Question about combined motor and pedaling speed

[lava]

This has probably been covered on this forum before, but I couldn't find it with the search function.

Let's say I have an e-bike kit, a 500W brushless hub motor w/36V battery, that gets my bike up to 20 mph with no pedaling.

Here's the question: Assuming my bike is geared high enough for me to pedal at high speeds, if I pedal while the motor is at top speed can I get up to 30 mph or is there a point at which the motor will start resisting me and prevent me from going that fast?

I ask because I have always used low-geared single-speed bikes for my e-bikes, which don't allow for effective pedaling at high speeds. But I might be switching to a different bike with gears. A recumbent, actually, that I can get to 30 mph on *without* a motor on the flats. Just how fast will I be able to go once the motor is in place, wind resistance notwithstanding?

 

[t_tberg]

On my 1000 watt bike I have a 48t chainring with a 12t freewheel on 26" wheels (not sure about crank length) and I can "out-pedal" my motor which tops out around 32mph hot off the charger; I can pedal with WOT to get to 35-36 mph but thats hard work. Contributing to your top speed really just depends on your gearing and how much power YOU can produce, a hub motor is not working against you after you add a measly 50 watts or so. I saw Dogman post about this the other day, I think he said ~150 watts or so the effectively cancels out the weight of most batteries and hub motors. Of course these numbers are to be taken with a grain of salt but I think it conveys the idea that cogging and the cumbersomeness of a hub motor is canceled out with the first little bit of juice ya give it. Then again, depending on the geometry and added weight of an ebike you'll probably be able to produce less power then you would on a good fitted road bike or recumbent. So you would be able to go faster then 30mph for sure, assuming you could go 30mph without a motor..

 

[lava]

Great, thanks for the reply. Honestly, if I hit 30 I'll be happy. Don't really need to go any faster than that.

But what is WOT? EDIT: Nevermind, wide open throttle...

 

[lava]

Reading your post again, it seems a little contradictory... you say it's hard work to pedal only 3 mph faster than your motor with WOT. Why? Because your bike isn't geared right? Or because the motor is resisting you? But then you state I should have no problem going from 20 to 30? It's a little confusing.

Is it easier to go 10 mph on a bike with no motor than it is to add 10 mph to a motorized bike at speed? Is that a different level of effort?

 

[teklektik]

A quick visit to the ebikes.ca simulator shows the answer.

Here's a plot of the power necessary to propel a 235lb mountain bike on the flat. The motor and drive system don't enter into it - this is just a reflection of the effects of aero drag, rolling resistance, and incline (it actually turns out that on the flat weight plays only a small role - the power is almost exclusively lost to aero drag):



It takes about 350W to go 20mph (this is delivered to the road - not battery power). A very fit racer can sustain about 200W output - more in bursts. You may not come anywhere near that amount.

If we assume perfect gearing and add your 200W to the baseline 350, then work back to a speed - well the answer is clear: you get skunked by the speed. As you can see from the plot, that 200W gets you about 16-17mph by itself, but at 20-23mpg it's only good for an extra few mph.

 

[lava]

Nelson, when you say the "motor cuts out completely" are you saying that it turns off or is no longer contributing any energy? Please explain.

Teklektik, since I already know I can hit 30 mph on the flats on my recumbent, and there's no way I'm generating 1000W when I'm doing that, I can be sure that my Watts vs Speed slope will not be nearly as steep, right? And therefore any additional pedaling should impart a much greater speed increase than what's shown on the mountain bike plot?

 

[wesnewell]

The motor will not provide any resistance until you reach the no load rpm of the motor. And then the resistance is not a lot. It's just once you reach top speed, you'll just have to pedal a lot faster to even stay up with the motor. It usually doesn't pay much to try and pedal past 20mph unless you have some serious gearing.

 

[teklektik]

I gave you the link to the simulator so you could answer these 'What If?' questions easily...

This is the premiere tool for figuring out what ebikes will do - worth the effort to get familiar with it (provided by Justin who owns Endless Sphere BTW)

...but - here's a quick plot of mtn bike vs bent using the canned profiles. You can enter your own drag coefficients, etc to tune up the results, but this portion of the simulator uses calculations identical to every other bike speed calculator - it's basic physics and the results are as good as the parameters you put in.


You can see the dramatic difference. Similarly, if you are on a mountain bike in a full tuck, things get better as well. If you play with the parameters you can see the effects - IMHO so much better than anecdotal evidence that doesn't tell about that 1% downgrade, posture, weight, running hot off the charger, etc.

 

[lava]

The motor will not provide any resistance until you reach the no load rpm of the motor. And then the resistance is not a lot. It's just once you reach top speed, you'll just have to pedal a lot faster to even stay up with the motor. It usually doesn't pay much to try and pedal past 20mph unless you have some serious gearing.

Is the resistance of the motor beyond no load rpm as much as the resistance when the throttle is off?

 

[dogman dan]

The faster the unpowered direct drive motor is spinning, the more it resists.

It's not a wall you hit at no load speed, it gradually increases as rpm increases, with no power to the motor. By no load speed, it's resisting very hard.

This all gets very hard to understand, and I often as not, have the concept right but am full of shit about exactly how the effect happens. It has to do with back EMF.


Bottom line though, if you are motoring along at 20 mph, and want to increase your speed solely by pedaling, you would have to pedal up 600 watts or more to reach 30 mph. It ain't happening.

But you can easily build a bike able to motor 30 mph, and be geared to pedal at 30 mph. Then you can motor at 30 mph, but use 100-200w less power than it would have.

Definitely, build your bike with a 52 tooth front chain ring. or bigger if it can fit without rubbing the frame. One of my bikes has a 56 on it, another 52.

Lastly, the planetary geared type motors have internal freewheels. So they don't resist any while coasting. But you can pedal so fast the motor is just spinning inside the shell, essentially idling, and all your speed is coming from pedaling.