Hub motors vs mid-drives: is unbiased comparison possible?

[Emmett]

Formula E doesn't have hub motors, they even have gears lol.

LOL, What's your point? Who cares about gears or motor configuration. I'm talking about motor technology, and where it is headed (already). Power to weight is just that.

Re Formula E motor: "The electric motor is built by McLaren Electronic Technologies. The motor weighs 26 kg and produces a maximum of 270 bhp with 140Nm of instant torque."

 

[riba2233]

Formula E doesn't have hub motors, they even have gears lol.

LOL, What's your point? Who cares about gears or motor configuration. I'm talking about motor technology, and where it is headed (already). Power to weight is just that.

Re Formula E motor: "The electric motor is built by McLaren Electronic Technologies. The motor weighs 26 kg and produces a maximum of 270 bhp with 140Nm of instant torque."

Sorry, I got you wrong, because the other part of your post.

Well, there is a reason why mid mounted motors have better power density than hubs, and that wont change with some magic.

 

[Emmett]

Sorry, I got you wrong, because the other part of your post.

Well, there is a reason why mid mounted motors have better power density than hubs, and that wont change with some magic.

No problem. Good answer. I assume the primary reason is better cooling options for mid drives.

It caught my attention to learn that the Formula-E motors appear to have 8 to 20 times the power density of common (inexpensive) e-bike hub motors in use today. Technology always filters down and becomes more affordable. For my application/needs, I'd be very happy with just 2x the power density from my hub.

I've been riding and maintaining dirt bikes for 20 years, we get ample rain, thus mud where I ride, and the maintenance time and cost of ownership of the drive train (clutch, trans oil, chain, sliders and sprockets) is obviously significant and IMO often depressing. In the past year I've also enjoyed riding an off-road e-bike (Stealth Fighter) and the simplicity of the (sealed and oil cooled) hub motor makes ownership a real joy. I clean the bike and charge it, and it's just ready to go again. I've got the rear suspension working very well considering the unsprung weight. But to have the same power with say half the weight would be incredible. How many years away is that?

 

[recumpence]

I am currently pulling 14kw from a single 4 pound motor at 14,000 RPM in bursts. That is controller limited. This same motor is capable of 25kw in short bursts with a different controller. I have pulled 10kw for 7 minutes with that same motor with fan and liquid cooling.

Motor power density is a matter of efficiency, cooling, and RPM. A hub motor will be very limited in power density due to the low RPM they are forced to run at.

Matt

 

[Emmett]

Thanks Matt. That explains a lot!

 

[100volts+]

I think it all boils down to the same answer it always has in ebiking. It depends on the application and rider preference. I commute 6 miles of flat paved road. I prefer a hub, but that would change if my commute was a rugged offroad commute.

 

[LewTwo]

One of the several reasons I try not to attract too much of the wrong kind of attention squirting around town, what the officials are not aware of they are not so inclined to make up silly rules for.

AMEN !

 

[MadRhino]

I think it all boils down to the same answer it always has in ebiking. It depends on the application and rider preference. I commute 6 miles of flat paved road. I prefer a hub, but that would change if my commute was a rugged offroad commute.

I ride off road in mountain trails, not technical, some steep and narrow sections, some root and rock gardens, but generally nice traIls that you can speed. So far not a single mid drive could come close to follow me, and there are quite a few here now. Many have broken trying, even more just gave up in the first Km. You can blame riding skills, but after 20 that have tried it is just impossible that none of them were able to pull the best of their bike.

The fact is, a mid drive could do better, but it would have to be built with motorcycle components. Bicycle chain and sprockets are not up to the task, and a real transmission would be needed for a mid drive to compete with my hub build bikes. Still, that would make it much heavier, thus handling and braking handicap that would require some serious acceleration advantage to catch up.

That is why I say mid drives are good only at both ends of the power scale, good when they are low power to build a light weight bicycle feel, and good when they have much more power than one could build with a hub. If your performance requirements can be achieved with a hub, no mid drive can be as good and reliable.

 

[LewTwo]

Can someone give me a reason why anything could be possibly be more efficient than a direct drive hub?

If you are referring to 'looses in the drive train' then no.
If you are talking about running a motor (ice or electric or steam)at its optimum RPM then that is what transmissions do most efficiently.

