hill climbing? 750W hub versus 250W mid drive

[m@Robertson]

Thanks! Maybe you can help me understand the oft-claimed mid-drive climbing advantage.

Look at enough official mid-drive+derailleur bike specs, and you'll see that most have chainrings larger than any of their stock cassette cogs. Torque-wise, this puts the mid-drive motor at a mechanical disadvantage in every gear. Also makes the both motor torque and mechanical power subject to drivetrain frictional losses.

The only reason this happens is because some mid drive motors have a secondary housing - that contains the gear reduction off the motor - which sticks out on the drive side and in turn widens the Q factor out to the right. Thats why BBSHD crankarms have an 18mm offset on the non-drive-side... to even out the pedals underneath you so they are centered again.

The existence of that secondary housing also means the chainring moves outboard. Which in turn means chain alignment is biased towards the outside of the cluster, onto the little cogs. You can't even reasonably get to the big cogs oftentimes. SO the solution to that is to have a chainring which covers overtop of the secondary gear housing like a lid on a jar thats a bit too big, and provides inboard offset to bring the chain line back to normal. This loose fit over the 'jar' can only go so small as a result. The limit is 42T. If you buy a Lekkie replacement motor cover for a BBS02 or a BBSHD they are a bit smaller and reduce the size limit to 40T. Thats the entire reason you see big chainrings on BBSxx motors, and would also see them for example on ToSeven motors.

My Smash has a Cyc X1 Pro on it, which does not have a secondary gear housing to fight with. If I recall correctly I put a 34T hiding behind the primary (kart) drive chain.

My Big Fat Dummy has a BBSHD, but thanks to the long stays on a longtail, the angles that come from that allow me to get away with murder on chain alignment, and I can use a 36T front chainring without alignment issues. You can see in the closeup how the ring is only barely bigger than that secondary housing just behind it. BTW when I finish with this post I am going outside and fitting an 11-51T rear cluster on that bike.

And here is a BBSHD with a replacement motor cover that allows me to bump down the front to 40T.

And here's a real good look at that inboard offset. This is a Luna Eclipse ring which has the most offset of any BBSxx ring on the market. It looks as if you could go smaller if you look at the secondary inside the chainring, but you can't.

Why is this better than applying the same motor torque and mechanical power directly to rear hub? What am I missing?

Gears help the rider climb a hill. We all learned this before ebikes, the first time we tried climbing a hill without shifting. They do exactly the same thing for an electric motor that is using the drivetrain to do its work just as you do.

Something @Rexlion related above pointed out a caveat I hadn't thought of, though. He test-rode a 250w Bafang mid drive motor and found it ... sucked. Which points out that mid drives are not repealing any laws of physics. If they have no power to start with, they aren't going to do much, just like a weak hub motor. Here in the USA our legal limit is triple that of a 250w EU motor (even though those EU motors typically peak at more than double that) so I'm not accustomed to seeing or dealing with such a weak mid drive.

I mentioned earlier that Bafang motor power is overstated. Here's what I mean: A BBSHD is rated for 160 Nm. Sounds enormous compared, for instance, to a Bosch CX's 85 Nm. But Area 13 put a BBSHD on its dyno and found that 160 Nm at the axle translated to 58 Nm to the ground. Its still a lot, but nothing like what you would think by reading the specs. If a 48v/28a BBSHD is only putting down 58Nm, whats a 20a / 36v BBS01 putting out? Not much. What I see very often in EU cargo bike conversions are 48v BBS02's, which from what I hear creates really good middle ground insofar as power is concerned.

 

[Stefan Mikes]

Just to make the discussion more complete, let me mention that the granny gear on the MTB is from 20 gear-inches down

 

[Jeremy McCreary]

34-105 sounds good with 500 W of assist ready. You could pedal at 20 mph with a cadence of 78, I think, but sitting upright, you'd want assistance against air drag. At a cadence of 85 in low, you'd be going 8.4 mph. Below that, the motor might not offer many watts of assistance.

Forgot to mention that I usually ride at low assist, even up hills. In most cases, my 500W, 65 Nm hub motor still has plenty of help to give at 5 mph. Usually in no hurry, but if I want to climb faster, I can up the assist and still get a useful motor response.

With a lot of huffing and puffing at way below preferred cadence, this hub-drive once climbed a half-mile 22% grade at assist 7/9 without overheating. Low gear then was 24 inches. Was still catching my breath a few minutes later when a big 50-something guy on an unmotorized 18 lb gravel bike joined me at the top, not the least bit out of breath.

Getting the feeling that there's a lot of gray area in hub- vs. mid-drive climbing ability, and some of it has to do with gearing, technique, and exertion tolerance.

