hill climbing? 750W hub versus 250W mid drive

[mschwett]

DC motor y potencia - Electronica

Here's what a DC permanent magnet motor would do without a controller, although a hub motor isn't wound to turn nearly so fast. At the low end, torque ( current ) would be so high that the motor would be wrecked in seconds.

Why Ebike Motor Watt Ratings Are Meaningless

Ebike motor watt ratings are almost meaningless: the same hub motor is sold as 250W, 500W, and 1000W, with no standard way to rate motor power. Here's what really determines a motor's power, and why torque and heat matter more than the number on the label.

ebikes.ca

Here are charts of a hub motor with two controllers. In both cases, maximum torque is about 3 times higher at 0 rpm than at max power; I've seen similar results with the two bikes I measured at different speeds. Note the knee in the torque at max power. That's where the controller quits restricting torque (amps) and lets back emf take over. If the controller didn't restrict current to the left of the knee, it would rise as steeply as it falls to the right of the knee, burning out the motor very quickly at low rpms.

A flat torque curve seems to be a feature of mid drive controllers. With a gearshift, a motor doesn't need extra torque to reach an efficient operating speed.

that’s interesting. essentially, the controller can be tweaked to produce different amounts of torque at different speeds, resulting in power peaking at a different speed than torque except by coincidence.

the torque curve also explains why electric motors are so good for accelerating from a stop, although i am not sure the torque continually
decreases from a peak at 0 rpm as shown. probably an oversimplification or programming intended to illustrate a particular result.

 

[Jeremy McCreary]

the torque curve also explains why electric motors are so good for accelerating from a stop, although i am not sure the torque continually
decreases from a peak at 0 rpm as shown. probably an oversimplification or programming intended to illustrate a particular result.

I've worked with a lot of brushed permanent magnet DC motors as a hobbyist. Those torque and power curves are pretty accurate for real-world motors of this type — including those with internal gearing. Peak torque and no mechanical power at the shaft at 0 RPM. No useable torque or power at no-load shaft speed (NLS). And peak mechanical power and ~50% of stalled torque at ~50% of NLS.

Allowing for controller-induced distortions, brushless DC motors seem to follow similar curves.

 

[m@Robertson]

I find gear inches very useful

Yeah there's nothing wrong with knowing what they mean. Especially as an academic exercise. But for comprehensive results, the tools we have available to us now are better suited to giving me a bigger picture at a glance.

Here's an example I just went thru over the last couple of days on my Big Fat Dummy. 36T front chainring and an 11-46T rear cluster. Because of the 2XL 5.05" tires I just put on, I have lost my two biggest cogs (the biggest one was already gone with the XL tires I had). This is an 11s so I am down to 9 speeds which is still good. Subtracting the two big cogs, this is what the bike's behavior is like.

So, that behemoth of a bike is still quite capable of being pedaled to 24-25 mph, and it should be throttleable to 26 or 28. The 36T largest cog in the back gives me 6 mph in the ballpark of my preferred cadence, which is about 70. I can probably get it up to 7-ish which is OK on a super steep road, and on the fast side for when the bike is really challenged in the woods where oftentimes there is no trail at all and I am going overland. So... room to improve at the low end. So what happens if I take a step up and change to a 11-51T cluster, where the biggest usable cog is still 3rd from the top, increasing to 39T?

No change on the high end of course, and on the low end we're closer to 4-6 mph, which is a little more like it on an unimproved hillside or forest floor, loaded with firewood in the back. But thats still a little fast, and riding the bike after the change, its working pretty hard on that lowest gear on the steepest stuff I can test it on. A mid drive is at its best when its never bogging. So... What if I went to the lowest front chainring I can put on this bike: A Lekkie 28T?

I've lost some of my top end this time and realistically this is an 18 mph street bike on flat pavement. Maybe 22 on throttle. Thats fine. More importantly at the super low end, when I need torque the most, I'm given what amounts to a wider available power band, both for pedaling and for throttling. Heck... with 4 36 paks of soda on the bike (about 125 lbs) on pavement... 8 mph is too much. Going 4-6 on forest floor is plenty.

So that told me to spend the $75 on the 28T cog and hopefully it will be here soon.

Fussing with gear inches just doesn't give a big picture like this. Again... nothing wrong with using them and knowing them but for me their use is no longer practical.

