For safety/overhead tolerances, I say 45 degrees is a good design margin, especially for the size of tire.
For MuK of 0.03 (arbitrary for knobby tire on dirt), on flat ground,
0.03 * 9.8 m/s^2 * 68kg = 19.992 kilogram-meter per second squared = 19.992 Newtons of thrust.
friction force going down = Mu * cos(theta) * m * g
thrust going up = friction force + sin(theta) * m * g
For MuK of 0.03 at 45 degree angle,
friction force = 0.03 * 0.707 * 68 * 9.8 = 14.13N
thrust force = 14.13N + (0.707 * 68 * 9.8) = 14.13 + 471.1448 = 485.27 Newtons
For MuK of 0.03 on 20 degree plane:
friction force = 0.03 * 0.93 * 68 * 9.8 = 18.59N
thrust force = 18.59 + (0.34 * 68 * 9.8) = 18.59 + 226.57 = 245.166 Newtons
For MuK of 0.03 on 5 degree plane (handicap ramp is 4.73 degrees or shallower per ADA)
friction force = .03 * 0.996 * 68 * 9.8 = 19.92N
thrust force = 19.92 + (0.087 * 68 * 9.8) = 19.92 + 58.080 = 78.00 Newtons
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Now that said, I o back to the designed spec for this motor. A 250 pound rider with a 200 pound dry weight is 450 pounds total- 204.5 Kg. I'm assuming they designed a minimum spec of going up a wheelchair ramp, ADA limited to 1 foot fo rise per 12 feet of run, or similar. Round off to 5 degrees.
friction force = 0.013 (arbitrary for smooth tire to concrete) * 0.996 * 204.5 * 9.8 = 25.95N
thrust force = 25.92 + (0.087 * 204.5 * 9.8) = 25.92 + 174.36 = 200.27 Newtons of thrust.
To generate that, 200.27 Newtons * (4.5" radius/39.25" per meter) = 22.96 Newton-meters of torque, through the 16.75 gearing, the electric motor had to generate 1.37 Newton Meters of torque to ascend that handicap ramp.
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1.37 Nm * 16.75 = 22.96 Newton-meters at the output shaft * (32/9) = 81.635 Newton-meters at the axle shaft. 81.635 Newton-meters / (11" radius/39.25" per meter) = 81.635 / .280255 = 291.289 Newtons of thrust
If that new formula and calculations hold,
friction force going down = Mu * cos(theta) * m * g
thrust going up = friction force + sin(theta) * m * g
thus, thrust force = (Mu * cos(theta) * m * g) + (sin(theta) * m * g)
finding theta for a Mu of 0.03 = (0.03 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 19.992cos(theta) + 666.4 * sin(theta) = 291 Newtons
19.992cos(theta) + 666.4 * sin(theta) = 291 Newtons
plugging that in my calculator, shows a 289.31 N force at 24 degrees and a 299.75 N force at 25 degrees. So a 24 degree slope would be the upper limit.
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using the 0.85Nm rating on the sticker,
0.85 Nm * 16.75 = 14.24 Newton-meters at the output shaft * (32/9) = 50.622 Newton-meters at the axle shaft. 50.622 Newton-meters / (11" radius/39.25" per meter) = 50.622 / .280255 = 180.63 Newtons of thrust
finding theta for a Mu of 0.03 = (0.03 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 19.992cos(theta) + 666.4 * sin(theta) = 180.63 Newtons
Calculator shows a result of 180.61N at 14 degrees. So with a Mu of 0.03 and a weight of 68Kg/150 pounds, It could climb up to 14 degrees, possibly up to 24.
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finding theta for a Mu of 0.018 (arbitrary value for knobby tire on concrete) =
(0.018 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 11.995cos(theta) + 666.4 * sin(theta) = 291 Newtons = 24 degrees
finding theta for a Mu of 0.018 =
(0.018 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 11.995cos(theta) + 666.4 * sin(theta) = 180 Newtons = 14 degrees---------------------
finding theta for a Mu of 0.013 (arbitrary value for smooth tire on concrete) =
(0.013 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 8.6632cos(theta) + 666.4 * sin(theta) = 291 Newtons = 25 degrees
finding theta for a Mu of 0.013 =
(0.013 * 68 * 9.8 * cos(theta)) + (68 * 9.8 * sin(theta)) = 8.6632cos(theta) + 666.4 * sin(theta) = 180 Newtons = 15 degrees
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So the Mu isn't very effective of the total angle- So with this system of equations, 14 and 24 degrees are the lower and upper inclination limits.
180 Newtons = 40.5 pound-force
290 Newtons = 65.2 pound-force
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The batteries of choice would be Group 26 size for a 2000-2001 dodge neon. Each battery is 28 pounds, 675CA, 530CCA. Based on an 18AH load, would be about 30-37 hours of runtime between charges.