 

[Jeremy McCreary]

34-105 sounds good with 500 W of assist ready. You could pedal at 20 mph with a cadence of 78, I think, but sitting upright, you'd want assistance against air drag. At a cadence of 85 in low, you'd be going 8.4 mph. Below that, the motor might not offer many watts of assistance.

Forgot to mention that I usually ride at low assist, even up hills. In most cases, my 500W, 65 Nm hub motor still has plenty of help to give at 5 mph. Usually in no hurry, but if I want to climb faster, I can up the assist and still get a useful motor response.

With a lot of huffing and puffing at way below preferred cadence, this hub-drive once climbed a half-mile 22% grade at assist 7/9 without overheating.

Incidentally, was still catching my breath a few minutes later when a big 50-something guy on an unmotorized 18 lb gravel bike joined me at the top, not the least bit out of breath.

Getting the feeling that there's a lot of gray area in hub- vs. mid-drive climbing ability, and some of it has to do with gearing, technique, and exertion tolerance.

 

[fooferdoggie]

Forgot to mention that I usually ride at low assist, even up hills. In most cases, my 500W, 65 Nm hub motor still has plenty of help to give at 5 mph. Usually in no hurry, but if I want to climb faster, I can up the assist and still get a useful motor response.

With a lot of huffing and puffing at way below preferred cadence, this hub-drive once climbed a half-mile 22% grade at assist 7/9 without overheating.

you sure it was 22% thats a hard climb for anyone. and for a bike really hard. here are pics of 20% and the last one at the top is 22%

 

[Jeremy McCreary]

you sure it was 22% thats a hard climb for anyone. and for a bike really hard. here are pics of 20% and the last one at the top is 22%

Not certain, but (a) it looked as steep as your photos, and (b) the rider who came up after me had a bike computer that put it at 22%. Several 15% grades near my house, and that climb was much steeper than those.

 

[fooferdoggie]

I calculated 23.6%. I was mystified! Then I realized that if the camera had been level, the corners of the barn would have looked plumb. Chain link fence posts on a slope? Who knows! (That bike has a seat position that would work for me.)

ya I realized it was not level it was hard to stand sideways on that slope and get it accurate.

 

[fooferdoggie]

here fixed it. fist time.

 

[AvalancheRun]

I mentioned earlier that Bafang motor power is overstated. Here's what I mean: A BBSHD is rated for 160 Nm. Sounds enormous compared, for instance, to a Bosch CX's 85 Nm. But Area 13 put a BBSHD on its dyno and found that 160 Nm at the axle translated to 58 Nm to the ground. Its still a lot, but nothing like what you would think by reading the specs. If a 48v/28a BBSHD is only putting down 58Nm, whats a 20a / 36v BBS01 putting out? Not much. What I see very often in EU cargo bike conversions are 48v BBS02's, which from what I hear creates really good middle ground insofar as power is concerned.

160 vs 58 nm is worlds apart. My two bikes are 75 and 50 nm and they are night and day in terms of climbing. Another 90 nm bike I had was again significantly better than the 75. Do you know how manufacturers typically measure torque for spec sheets?

Another example. The Priority Current is claimed to have 140 nm, but owner reviews liken it to the 85 nm CX in terms of climbing ability.

IMO the bicycle manufacturer must know this. If that were my product I would reevaluate those numbers and come up with something equivalent to how Bosch/Brose, etc. measure torque.

 

[mschwett]

Ah, that makes a lot of sense. I do everything I can to keep a cadence of 85±5 RPM just to keep my knees happy. Sounds like a mid-drive would be happy with that, too.

yes, this is the issue. all of these motors are geared way down - they spin internally much much faster. on the mid drive, you can gear them down internally with that roughly 80rpm cadence in mind regardless of the speed of the bike, because the 4-5x range of th e-bike‘s drivetrain takes care of the wheel speed. on a hub drive, you need to do the internal gearing in a way that produces usable torque all the way up to 300 rpm or so. so while there is very rarely a torque advantage (except on mountain bikes) between chainring and rear cog, the internal gearing of the motor can be designed to greater advantage on a mid drive. small mid drive motors produce much more torque than small hub drives - compare the mahle x20 to the specialized SL - 23nM vs 35 or 50nM for motors of similar weight and power rating. mahle could have geared it down another 2x Internally to make it 46nM, but then it probably wouldn’t work well at 300rpm!

 

[mschwett]

Because most bikes sold are wildly geared too high for normal people. And electric bikes make this even worse.

I think most bike manufacturers think all of their riders have thighs and quads that are bigger than most tree trunks.

nah, the real reason is that most people don’t ride up steep hills, and the people who make bikes know this.

 

[Stefan Mikes]

I was riding my Vado with the 36-46T gearing (250 W nominal, 520 W mechanical peak power 85 Nm mid-drive...)

