"Efficiency" of Brose mid-drive vs hub

[ferrellms]

So, the Specialized marketing guy says that the Brose mid-drive is much more "efficient" than the hub drives they use now (he is a marketing guy, so what else would he say?). Leaving aside things like weight distribution for off-road riding, riding feel, and so on and focusing on the efficiency of the new mid-drives - What does "efficient" mean? How can one motor of a certain power be more "efficient" than another? How can a motor with internal mechanics like gearing be more "efficient" than one without?

I am skeptical of the efficiency claim.

 

[Charlie Rohlfing]

Well, I have two thoughts on this question ... but not an answer. On the issue of gearing, it's a known fact in cars and motorcycles that power is lost as it travels through gear systems. There is a factor of friction and power loss that's just basic physics. On the other issue, some motors (especially internal combustion) operate more efficiently at higher RPMs, so a gear system that lets the motor work at it's optimal RPM would actually enhance efficiency. Is this true for electric motors? I don't know. I do know that hub drive motors operate at extremely low RPMs, relatively speaking. A geared mid-drive system would allow for a motor to spin at a much faster (more efficient?) speed. ALSO, and this is perhaps the most pertinent factor ... a hub drive system operates through the gearing of the bicycle (derailler, cassette, etc.) which allows the motor to operate more in its optimal range at all times, regardless of terrain or load. Hub motors, by necessity, have to operate very slowly up hills, probably at compromised efficiency.

 

[bazzapage]

There will be an electrical engineering explanation to it probably related to current, reverse flux blah blah. Bottom line is that direct drive hub motors eat battery in comparison to mid drive motors.

 

[Douglas Ruby]

Couple of issues...

1. Direct drive require very large armatures with heavy magnets and windings to get good torque at very low RPM. Geared drive (whether hub or mid-drive) can have smaller, higher RPM motors that are lighter in weight. Further, they can have free wheeling connecting the motor to the wheel so that there is no (little) drag when not pedaling. So in this respect, they are "more efficient".
2. Direct hub drives have little if any frictional loss because of no gear system, and can be optimized for a particular speed (20-28 mph) so in this respect, direct hub drive is more efficient.
3. However, there is more torque multiplication due to using a high rpm motor connected to a reduction gear (in a mid drive unit) then connecting through any where from 7 to 22 speeds via a derailleur or internally geared hub of a mid drive. Thus in this sense the mid drive is "more efficient".
4. Weight is lower and more centralized with a mid drive and downtube battery...blah blah

Having spent many years as a product manager in marketing myself, I could probably say "more efficient" with a straight face using any of the three basic systems in use today (mid-drive, geared hub, direct drive hub). The only systems that definitely would not qualify for me would be a friction drive or the old kludgy Currie system with the motor hanging behind the rear axle on one side.

 

[Ravi Kempaiah]

I agree with @Douglas Ruby .

Mid-drives shine in slow, technical conditions. Also, some of the motors like Bosch or Brose have very accurate torque sensing and they don't over exert themselves.
Their controllers are often limited to ~15A max and they operate very efficiently in the sweet spot of 12mph to 20mph.
Above 20mph, they simply lose their edge to DD hubs. I have very strong evidence to this.

On my Stromer ST2, I was able to ride 53 miles on a single charge (stopping at 15% battery ) at a fairly high avg speed of 25.2mph.


I have tried to emulate that kind of efficiency on a Haibike Super Race (multiple times) and I could only do ~50 miles before running 2 Bosch batteries down to empty.
May be it's the aerodynamics or the wind but high speeds, geared mid-drives lose efficiency because of friction and electrical losses. Most mid-drives run at 36V but I am sure BBS-HD can handle the speed.

I remember on one stretch in Grand Tetons national park, I was able to get 80 miles per charge averaging 23mph (ideal conditions, light tailwind etc). I have never seen that kind of efficiency on a Bosch speed pedelec.

Considering Specialized is switching to Brose, you will see range improvements in city riding but not high-speed cruising.

 

[Trail Cruiser]

I agree with you Ravi,

The DD hub drive is my choice if the goal is for high speed rides (above 20 mph) over long distances. The DD motor is most efficient on those speed range (20-30 mph) plus the fact that there is no mechanical efficiency loss from gearing/chains. There are also no added stress and premature wears to the chain/chain ring/cassette so they last a very very long time (complete opposite to mid drive). DD is also very very silent.

However, if I ride at higher speeds above 20 mph, all the bumps are magnified so a full suspension would make sense to me. The body float is OK but not as good as rear suspension in road isolation and comfort. The body float also does not isolate your feet (or the whole bottom bracket) from road impacts.

 

[jwb]

Mid drives are noisy and ugly. They'd have to be really efficient to make up for that.

 

[bazzapage]

Hub drives tend to be one-trick-ponies whereas mid drives can be optimized for a wider range of situations. It is even possible that they will be better at high speeds than the GoSwiss. Let's wait and see!

 

[Nirmala]

Clearly electric motors are more efficient at higher RPMs, but that is just one factor in the entire system of a bike and rider. But to illustrate the point, any hub drive is 0% efficient when it bogs down and comes to a stop on a steep hill. All that current going to the motor is having zero effect on moving the bike forward. At their peak efficiency, a good hub motor might reach approximately 80% efficient.

