geared hub motor ratios versus mid-drive

[toledobythesea]

While trying to decide whether to replace a no-name motor that looks to be a 250w having a reduction ratio I calculate to be 3.84:1, I read about larger motors such as Ezee as well as mid-drive motors. This number is important to me as I am in such poor physical shape that pedalling even half a kilometer up a small 3% grade risks me blowing a heart valve or brain artery. Although I haven't measured my own pedal power, I doubt whether I could produce more that 50 watts for more than a minute or so.

Various websites seem to tout mid-drives as superior for hill-climbing. Although I don't remember seeing the reduction ratio quoted for any mid-drives, some photos of their insides suggest to me that their ratios are probably around 5:1 or even 5.5:1. Now the 500w Ezee is advertised as having a 5:1 ratio and I guessed that the Mac and 8Fun hub motors are similar. However, it seems to me that pretty much every outer chainring-rear sprocket combination is what automotive people call overdrive, which will reduce a mid-drive motor's reduction ratio, which will reduce the motor's torque advantage. Whereas the geared hub motor has its nominal torque advantage regardless of chainring/sprocket size. The mid-drive advantage therefore seems to be higher top speeds at the expense of slightly less torque. Have I got this right?

Secondly, can anybody quote exact reduction ratios for the various mid-drives?

The reason I ask along these lines is that for me and I think some other fairly elderly people, top speed isn't what matters, nor long-range either. For me my two cheap no-name bikes are utilities, for carrying as much as thirty pounds of groceries for 3 or 4 klics. I just want to get up the hill! I've measured some of my routes fairly carefully and they all include segments that exceed a 9.6% grade for at least 0.3 kms (which caused phase wire insulation melt on one bike) and there are a couple of routes that have segments ranging between 8.4% and 13.4% for a full kilometer that I would use if I had enough motor boost.

Some kids around here have 1000w+ mid-drives and I was very impressed after riding one without pedalling at all. However I always pedal except when braking - which I do a lot of (being on the lower reaches of a 4000' mountain, my routes are all downhill going out and uphill coming home). Pedalling might help might blood circulation a bit and it certainly keeps me warmer in the winter. I think there are plenty of seniors like me who compare a bike not to a car but to walking. Five times walking speed on the flat is fine by me and two times walking speed uphill beats the alternative of walking my loads uphill.

 

[Drunkskunk]

Welcome to the forum. Sounds like you were an engineer of some kind.

The gear ratio of a hub motor is meaningless, unless you are comparing to identical motors. One 350 watt motor with a 2:1 ratio might produce more torque than another brand 350w motor with a 5:1 ratio. There is no way to make a comparison between brands this way. Only the actual output should be compared. And generally, their output is all fairly close.

Generally speaking, any 350 watt motor is going to be able to climb a short 10% grade hill. But a 500W would be better for climbing, as the larger gear surfaces will be more reliable. For even better climbing ability, a mid drive works best, since the gear ratio isn't fixed and you can gear down as much as needed.

The motors used in mid drives aren't the same as the motors used in hubs. They are designed to run optimally as mid drives. In theory, you're losing some efficiency by having part of the drive be in overdrive, but in practice, there is no real loss of power or tangible range from those systems.

You should check out this motor simulator. not all motors are there, but many popular ones are. http://www.ebikes.ca/tools/simulator.html

If you are looking for the best possible reliable hill climbing drive system, there are 2 ways to go. One is a mid drive like the BBSHD, powering an IG hub like the Shimano Alfine 8 by way of a BMX chain. The other is to simply throw more watts at the problem, and use a direct drive. While a direct drive isn't as efficient at climbing, it's easy enough to just keep adding power until you get the torque you need. The direct drive will be more efficient on flat ground than any other system.

 

[toledobythesea]

Thanks for those comments. To try to clarify the point I was trying to make: if you could fit a given mid-drive motor and gears in a hub, I think it would produce somewhat more torque than it would when mounted at the crank on the same bike.

