HUB vs MID DRIVE - how can I compare?

[ferrellms]

A while back there was a cyclist (Adam Alter) who owns both the 350 watt mid drive (Focus Aventura Impulse speed) and 500 watt direct hub drive (Stromer st2) and he compared the two going up a steep uphill road. His video got lots of views but for some reason, it was taken down.
https://plus.google.com/101834765168487054831/posts/XdyFv49Jyd3
He exerted basically the same effort for the 2 ebikes but the Stromer was way faster than the Focus. he also mentioned that the Stromer's motor was a lot hotter compared to the Focus. There were lots of comments and theories on the posting. However, there was a big white elephant that they never talked about. And that was the raw power of the motors themselves (500 watts vs 350 watts). So I asked in the comment section on the energy consumed from the battery between the 2 ebikes, since the more energy you use the faster your ebike runs (isn't that obvious?). I did not get a response but the video was gone since then. And also, the more power exerted by the motor the more heat is generated, which supports the scenario where Adam mentioned the Stromer's motor was hotter.

As long as the motor can spin, the more significant factor in the performance is the wattage rating. The mid drive has the advantage on very steep hills but it is still slower overall (due to lower wattage), but it also has more range per watts consumed overall. For the same size battery the 350 watt mid drive ebike can go farther but a little slower.

The Hub drive's disadvantage is that it needs to be moving a little faster before you can take take full advantage of the motor and it is has an efficiency window to which speed it operates. Many direct drives tends to have the efficiency widow in the higher speed range 15 mph - 26 mph, while the geared hub drives tend to have the efficiency window at the lower speed range 5 mph - 18 mph (except for the speed hubs).

The misconception repeated constantly in all these posts is that slower motor spinning speed results in less torque, thus torque multiplication is good via mid-drive or that the hub needs to spin up blah blah.

As a matter of fact, DC motors have the same torque at 0 rpm as at all other rpms they can do. How many gears in a Tesla? 1.

 

[Alex M]

The misconception repeated constantly in all these posts is that slower motor spinning speed results in less torque, thus torque multiplication is good via mid-drive or that the hub needs to spin up

Let's not make it more confusing than it is
Slower motor spinning results in lower EFFICIENCY. Hub burns more energy per same amount of "work", in mechanics terms. This is why 750W mid-drive is plenty for off-roading, and 750W hub is barely enough.

 

[Trail Cruiser]

The misconception repeated constantly in all these posts is that slower motor spinning speed results in less torque, thus torque multiplication is good via mid-drive or that the hub needs to spin up blah blah.

As a matter of fact, DC motors have the same torque at 0 rpm as at all other rpms they can do. How many gears in a Tesla? 1.

You have discounted the big factor that fuels the present trend on ebikes.

Weight.

Haven't you noticed that even the Specialized brand that used to make state-of-the-art speed hub drives is switching to mid drives?

The Optibike company owner (Jim Turner) once hired an engineer to design a hub drive that can perform all the criteria he asked for (such as climb a steep hill and can cruise above 20 mph on the flats). The engineer eventually quit and said that the hub motor would be too heavy and impractical. Since then, Jim did some brain storming and concluded that the motor has to be mid mounted and operates in the same window of efficiency as the cyclist. Other big players such as Bosch, Impulse, Trazx, Brose Yamaha, etc were also doing there own independent R&D and basically came up with similar conclusion.

The Tesla car is a different story. It is a very powerful car and is designed to last hundreds of thousands of miles. And the weight is not that much of an issue. The Tesla originally had 2 speed gearbox but it suffered reliability issues. So instead of a fragile gearbox, why not put a more powerful motor (heavier) so it does not need a 2 speed gearbox anymore? And that's what Tesla did. In contrast, an ebike has different characteristics and goals for designing it. It has to to be light and the power is not overwhelming so as not to destroy the rear cluster.

 

[emco5]

....Ignore all advice that talks about torque and mechanical advantage of mid - there is none...

For all of you who like to tinker with bikes, or want to understand mechanical advantage, here's a fun pdf from MIT about gearing https://tinyurl.com/yb9hlq5m

 

[Bicyclista]

In all this interesting discussion about hub vs mid drives very little has been said about the effect of the added mass of the motor. Yes, some comments have correctly pointed out that the weight of the motor is better balanced with a mid drive, but few or none have pointed out that adding mass to the wheels of a bicycle is a bad idea. Yet, ideally, wheels (of any kind, really) should be as light as possible. This ideal goes back to the time when scientists and engineers started to analyze motion and wheels with math. This article, titled We Can Prove that Adding Mass on Bike Wheels is Your Worst Enemy, does just that.