An interesting approach that might eventually be the best of both worlds are the two speed hub motors as mentioned in another thread on here:
https://electricbikereview.com/forums/threads/which-is-better-mid-drive-or-two-speed-hub.1195/
But they are not available widely yet, and of course would have to prove to be reliable.

 

[Trail Cruiser]

The hub motor operates ideally on a 2 speed mode. The problem is the logistics of space. There is not enough room so they end up with small motor/low power. There was the SRAM ematic that was used by Townie and Ohm at one time. There's also the Xiongda that suffered durability issues. However, it looks like it has been beefed up lately and Luna is hot rodding it and installing thermal cut off protection. You might want to look at Luna.

 

[bazzapage]

I love the mechanical aspect of those Xiongda motors. Very clever. They will likely have a niche.

 

[pxpaulx]

Mid drives are noisy and ugly. They'd have to be really efficient to make up for that.

Uh what? I can't hear my bulls brose motor at all 95% off the time...and also can't really call it an eye sir since it is completely in the frame!

 

[Douglas Ruby]

Uh what? I can't hear my bulls brose motor at all 95% off the time...and also can't really call it an eye sir since it is completely in the frame!

Brose has an internal reduction belt drive system instead of reduction gears. This is why it is much more quiet than the typical gear driven mid-drive.

 

[ferrellms]

OK, thanks for the replies. My own research does contradict some of the assertions and speculations, however. Here is the treatise

Efficiency refers to the ratio of electrical input watts to mechanical output watts. Different motors differ in this regard due to various internal power losses that are well-understood. Rotation speed is irrelevant as to efficiency, it is a matter of torque load. If torque load is greater than about 50% you will typically get the maximum efficiency from the motor, regardless of the motor rotation speed.

There is no good reason to believe a mid-drive or hub-drive motor would be more efficient; you would have to understand the characteristics of the motors. Hub drive would have less mechanical loss but that doesn't mean a bike with it would be more efficient, if its motor were less efficient. But, efficiency differences for motors with the same power output are not huge; typically less than 10% between worst and best.

I remain unconvinced that a Brose mid-drive is "much more efficient" than the current hub-drives. I would expect them to be pretty similar. If this is correct, you could justify the mid-drive vs hub-drive decision on noise, off-road balance, handling, looks, price, freewheeling ability, what your buddies use, weight, what the marketing people cook up, etc. - but not on efficiency.

https://en.wikipedia.org/wiki/Electrical_efficiency
http://www.thor-power.com/technique/motor-efficiency-vs-torque/
http://www.reliableplant.com/Read/7393/motor-efficiency-standars
http://slideplayer.com/slide/9477186/
https://en.wikipedia.org/wiki/Electric_motor
http://www.novatorque.com/aboutus/white-papers/060_John_Petro_final_paper.pdf

 

[pxpaulx]

That was...a delayed response! Also, I don't think anyone was really arguing anything here anyway.

 

[Allan47.7339]

I don't think efficiency was the only design requirement. This is one of the best balanced ebike review by dedicated mountain bike riders.

If the link doesn't, search YouTube for:

we:Ride- Specialized Turbo Levo FSR by Flow.

 

[ferrellms]

Well, if you look at the name of the thread and the first post, I was specifically calling out ("arguing with" if you will) the claim of the Specialized marketing guy that the mid-drive was "much more efficient" which, given my admittedly sketchy knowledge of electric motors, did not make sense. So, the many other criteria that may be important still stand, of course, as I listed in the post.

I spent a couple of hours researching all this and came to the conclusion that there was no reason to believe that mid-drive is "more efficient" than hub-drive. I could be wrong, check the links, or better, explain it all in a post. I have a feeling that this is a topic of general interest; therefore the "delayed response".

 

[Allan47.7339]

The review illustrated it's about the design choice and not raw power. I am in the "who cares camp" if the design works, it's good.

 

[pxpaulx]

Hmm, so I did realize I never actually contributed to the original discussion point! Here are my two cents as a larger guy (hint, exceed bike weight limits).

For me, mid drives are noticeably more efficient. I contribute to my riding, and have always used assist only (no throttle). I know how to use the bikes gearing properly and efficiently. For both my haibike and bulls bikes, I get around 10wh per mile riding in mid level assist.

On the first easy motion I owned (neo xtrem) I could only get about 18-20 miles (326wh battery) and on the evo snow probably 25-30 (400wh battery). I do think the evo snow was more efficient than the older model, and have no doubt improvement will continue for efficiency in both types of motors.

My final thought is that a mid drive can run at better peak efficiency with proper gear usage, in my experience. If you are going long distance at a hub motors most efficient speed though, there is much less difference between these motor types.

 

[Alphbetadog]

I've got both hub (geared) and mid-drive bikes and both are comparably efficient. I do believe that mid-drives such as the Bosch and Brose are better suited for emtbs with their more sophisticated cadence and torque sensing, but at a higher price. Hub drives work perfectly well in the paved and smooth dirt trails. This is just my opinion based on my experience.