Just more than a bike (low power / low consumption)

[donn]

But a low wattage motor can pull a very fit guy up a hill, folks that can pedal up 8% at 10 mph only need 5 mph of assist to get into that efficient hub motor rpm. Most of us though, walk a bike up 8%, that's why we bought e bikes.

Well, of course there's a lot of bicycling that falls somewhere between 10mph and getting off and walking, thanks to the widespread adoption of variable gearing in bicycles. I don't get off at 8%, but I probably don't exceed 4 mph.

The reason most widely cited why we don't universally use that gearing in bicycle motors - crank drive - is that the motor delivers power considerably in excess of normal human effort, and that can easily mess up drive train components that are engineered for human forces.

The reason we can get away with not using the gearing, is that the motors are so powerful that they can operate rather inefficiently and still get going fast enough to stay out of trouble.

In the very eccentric case where you really want a more feeble motor, then, a crank drive configuration seems to be a much more obvious choice. I think it's kind of funny how rare that configuration actually seems to be, because it seems like it would be very desirable, not to mention legal worldwide. Maybe it's because we so often go looking for motor solutions for the extreme case - "oh, by the way, I want to pull 100 lbs of camping gear up long 15% grades."

 

[biphaeboh]

Last year I rode a gravel road tour on the Forestry Trunk Road in Alberta, on my non electric touring bike. I was carrying about 40 lbs of gear. The steepest hill was a maximum grade of 13% for about 9km. I think I was going about 5 km/hr. Any slower and I would have fallen over.

I was wondering how my GMAC 10T on my vintage Stumpjumper would have fared. Could I find a level of assist low enough to not stress the motor and still be worth while to push me up the hill faster? The whole trip consisted of steep loose gravelly uphills, followed by steep down hills. So re-gen would have had some effect.

A bit of a moot point as the next charging point was 180 km ahead and would have involved staying in a fairly expensive lodge rather than camping for free.

I just can't quite figure if an ebike could be used effectively on such a tour. Maybe, but you would have to be fairly miserly about the level of assist.

 

[2old]

I rode a Faraday front hub bike for about a month and really enjoyed it's light weight (40 or so ponds AIR) and maneuverability. Currently ride off road on a BBS02-equipped hardtail. but have a 23 pound Niner hardtail just waiting for a light weight mid motor in order to produce a 30 pound (or so) bike using a 52V, 4ah (3 1/2) pound battery.

 

[dogman dan]

GMAC 10T would overheat under that condition. Mostly because the grade would take a long time to climb.

But if you are fit enough to ride up that grade with no assist, that loaded up, of course you would go up that grade faster than 5km/hr. 9km would be plenty of length of that load to get the motor very hot. But if you kept the assist low enough, it might not be enough heat to melt your hall sensors by the top. And weather matters too. When I set out to test motor limits, I would ride on a 105 degree day.

Think though, about what happens in either case if you are riding in an rpm that makes 50% of the wattage into heat. Feed in 200w, and you have a 100w heater. 100w takes a bit longer to heat the mass of the motor than if you feed in 1000w and have a 500w heater.

But its quite clear, if you want to climb mountains with a load of camping gear, with a 350w or less capable system, its gotta be a mid drive, or a hub so small its gearing is similar to a low gear in a mid drive.

I have only actually owned one mid drive. It was a 250w brushed motor. Even in its lowest gear on flat ground, it tended to blow its controllers. It was too small a motor, causing such a surge of power on each start that the controllers would blow. It was just overloaded for the components that existed back then in the brushed motor days. It's all much better now, but it shows how too little power can stress the whole system. Better to have 1000w system and run it at 200w in my opinion, than have a 250w system. I mean for cargo + huge hills.

I got aimed in that direction right away when I started riding e bikes. I live 1000 vertical feet above where I worked, and used to take a bus half way home all summer, so I would not melt down the brushed motors you got back then. Jut me at 200 pounds, a few groceries, and that hill to home would melt the motor if the temp was above 90F that day.

Others don't live in the desert, or have mountains in town. I did, and started needing more than 500w immediately, and better motors than brushed crap.

 

[biphaeboh]

I think you may be right, Dan, but I have been playing with the motor simulator and I think perhaps I could do it.
With a low level of assist and pedaling hard I could get up the hill and not overheat the motor. If the simulator is to be believed.

I'm not going to try it though.

I am trying to imagine taking an ebike down the Forestry Trunk Road. You would have to have a lot of batteries even if you had a setup that wouldn't melt the motor. The combination of steep gravelly hills and large distances between charging points would make it challenging.

It is a good thought experiment.

 

[dogman dan]

One thing about the geared hub motors, you can't just stop and cool one very easy. All kinds of quick ways to cool a DD motor, like put a rag on it and soak it in water. A few very small holes can help air cooling happen quite fast when you stop. Stop to cool a geared motor, and the shell cools off while the inner core remains quite hot, for a really long time.

Change that rig to a geared motor in front, and another in back, and now you can climb anything. Because with double motors you never have to ride slow enough to make them hot. Once over the hill and on the flat, you just let that front motor coast.

In general, once you are talking hub motors, the best bet is to have lots of power up the hill. This gets you up with max efficiency if you ride up the hill about 15 mph. Not fast enough for wasting much on aero, not too fast to pedal hard to help, and not too slow so the motor makes heat. Quick up the hill saves since you take less time drawing more than 200w. Once over the top, resume using only 200w.

