, and the theoretical advantage appears in the results, but doesn't seem enough to be worth much on the road. Are low power setups used off road?

 

[Chalo]

I wrote an article where you can read that a mid-drive motor hardly outperforms a hub motor.

Meanwhile, back in the real world, some of us have had both. I've had both. My current hub motor bikes both peak near 1900W (electrical) on a freshly topped up battery. My former mid drive bike peaked at about 1300W. Yet the mid drive could climb much steeper grades, accelerate across an intersection more quickly, and reach a higher top speed on flat ground. Maybe you're talking about some other kind of performance?

If your car were stuck in one gear at all times, that would affect its performance, would it not? With any single-ratio, direct drive bike, you have to trade off top speed against climbing and acceleration. With a (crank) mid drive, you don't have to do that. You use the gear that suits the situation.

Hub motor bikes can be more efficient overall. They have lower maintenance requirements and fewer failure modes. You can, if you want, use regenerative braking with them. You can have two wheel drive with them. But in return for these things you sacrifice performance.

 

[donn]

With which he doesn't disagree - hub drive sacrifices performance. Everyone understands that, and why it is. But how much? Apparently not very much. Maybe it would be clearer if the graphs were superimposed? Maybe his analysis is bogus?

 

[MadRhino]

I mean, if you are building a low power bike, a mid drive does let you build it much lighter and still give it good torque.

Would you consider his "250W" motor to be "low power"? He uses the same motor for both, in the simulation, and the theoretical advantage appears in the results, but doesn't seem enough to be worth much on the road. Are low power setups used off road?

Of course 250w is low power, likely the lowest powered ebikes on the market. I consider 1500w low power, below which I would build mid drive in order to obtain the acceleration that I like. I don’t know where your comparison come from, but I assure you that the acceleration of a 250w hub is desperately lame as compared to a 250w BB drive on the low cog.

The best manufactured MTB ebikes on the market are feeding about 750w peak to a (mid) BB drive. They have a wonderful bicycle feel and handling in the mountain trails because they come as light as 35 lbs and equipped with the best components. Light weight and suspension efficiency are the most important advantages of mid drives.

 

[donn]

I don’t know where your comparison come from, but I assure you that the acceleration of a 250w hub is desperately lame as compared to a 250w BB drive on the low cog.

There must be some problem with his simulation, then. It's in the article he links to.

We should bear in mind that your kind of bicycling is motorcycling, in that European world where 250W is the norm. They're not going to develop a taste for anything like that, luckily for them.

 

[MadRhino]

I don’t know where your comparison come from, but I assure you that the acceleration of a 250w hub is desperately lame as compared to a 250w BB drive on the low cog.

There must be some problem with his simulation, then. It's in the article he links to.

We should bear in mind that your kind of bicycling is motorcycling, in that European world where 250W is the norm. They're not going to develop a taste for anything like that, luckily for them.

I guess you are right. The proportional difference is much smaller if you add 500w of human power to the 250w motor. :wink:

But about the taste, I know many french riders who had built fast ebikes. In EU too, there are many places where the police don’t care about ebikes power, and apply the trafic laws instead of ebike specific regulations. Being able to blend with trafic is the natural future of ebikes if we want them to retire larger, environmentally costly vehicles. Governments who are promoting alternate energy, will soon realize that it is the size of the vehicles that need to be cut in order to significantly reduce environmental impact of transportation.

 

[john61ct]

In places where 3kW is "small"

especially my use case where 500+ pound loads need to get up long steep mountain roads

this thesis is obvious

mid-drives just don't cut it on their own.

Yes extreme gearing will help you get rolling when at a standstill facing up hill

but powerful hub motors like GMAC (also geared!) and Cyclone are needed to get you to the top.

These 250W numbers don't hardly count at all in that scenario.

 

[Chalo]

With which he doesn't disagree - hub drive sacrifices performance. Everyone understands that, and why it is. But how much? Apparently not very much. Maybe it would be clearer if the graphs were superimposed? Maybe his analysis is bogus?

In my observation, 2/3 as much peak power resulted in indisputably better overall performance when using a crank drive. That's, umm...very much.

It wasn't enough to keep me using the crank drive, because I was wasting plenty of my own time and lots of decent bike parts to keep it running. My hub motors do what I want them to do with minimal input from me. Performance isn't high on my list of what I want them to do.

 

[dogman dan]

I was only considering performance in terms of can it cruise 25 mph, and carry 300 pounds up 10% grade without motor damage. But yeah, gearing could definitely improve acceleration across an intersection.

Not that a 500w rated dd running on 1000w from the controller accelerates all that lame. It was enough to blow all the 50 cc gas bikes and many of the 90cc bikes away leaving the start line at the death race.

 

[ferret]

In practice there is no significant difference in performance. You can clearly experience this yourself during test drives with both kind of motors, I did this myself too. Because I wanted to know the cause, I researched this subject extensively. The conclusion is that the advantage of mid-drive motors is only noticeable in extreme situations, on steep slopes while the cyclist pedals only a little or not.


I would like to know how others think about this.

You simulated the mid drive only in one fixed gear.
If the motor is the same and the gearing (inches/turn) is the same, installing the motor in a rear or a mid configuration should give almost identical performance.

If you simulate the mid drive in each gear, you will see that if you select the optimal gear for the speed you are riding according to the simulations, you can get better acceleration, better top speed or better efficiency. Since you can change the gears at will, on most rides you can get all three advantages.

Avner.

 

[donn]

You simulated the mid drive only in one fixed gear.

Where do you see that?

In the simulation, I assume the ideal situation, that the bicycle has an infinite number of gears. So, by switching, the mid drive motor speed remains constant at all travel speeds.

... which speed he identifies on some of the figures as 320rpm, which I believe would be an efficient speed for the motor.

 

[ferret]

You simulated the mid drive only in one fixed gear.

Where do you see that?

He posted only one graph from one simulation. A simulation should be run for each gear, each of these simulations will produce a different graph. Each of these graphs will show peak efficiency at a different speed.

In the simulation, I assume the ideal situation, that the bicycle has an infinite number of gears. So, by switching, the mid drive motor speed remains constant at all travel speeds.

... which speed he identifies on some of the figures as 320rpm, which I believe would be an efficient speed for the motor.

As I wrote before, running 2 identical motors at identical RPM at identical road speed will give nearly identical efficiency regardless of where they are installed.
According to the 0 degree slope example, this motor's peak efficiency is at 320 RPM. The rear motor setup will only operate at peak efficiency at 30 kmh. The mid motor's peak efficiency at 320 RPM can be used at several different speeds, Therefore the amount of time spent riding at peak efficiency is potentially greater with a mid motor.

Efficiency is not the only parameter in which one setup outperforms another. Acceleration, top speed, steepest climbable slope and temperature handling capacity are performance parameter that matter for most riders.

Avner.

 

[neptronix]

The appropriate motor depends on your terrain and conditions.

On relatively flat ground ( average of 1-3% grade ), a direct drive immediately wins every efficiency contest due to it's lack of friction from two sets of gears and chain. But loses the power density contest.

Once you start climbing major hills frequently, the numbers start favoring the mid drive because the motor can be kept at peak efficiency for a long duration of time.


The motor inside mid drives and dual reduction geared hubs are starting to close the efficiency gap because DD hub makers haven't bothered to implement things like using razor thin laminations..

 

[john61ct]

The assumption that a hub drive has no gearing is the over generalization

as is the idea that 500W is enough power

 

[spinningmagnets]

After some thought. I realized I didn't actually have any experience with 250W hubs or mid-drives, so...please disregard anything I wrote previously.