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

Posted: Apr 17 2014 6:12am
by wayover13
I've looked around the web for a bit now, hoping to find some more or less unbaised assessment of how hub motors compare to mid-drives. So far I've not turned up much.

I have no experience with mid-drives, but do have some familiarity with hub motors. My original upgrade plan from my first DD hub motor was to go with a mid-drive, since these drives seemed to me at that time to answer better to my needs. Because of cost and simplicity, however, I ended up going with a geared hub motor for my upgrade. My new system is functioning quite well so far, incidentally.

This should give an indication of my own opinion about these two drive systems: namely, that both have their advantages and disadvantages as compared to the other, and that there is not going to be a universal answer as to which is going to be better in every possible scenario. Each individual will have to decide, rather, based on his own circumstances and needs, which will be the better option.

To speak to my own circumstances, since my main application for electric assist was to get some extra propulsion when going up hills, a mid-drive seemed like it would work best. It could perform well the job of helping get me up those hills without pushing myself to total exhaustion, but it could also be brought to bear, in rare circumstances when I might need it (think unexpected approaching thunderstorm), to increase my top speed as well. So it would meet the main need as well as having some flexibility to meet other, less pressing needs.

But, like I said, I ended up, for the sake of simplicity and economy, going with a geared hub motor. It does a great job of meeting my main need (assistance on hills) and I'm quite happy with it for that reason. I have yet to experience a need for it to increase my top speed, but since its maximum speed is only about 12 mph, it does have a bit less flexibility than would a mid-drive.

I still may, on one of my other bikes, end up installing a mid-drive, though, so I'm keeping track of developments. And as I continue to consider that option, I've wondered whether I might find some resource on the web that offers a relatively unbiased comparison of the relative advantages/disadvantages of hub motors as compared to mid-drives. By unbiased, what I mean is a comparison premised on the fact that either might be better or worse for any given real-world scenario, and a comparison that does not try to argue that one or the other is going to be the better option for all scenarios.

A lot of what I've found in my searching is marketing-oriented and represents the viewpoint of someone whose final aim is to sell the reader one or other of the systems. So, of course mid-drive sellers are going to be touting all the advantages of their systems over hub motors, and vice versa. Take, for example, the discussion initiated by Rakesh (of Falco Motors) that I found at http://www.linkedin.com/groups/Mid-Driv ... 5503069184. Though there are some good points being made in the discussion, it starts off from a faulty premise; namely that mid-drives are being overhyped and are actually inferior to hub motors--the latter of which, <begin sarcasm> believe it or not, are produced and sold by Falco Motors</end sarcasm>. A reasonable discussion would, instead of starting from a flawed premise like that, proceed from the frank recognition that either might be better or worse for a given application. So, though that discussion contains some pertinent points for a fairer comparison, the points would have to be identified and excised from that sort of partisan context.

As that may be, I thought this forum might be a good place to try and generate something more like an unbiased comparison of the relative strengths and weaknesses of the two drive systems--one that could be of help to someone like myself, who is interested in finding out which might be the better choice for their own application. The closest thing I found so far to an unbiased comparison, is a page that lists the pros and cons of a recent (2012, it seems) GNG mid-drive kit. Some of the cons from this write-up are, or could readily be translated into, pros on the hub motor side. It therefore seemed like it might offer a good starting point for putting together a real unbiased comparison, so I'll re-post it here (original located at http://topsecretev.blogspot.com/2012/10 ... -hype.html )
PROS:
  • * Great hill climber
    * Lower center of gravity
    * Very good traction in soft sand (perfect for the 2 biggest e-bike markets in the US (S. Florida and S. California.)
    * Allows you to cycle the motor's power through all the bicycle gears
    * More efficient (from my personal testing a mid-drive kit is nearly 40% more efficient than a hub motor in some cases.) So it is safe to say a 750 watt hub motor is nearly equivalent to a 450 watt mid-drive in terms of current draw per Nm torque output.
    * Motor is less prone to current saturation over hub motors. It just climbs hills and goes through soft sand never bogging down.
    * The motor draws less current per Nm torque output so the system runs cooler
    * You can change rear flat tires easily as with any normal bike.
    * You can use any cassette and or disc brake caliper without clearance issues
    * Saves your frame's drop outs and omits the use of a torque arm.
    * Can use a smaller/ lighter amp hour battery to achieve a similar range and speed.
    * Easier to use off the shelf DC motors to lower tooling and replacement costs.
    * Gives new meaning to the term "Freeriding" hitting a trail at 25~30 mph on a DH mid-drive is absolutely amazing and will open up the sport to an older crowd.
CONS:
  • * Chain drive is extremely noisy (would like to see all belt drive)
    * A chain rear derailleur and or front derailleur is for the most part out of the question with anything over 350 watts. You almost need to cut power to the motor like a clutch to shift or else you hear a large grinding and thumping noise.
    * Derailleur must be dialed in perfectly and needs adjustment more often
    * Chain is prone to stretching and or premature wear/ breakage.
    * The entire pedal drive train is put under extra stress and it's lifespan will be greatly diminished.
    * Riders have belts or gears to worry about slipping, and or wearing out.
    * The entire crank system spins the chain-ring much faster so if a pant leg gets caught, the effect can be devastating.
    * The bottom bracket, and chain stays are put under tremendous stresses causing excessive flex (even my downhill mountain bike above has 4 bearings and 1 bushing in the "main pivot" and the flex is really noticeable under load.)
    * All the added freewheels mean no-regen, and the bike is difficult to push in reverse.
    * The initial start-up torque is harder to dial in than a hub motor in the controller programmer settings (it just takes off like a rocket.)
From what I'm reading, a con for the mid-drive as compared to hub motors that might be added to this list is the inability, using the standard rear derailleur, to shift while the mid-drive motor is under power. That could be a big downside for me, since I would be bringing the motor into play primarily when climbing grades and might thus want to shift down a gear or two.