No, I'm not an engineer so the ebikes.ca simulator was a great help when I was trying to decide what to blow my small budget on. Your point about gear surface also seems interesting too. I note that a 500 watt hub motor looks to have more possible room for larger sun and planet gears so maybe it's good to keep those two gears as close together in size as possible. I remember reading somewhere that Bafang had a sun gear with 11 teeth at one point which seems pretty small to me. If any of that makes sense maybe motors with thicker (as opposed to wider) gears last better.

(I won't be bothered if the Ezee front hub motor I chose needs gears after two years because I'll probably have worn out the rim by then from all the braking I do.)

 

[toledobythesea]

Meant to say the hub version would produce somewhat more torque at the wheel than the crank version would. Didn't mean to imply the motor torque would be different.

 

[Hillhater]

I think you are overlooking the gearing between the mid drive and the rear wheel, which can be varied considerably.
Also , little point in contemplating altering the gearing or motor in a hub, unless you are very mechanically skilled and have engineering workshop facilities,
The higher powered mid drives, 750W + , will power you up any grade you are likely to meet.
Or consider a 1000W+ hub motor laced into a 20" wheel !

 

[dogman dan]

The fact that a mid drive can use the largest cog rear gear you can find is what makes it so superior on a hill. Even with heavy weights, 350w mid drive can get in a 6 mph gear, and grind up the hill while keeping the motor rpm high, running efficient and cool. Meanwhile, even with 5-1 ratio in a geared motor, and 1000w, you can load it down to a point where a 10% grade of any length is a problem. It sounds like your load will not be that heavy,, but my rule of thumb for load and geared motors is keep it below 300 pounds for rider, bike including the motor and battery, and cargo. Too much load and you will slow to less than 10 mph, which puts the motor in a rpm that makes heat with that much load.

In other words, if you weigh over 250,, go for the mid drive for sure. Your hills are not extreme, but since you will not be pedaling, best to err on the ok side.

Get a bafang BBs02 kit, which will have 750w of power on 36v. If that much power is illegal where you are, the 350w one will still get up those hills. Mostly this is because you cannot help much with the pedaling, and even short hills close to 10% is plenty steep. If you only had 6% hills, a 500w rated geared motor would work fine for you.

All this applies assuming you have a 26" wheel or larger. If your motor will be in a 20" wheel, then the geared motor can haul more up the hill, since the small wheel effectively changes the motor wheels gear lower,, motor rpm will be higher when chugging up a hill at 8 mph,

 

[toledobythesea]

Sorry, I've edited out all of what this post used to say after I realized I had made a major blunder. Will try to correct it in a day or so. Sorry if I wasted anybody's time.

 

[toledobythesea]

I deleted this mixed-up post and put what I hope is a simpler one at the current end of the thread which might help indicate that I've read all the other posts so far, with appreciation for the experience they reflect, Tried to be a little more precise, realizing that sometimes I may have misled, for example by saying 'motor' when I should have said 'rotor',

 

[DAND214]

Try tis, the mid-drive motor runs at a high rpm all the time. I t is geared down inside the housing so the chainring can run at normal speed most of the time. You change the rear gear to adjust SPEED or TORQUE. Climbing you use the largest rear cog and to go faster you use the smaller one. All done not changing the throttle. I too get lost in longer posts. I don't own a Mid-drive but hope to get one soon. Only draw back with them is the chain and rear gear wear, other they are much better for your application.

Also long retired auto mechanic, not technician.

Dan

 

[dogman dan]

It's definitely all about keeping motor rpm high enough when going up hill heavily loaded.

One of the gripes I have about mid drives, is that riders may think the motor is happy, in a low enough gear when its not. Shimano is developing auto shift, where the motor will tell the rider to shift down. Not a new idea really, but a light the rider ignores is not enough, now they are looking at the bike shifts down for you when the rpm is wrong for the motor.

The other way to solve the problem,, a pretty popular one here anyway,, is just put a big honking direct drive motor on the bike, and let er rip. I confess to doing this myself. My monster touring cargo bike had a 2000w motor that could get me up my local mountains at 15 mph, in an efficient motor and wheel rpm.