 

[Alex M]

This article, titled We Can Prove that Adding Mass on Bike Wheels is Your Worst Enemy, does just that.

Their math considers unrealistic scenario when all the mass of the wheel is in the rim. OTH, hub motor is small, especially a geared hub, so you basically add weight to the axle. In this case, the effect on the bike efficiency will not be much different from adding small weight to the rear rack (think grocery bag under 10 lb). Battery often weighs more than a hub motor, and many manufacturers put it on the rear rack, thus adding more weight and wrecking the balance too. Smarter designs have battery on the downtube.

 

[Bicyclista]

Their math considers unrealistic scenario when all the mass of the wheel is in the rim. OTH, hub motor is small, especially a geared hub, so you basically add weight to the axle. In this case, the effect on the bike efficiency will not be much different from adding small weight to the rear rack (think grocery bag under 10 lb). Battery often weighs more than a hub motor, and many manufacturers put it on the rear rack, thus adding more weight and wrecking the balance too. Smarter designs have battery on the downtube.

While adding mass to the rim is certainly the worst place to do it, adding mass to any part of the wheel is bad. You can simply reduce the "R" in the equations to the radius of the hub motor. You still get a negative result.

And no, it is not the same as adding weight to a rack for the simple reason that a rack does not rotate.

 

[ferrellms]

Let's not make it more confusing than it is
Slower motor spinning results in lower EFFICIENCY. Hub burns more energy per same amount of "work", in mechanics terms. This is why 750W mid-drive is plenty for off-roading, and 750W hub is barely enough.

You are incorrect. Electric motor efficiency has nothing to do with rpms.

 

[ferrellms]

You have discounted the big factor that fuels the present trend on ebikes.

Weight.

Haven't you noticed that even the Specialized brand that used to make state-of-the-art speed hub drives is switching to mid drives?

The Optibike company owner (Jim Turner) once hired an engineer to design a hub drive that can perform all the criteria he asked for (such as climb a steep hill and can cruise above 20 mph on the flats). The engineer eventually quit and said that the hub motor would be too heavy and impractical. Since then, Jim did some brain storming and concluded that the motor has to be mid mounted and operates in the same window of efficiency as the cyclist. Other big players such as Bosch, Impulse, Trazx, Brose Yamaha, etc were also doing there own independent R&D and basically came up with similar conclusion.

The Tesla car is a different story. It is a very powerful car and is designed to last hundreds of thousands of miles. And the weight is not that much of an issue. The Tesla originally had 2 speed gearbox but it suffered reliability issues. So instead of a fragile gearbox, why not put a more powerful motor (heavier) so it does not need a 2 speed gearbox anymore? And that's what Tesla did. In contrast, an ebike has different characteristics and goals for designing it. It has to to be light and the power is not overwhelming so as not to destroy the rear cluster.

Just because a super powerful hub motor needs to be heavy does not mean it is the wrong choice. For example, a 250 watt hub will be heavier than the mid drive but has other advantages. Your remarks on the Tesla are simply incorrect.

 

[Trail Cruiser]

I am researching the heck out of a bike I can use as an overland trail bike. We have a Winnebago Revel on the way and need to match it up with a bike that can get us around some cool trails.

I've noticed some bikes have the option of a 350 w Mid Drive and a 500 w Hub Drive. Can someone help me compare these? I know there are some positives and negatives to both.....

BTW - such a great forum ! This site rules!

Going back to the original issue of the thread where the person who started it was interested on the comparison between 350 watt mid drive and 500 watt hub drive factory ebikes. Having both versions, I think I can provide a credible comparison especially that both my ebikes also happen to share the same batteries. So I can also tell which one is more efficient.

Note that to maintain average speed of 22 mph I use PAS level 2 on the 500 w IZIP but I use PAS level 3 on the 350 w Raleigh.
The Izip was a large frame while the Tekoa was a medium frame but both had similar standover heights.

 

[Ravi Kempaiah]

Just because a super powerful hub motor needs to be heavy does not mean it is the wrong choice. For example, a 250 watt hub will be heavier than the mid drive but has other advantages. Your remarks on the Tesla are simply incorrect.

I agree that the motor efficiency is not dependent on the motor RPM.

