Flecc can you give some guidance

[shep]

I am trying to get my head around how the various systems work. Now my understanding is that with front or rear hub drive the motor is allways on at the various power level selected, and is started or stopped by either pedal movement or a throttle.

With the Panasonic crank drive unit, the power is provided when you start pedalling and the electric power provided to the crank is governed by what power level is selected and is then in direct relation to how hard you actually pedal.

With the Tonaro bevel drive crank system this is similar to the hub drive whereby the power is allways on at the various power levels selected and is started or stopped by either movement of the pedals or the throttle.

So when faced with a hill, with the hub system this can be efficient as long as you can maintain pedalling to keep the bike at the optimum speed for the hub motor, if you cannot then the motor becomes inefficient as the motor will not have sufficient gearing to "torque" you up the hill.

With the Panasonic system the limiting factor is having a low enough gear, as the motors power is always multiplied as you change to lower gears. So the motor can always stay in the sweet spot. Hence this systems ability to defeat nearly any hill you are ever likely to encounter.

With the Tonaro system this is very similar to the Panasonic but without pedal sensing, what i find with the Tonaro is going up a steep hill if i push the pedals hard, then if i tire and back off the motor seems to take over, so that we maintain a similar speed we do not go slower, then if i start pushing on the pedals hard again, the motor eases up and we do not go faster.

Also i have noticed by slowing the cadence on a hill the motor seems to provide more power, is this a function of an electric motor simply providing more torque at a lower RPM? is this why the Panasonic system seems to work best with a lower cadence?

And that by effectively lowering the RPM of the electric motor on a hill, you effectively get "torqued" up the hill by the slow running electric motor? (although you will now have a high amp draw) and that this torque is then multiplied by the gears to stop the motor from stalling.

With a hub system you cannot stop the motor stalling out on a steep hill because it is effectively stalled out by trying to drive too high a gear? Does this explain why hub motors use more electric, because as they come under load as they come out of the motors "sweet spot" the amp draw really shoots up? and there is no way to stop this? Because on a crank drive you can "help" the motor by changing down a gear? and therefore the torque produced by the motor and the cyclist is then multiplied? at the expense of speed.

I am just very interested to know how this all works

 

[flecc]

Yes, you've got all the factors right, the main difference with the Tonaro being the throttle override. Even there though, when you pedal harder you are probably increasing the cadence which in turn is reducing the motor assistance.

Although hub motors do indeed lose efficiency as they drop below optimum speed, there is the intrinsic compensating factor that slower climbing needs less power, and this is linear, half the speed needs half the power.

I've got two website articles that you'll find of interest:

Panasonic power delivery explanation decribes the system's working in detail.

Drive through gears on e-bikes argues for hub motors but ends with the ultimate limitations.

 

[shep]

Thanks Flecc, really appreciated. Such an interesting subject this, i find it fascinating. Also i think it helps with your cycling if you know what is going on.

 

[tangent]

Although hub motors do indeed lose efficiency as they drop below optimum speed, there is the intrinsic compensating factor that slower climbing needs less power, and this is linear, half the speed needs half the power.

Sorry to be pedantic, but this not quite right. When going up hill you have 2 retarding forces - gravity and friction of which wind resistance can be a major factor. Assuming the slope does not change, gravity is constant, so halving speed will require half the power to move against it. Wind resistance though increases with the square of the speed so halving speed will reduce the wind resistance by 1/4. This means only 1/4 of the power is needed to move against this part of the retarding force.

 

[flecc]

Sorry to be pedantic, but this not quite right. When going up hill you have 2 retarding forces - gravity and friction of which wind resistance can be a major factor. Assuming the slope does not change, gravity is constant, so halving speed will require half the power to move against it. Wind resistance though increases with the square of the speed so halving speed will reduce the wind resistance by 1/4. This means only 1/4 of the power is needed to move against this part of the retarding force.