Currently that motor is on this bike, which fears no hill even though I live in the rocky mountains. But when I take it out, I still pedal all the way, and typically actually run below 300w. But over San Augustine pass or Emory Pass, I can let er rip with 2000w.

So there are many ways to kill the hill, is all I'm saying.

 

[toledobythesea]

Thanks again for all of the comments and simplification suggestions. Rather than try to prove a grander theory, let me try to describe just the observation that led me to posting this thread in a way I hope is simpler than my previous posts..

You could say my motive is to compare apples to apples. I mean comparing the choice of a hub-driven bike versus that of a mid-driven one solely in terms of the two different drive trains, keeping all other factors equal. So to compare mid versus hub, I assume both have an identical motor except that on one bike the motor housing is connected to the wheel by spokes and on the other bike the housing is connected directly to the crank by a shaft. Otherwise the two bikes are identical, for example they have the same wheel diameter.

When people talk about a higher motor rpm being generally desireable both for torque and efficiency aka endurance, I presume they really mean the motor's ROTOR rpm. When they talk about speed I presume they mean wheel rpm.

On a mid-drive bike the motor drives the crank and indirectly through the chainring it drives a rear cog which in turn drives the wheel. For argument's sake suppose a 52T chainring and 31T large rear cog so approximately 3 crank turns gives 5 wheel turns. Now assume both bikes have the same wheel speed. The mid bike motor's rotor must turn at 3/5 or 60% of the speed of the hub bike's rotor.

So if the goal is solely to maximize rotor rpm, obviously the hub drive bike has the higher rotor rpm for a given wheel speed.

With regards to torque I would think the mid-drive bike is in effect turning a larger wheel so it must make up for the torque it loses because of its lower rotor rpm, which would mean higher current and heat and probably lower efficiency, at least for the lower wheel speed ranges I'm interested in (less than 20kph). The ebikes dot ca simulator seems to indicate similar, also showing showing close to twenty percent extra battery power use on the mid-drive for the scenarios I've looked at.

 

[Hillhater]

You need to do more research on how a mid drive motor is constructed.
None of them drive direct to the crank at 1:1 ratio, they all have reduction gearing built in such that the motor rpm is many times higher than the crank rpm which needs to be kept low at 80-100rpm such that the rider can add pedal power if necessary.
So mosr mid drive motors will always have a much higher rpm than a dd hub motor

 

[Alan B]

By way of example, the Bafang BBSHD mid drive has two stages of reduction between the motor and the crank, as most mid drives do. So the overall ratio from motor to crank is far more than 5:1. Bafang has stated 21 and 30 to one reduction in the drive. Not sure what the variation is for, but they do have several models. On my RidgeRunner the chainring is 42T and the sprocket is 34T. So in first gear you don't lose much.

 

[tomjasz]

It's definitely all about keeping motor rpm high enough when going up hill heavily loaded.


But over San Augustine pass or Emory Pass, I can let er rip with 2000w.

So there are many ways to kill the hill, is all I'm saying.

What is your hill killer motor!
Thanks!

 

[dogman dan]

It's rarely apples to apples, because the mid drives usually do have a reduction gear. There are some that don't, they have a coaxial motor on the crank, so when the motor axle turns, it's the crank spindle turning, with a freewheel on each pedal. In that instance, it's 1-1. However they still tune the motor winding rpm so the efficient rpm is not the same as the efficient rpm of a hub motor. So you see,,, its always apples to arugala, not even apples to oranges.

But nevertheless,, there is no question that hub motors in wheel need to reach a certain wheel speed to run efficiently WHEN THE LOAD IS HEAVY. When the load is light, they can run efficient slow as you can wobble along. It's dang complicated, and to confuse you even more, low rpm hub motors do exist, and do work good for hills since when they lug down to 10 mph, that can still be in a fairly efficient rpm compared to a faster wind motor in 26" wheel.