 

[Daffyh]

For those who have ridden up hills on both a hub and a mid drive, how do they compare in terms of how easy and quick they are to make it up the hill? I have a 750W geared rear hub with (IIRC) 90 Nm torque, and torque sensor at the crank. Thinking of getting a Bosch mid drive with 250W, 65 Nm torque. Assuming the same gear inches for the gearing (and let's suppose we use the peak PAS since it's a really steep hill), which one will pedal easier up the hill? TIA. Mainly I just want to make sure I don't buy the mid drive and then regret it, which I would do if it made life harder on the inclines.

I enjoy bicycling, but if I wanted to work my tail off during the climbs I'd just ride an acoustic!

Mid drive any day of the week with decent gearing

 

[Jeremy McCreary]

You have to count and input the number of teeth on each of 10 sprockets. I just count revolutions. You have to input the rim diameter and tire width. I just measure the height of the axle.

If I count chainwheel teeth, a spreadsheet can show the cassette teeth in each speed without the fuss of counting all those sprocket teeth.

Many ways to skin this cat. All I really to need to tweak an existing drivetrain for my knees and riding conditions are the teeth on the chainring and the largest and smallest cassette cogs. All that's known from readily available specs. No counting involved.

Once I calculate the existing top and bottom gear-inches, I do many varied test rides with those 2 figures in mind. If the bottom gear isn't low enough, or the top isn't high enough, I can then make reasonable guesses as to the top and bottom gear-inches to try next.

If you're not too picky about cadence, you could probably get by with pedal feel in the testing phase. But my knees are very picky, and a real-time cadence readout on the handlebars helps me know just what they'll tolerate. I use a cheap Bluetooth sensor strapped to my left crank. RideWithGPS on my phone provides the readout.

Back home, I put the guesses into my gearing spreadsheet and predict the top and bottom speeds they'd give at acceptable cadences. The tables @m@Robertson linked would also work well here. This step prevents glaring errors, but as he pointed out, you still have to confirm in the saddle.

As for the cogs in between, I've been happy to let Shimano space them. From the online cassette data I've checked, they do it pretty much exponentially to give an evenly spaced feel at the pedals. Within rounding error, that means a 16% jump in teeth from one cog to the next larger on my 11-42t Deore cassette.

 

[teskow]

If you gear a bike like a tractor it will clime any hill easily able it very slowly. That is where a transmission comes int play.

 

[m@Robertson]

You have to count and input the number of teeth on each of 10 sprockets.

No you don't. You just cut and paste from the specs. Since the exercise described is shopping for clusters you'll already have them in front of you. Like this:

Amazon.com

I just count revolutions. You have to input the rim diameter and tire width. I just measure the height of the axle.

I click on "26" and 2.0" from the dropdowns on the page (I will also say I have to click Enter so you don't have to make another post saying I left out that additional step).

Once again like earlier in this very same thread, you are bending over backwards to make things up that you pretend are significant so you can start and then continue a meaningless argument.

I think a little picture of 14 speeds is better than a big picture of 117 cadences.

Thats nice. I think everyone else knows implicitly they can ignore the stuff in those tables that isn't relevant, like most all of the boxes that show cadences that are not humanly possible (or so low they are undesirable). I wouldn't try to reach 26 mph on the big cog so I don't have to burden my dim intellect with wondering what the consequences of a cadence of 383 are.

But by all means, as I already took great pains to say more than once, there's nothing wrong with using these if thats what you want.

 

[m@Robertson]

As long as I'm noting results-oriented stuff here, this is the Sheldon Brown gear-inch calculator I used for years before leaving them behind.

Sheldon Brown's Bicycle Gear Calculator

A JavaScript calculator for analyzing bicycle gearing

www.sheldonbrown.com

and BikeCalc has one too.

Bicycle Gear Inches Table | BikeCalc

Calculate gear inches given chainring, sprocket, tire and wheel sizes.

www.bikecalc.com

You will have to type into the form or pick from some drop-downs, and press Enter.

