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How many in/lbs - n/m - watts are people ?
#1
I have what should be a reasonably simple question to answer,
and this looks like a nice in depth forum full of experienced people.

All I'm looking for is how much "Energy" is needed
to maintain a speed of 20-MPH on a simple bicycle...

Leaving out the many many variables of a lessor degree,
and utilizing some generic major values for an approximation.

And forming an answer in an
accepted measurement format
like the topic heading indicates:
Pound/Inch Torque
Newton/Meter Torque
Watts (IMHO too vague)

1 - A standard 180-Lb person
2 - Standard street tread 26" wheels
3 - 60-70-SPM pedaling cadence
4 - Standard bike/handlebars/clothing
5 - No additional wind force or road drag (level grade)

The problem is I'm having a hard time pinning this down
to any reasonable average value of some kind to utilize,
and none of my Google hits agree on where to test for this.

The closest, and possibly most accurate I found indicated
that it takes about 280-watts of equivalent 85%-Eff motor.

This was determined by building in a tensor sensor into a
an emptied out hub motor (by a hub motor manufacturer)
to sense the hub force required to maintain the US 20-MPH limit.

I think more of this test because it eliminates the gear ratio factor,
as it was a direct test of the force applied to the hub via freewheel.

When that same tensor sensor was connected to their hub motor,
and the rim's RPM was held at the same speed needed for 20-MPH,
the required wattage to maintain the same force was 280-Watts.

It was not one of the planatary-reduction hubs to be clear on this.



I took an 11-tooth gear and put it on an old crappy (China)
MY-1016 24-Volt 350-Watt motor I got on eBay for about $16,
and fed a 65-tooth excer-cycle sprocket on a 26" rear wheel.
(1523-RPM = 20-MPH for this setup)

I only needed ~200-watts (24.81-Volts x ~8.1-Amps)
to maintain a 20-MPH average speed in my tests.

Don't get me wrong, this motor sucks,
and I will buy a real one next, heheh...

But is there a known value that people agree upon (for the most part)
that equates to a given speed and weight of rider perhaps please ?

Thanks
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#2
Wow nice formula Smile
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
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#3
(12-05-2011, 07:15 AM)Bill Wrote:  Wow nice formula Smile

Thanks, but that is just an observation
from a couple of real world (motor) tests.

It isn't really a final definitive answer
of a power to weight ratio/constant really.

I've since found a nice 4800-RPM scooter motor,
it is wound for a higher RPM by its initial design,
and the commutator is also slightly advanced.

So it may spin in reverse poorly,
but bicycle's don't need reverse...

It is rated at 250-Watts at its
ideal rpm verses torque natively.

But with a huge 65-Tooth excer-cycle sprocket as a rear,
and a 6-Tooth motor sprocket for a overall rollout of 75.4
(Gear ratio of 10.83 for those more comfortable with that).

I can use a PWM (Pulse Width Modulation) motor controller,
and still maintain 22-MPH at just over 180-Watts averaged.

I weigh a sad 180, about 20 over what I should be,
the bike I used was a sacrificial old steel tank too...

So I consider these worst case scenario numbers to go by.

But thanks for recognizing my efforts sir. Smile
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#4
(10-25-2011, 11:57 AM)WeThePeople Wrote:  I have what should be a reasonably simple question to answer,
and this looks like a nice in depth forum full of experienced people.

All I'm looking for is how much "Energy" is needed
to maintain a speed of 20-MPH on a simple bicycle...

Leaving out the many many variables of a lessor degree,
and utilizing some generic major values for an approximation. . . .
But is there a known value that people agree upon (for the most part)
that equates to a given speed and weight of rider perhaps please ?

Thanks

What? no specific tire air pressure? I think you answered your own question, there are too many variables to make an accurate calculation. So an estimate should be fine. Smile

An example from Wikipedia: "A person having a mass of 100 kilograms who climbs a 3 meter high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power. A laborer over the course of an 8-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes"

Steve
Junkyard Tools rescued from the junkyard!
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