@Alex M

There is a misconception among ebike community that when a motor is spinning at slow RPM, it loses it's efficiency. It's understandable because they get confused with the motor efficiency Vs the transfer of torque at any given RPM and that of the combustion engines. In a combustion engine, the usable torque occurs only in certain RPMs. Here is a pic of that. The transfer of torque on a hub motor at very low RPM cane be limited.

(Link Removed - No Longer Exists)

 

[Trail Cruiser]

Your remarks on the Tesla are simply incorrect.

Dear ferrellms, allow me to quote this article and you tell me if my remark about Tesla is incorrect.

Tesla had to beef up the motor to compensate for the single speed, as a result...
"One of the changes being developed for WhiteStar, which will be a larger heavier vehicle than the Roadster requiring more power, is a liquid-cooled motor. The Roadster will now also get a liquid-cooled motor. This will allow to produce more sustained power than before."
https://www.autoblog.com/2008/01/23/breaking-tesla-has-a-solution-for-their-transmission-woes-get/

Now you tell me if I am incorrect or you just did not know about Tesla's early engineering hurdles?

 

[Trail Cruiser]

I agree that the motor efficiency is not dependent on the motor RPM.
@Alex M
There is a misconception among ebike community that when a motor is spinning at slow RPM, it loses it's efficiency. It's understandable because they get confused with the motor efficiency Vs the transfer of torque at any given RPM to the combustion engines. In a combustion engine, the usable torque occurs only in certain RPMs. Here is a pic of that.

(Link Removed - No Longer Exists)

I beg to disagree, when Bosch extensively did their research and development of their ebike motors, they actually engineered it so the efficiency window also falls within the efficient operating range of a human cyclist. There is an efficiency window for an ebike motor.

 

[indianajo]

My geared hub uses less watts on 15% grade if I go faster (about 10 mph & 3-400 w) than if I go slower (3-5 mph & 6-700 w) . I don't like going 10 up steep grades due to the habit of the testosterone poisoned of popping over crests in the middle of the road at 35 mph, but I suppose I'll get used to it.
My unpowered mountain bike weighed 85 lb rear 20 front just with the baskets and tire tools, so some extra weight on the front end will be a positive good. 50 lb of supplies probably runs the weight up to 135 rear 15 front, without me on it. I've had trouble with dogs knocking me over due to not enough traction on the front end to allow for quick evasive turns. So I view 12 lb of motor and 13 lb of battery on the front as a positive good. The whole weight thing about bicycles is a fantasy IMHO to sell carbon and titanium to people wearing a 50 lb spare tire of fat above their belt. I've been pedaling about 2500 miles per year at a net bike+supplies weight of 150 lb including grinding up 15% grades without stopping. The whole point of the electric drive is to push me my 30 miles in less than 5.5 hours if there is a 25 mph headwind. 3.5 hours is the right amount of exercise IMHO. Plus the electric with hand throttle could carry me home without pedaling if I sprain an ankle or pull a tendon - there is no phone service out at my camp. It has happened, and fortunately I had enough food and water that day to wait a couple of days without going to town. Mid drives force you to pedal, or hire a car. I seriously doubt if Yellow taxi or enterprise rental could find me out at my camp, even if I had a $70 a month verizon phone.

 

[harryS]

I own both mid-drive and hub drive (geared), although these are kit motors. I also went to a Bike Expo and test rode a bunch of commercial e-bikes, some quite sophisticated and others probably worse than my home kits due to sloppy setup/maintenance. As long as the hub motor is reasonably light, I couldn't see much difference in handling, whether front wheel or rear wheel, for riding about town. I couldn't detect much power robbing cogging in the Evelo and Stromer direct drives. At 16 mph, all seemed to have enough power for me. Got to see what torque sensing felt too. Bottom line, all ebikes are fun on a demo day.

No single solution here. The Winnebago poster should tell us how he plans to use his bikes. I thought he was going to have a 40 foot monster, but the Revel isn't big . I would want a strong platform rack on a 2" hitch. A pair of fat tired 20" bikes for easier carrying, although they are heavy. Two monster fat bikes? A pair of mid drive mountain bikes. Light street bikes if he never goes into the woods? Range, ease of pedaling (if desired) and comfort matter first, along with price. Given a choice between throttle/pedal to 20 mph or pedal only to 28 mph, I would pick the throttle.