But even so, the winner of the hill climb contest if power is limited to 1000w, is a very geared down mid drive, so the rotor of the motor can wail at it's most efficient rpm while crawling up a hill heavily loaded at 5 mph.

More ways to skin the cat than you can imagine.

My big motor is just an old crystalyte 5304. People have run them at 5000w, 3000w is common too, but I keep mine "stock" with a 48v 40 amps controller, as many were sold back in the day. 2000w is quite adequate for emory pass, which is 10 miles long of 8%, with 10% the last mile. I climb efficiently at 15 mph no sweat. I pedal, but only 100w worth, so 5% is pedal.

The more common choices now, since the 5304 is not made anymore are the current large crystalyte, the 3000w muxus, and a large Leaf motor.

The common factor here with those big motors is that they have a ton more copper wire in them, and much wider magnets. It's just that simple, like a v8 compared with a 4 clylinder. Twice the power with double the magnets and copper. These big motors can run very effieicnt, unlike aV8. But like a V8, if you hammer down the throttle, it sucks down all your gas/battery fast.

 

[toledobythesea]

With so many people making the effort to say in effect that I was wrong although not exactly in so many words (you've all been more polite than that), I had to revisit my assumptions. Now I see that those assumptions were dead wrong - given the available geared hub motors which don't appear to exceed a 5:1 rotor to output rpm ratio, there is no apples-to-apples equivalent to a mid-drive, at least to one of the Bafang-style design, so thank you all again.

My chief blunder was discounting the mentions of the double or two-stage reduction from rotor to crank. Also I hadn't realized Bafang even had an English-version of its website and now I see it quotes total reductions ranging from about 20:1 to over 30:1, which are reductions that I believe no geared hub motor can come even close to. The potential efficiency of those striking reduction ratios seems especially advantageous for my low pedal power/slow pedal speed.

So I take it all back, all of you were right and I was wrong but this is still progress for me because I can now see the difference in the basic physics of the two kinds of motors in terms I'm comfortable with.

 

[Voltron]

Being able to see where ones thinking went wrong is the best kind of human progress

 

[dogman dan]

You are still on the right track though, and always were. if you want to keep power on the low side, then you need the lowest possible gear for both the pedaler and for the motor, to climb steep hills when the bike, you, and any cargo will be much over 300 pounds.

The only way to lower the actual gear ratio of a hub motor is to make the wheel smaller. Which results in many of us building bikes that look a bit like this.

In this bike, I wanted to still get up the big mountains easily and efficiently with a hub motor that only has 1000w of power. So I chose a low rpm hub motor, put it in a 20" wheel, and then built the bike around it.

It's low geared then, and has a motor that likes a lower rpm. it gets up the mountains fine, but slower than the high powered bike. It's top speed is only 18 mph, so it's able to run under a heavy load at 8-10 mph, while the same motor in 26" wheel would be inefficient at 8 mph under heavy load.

This bike works well, but it's hardly a substitute for a mid drive for getting heavy weights up a mountain, or short but very steep hill. it's a bit like having a car that has only second gear. Works great on the farm, but don't take it on the freeway. It's limited by it's low gear. Just another experiment done because I have a garage full of hub motors, but no mid drive. It does work well though, for what I do with it. Mostly it goes down a long steep hill to the Walmart, and comes back towing a trailer loaded with groceries. Most of my other 1000w ebikes can do this, but struggle on that hill. This one doesn't struggle.

Virtually all that choose the20" wheel solution run a faster motor, and do have a bike that still goes fast, and climbs a hill fine. With the 20" wheel, you don't really have to lower the motor winding rpm. The lowering of the gear by using the 20" wheel is plenty.

But let me make this clear,, with 750w,, most people have all they need with a regular hub motor of either type. Most don't have that heavy a load to carry. Most don't have that steep of a hill that is long enough to be a problem. Only because you are 250 pounds, and would like to climb very steep hills is it really going to be best for you do do a mid drive. If your hills were more like most areas, around 5% grade, and not over a mile long, then you could use any hub motor, including the very small ones.