 

[Fingers]

For those who have ridden up hills on both a hub and a mid drive, how do they compare in terms of how easy and quick they are to make it up the hill? I have a 750W geared rear hub with (IIRC) 90 Nm torque, and torque sensor at the crank. Thinking of getting a Bosch mid drive with 250W, 65 Nm torque. Assuming the same gear inches for the gearing (and let's suppose we use the peak PAS since it's a really steep hill), which one will pedal easier up the hill? TIA. Mainly I just want to make sure I don't buy the mid drive and then regret it, which I would do if it made life harder on the inclines.

I enjoy bicycling, but if I wanted to work my tail off during the climbs I'd just ride an acoustic!

Hub motor vs mid-drive...it's a fascinating discussion. Let me give you an engineer's point of view. When it's torque vs RPM, the mid-motor is on your side. If you're chugging up a lot of hills (like we do the minute we pull out of the driveway here in Sicily) you usually want to be pedaling at your normal cadence (usually 50-70 RPMs) especially if you intend to go more than 1-2 miles. When I first got here I had a Prodecotech Phantom X with a monstrous 500W hub motor. When it had the factory installed 42-tooth single speed front sprocket it could fly down the road, but it couldn't make it up some of the hills here; at all! I had to get off and walk the bike up. I changed the front sprocket to a 32-toothed and I found that I could make it up most of the hills, but I still had to stand up [on the pedals] to make it all the way up some of them. I would make it to the top - going about 5 mph, huffing and puffing the last 300 yards. My wife, on her Giant 250 watt mid-drive, would just cruise by me at 12 mph, pedaling at about 50 rpms (she has a 30-tooth front sprocket and our rear sprockets have about the same range). At 0-5 mph, the hub motor has no torque because it is turning at such a low rpm. If you look at any DC motor power curve you will see that they don't have much torque at low RPMs. That's why my wife's 36 volt, 250W mid-drive could go further, faster and up steeper hills than I could on my 36 volt, 500W hub motor. There are those that would tell you that if you kept up your speed at the base of the hill (going at least 25 mph) chances are a hub motor would get you up the hill...but it has to be smooth pavement and a short hill, two things not very common here. (And, her bike is much lighter too)
After the second controller on my hub motor finally burned up, I bought a 250W, mid-drive, off-road [rated] bike on huge, 29" knobbies and I have never looked back. We can go about 25 miles over solid hills on a single charge (depending on where you set the power at) and 50+ miles on the flats against a head wind (power settings 2-3). Bottom line, if you have a lot of hills, and could afford it, go mid-motor. Also note that mid-motor bikes don't have throttles...no pedal, no go. This does make it tough to start going up a hill.

 

[gromike]

Non-Euro mid-drive motors do have throttles. There are advantages and disadvantages to both motor types, so it's up to the individual. I have a 750w Bafang mid-drive bike, and a 500w direct-drive hub motor bike, and both are great ebikes.

 

[Chargeride]

The 250w limit is a nominal figure designed for legal reasons, midrives can go up to 1000w and still be legal, in fact there is no limit to 'peak' power.

The enforcment of the regulations surrounds speed and pedal assist in practical use.

My 2200w geared hub will beat my 800W mid drive up to the point where the steepness of the incline reduces rpm enough to make the hubs power drop off drastically.
Hubs are single speed, this had advantages and disadvantages.

 

[Stefan Mikes]

midrives can go up to 1000w and still be legal, in fact there is no limit to 'peak' power.

Europe limits the peak power to 600 W mechanical because of Austria, the only EU country to limit the actual but not nominal e-bike motor power.

 

[spokewrench]

At 0-5 mph, the hub motor has no torque because it is turning at such a low rpm. If you look at any DC motor power curve you will see that they don't have much torque at low RPMs. That's why my wife's 36 volt, 250W mid-drive could go further, faster and up steeper hills than I could on my 36 volt, 500W hub motor. There are those that would tell you that if you kept up your speed at the base of the hill (going at least 25 mph) chances are a hub motor would get you up the hill...but it has to be smooth pavement and a short hill, two things not very common here. (And, her bike is much lighter too)
After the second controller on my hub motor finally burned up, I bought a 250W, mid-drive, off-road [rated] bike on huge, 29" knobbies and I have never looked back. We can go about 25 miles over solid hills on a single charge (depending on where you set the power at) and 50+ miles on the flats against a head wind (power settings 2-3). Bottom line, if you have a lot of hills, and could afford it, go mid-motor. Also note that mid-motor bikes don't have throttles...no pedal, no go. This does make it tough to start going up a hill.

