Most powerful eBike for long uphill journeys?

[GLJoe]

In a word, no, not in a practical way for a legal ebike.

But why wouldn't it work?
And what is the relevance of the 'legal' bit? (bearing in mind that the nominal 250w of the current 'legal' ebikes seems extremely suspect/flexible in the way its already defined/implemented)

 

[Woosh]

But all the information I've been seeing recently, indicates that the ebike controllers already perform the power control by pulse width modulating MOSFETS that are driven in switching mode. That in effect is what a switched mode power supply does.
??

that's true to a large extent. What I meant to say is there are limits at both ends of the RPM spectrum, otherwise, you would have infinite torque at the zero RPM end and infinite RPM at the other end.
At the high RPM end, the motor is limited by its Kv constant. Kv is the number of noload RPM per Volt. Take the CX motor for example, if the graph is correct and the controller does not intentionally cut out at 120RPM, and you extend the plot, the noload speed will be somewhere in the region of 220 RPM against 105 RPM for the Yamaha, so the Kv of the CX is roughly twice the Kv of the Yamaha.
At the low RPM end, the motor is limited by its Kt (torque constant), so you have the familiar mountain shape of output power.
The controller can limit the torque to 75NM if the motor can output 120NM, therefore flattening the power curve to 75NM. The CX has roughly the power of the Bafang BBS02B 36V 25A 500W but the BBS02 does not limit the maximum torque, thus flattering for Bafang but not as well thought out as the CX.
I sell the BBS02B here:
http://wooshbikes.co.uk/?cdkit

 

[brightonebikes]

that's true to a large extent. What I meant to say is there are limits at both ends of the RPM spectrum, otherwise, you would have infinite torque at the zero RPM end and infinite RPM at the other end.
At the high RPM end, the motor is limited by its Kv constant. Kv is the number of noload RPM per Volt. Take the CX motor for example, if the graph is correct and the controller does not intentionally cut out at 120RPM, and you extend the plot, the noload speed will be somewhere in the region of 220 RPM against 105 RPM for the Yamaha, so the Kv of the CX is roughly twice the Kv of the Yamaha.
The controller can limit the torque to 75NM if the motor can output 120NM, therefore flattening the power curve. The CX has roughly the power of the Bafang BBS02B 36V 25A 500W.
I sell the BBS02B here:
http://wooshbikes.co.uk/?cdkit

Good example.... BUT, the CX is rated at 250W and as such is considered UK EAPC road legal, I don't think the BBS02 36V 500W would be?
It is all a bit ridiculous, but them's the rules!

 

[Woosh]

the 500W is factory rating, not nominal rating.
You can ask for OEM labelling to BBS02B 36V 25A 250W.
I like the way Bosch make their motors. They reduce their exposure to abuse by imposing a maximum assist ratio and limit the maximum torque.
I think it's possible to emulate their philosophy by adding a torque sensor to the BBS02 kit though.

 

[Deleted member 4366]

But why wouldn't it work?
And what is the relevance of the 'legal' bit? (bearing in mind that the nominal 250w of the current 'legal' ebikes seems extremely suspect/flexible in the way its already defined/implemented)

You could take a massive powerful motor that gives power all the way to 200rpm, then control it down to fit that Bosch curve. The Bosch motor isn't a massive powerful motor. It's a tiny 250w motor, probably already running close to it's maximum.

Why would you want to run it past 120 rpm, when most people can't pedal that fast?

 

[brightonebikes]

the 500W is factory rating, not nominal rating.
You can ask for OEM labelling to BBS02B 36V 25A 250W.
I like the way Bosch make their motors. They reduce their exposure to abuse by imposing a maximum assist ratio and limit the maximum torque.
I think it's possible to emulate their philosophy by adding a torque sensor to the BBS02 kit though.

Niceness

Sent from my E5823 using Tapatalk

 

[GLJoe]

You could take a massive powerful motor that gives power all the way to 200rpm, then control it down to fit that Bosch curve. The Bosch motor isn't a massive powerful motor. It's a tiny 250w motor, probably already running close to it's maximum.