Mid drives are time consuming, both in building and maintenance. 

I hope that you are mistaken on that last bit :?

I've looked around the web for a bit now, hoping to find some more or less unbiased assessment of how hub motors compare to mid-drives. So far I've not turned up much.
...
Can anyone else add additional pros/cons for either and help to fill out the list so we can come up with a more comprehensive comparison?

I think a person's specific needs and/or desires are a big part of the equation. The perfect solution for bunny hopping over hill and dale all day long is probably not even close to what is most suited to go pick up a bottle of milk and a loaf of bread at the local quicky-mart.

Then there is the cost and complexity of the build to consider as well. A pre-built front wheel with a small hub motor is by far the simplest and easiest to install in most cases. For a simple steel frame bike it can take as little 1/2 of a hour from start to finish. If however there are other parameters to complicate matters (suspension and/or non-steel forks, matching front and rear wheels, the need for speed, heavy payloads, or desire for way too much power) then the alternatives bear consideration. Also an individual's mechanical and electrical abilities as well as access to the proper equipment comes in to play. My current project is rebuilding a vintage touring frame. For that task I have to acquire some specialized tools for the bottom bracket and head set ( ... when I was a kid we got by with a screw driver, spoke wrench, crescent wrench, water-pump pliers and a hammer).

I just completed an evaluation for my next project between a 36 volt 250 watt front hub and Bafang 36 volt 250 watt mid drive. I concluded in this case that the advantages of the mid drive out weighted the additional cost and weight. However there were some additional parameters that influenced that conclusion. I am retired and will be disposing of my Honda Insight electric hybrid. I am cheap and it needs a new battery pack, yearly inspection, registration, insurance, oil changes, ....etc. The bike will be a backup for the 'grocery getter' utility ebike. It is to be an urban leisure bike to be ridden exclusively on paved streets. Additional Parameters:

(1) Max required speed 15 to 20 mph
(2) Max required range 5 miles
(3) very small battery for lighter weight
(4) I am anal retentive about matching wheels
(5) self built wheel set for double butted spokes for shock absorption of (something NOT offered in pre-built wheel kts).
(6) both front and rear drop-outs are forged Aluminum
(7) have to acquire and install a bottom bracket and crankset anyway
(8) three speed IGH rear hub (less gear changes to deal with)
(9) more time than money
Full thread here: http://endless-sphere.com/forums/viewtopic.php?f=3&t=70586

My point is that the pros and cons vary according to the application.

 

[Emmett]

That is why I say mid drives are good only at both ends of the power scale, good when they are low power to build a light weight bicycle feel, and good when they have much more power than one could build with a hub. If your performance requirements can be achieved with a hub, no mid drive can be as good and reliable.

I agree 100%. I look at my bike and think how much I like the hub motor for off road use.

I also ride trails which seem similar to yours. Typically in the speed range of 15 to 45km/h. Mix of terrain between tight/twisty, rough/fast and sandy/hard or rocky. Using a sealed hub motor with oil to keep the stator from overheating. Rear 19x3" trials type tire to add suspension damping for the hub weight. On/off throttle, peak 3700 Watts. Typically using about 2500W on the flatter ground. Lots of wet or mud sections to cross!

I only pedal lightly to extend my range, but my front alum sprocket is nearly spent after just 1700kms. Much of my post ride maintenance routine is spent cleaning and re-oiling the chain and sprockets. I occasionally see MTB riders on the trails with broken chain or sprocket systems, and that's with just light duty (human power). No bicycle spec chain system could cope with my power needs off road.

I'm always replacing 520 X-ring chains and steel teeth sprockets on my dirt bikes. So for my e-bike application I'm loving my hub motor. The only time I wish for a mid drive is when I think about mass centralisation for flat whips on nice jumps with easy air time. Right now it's a compromise. So I dream about a 4000W from a 4Kg hub which can also be kept cool enough for an hour of heavy duty.

 

[rsilvers]

Some of the attributes listed for hub motors only apply to geared or direct drive but not both.

 

[speedmd]

I have not seen a production electric that comes close to a top level ice trials bike in terms of getting though near impossible to ride through sections. I am sure such a bike can be made, but not seen one yet. I see the new e bultaco, but not followed reviews/ tests. The kids 20 and 24" e trials bike does look good. Only a matter of time.