Why Ebike Motor Watt Ratings Are Meaningless

Ebike motor watt ratings are almost meaningless: the same hub motor is sold as 250W, 500W, and 1000W, with no standard way to rate motor power. Here's what really determines a motor's power, and why torque and heat matter more than the number on the label.

ebikes.ca

The torque of a permanent-magnet motor varies with the current, and current is maximum at zero RPM. If you scroll down to the first pair of graphs, they show the idea. If it were just a battery powering windings, the torque would be a straight diagonal line, from the peak at a stop down to zero torque at the motor's maximum speed. The curves and knees on those lines come from the controllers.

Suppose that if fed 36 volts at 6mph, your motor could draw 14 amps, and you had a 14-amp controller. That would mean that instead of increasing as a hill slowed you below 6 mph, the torque would stay the same from 6 mph down to zero. If instead got less torque at 3 mph than at 6 mph, that sounds like a defect in the controller.

My first e-bike had a hub motor advertised at 80 Nm. I tested it by finding a driveway it would climb only at walking speed, where torque would be near its maximum. Calculating my gross weight and the steepness of the hill, I found I was getting only 53 Nm. Bafang had obviously tested the motor with a 25 A controller, but the e-bike manufacturer had installed a 16 A controller. I put on a 35 A controller, which gave me plenty of torque for hills, and the motor never felt hot.

If the motor couldn't handle more than 16 A, then I agree: it could have put the necessary torque on the wheel as a mid-drive.

 

[gromike]

I think he's confusing torque with power. If you have 100 Nm torque at zero rpm you have zero power, at least as far as the wheel is concerned in making you go. You want the motor turning at peak efficiency, and yeah, that's where a mid-drive can shine, especially in comparison to a direct-drive hub motor.

 

[gromike]

And I'll add to the above that I prefer a rear direct-drive hub motor. The only downside for me is that dang heavy rear wheel. With pedaling I can fly up the hills. My biggest hub motor is labeled at 500w, but is fed up to 1440w. It helps that it's in a 3x9 gearset bike.
With direct-drive hub motors the mantra is more amps!

 

[gromike]

....that is until you smell that smell.

 

[Fingers]

Back to my engineering point of view...power is power, and, as you might already realize, if you want to do less work, or go faster, more power is better! No argument there.

Now, for some simple math. 1 hp = 745.7 Watts This means that a 750W motor is about1 hp, so a 500W motor is about 2/3rds of a hp, and a 250W motor is about a 1/3rd hp. The average human (in decent physical shape) can put out about 1.5 hp instantaneously (like a dead lift), 1 hp for about 1 minute and .5 hp indefinitely. [I am familiar with these numbers because for many years I was heavily involved with HPVs of all types.] And hp = torque x RPM. What does this mean? If you want more hp going to the rear wheel, just pedal harder [more torque] or pedal faster [more RPMs]. This also means that with limited power, and you want to increase torque to the rear wheel, you have to lower the gear ratio. This also means that if you want more power than you can provide, just get a bigger motor. I [myself] just use longer cranks to increase my torque to the rear wheel. (It also doesn't help that I weigh more than 25 kg than my wife!)

I do wish we had a throttles on our e-bikes, because without one it makes a huge difference where and when you come to a stop. Stopping on a steep, uphill grade can make your ride interesting when it comes time to get back on and to get going again. Some of the better mid-drive e-bikes do have a 'temporary motor boost' on their controls (just for starting up hills) but [as far as I can tell] ours do not.

All this really means is that any e-bike is what you want to make of it. We prefer the 250W motor because it means we still have to do some work, and after the age of 65, the amount of physical exercise you get in on a daily basis makes a difference. I also like to brag that 'no matter how I feel first thing in the morning when I get on my bike, I have a button [power setting] for that!'

 

[spokewrench]

I do wish we had a throttles on our e-bikes, because without one it makes a huge difference where and when you come to a stop. Stopping on a steep, uphill grade can make your ride interesting when it comes time to get back on and to get going again. Some of the better mid-drive e-bikes do have a 'temporary motor boost' on their controls (just for starting up hills) but [as far as I can tell] ours do not.

