I think your mistake is in the diameter of the tire. A 170/xx tire is
about a 22 inch diameter. To get the circumference multiply by pi or
3.14. The circumference is how far the bike travels for one rotation of
the tire. Call it 70 inches.
To adjust your calculation:
tire rpm = (3500 rpm / 4.24) = 825.47
MPH = (825.47 rpm) * (70 in.) * 60 min per hr = 3466974 inches per hour
/ 63360 inches per mile = 54.7 MPH
Sound better?
jv
lyle sloan wrote:
Youre right, bobcat. Maybe I need to go back and show
my work.
Here is Mike's specs from previous posts:
Motor: Etek
Motor Voltage: 48V = max RPM 3500 (using posted info
from electricmotorsport.com)
Max Motor Amperage: 330 amps @ 1 min
Continuous HP: 8 hp = 48V * 124.3 A / 746 (barring the
efficiency of the motor, I think I had it written down
as 6 hp = 48V * 93.25 A / 746 so taht is why the green
line is lower.)
Torque: 1.14 lb-in/amp (0.13 nm/amp)
Gear Ratio: 1:4.2352941176
Rear Tire: 170/60
Bike Weight: 377
Mike Weight: 165
I used an excell spreadsheet to do most of the math so
that it could be copied onto the graph.
Doing the math for the y-axis was easy, I used the HP
conversion and multiplied 48V to each amp value and
divided by 746. Then multiplied taht by the gear
ratio to get the HP @ gear. HP= 48V * Amps / 746
Torque was available from the listed specs so I found
the values at each amp value again and multiplied by
the gear ratio to find the Torque @ gear. [EMAIL PROTECTED]
(1.14 * Amps) * Gear ratio
The x-axis to find the max speed is probably where I
messed up. I figured I would start with the RPM of
the motor so I took that and divided it by the gear
ratio to find the RPM @ wheel. [EMAIL PROTECTED] = [EMAIL PROTECTED] /
gear ratio Then took 170 which I believe is the
circumference and divided that into 63360 in/mi to
find the revolutions per mile 372.70. Finally, I took
the [EMAIL PROTECTED] and divided by revolutions per mile and
converted the minutes to hours by multiplying by 60.
In summary, MPH = ([EMAIL PROTECTED] / Gear Ratio) / (63360
in. / Circumference) * 60min per hr
MPH = (3500 rpm / 4.24) / (63360 in. / 170 in.) * 60
min per hr
MPH = (825.47 rpm) / (372.71 in.) * 60 min per hr
MPH = 132
So double check my math and see if I messed up for
setting the max speed per the gearing and motor rpm.
--- Bob Martin wrote:
--
Blog: http://volcano.newts.org