I climbed my test mountain many many times with different hub motors. 5 mile climb, the last 3 miles 5-8% The wh took was always nearly exactly the same. It was after all, the same weight lifted to the same height each time. So the 1000w setup still took the same wattage as the 2000w setup. But the 350w setup took more, since the motor was nearly overheated by the top. It lost more because it was unable to go 15 mph up that hill. All these tests, moderate pedaling. I was never testing for how hard I could pedal.

But of course, a really fit likely 40 years younger rider could just pedal up that hill at 15 mph, and not overheat a small motor. I was doing tests for the average customer of E-bikekit. Typically at least 40, rarely under 200 pounds, and generally not pedaling more than 75 watts. I was 52 when I was doing all this testing, in 2010.

 

[Jil]

Test done yesterday on my usual route to the office :
17km, more or less flat, 50% road and 50% city.

Usually I do this with full power (i.e. between 800 and 1300 watts), 45-50 kph max speed. It takes me around 30mn and consumes around 20-22 Wh/km.
Yesterday evening I've done it with the same bike (Surly Krampus with GMAC) but minimal assistance : 75% of my pedaling power (torque sensor).

Result :
- 45 mn
- 22 kph average speed
- 107 human Wh
- 71 electrical Wh
--> consumption of 4.2 Wh/km.

I've divided my consumption by 5 with a trip duration 50% longer.
The light assistance was agreable.

 

[Jil]

In general, once you are talking hub motors, the best bet is to have lots of power up the hill. This gets you up with max efficiency if you ride up the hill about 15 mph. Not fast enough for wasting much on aero, not too fast to pedal hard to help, and not too slow so the motor makes heat. Quick up the hill saves since you take less time drawing more than 200w. Once over the top, resume using only 200w.

It's not so clear.
If we consider one of the hardest pass of the Tour de France (col du Tourmalet, 7.3% average), for consumption and even heating it's better to climb it with a small G310 at 10 kph, than with a powerfull GMAC at 20 kph. And despite the lower efficiency of the G310 due to lower speed :
https://ebikes.ca/tools/simulator.html?motor=MG310_STD&cont=BRZ9&wheel=650b&mass=110&hp=125&grade=7.3&autothrot=true&throt=27.15785395319042&autothrot_b=true&cont_b=BRZ9&motor_b=GMAC10T&wheel_b=650b&mass_b=110&hp_b=125&throt_b=44.11095027995037&grade_b=7.3&bopen=true

 

[Mclewis1]

In general, once you are talking hub motors, the best bet is to have lots of power up the hill. This gets you up with max efficiency if you ride up the hill about 15 mph. Not fast enough for wasting much on aero, not too fast to pedal hard to help, and not too slow so the motor makes heat. Quick up the hill saves since you take less time drawing more than 200w. Once over the top, resume using only 200w.

It's not so clear.
If we consider one of the hardest pass of the Tour de France (col du Tourmalet, 7.3% average), for consumption and even heating it's better to climb it with a small G310 at 10 kph, than with a powerful GMAC at 20 kph. And despite the lower efficiency of the G310 due to lower speed :

Yes but could you maintain that 125w human output for 50% longer?

I have to say that reading back and forth between Dan and Jil's posts I'm confused and my head hurts a bit ... something doesn't add up for me. But for me there is one reality when considering dropping speed to increase range, that is the endurance of the human motor. Or more specifically the mechanicals ... which affect the ability to handle the longer time in the saddle. My hands/wrists, shoulders, and seat all contribute to limit my endurance more than my heart and legs.

 

[Jil]

Yes but could you maintain that 125w human output for 50% longer?

As per my Cycle Analyst I can maintain without too much effort 160 watts of human power during 1 hour. But it depends on the rider of course.
I would say that 100 watts human power is the minimal value that anybody can maintain for a long time.

 

[dogman dan]

Heh heh, I could do that too, once upon a time. Used to break chains from never shifting out of the highest gear on the bike up big hills. How big? Google the road to Santa Fe ski area. But to be fair, I weighed 120 pounds then, and wasn't even 20 years old.

The reason I got into e bikes in the first place, was I got fat and old and didn't like riding 10 mph. 75 watts is a realistic number for folks that are no longer young. Sure, lots of exceptions, but I'd say 90% of people I sold an E-Bikekit to were unable to put out 50w. I am talking about what a typical buyer of an e bike can do. Jil is talking about what a typical rider of non electric bikes can do. A guy who rides his bike, not a guy who's bike gathered dust in the garage like mine did.

The reason Jil and I have such different expectations is he can still ride up the TDF routes. I have similar routes too, Emory Pass has just about the same profile as Alp D Huez. With my weight, 200 pounds, I have rode up it many times using 500w rated motors, and pulling about 800w. That's what it took, with my weight, my pedaling effort of less than 100w, and enough throttle to keep the bike moving 15 mph.

When I talk about needing more wattage than that, I'm talking about packing additional 30 pounds of battery than most e bikes, and 50 pounds or so on a cargo bike, or pulling a trailer.

But hey, if you can pedal 10 mph up that route, 10 miles long of non stop hill, then it won't take much more to keep it going 15 mph. But I can't, and 90% of people who bought an e bike can't either.