In addition to the above list, I also found this page--http://www.avdweb.nl/solar-bike/hub-mot ... ation.html--which offers some simulations of both hub motors and mid-drive systems. So some technical specifications for the sort of comparison I'm hoping to create could be extracted from a page like that.

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? If there already exists some resource like this that I've overlooked, I beg pardon here and will ask that this posting be removed.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 6:50am
by spinningmagnets
For the majority of E-bikers, I think cost is a big issue. I believe hubmotors will continue to cost less than mid-drives (unless the cost of copper shoots up, then they might cost the same).

Most people can be satisfied with a hubmotors performance, and save a few bucks.

Also, hubs will likely continue to have an easier installation.

The mid-drives only requiring a smaller battery is more significant to new E-bikers than it might at first seem it would be. A direct-drive hub might require LiPo to get very high peak amps at an affordable price. High current batteries that are not LiPo are available, but they are pricey...

In spite of the unsprung weight of a large hub in the rear, I keep reading that some guys just love the near-silence.

Image

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 7:25am
by teslanv
As I have yet to install my first dd hub, I can't compare it to my Bafang BBS02. But you need to add one "CON" to your Mid-drive list. Constantly shifting. And with a mid drive, shifting means you have to kill the motor briefly. Its just... Annoying.
I can get up any hill though, and since I live in the great pacific northwest, there are many hills to traverse. My first Yescomusa Hub arrives tomorrow, and I will be modding for extra power. Then I should have a good feel for a comparison. :)

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 7:29am
by Bluefang
I would say mid drives are king of the hill if you want to push the absolute limit of what a particular motor is capable. A perfectly designed hub motor would come close but most hubs are not even close to designed perfectly for everyones different applications.

Hub drive is the easy way out, not really high power for long runs but definitely enough power for 99% of people, and simpler. :)

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 9:08am
by d8veh
All the theory says that mid-drives should be more efficient. I have three mid-drives, and from the tests I've done, I can't prove that they are. In theory, they also have better versatility like you mentioned, being able to give both high and low speed from the same motor, which is easy to prove, so that is one advantage they have. From my own experience, for general commuting and touring, a hub-motor is difficult to beat provided that you choose one with characteristics that match your riding requirements. If you ride off-road tjrottle only, a middle motor behaves just like a lightweight motorcross bike. The gear-changing will keep you busy.

The European crank-drives like Bosch and Kalkhoff don't have throttles and they use torque control rather than speed control with their power algorithms based on torque multiplication, so you have to do a lot of work with the pedals to get the power out compared with the cheaper Chinese ones that use speed control.

At the Shanghai show, there were at least five new crank-drives all about the same size as the 8fun one. Don't expect a lot of power though. They all looked about 250w.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 9:39am
by MadRhino
Basically, build a mid drive if you like to build, or a hub if you like to ride.
Mid drives are time consuming, both in building and maintenance. 
There is a certain advantage into gear reduction and transmission to use a smaller motor, but at the end gears are not gonna change 3hp into 15 when you need them.
Building with a hub, it is easy to make a reliable high powered bike.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 9:55am
by parajared
I own both,
The primary problem with hubs is they suck at "real" off-road. They make your bike tail heavy and awkward and they overheat on the slow steep grinds. I love hubs and use them for everything but steep rugged off-road.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 10:09am
by ebike11
Why not install both?? Doable with a rear hub installed with a GNG??? Assuming its mostly paved riding and not as much concern of the weight

Then youd always have a back up. Of course youd need 2 controllers but that shouldn't be hard to find a way to mount