...could stop at the road-sign then restart the ride... and was actually singing while climbing!

I was on a 58 mile ride with 5,000 ft elevation gain that involved six steep climbs...

...was taking pics of Slovak kids practicing mountain riding...

...and was racing against a Slovak roadie

All in high mountains of Slovakia.

 

[kevinmccune]

ah,vados! give it up Stef! just admit you are one those double strong east bloc superlative humans,dont believe me watch ufc?

 

[AvalancheRun]

The front belt wheel looks twice as big as the rear belt wheel, so 140 Nm from the motor would be about 70 Nm on the rear belt wheel. From there, the torque on the rear tire would depend on the gear range in the hub transmission. They call it a commuter bike, so I guess it would be geared for speed, not torque.

I found Radpower egregious. A 500 W direct drive bike of another brand would take me up a 6.1% a slope as fast as a 750 Watt geared hub motor Radrunner. I've seen a published disclaimer that their power and torque claims are what Bafang says a motor is good for, not what it produces with a Radpower controller.

Are the Bosch and Brose mid drives measured this way?

FWIW my butt dyno found perfect agreement with the way all of the Specialized, Trek, Gazelle, etc. bikes are with regards to advertised torque spec.

 

[m@Robertson]

@Jeremy McCreary says 34 gear inches is too high for some grades he pedals up. He must be working at speeds below 10 mph. A 250 W mid drive might do better than a 750 W hub if, as @Stefan Mikes says, the chainwheel is suitable. When I encountered a steep grade on soft ground with a heavily loaded Abound, I needed torque on the wheel, not speed. In that case, my direct drive hub motor was adequate.

I dropped my car (what we call a station wagon here in the States... an Estate everywhere else) to the shop to get some work done. I had that little red bike I pictured in the earlier post in the back so I could ride home. A single BBSHD motor with a middling 12ah 52v battery pack.

My normal ride home includes a 16% grade, and since I don't ride the red bike so often, I paid attention to my speed/gearing. On pedal assist level 5 (of 9) and the fifth cog up from the bottom (a 21) which gives me dead straight chain line, I traveled between 6 and 7 mph pedaling, which was easy and relaxed (probably 40-50) on that 16% segment. To see what I was leaving on the table, I hit the throttle halfway up and jumped right up to 16 mph. It had more in it - I was still accelerating - but I ran out of hill.

Gears were 40 in the front and 21 in back. Bikecalc tells me I was running 51 gear inches, and I still had 4 bigger cogs that I didn't need to try and tap into (24, 30, 37 and 46, all of which I can get to with the Box 2 derailleur that can go up to 51T). So I had loads of surplus power untapped. My cadence up that hill was fast and while I don't ghost pedal, this was my first decent ride in over a week after a light injury so I kept the workload light.

At a stoplight further on I forgot I wasn't riding a big cargo bike today and used throttle when the light turned green... On that 21T rear cog I still popped a wheelie right up off the ground and had to let off or lose it. And I weigh 235 lbs. If instead I was on say the 37T cog ...

 

[m@Robertson]

Because most bikes sold are wildly geared too high for normal people. And electric bikes make this even worse.

ABSOLUTELY this. Storebought bikes flat out suck when it comes to gearing. I think this is to cater to people's perception that if the bike is not fast on flat ground, it sucks somehow. So they gear it to go fast on flat ground. I saw a lot of first time mid drive riders in the Sondors world take the already overgeared Rock Star and want bigger chainrings on it. To make it go fast. Which also has the effect of creating chainring tacos, bogged motors etc.

When doing a bike build, fussing to get the right gearing is a huge deal. And I seldom got it right the first time on paper. Usually I had to actually ride the thing to shake out what the lab results said I should experience versus what actually riding the bike in the real world showed me.

The BBSHD: Musical Chainrings

It seems inevitable. When I build a bike, I go through front chainrings trying to get the gearing just to my liking. My Mongoose Envoy build has pretty much set the world record for tweaks in thi…

talesontwowheels.com

I wrote that back in 2020 and am still only marginally better at gearing a bike right on the first try without riding it. One thing I learned though is how to not have any of the alignment wear/tear issues I noted there.

What's too high in gear inches?

I'll argue you should ignore gear inches. Its a unit of measure everyone swore by... decades ago. In the modern world we have much better tools that give much more detailed performance answers. In particular when building a bike I would spend a lot of time with this table

Bicycle Gear Speed Cadence Table | BikeCalc

Calculate cycling speeds at a specified cadence given chainring, sprocket, tire and wheel sizes.

www.bikecalc.com

and this one.