Well, yes, you probably wouldn't describe it as a 'massive powerful motor'. But it doesn't seem to be particularly 'tiny' either when compared to other offerings, and if my understanding is correct, the internal gearing is such that it runs at something like 2.5 times the rotational speed of say the yamaha motor. How this design decision affects the relative power delivery, is yet another mystery that I need to investigate! but I imagine they did it for a very good reason.

Why would you want to run it past 120 rpm, when most people can't pedal that fast?

I imagine cycling at >120 rpm is not needed. However my interest in this topic has been vastly increased because a couple of days ago, the wife (yamaha) and I (bosch) went out for a ride, and I could not understand how she was struggling so much at certain points when at other times, she was doing so well. So the next day, we repeated the route, but we swapped bikes. I had a real eyeopener when I started to experiment with cycling at different cadences in real life hilly terrain.
Basically, I'd say that the performance of the Yamaha motor followed almost exactly the curve depicted in the graph shown in this thread. I was shocked at how the power fell off to virtually nothing (and really quickly!) once you went up to around 100rpm - and this is NOT a particularly unreachable figure. Its well within what keen cyclists would class a normal operating range, especially for a burst now and again.
I felt really guilty as I'd been advising her to increase her cadence, but I had no idea what bad advice this could be with the Yamaha motor! I'd never experienced this with my Bosch, and when we swapped bikes and she rode mine, she flew up the areas that she previously struggled with (when going for a high cadence).
I'd seen some people mention how the Bosch system allowed a higher cadence range, but I'm a bit gobsmacked as to how little seems to be discussed as to how nasty a steep power falloff can be. For people tootling around at 60rpm or so, its probably no big deal, but for some people who want to ride their ebikes in the same basic style as their non powered bikes, that could be a huge consideration.

 

[Woosh]

For people tootling around at 60rpm or so, its probably no big deal, but for some people who want to ride their ebikes in the same basic style as their non powered bikes, that could be a huge consideration.

there are always the strong geared hub motors - they don't make you feel like having bionic legs but they climb hills just as competently without requiring you to up your cadence.

 

[Deleted member 4366]

Basically, I'd say that the performance of the Yamaha motor followed almost exactly the curve depicted in the graph shown in this thread. I was shocked at how the power fell off to virtually nothing (and really quickly!) once you went up to around 100rpm - and this is NOT a particularly unreachable figure. Its well within what keen cyclists would class a normal operating range, especially for a burst now and again.

The Yamaha is designed to have different characteristics. They designed it to give its high power at a lower cadence than the Bosch, probably because the average unfit cyclist has a cadence of around 60. Designing for 120 rpm doesn't make sense to me because most people wouldn't be pedalling it in its efficient zone. Anything lower than 60 rpm and the efficiency drops down rapidly below 60%, so you lose a lot of the potential power.

that graph seems to show the Bosch CX power going on to about 140 rpm, which would mean an efficiency of about 50% at 60 rpm. There's no way that the graph is correct.

 

[Woosh]

The main attraction for me is that the BBS01 output maxes out at 70 rpm, the CX at 90 rpm. The CX clearly targets fitter e-cyclists.

 

[anotherkiwi]

I have been playing with cadence and have come to the conclusion that I need a 44 tooth big chainwheel. The 42 enables me to spin past 90 rpm and changing gear on the cassette doesn't "feel" right, cadence does come down but I am not comfortable. The standard 46 tooth was just too big on my local hills but would probably be ideal with a 26" wheel. The 32 stays on for the hills over 15% that I keep running into on my travels in Navarra.

 

[GLJoe]

The main attraction for me is that the BBS01 output maxes out at 70 rpm, the CX at 90 rpm. The CX clearly targets fitter e-cyclists.

If its giving maximum power at 70 rpm, that is indeed a really great and useful cadence to do so at.
If however the graph that someone else posted on this thread is correct, and the power falls off so sharply that at around 85rpm you could find yourself with virtually zero assistance, that's really, really bad for a pedelec! Surely??

 

[Woosh]

If however the graph that someone else posted on this thread is correct, and the power falls off so sharply that at around 85rpm you could find yourself with virtually zero assistance, that's really, really bad for a pedelec! Surely??