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 11:49am
by Jonathan in Hiram
The two speed hub motor being discussed in another thread sounds like the all around "bolt on" solution. From playing with the Grin simulator with different tire sizes it's clear that surprisingly low powered motors don't need more than two speeds to cover a wide range of conditions if the ratios are chosen well. A rear hub motor built around a three speed hub that took both motor and pedal input and geared it up or down with a manual or automatic selection would be great. Get a lot of the advantages of a BB drive all in a rear hub.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 1:20pm
by MadRhino
2 speed of a small hub will never make for the power of a big one. This is only good for slow bikes, it is not because there is a 2nd speed that 1000w will push you to 60 Mph. A powerful hub does not overheat as much as a low power one in the mountain, because its size sheds the heat better but also because it doesn't stall as much. Nothing is worse than being short of power to turn most of it into heat.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 2:39pm
by Jonathan in Hiram
Sooner or later only "slow" ebikes will be allowed on the road without the usual license, registration, insurance, inspection, tags, taxes et pain in the bureaucratic cetera.

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.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 4:09pm
by John in CR
Mid-drives don't have to go through the chain rings or the derailleur or other gears. That's only appropriate for low power, and only under-powered bikes need multiple gears. When I do a mid-drive it will drive the rear wheel directly and won't have variable gears. The purposes will be to:
1. Get the motor out of the wheel for off road use.
2. Gear the motor lower than is practical with a wheel for better low speed high load performance.
3. Mount the motor higher and protect it more from ingesting debris, so I can provide maximum cooling.

The compromises in doing this are:
1. The motor will use up space, which is at an extreme premium on our ebikes.
2. Increased noise- even though I'm going with a belt drive, it will be noisy in comparison to the hubbies I use.
3. Increased complexity- I'm a KISS principle guy, and this is probably the biggest reason I don't already use a mid-drive.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 5:37pm
by lbz5mc12
I've got a front hub kit and a rear currie kit. I'm running the front at 48V and the rear at 36V. The top speed is about 25-30 mph and it can climb pretty steep street and dirt roads with very little assist from me. The acceleration is awesome and I can shoot across most intersections in a couple of seconds. I've owned the R-10 mid-drive bike but it was kind of crappy and parts kept failing. What I really want to do now is add a mid-drive to the mix and set it up to work with pedal assist and no throttle. I could use the mid-drive for street riding and then switch to the other two motors when I ride off road.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 5:40pm
by wayover13
John in CR wrote:Mid-drives don't have to go through the chain rings or the derailleur or other gears.
Interesting point. I hadn't thought of that type of a drive as a mid-drive, but I guess it does qualify, doesn't it?

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 9:55pm
by waynebergman
I think John in CR has some great points regarding the main benefits of a mid drive. Which route to go really starts with I think how much battery weight you are willing to lug around and what kind of riding you want to do and also your willingness for tinkering with the drive train.

For me I think its come back to the silence, smoothness of power and the simplicity of the hub motor in the wheel.These three things are what originally got me thinking about an electric bike. A quiet pedal assist bike is very cool and fun to ride.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 10:13pm
by Josh K.
Automatic, vs stick shift...

Hub motors are great for folks who just want to get on, and ride. Pretty efficient sometimes too.

Pedal drive, or true mid drives, are more efficient, especially when starting up, up hill, and or pulling a load.

Hub motors over compensate by drinking up the amps. Shifting systems really need the rider to be on the ball, to downshift at a stop.

Overall, you can achieve higher efficiencies with a mid, or pedal drive. But, you can just get on and ride with a hub motor.

So, are you a scientist, or a happy go lucky rider? Care about going long distance, or recharge more often?

Peace, Josh K.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 17 2014 10:54pm
by liveforphysics
Can someone give me a reason why anything could be possibly be more efficient than a direct drive hub?

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 1:31am
by Geebee
liveforphysics wrote:Can someone give me a reason why anything could be possibly be more efficient than a direct drive hub?
Easy, in hilly terrain it will amost always being running at an inefficient rpm.

2 ways a mid drive beats a hub, one is that it is more like riding a normal bike if setup with PAS, you pedal and change gears for hills or speed, once you adapt to hitting the brake on shifts which only takes a copuple of rides.
If you are trying to stay legal in 250w markets a 250w mid drive will out climb any hub and especially so if have low/limited rider input.

The riding like a bike is a biggy if you are coming from a normal bike as other than tapping the brake to shift it is the same as riding any bike.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 2:14am
by liveforphysics
Geebee wrote:
liveforphysics wrote:Can someone give me a reason why anything could be possibly be more efficient than a direct drive hub?
Easy, in hilly terrain it will amost always being running at an inefficient rpm.

It can have excellent stall rotor torque performance, all the way up to full speed. All drivetrains start at 0% efficiency at 0rpm, but a properly designed hub would be making that torque directly with greatest efficiency due to no additional power transfer stages.

For easy evidence, you can look at solar cars. Most efficient electric drive-trains on earth, and they use direct drive hubmotors almost exclusively.