All this really means is that any e-bike is what you want to make of it. We prefer the 250W motor because it means we still have to do some work, and after the age of 65, the amount of physical exercise you get in on a daily basis makes a difference. I also like to brag that 'no matter how I feel first thing in the morning when I get on my bike, I have a button [power setting] for that!'

When I bought my first e-bike, I thought of electric motors as being nearly 100% efficient. I found a hill where on motor alone it would accelerate to a certain speed and no faster. With no speedometer, I marked 50 yards and used a stop watch to see what that speed was: 11 mph. That was a good test speed because in still air, there would have been little aerodynamic drag, and it was fast enough for the 20mph motor to produce nearly its maximum power.

I knew my gross weight, measured the grade, and found that the motor was producing about 2/3 hp or 500 watts. I believe a motor rated at 750 watts is supposed to produce 750 mechanical watts indefinitely without overheating, but Radpower meant their controller was feeding the motor 750 watts. That’s like saying a 100 hp gasoline engine is one that can burn 2.23 gallons of gasoline an hour.

I couldn’t climb that hill faster because going faster would mean less torque. An e-bike motor produces maximum horsepower at a speed where torque has dropped to about a third of its maximum. If you ride where hills often slow you way down, a mid drive would, by letting you maintain an efficient motor speed, greatly increase battery range and reduce heating.

My newest e-bike came with torque-sensor PAS. At its lowest setting it gave me much more assistance than I wanted, and if I turned PAS off, the throttle didn’t work. When the BB bearings acted up, I installed a bottom bracket with no torque sensor and changed the lever throttle for a half twist-throttle. It’s much better than PAS. Mostly, I pedal without assistance, but if I’m slowing on a grade, I can use the throttle momentarily to bring my speed back up or continually to add just a little assistance.

Throttle without PAS is very helpful when pedaling for exercise. In April, I was going about 300 miles per charge, meaning my legs were generating almost all the energy. In July, when the pavement could be 145 F and the dew point 74 F, I was getting about 150 miles per charge. I depended more on the throttle to stay cooler instead of producing all that kinetic energy myself. Even if I were to use the motor continuously, pedaling would let me minimize the amount of throttle and still maintain an efficient speed for the hub motor.

 

[Jeremy McCreary]

My newest e-bike came with torque-sensor PAS. At its lowest setting it gave me much more assistance than I wanted, and if I turned PAS off, the throttle didn’t work. When the BB bearings acted up, I installed a bottom bracket with no torque sensor and changed the lever throttle for a half twist-throttle. It’s much better than PAS. Mostly, I pedal without assistance, but if I’m slowing on a grade, I can use the throttle momentarily to bring my speed back up or continually to add just a little assistance.

Throttle without PAS is very helpful when pedaling for exercise. In April, I was going about 300 miles per charge, meaning my legs were generating almost all the energy. In July, when the pavement could be 145 F and the dew point 74 F, I was getting about 150 miles per charge. I depended more on the throttle to stay cooler instead of producing all that kinetic energy myself. Even if I were to use the motor continuously, pedaling would let me minimize the amount of throttle and still maintain an efficient speed for the hub momotor.

Sounds like we have similar riding habits. The torque-sensing assist on my 500W, 65 Nm hub-drive is nicely implemented. But like you, I wanted less help at lowest assist.

Luckily, my display menu offers an easy solution. The bike comes set for 5 non-zero assist levels with options for 3, 6, and 9. Stock lowest assist 1/5 was too much, but significantly lower 1/9 is just right.

The "throttle on top of pedaling" option you mentioned deserves more attention. To take full advantage, you need torque-sensing assist and a progressive throttle that simply adds metered power to whatever the assist is doing at the time.

Yes, you could just up the assist, but the throttled power boost is instant, precise, and totally predictable in both timing and amplitude. Often more convenient as well. And pedaling all the while.

Among many other things, great for keeping cadence and speed over brief ostacles like steep driveways or roots on trail climbs. And as you noted, for keeping up hub motor (= wheel) speed on climbs that would otherwise bog the motor down. A few seconds of throttle on top of pedaling is usually all it takes.

In short, I see my throttle as an occasional adjunct to pedaling, not as a substitute. With knees that don't like strong accelerations, giving up this functionality for the advantages of a mid-drive is a tough sell.