That why Chinese motors can't still compete with Bosch - they produce big power but in the same shape as the Yamaha's whereas it would be better to refine delivery. You can reprogram the 48V BBS02 to make it a bit more like the CX but without a torque sensor, fitter e-cyclists won't be satisfied.

 

[jarob10]

If its giving maximum power at 70 rpm, that is indeed a really great and useful cadence to do so at.
If however the graph that someone else posted on this thread is correct, and the power falls off so sharply that at around 85rpm you could find yourself with virtually zero assistance, that's really, really bad for a pedelec! Surely??

In my experience, on an 11-52 setup with 700 wheels, 85rpm = about 29mph on the flat, so plenty fast enough. Hit a headwind / incline, of course cadence drops BUT then the motor rides back up the curve and you can feel the motor delivering more power to assist - downshift then (if necessary) to maintain 70rpm to work the motor & maximise road speed. Its a case of adapting your riding style to suit the motor, which wouldn't suit everyone.

 

[Zlatan]

My Haibike , no matter what graph says, is giving plenty of power at 85...but only if you can still load pedals ...
Lets say for example you have to exert 10kg force to trigger max current ( no idea what actual figure is) A normal cyclist will start to struggle putting same force on pedals as they reach high cadence.

That's why I,m not sure unit needs to use your cadence to calculate anything, force on pedals must diminish but because you are pedalling faster not necessarily the power.( from you) Would be interesting to look at force applied on pedals by various cyclists throughout cadence range. ( I,ll bet Woosh has a formula for it)

 

[Woosh]

( I,ll bet Woosh has a formula for it)

this diagram may look complicated but it's not really.

http://www.analyticcycling.com/PedalModelConcept_Page.html

The torque at each point is described by a function here called a strength function.

note the theta2 angle between the thigh and the horizontal. The maximum strength is between 30 degrees to 50 degrees.

 

[Izzyekerslike]

All crank motors have a power band. After a certain pedal speed, the power ramps down due to the back emf, so you have to be careful about pedalling too fast if you want a lot of assistance. It's not the control system that does that. it's just the way electric motors work.

EMF ?

 

[Deleted member 4366]

EMF ?

You should have paid attention in science at school. Electro-motive force. It's what makes electricity give power. The units are volts. The volts push the electricity down the wire like water pressure in a hose pipe. The more volts, the more the push.

As soon as a motor turns, it becomes a generator. The faster it spins the more volts it generates. The volts are in the opposite direction to the battery's ones, so the battery's volts are pushing electricity into the motor and the motor's volts push back. When the motor gets to a certain speed, it makes the same volts as the battery so you have a stand-off and no electricity can flow. That's why every motor has a maximum speed with any battery. The more volts in the battery, the faster the motor can go.

The next mystery you need to solve is why you have to steer a bike left to go right. For your homework, read up on it and I'll test you tomorrow.

 

[rich_r]

It is in reality a little more complex than that with the brushless motors that we tend to use on bikes, but in essence you do get the same limiting effect on maximum speed due to back emf. d8veh's explanation is spot on for brushed motors though, and the general idea still applies to brushless.

There are other limiting factors too such as the switching rate being limited by the rise rate imposed by the inductance of each coil and the back emf when a coil is de-energised causing a big spike in current that may overload a switching transistor etc.

That last point is particularly important as it can (and does) cause real heat problems with the speed controllers in radio controlled aircraft. Suddenly reducing the load on an RC helicopter's rotors (eg aerobatics) can cause a huge current to flow can the speed controller to overheat and not uncommonly catch on fire. To be fair, the currents involved in RC helicopters can be massive and the controllers only have small heatsinks. One of my RC planes suffered a small fire after the prop came off due to the controller overheating. Not great when the fuselage is made of polystyrene foam!

Hence why the bike motor controllers go to great lengths to limit current output to the motor, and faster motors use a higher voltage (and hence lower current) to achieve a higher power output.

As for counter-steering, that's all to do with weight transfer

 

[Deleted member 4366]

Brushed or brushless makes no difference.

Counter-steering is because of gyroscopic precession according to the right-hand rule - nothing to do with weight transfer.