 

[Bullfrog]

To over simplify things a little bit....

For a hub motor, the only way to effectively change the gearing between the electric motor and the ground is to change the diameter of the wheel/tire the hub motor is mounted in. I am currently running a MAC 12T motor in a 20" rim just to speed the motor up enough to keep it from overheating while riding off road. Advantage is if your motor fails you can still pedal your bike to get home. Disadvantage is you can't change the effective gearing without changing the wheel diameter.

A mid drive uses the bikes gears to transmit power and you can utilize the bikes gearing to change the gear ratio between the electric motor shaft and the ground. Advantage is you can use the bikes gears....disadvantage is you use the bikes gears and they wear and you can break components if you transmit too much power for too long.

Why does all this matter....well you want the electric motor to operate at an efficient rpm or it will overheat. An electric motor will produce the maximum torque at zero rpms but if you keep it at zero rpms or relatively low rpms while it is producing high torque i.e. wide open throttle, then it will produce a lot of heat and you will quickly start to melt components in your motor.

I have both.....a MAC powered bike with a 20" rear wheel and a Bafang BBSHD with a 27.5 rear wheel. I love the simplicity of the MAC hub motor but In the big picture the hub motor works best on pavement and the BBSHD works best off road. At least that is my opinion based on my experience with both the MAC hub motor and the Bafang BBSHD mid drive.

 

[spinningmagnets]

if you could fit a given mid-drive motor and gears in a hub, I think it would produce somewhat more torque than it would when mounted at the crank on the same bike

Mass-production has worked its magic by making certain products very affordable. I have a direct-drive rear hubmotor I use on flat land to impress new friends who are not familiar with electric bikes. However, the ebike I ride the most is a BBSHD mid drive using about 1500W. But even with a mid drive, I often find that I only use two gears [out of seven gear options], because the mild hills around here don't require much more than that.

There are two trends I have noticed. Hubmotors [ostensibly a one-speed], remain popular for relatively flat streets [with only mild hills]. For off-road, middrives are popular and gaining in popularity.

I don't see the off-road "mid drive trend" changing, and I foresee mid drives becoming dominant there. But....if mid drives have some type of benefit, why are they not more dominant for street ebikes? It's true that street mid drives are becoming more popular as more models become available to choose from, but...whether a kit or a turn-key ebike, hubmotors are not going away.

Sondors sold over 10,000 geared hubmotor ebikes, and last year? Pedego sold roughly 8,000 beach cruisers with a DD hubmotor. The American market is different than the EU. The land here [on average] is flatter than the EU, and compared to the EU, gasoline is much less expensive, along with the cities here being set up for cars. Two places that seem to have the most ebike retail stores is southern Florida and southern California, especially the beach communities. Property there is expensive, so parking a car anywhere near there is also expensive as a result.

In that paradigm, a hubmotor is cheaper to buy, easier to maintain, and easier to fix when there's a problem. Of course I realize that if someone lives in San Francisco or Portland, there are very significant hills. Ebikers in those locations will [of course] put up with greater complexity and cost, in order to have a powerful mid drive.

The conventional mass-production methods have resulted in a common geared hubmotor reduction of 5:1 inside the shell. The Q100H is popular and it has a double reduction which results in a higher rotor speed [more efficient?], but...doing that also limits how wide the stator can be (if we want it to fit onto standard 135mm drop-out widths).

You might be interested in reading up on the Xiongda 2-speed hubmotor. My most-desired new product for the ebike world is a larger version of that design, which uses 750W when using 36V X 21A, and has a Kv that runs 20-MPH when in a 26-inch wheel. Such a design would run at around 25-MPH when using 48V. If designed from the outset to allow oil-cooling, it is simple and affordable to avoid oil leakage.

Such a hubmotor would cost less and be easier to install than a mid drive kit. Of course, adding the second gear (activated by reversal of the motors' rotor) would make it more expensive than the common-style DD hubmotor. In this way it would occupy a middle ground between hubmotors and mid drives, in relation to cost and benefits.