Bicycle Gear Cadence Speed Table | BikeCalc

Calculate cycling cadences at a specified speed given chainring, sprocket, tire and wheel sizes.

www.bikecalc.com

Using these you can give yourself a very near idea of how the bike will behave across a range of its gears, taking into account wheel and tire size. It is still not perfect, but its going to get you a whole lot closer to a correct result than trying to picture gear inches, which even if you master them which is not hard to do, will still not tell you speed at cadence, or cadence at speed which I would argue are actual end results and what you really want to know.

ya I realized it was not level it was hard to stand sideways on that slope and get it accurate.

I have a clinometer app for my phone that does a pretty good job. You just lay it on the ground and read what it says. But such a short sample of a hill is bound to have a pretty decent margin of error.

 

[m@Robertson]

160 vs 58 nm is worlds apart. My two bikes are 75 and 50 nm and they are night and day in terms of climbing. Another 90 nm bike I had was again significantly better than the 75. Do you know how manufacturers typically measure torque for spec sheets?

Its all over the place and nobody is giving a straight answer. Your two bikes tell you they are 75 and 50, but is that torque at the motor axle or to the ground at the rear wheel, after its been run thru the drivetrain? This is exactly like the horsepower numbers you see on automobiles. The manufacturer tells you one hp and torque number ... at the crank. But get onto a dyno and you will get RWHP and RWTQ, which any hot rodder knows are the real numbers that matter (and need to be measured rather than taking the old-standby rule of thumb of 17% loss thru the drivetrain).

My Bafang fat motors are rated at 80 Nm and I don't believe that, either. Neither should anyone else for ANY motor measurement that hasn't been independently tested. You could argue Area 13 is a seller and thats not independent, but I know the owner well enough to be confident he is doing everything he can to be as consistent as possible in obtaining his results.

Dyno Testing Ebikes and more!

Electric Bike Dyno Testing

area13ebikes.com

Another example. The Priority Current is claimed to have 140 nm, but owner reviews liken it to the 85 nm CX in terms of climbing ability.

See above I think this is exactly like the ebike range numbers you see... if you've been around long enough you know that every one of them is a lie.

IMO the bicycle manufacturer must know this. If that were my product I would reevaluate those numbers and come up with something equivalent to how Bosch/Brose, etc. measure torque.

Who is to say Bosch/Brose is any better than all the rest of the gang of thieves/cutthroats? The engineers know, but the marketing department writes the ad copy.

 

[Stefan Mikes]

Who is to say Bosch/Brose is any better than all the rest of the gang of thieves/cutthroats? The engineers know, but the marketing department writes the ad copy.

All the big mid-drive motor manufacturers first take their mechanical motor peak power, and then divide it by 6.28 rad/s (it is 60 rpm) to get the torque figure. If the number is not satisfying for the e-bike manufacturer marketing department, the latter is advertising any number to make better sales.

For instance, Specialized gives 50, 70, and 90 Nm for their three motor models. In fact, the two weaker models have far higher torque than advertised (68 and 75 Nm). It is done to attract the buyers to choose the most expensive e-bike model. (The strongest motor is indeed 90 Nm).

 

[tomjasz]

(It looks @Jeremy McCreary has Ignored me).

Just one of many...

 

[Mr. Coffee]

ABSOLUTELY this. Storebought bikes flat out suck when it comes to gearing. I think this is to cater to people's perception that if the bike is not fast on flat ground, it sucks somehow. So they gear it to go fast on flat ground. I saw a lot of first time mid drive riders in the Sondors world take the already overgeared Rock Star and want bigger chainrings on it. To make it go fast. Which also has the effect of creating chainring tacos, bogged motors etc.

I think a lot of it is because the bicycle industry is really bad at listening to customers who aren't racers. And bluntly in a lot of cases they don't care about your opinion unless you are a super athlete.

The near-extinction of 2x (and to a lesser extent 3x) drivetrains is a symptom of this problem. A strong, fit, and experienced cyclist can make a narrower range 1x drivetrain work for them. A beginner or weekend warrior will struggle on all but the gentlest hills. Pushing a bike up a hill is not very fun and if your introduction to cycling includes a lot of pushing your bike uphill you aren't very likely to stick with it.

E-bike manufacturers, in a lot of cases, basically have engineered systems where you'll have to push your 50lb bike up the longest, steepest hills where with an acoustic bike at least you're only pushing 25lbs (or maybe quite a bit less).

One reason I go with alternate drivetrains like Pinion and Rohloff is at least there you can get a decent gear range. I was working with a LBS on a modified Surly Bridge Club build for a friend and we got totally stuck on finding a suitable 2x drivetrain, and it was a deal breaker for her to not have one. Which is a shame because that is an otherwise excellent all-around bike at a reasonable price.

Another bike I really kind of like is the Otso Fenrir, but it suffers from a similar issue with respect to drive trains and gearing.