Mid drive who ride 25 mph on throttle only (although you can't buy this bike) report high gear/chain wear. I never take mine past 18 mph, and it's old bike with already worn gear/chain. They haven't gotten worse, although I put on a new chain this summer. I see posts from road bikers who do ride that fast on legs alone, and their chains don't last either. I think we can ignore this issue for mid drive.

I also figure we aren't supposed to ride up hills that we couldn't pedal in the lowest gear, but owners try to do this with a motor in the highest gear and smoke the notors Use the gears, go slow, and I think a 500W motor will do the job.

 

[Ravi Kempaiah]

I beg to disagree, when Bosch extensively did their research and development of their ebike motors, they actually engineered it so the efficiency window also falls within the efficient operating range of a human cyclist. There is an efficiency window for an ebike motor.

I ride a Bosch speed bike and have over 5000 miles on that system. There are benefits and downsides to the system.
The efficiency window you are quoting is for human cadence. At certain cadence you can sync your input with the mechanical input from the motor better. It is not the efficiency of the motor itself.
At the current technological level, mid-drives still have lot of room for improvement. People who simply think mid-drives are always better have no clue what they are talking about. They are just some fan boys or have developed attachment because they spent money on it.
It is not uncommon to see armchair pundits on social forums.

 

[Trail Cruiser]

I ride a Bosch speed bike and have over 5000 miles on that system. There are benefits and downsides to the system.
The efficiency window you are quoting is for human cadence. At certain cadence you can sync your input with the mechanical input from the motor better. It is not the efficiency of the motor itself.
At the current technological level, mid-drives still have lot of room for improvement. People who simply think mid-drives are always better have no clue what they are talking about. They are just some fan boys or have developed attachment because they spent money on it.
It is not uncommon to see armchair pundits on social forums.

The blue window represents a cyclist's normal cadence range, the curve represents the motor's efficiency. You can play around the Bosch's website on their range calculator and you will find out that the cadence that provides the best range is from 70 RPM to 89 RPM, exactly within the window of a human cyclist's normal cadence range. How cool is that?
https://www.bosch-ebike.com/us-en/service/range-assistant/

Note, the efficiency chart was about the early version of the Bosch motor. The current efficiency can be extrapolated from the current Bosch range calculator website. My field of interest has been on efficient automotive propulsion system design, especially hybrid vehicles but also in ICE and electric vehicles. So I am familiar at charts and curves especially when it comes to torque, power, and efficiency.

 

[flymeaway]

There is a misconception among ebike community that when a motor is spinning at slow RPM, it loses it's efficiency

I use the ancient standby method; a hot motor indicates inefficient operation. Electric motors reach zero torque at maximum RPM, and maximum torque at zero RPM. Best practice keep the motor as cool as possible under all riding conditions.

Court J.

 

[Ravi Kempaiah]

Note, the efficiency chart was about the early version of the Bosch motor. The current efficiency can be extrapolated from the current Bosch range calculator website. My field of interest has been on efficient automotive propulsion system design, especially hybrid vehicles but also in ICE and electric vehicles. So I am familiar at charts and curves especially when it comes to torque, power, and efficiency.

I hope you're familiar with Bosch. Because Bosch doesn't make just mid-drives. They cater to one of the biggest scooter markets in Asia using their hub drive system. For high-speed, heavy load, a properly designed hub motor & controller is more effective.
I suggest you study this before concluding that Bosch = mid-drives.

Now, they are entering the Japanese market.

(Link Removed - No Longer Exists)

The traction in the scooter market is tremendous, especially for Asian, EU markets.

https://techcrunch.com/2017/01/27/niu-china-electric-scooter/

(Link Removed - No Longer Exists)

 

[Trail Cruiser]

I hope you're familiar with Bosch. Because Bosch doesn't make just mid-drives. They cater to one of the biggest scooter markets in Asia using their hub drive system. For high-speed, heavy load, a properly designed hub motor & controller is more effective.
I suggest you study this before concluding that Bosch = mid-drives.

Now, they are entering the Japanese market.

(Link Removed - No Longer Exists)

The traction in the scooter market is tremendous, especially for Asian, EU markets.

https://techcrunch.com/2017/01/27/niu-china-electric-scooter/

(Link Removed - No Longer Exists)

Hi Ravi,

Relax man, I have nothing against you. I was responding (and I am very angry) to another guy who insulted me and other people, left and right by telling us that we are incorrect (or being wrong) without citing valid supporting references. Yes, I saw the Bosch hub drive at the Patent record a while back and also mentioned at greencarcongress in 2012. And I don't want to start an argument with you.