I look at it like this, if you want 100ft-lbs of torque on the rear wheel, you can make 10ft-lbs at the motor and use 10:1 gearing, but this requires a gearing loss stage due to no power transfer being perfectly efficient. The alternative is to simply generate 100ft-lbs directly. As torque grows at the square of motor radius, you can grow radius until you've achieved the desired torque.

Think of growing radius on a motor like adding gearing. The magnet passes by faster even at low speeds which enables it to reach decent efficiency quickly.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 2:41am
by Geebee
http://www.ebikes.ca/tools/simulator.html
Default motor, I ride up a 12~13% hill to get home with over 20% ramps, on the default 9C motor efficiency varies form 25% to zero, 12% hills are real common around here and 20% isn't that unusual.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 2:44am
by cwah
liveforphysics wrote:
It can have excellent stall rotor torque performance, all the way up to full speed. All drivetrains start at 0% efficiency at 0rpm, but a properly designed hub would be making that torque directly with greatest efficiency due to no additional power transfer stages.

For easy evidence, you can look at solar cars. Most efficient electric drive-trains on earth, and they use direct drive hubmotors almost exclusively.


I look at it like this, if you want 100ft-lbs of torque on the rear wheel, you can make 10ft-lbs at the motor and use 10:1 gearing, but this requires a gearing loss stage due to no power transfer being perfectly efficient. The alternative is to simply generate 100ft-lbs directly. As torque grows at the square of motor radius, you can grow radius until you've achieved the desired torque.

Think of growing radius on a motor like adding gearing. The magnet passes by faster even at low speeds which enables it to reach decent efficiency quickly.
I'm using a 2 times and bigger DD than my BPM:
http://www.leafbike.com/products/e-bike ... l-892.html

but torque is still not as good for the same amount of power. So if the motor has to be 3 times heavier in order to achieve the same torque, not sure that DD still win

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 5:55am
by Jonathan in Hiram
cwah wrote:
but torque is still not as good for the same amount of power. So if the motor has to be 3 times heavier in order to achieve the same torque, not sure that DD still win
The copper wire, the magnets and the magnetic armature material are the expensive bits in these motors, geared motors use less of those expensive bits and more of the cheaper stuff like aluminum, steel and plastic so it's less expensive to squeeze relatively good numbers out of them.

A hub motor is ultimately more efficient if you take costs out of the equation because any sort of gearing always has associated losses.

A bicycle is a hard vehicle to make a perfect motor for, simultaneously going for high power, low weight, high reliability and good aesthetics (because it's hanging out in the open) makes for a real challenge, doing it cheaply is yet another level of challenge.

It's hard to get by the mentality that a bicycle is a toy which shouldn't have a lot of money put into it (unless it's for "sport"), everyone has two or three hanging on the wall in the garage with tires so dry rotted they crumble if you put them on the ground. I find myself fighting that mentality personally and if I tell people how much I have invested in my bike they cringe (particularly if I add in what my hourly charges would be to study this stuff for work purposes).

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 11:22am
by MadRhino
A friend did ride with me in the mountain with his (relatively) high powered mid drive that was about 7000 w peak with gears, compared to my 15 000 w peak DD. On nice trails he was lost because of my much higher top speed, on bad sections of trails he was lost in shifting delays, on bad steep grades he was good but broke on half of the rides. Result: He did not complete half of the rides we did together, and was left far behind on all of those rides that he completed.

I see the real use of mid drives at both ends of the power. Either to make a very low power build, or to use a motor that can be fed better power than a DD hub. I agree that anything but a very low power mid drive shouldn't use the bicycle drive train, for this is making it unreliable, sometimes to the point of spending more time fixing it than riding.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 11:54am
by wesnewell
cwah wrote:I'm using a 2 times and bigger DD than my BPM:
http://www.leafbike.com/products/e-bike ... l-892.html

but torque is still not as good for the same amount of power. So if the motor has to be 3 times heavier in order to achieve the same torque, not sure that DD still win
Look at the specs on that motor. It's an extremely fast wind with a 731rpm no load speed on 50V. This is not a motor one would want if they are heavy like me or they are riding in a hilly area. It will not have much torque, as the no load speed is over 56mph, compared to ~36mph on a motor rated for 470rpm at the same voltage. One person already melted that motor trying to climb hills with it wot, and then came here and complained. If you have this motor, be extremely careful with it on hills. On level ground it should get over 40mph on 48V though unless your heavier than normal weight.

Re: Hub motors vs mid-drives: is unbiased comparison possibl

Posted: Apr 18 2014 1:45pm
by 999zip999
D.D. at 72v. or a geared mac at 48v.30amp. for the street as a commuter. Maybe the new middrive when they get the bugs out or have a programable controller. Do you want a mid drive hobby setup ?