EV Power vs Speed

[Farfle]

durp, accidentally copypasted the "mph" into the last entry .

 

[Russell]

Most e bikes particularly hub motor driven that can do 80 - 100kmh have 80 - 100A or more of controller and 100v or more of battery. So if a car can hold 60mph on 15kw why are we pumping 10kw into our bikes to come close to the same thing. The only thing I can think of is we are pumping 10kw and receive 5kw or less at the wheel ?

Kurt

Nah, it's just an upright rider on a bike has very poor aerodynamics :|


-R

 

[lazarus2405]

To hold 60mph on an upright, unfaired bike you need about 7kw at the wheel, which, yes, will be about 9kw at the battery. Any hub motor setup with an intelligent choice of voltage/gearing/winding for that speed will probably be slightly over 80% efficient.

Extremely poor aerodynamics is the answer you're looking for. An upright mountain bike has an abysmal coefficient of drag. Going recumbent doesn't help much with the Cd, but it cuts your frontal surface area in about half. An upright mountain bike will have a Cd*A of about 0.57m^2. Wikipedia says an aerodynamic car like a 2004+ Prius will have a Cd*A of about 0.15m^2 (.26*.58m^2). That's a huge difference. If my bike had the aerodynamics of a Prius, it would only take me 2.4kw to hold 60mph (2.0kw at the wheel).

Also, the people hitting 60mph and pulling 100a aren't pulling >100a while riding at 60mph, they're pulling enormous currents while accelerating. A hub motor that has enough torque at 60mph to sustain 60mph has stupidly fun acceleration, and it's just silly not to lay into that.

 

[Kurt]

Interesting, sounds like one of the clear perspex fairings could have a noticeable impact on my recumbent trike.I feel they are a wast of money for peddle speeds but under power its a different story.

kurt.

 

[NeilP]

I am finding it is difficult to get accurate figures to enter in the sheet.
Finding a piece of road flat and long enough to get, a steady speed , no wind etc is a quite a challange

 

[Jeremy Harris]

It's the effect of the cube law relationship between power needed to overcome aerodynamic drag and speed that's the culprit.

Double the speed and you need eight times more power.

Halving the projected frontal area, or halving the drag coefficient by streamlining, will halve the power needed to overcome aerodynamic drag at any speed, as both area and drag coefficient are directly proportional to power. This is why recumbents are so much faster than uprights, they often need less than half the power for any given speed.

 

[dogman dan]

Some cars likely have less drag than a bike rider sitting up, coat a flapping.

But on the bike, 10 kw is not their wattage while cruising. 40 mph can take only about 2kw ( for sure less than 3kw) to maintain. Dunno about much faster. 47 mph breifly is my personal best, and I don't recall looking at the CA at that moment.

 

[Kurt]

Some cars likely have less drag than a bike rider sitting up, coat a flapping.

But on the bike, 10 kw is not their wattage while cruising. 40 mph can take only about 2kw ( for sure less than 3kw) to maintain. Dunno about much faster. 47 mph breifly is my personal best, and I don't recall looking at the CA at that moment.

It is amazing when you think about it then that a car can have less wind drag than a person on a bike. I wonder how much of the energy is overcoming friction (rolling resistance) in a car compared to a bike? I know its a hell of a lot easier to push a bike than a car along the road even on perfectly flat ground.

Kurt

 

[dogman dan]

I doubt I own a car with that low drag, and perhaps nobody does, but it is pretty amazing how low drag a good car body can be compared to a fully erect human body. Riding a motorcyle, a very low tuck can add quite a few more mph. Hang you chest into the wind, and it's a lot of drag.

 

[John in CR]

Last week at the beach I got on a lonely 1 mile stretch of good road with no cars or side streets. I switched the motor to low speed, which held steady at about 31mph. I experimented with different body positions while watching the CA, and the difference between upright and a comfortable tuck was 600W. Considering that one of my bikes will do 30mph on just under 1kw, 600W is huge at that speed. I can't wait to try some real aero treatments on a recumbent, because I'm pretty sure that 40-45mph on 1kw is readily achievable, especially since the record is over 60.

Kurt,
My cargo bike will do 60mph with me tucked in as small as I can make myself on about 4.5kw from the battery, so just under 4kw at the wheel. That bike is a lot lower than a regular bike though, and definitely more aerodynamic than my more upright bikes.

 

[Hillhater]

I dont see anything from these practical "on road" test data reports that is significantly different to the results from the line power/speed calculator.
http://www.kreuzotter.de/english/espeed.htm

 

[mylittlepony]

Hey Farfle, I fixed your entry.