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• # gear ratio confirmation?

Discussion in 'Calculations' started by jawnn, Apr 21, 2015.

1. ### jawnnWell-Known Member

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I just need to know if I did all this correctly

[FONT=&amp]I used this calculator [/FONT][FONT=&amp]http://www.kreuzotter.de/english/espeed.htm to estimate that 900 watts would drive 550lbs up a 9% grade at 38.8mph. But it says nothing about gears. I can only assume that it reduced the energy by the 15% lost into the bicycle gears. And I have to assume that timing belts would not loose more energy.[/FONT]

[FONT=&amp]What gear ratio to use? [/FONT]
[FONT=&amp] If I knew what speed the motor would turn at the 900 watts I could make a better estimation.[/FONT]

[FONT=&amp] Maybe 2000 rpm with a 1:3.75 ratio reduction(4” pulley on the motor / 15” rim pulley on the drive wheel) on 24” drive tire? Or a 3.2” pulley on the motor / 12” rim pulley. I do not know the exact sizes of available pulleys.[/FONT]
[FONT=&amp] 24”tire: 75.36” circumference x 533.3333rpm= 4019.9999”/12=3349.3332 x 60 = 200959.99 ft per hour /5280 = 38 mph. With a 3” pulley/15” (1:5 ratio) it would be about 28.54mph. That would give energy to spare.[/FONT]

[FONT=&amp]On a 26” tire: 1:5 ratio (3”/15”) 2000/5 = 400rpm still that come out to about 31mph. I could use a 2.4” pulley on the motor and a 12” rim pulley on the drive wheel.[/FONT]

[FONT=&amp]If you want to use a 12” sprocket or rim pulley on the drive wheel, better to stay with a 20” drive wheel (tire); use a 3”pulley on the motor for a 1:4 ratio = 29.73 mph[/FONT]

2.
3. ### ErichWell-Known Member

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Feb 2012
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I do not get your results at all.

I chose a roadster bike and then entered a rider weight of 510 pounds. Enter 9% grade and 900 watts.
Speed I get is 8.1 mph. No where close to 38 plus.

Using a back of the envelope calc I estimated that 900 watts could push 550 pounds at 9 mph.
So I believe the calculator, just not your results.

To choose the gear ratios You MUST know at what rpm the motor makes maximum hp.

If you are using a Permanent magnet DC motor Max hp occurs at 1/2 the no load rpm.
Apply your max voltage to the motor and measure the rpm. Do this on the bench with the motor not driving anything.

Use 1/2 that rpm as your design point for hill climbing.

You know the max speed you can go (8.1 mph) You know the rpm of the motor for max power.

You know the circumference of your wheels.

With that info you can calculate the gearing you need.

Last edited: Apr 21, 2015
4. ### jawnnWell-Known Member

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Jan 2012
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OKI think the calculator gave me the wrong answer....

Ineed to know how much wattage it will take to drive a 400lb and a550lb vehicle up a 9% grade.

Thesecond time I used the calculator I got 900 watts for 400lbs up a 9%grade at 10.7 mph.
Idon't think that is right at all. but maybe it is if you came that close for 550lbs.

Itwould really good to find a calculator that works for e-bikes.....gosh foundseveral of them....thanks for the help

Last edited: Apr 23, 2015

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