That's a good example of theory being right but useless in practice. At common e-bike climb speeds wind resistance in static air conditions is so slight as to be negligible in practice. The general assumption in the cycling sphere is that up to 12 mph wind resistance isn't worth bothering with, and legal e-bikes don't climb hills at higher speeds than that. Most are much slower.

So what I posted is correct and needs no adjustment.

 

[tangent]

At 12mph I agree. I was just being pedantic.

There is an interesting web site which models bike dynamics here:

Analytic Cycling, Interactive Methods for Estimating Cycling Performance Parameters. Tom Compton

This allows you to vary all sorts of properties, including things like air density!

 

[Hackney Andy]

Aerodynamic drag increases with the square of velocity, but work done is force times distance, and power is the rate of doing work: therefore power required to overcome aerodynamics drag varies with the cube of velocity.

 

[indalo]

Blimey! This thread is like being back at technical college 40-odd years ago, Stow College of Engineering in my case. Most of the stuff I learned there is deeply embedded in the darkest recesses of my mind, so deep in fact that it's easier for me to ask my other half if I need to know anything. She's a mathematician, lectures in it and just lives and breathes science and calculations.

I'm worried I might have Alzheimers but when i saw my GP about it, he told me just to forget it.

Regards,
Indalo

 

[Cyclezee]

I'm worried I might have Alzheimers but when i saw my GP about it, he told me just to forget it.

Regards,
Indalo

Like it Indalo The fact you remember what your GP said, proves that they were correct
Can you remmere the name of your GP

 

[flecc]

There is an interesting web site which models bike dynamics here:

Analytic Cycling, Interactive Methods for Estimating Cycling Performance Parameters. Tom Compton

This allows you to vary all sorts of properties, including things like air density!

Yes, that one is interesting. The book "Bicycling Science" is also stuffed with graphs on all manner of factors around cycling, but fortunately almost all rank with wind resistance where e-bikes are concerned. In a way the legislators have done us a favour in keeping our interest simple, though the speed freaks won't agree!

 

[flecc]

Can you remember the name of your GP

Yes, it's Doctor.

 

[Deleted member 4366]

Also i have noticed by slowing the cadence on a hill the motor seems to provide more power, is this a function of an electric motor simply providing more torque at a lower RPM? is this why the Panasonic system seems to work best with a lower cadence?

And that by effectively lowering the RPM of the electric motor on a hill, you effectively get "torqued" up the hill by the slow running electric motor? (although you will now have a high amp draw) and that this torque is then multiplied by the gears to stop the motor from stalling.

I am just very interested to know how this all works

As the motor speeds up it produces an increasing back e.m.f. that opposes the supplied volts which reduces the power until they are equal and the motor can't go any faster. If you fitted a wattmeter to your bike, you could see this very clearly: The faster you pedal, the less power you use, and, as you have found out, as you slow down your pedalling, the motor produces more power until it takes over completely. Some controllers can measure the speed of the motor and can use this to control the power, so that when you pedal faster than the designated speed, they reduce the power and likewise increase it below that speed.

Scotty if you're reading, or anybody else with cruise control, you can use this effect to make your battery last longer. Just set the cruise control to 12mph and leave it like that as long as you can. When you slow down up the hills, it'll feed in increasing power in an attempt to maintain your speed, but as long as you are over 12mph, it'll use hardly any power. The higher you set the cruise speed, the faster your battery goes flat and vice versa.

 

[tangent]

Aerodynamic drag increases with the square of velocity, but work done is force times distance, and power is the rate of doing work: therefore power required to overcome aerodynamics drag varies with the cube of velocity.

Quite right Andy - momentary lapse of reason on this. If you double the speed, drag goes up by a factor of 4, but you are heading into it twice as fast so 8 times more power required.

I have certainly been noticing the effects of wind resistance the last few days. My journey to work (wind behind me) draining around 65Wh from my battery. Journey home about 90Wh! Under still conditions they are always about the same.