Determining gearing and final speed

[Neovin]

HI all, I was helping and fellow member and thought I would share this with all of you most of you don't need it but here it is.



Take you kv rating and multiply by the volts you are running, lets say 170kv x 24v = 4080.

Now you need your sprocket sizes, you will use this formula calculate the final rpm after 2 sprocket gear down.

(sprocket1 / sprocket2) x MotorRPM = final drive

now replace the values with you own, if you want to drive through the BB/Chainwheel you want to aim for an RPM of a max of 200 ish that will be faster than you can pedal but not to fast it wears everything out quick,
Remember you must factor in the gear down of the freewheel (motor side) to chainring

(10 / 60) x 4080 = 680 --- 10 tooth on the motor and 60 on the jackshaft

(13 / 44) x 680 = 200 --- 13 tooth freewheel to 44 tooth on the chainring

if you want to calculate your speed use the formula again from the chainring to the rear wheel and use this:

WheelRPM x WheelCircumference (in inches) x 60 (mins of time) /5280 (feet) = Speed in MPH

,Neo

 

[REdiculous]

WheelRPM x WheelCircumference x 60 /5280 = Speed in MPH

[edited for improved clarity]

1000rpm * 31.4 inches * 60 / 5280 = 356MPH (WRONG!!!)
1000rpm * 79.75 centimeters * 60 / 5280 = 906MPH (ALSO WRONG!!!)

31.4 inches is the same as 79.756 centimeters - you can see I rounded slightly.

Use this instead...
drive wheel circumference * drive wheel rpm * 60 = distance per hour @ wheel rpm

31.4 inches * 1000rpm * 60 = 1,884,000 inches per hour @ 1000rpm. (CORRECT!!!) :wink:
79.75cm * 1000rpm * 60 = 4785000 centimeters per hour @ 1000rpm (ALSO CORRECT!!!)

Here's where I use me calculator's conversion function and both of those results are equal to 29.73mph. If you measure your wheel in inches, you can divide the resulting inches by 63360 (inches per mile) to reach MPH. If you measure your wheel in centimeters and you want MPH, divide the result by 160934.4 (centimeters per mile).

Now that we can work out our desired wheel rpm...
max_motor_rpm / desired_max_wheel_rpm = gear ratio.

If I need 1k rpm at the wheel and my motor does 7000rpm, I need a 7:1 reduction in order to hit my target. Simple.

 

[Thud]

I am confused LOL

WheelRPM x WheelCircumference x 60 /5280 = Speed in MPH

^^ THIS IS CORRECT^^

REdiculous wrote:

1000rpm * 31.4" * 60 / 5280 = 356MPH (WRONG!!!)
31.4" ? or is that 31' 4" ? either way, a huge wheel or a misque. My math has a 31.4" circumferance wheel @1000rpms traveling at 29.73mph.

Use this instead...

drive wheel circumference * drive wheel rpm * 60 = distance per hour @ wheel rpm
exactly the same formula as OP.
31.4" * 1000rpm * 60 = 1,884,000" per hour @ 1000rpm. (CORRECT!!!) :wink:
that wheel is 10" in diameter......?

v v I agree with all of this v v
Now that we can work out our desired wheel rpm...
max_motor_rpm / desired_wheel_rpm = gear ratio.

If I need 1k rpm at the wheel and my motor does 7000rpm, I need a 7:1 reduction in order to hit my target. Simple.

will there be a test? I re-read the OP & can't find a referace to the 31.4 (feet ,inches, meters, phathoms, yards, chains, jiggers, bushels or anything)

 

[Neovin]

Ok my bad, i've added the units of measurement, this formula works as i use it on my arduino to calculate speed.

Cheers for the input though that's what its all about.

,Neo

 

[REdiculous]

will there be a test? I re-read the OP & can't find a referace to the 31.4 (feet ,inches, meters, phathoms, yards, chains, jiggers, bushels or anything)

There was a test - I used my wheel's circumference to test his math. A 10 inch wheel has a circumference of about 31.4 inches, and it shouldn't be doing 356MPH @ 1k rpm. :wink:

 

[REdiculous]

WheelRPM x WheelCircumference (in inches) x 60 (mins of time) /5280 (feet) = Speed in MPH

That gives you feet per hour, not miles per hour.

"5280 (feet)" should be "63360.000000068429 (miles)" :wink:

 

[E-racer]

i use this extremely simple applet from back in the TZI scooter world days. http://scooters.tziworld.com/progs/ratiocalc172.zip
it is setup for 1 stage, 2 satge, friction drive and even geared hubs :-D

 

[REdiculous]

exactly the same formula as OP.

It's not the same because my method will always, barring math/measurement errors on your part, produce proper results. :wink:

drive wheel circumference * drive wheel rpm * 60 = distance per hour @ wheel rpm

The result can be inches-per-hour@rpm or centimeters-per-hour@rpm, or you could measure the wheel circumference in feet and find feet per hour@rpm, or whatever other unit(s) you want.

 

[Thud]

Ahh, I see it now.
The units of measure are not singularly quantified....I am so used to running the equations, I often see whats right & not whats written.

I guess i asumed he had the circumfrence in feet as the mile units were stated in feet.......ASSUMING is dangerous.
potato-potato' as long as we get there eventually
T

 

[REdiculous]

As long as we get there eventually..yeah.. 8)

I've done some object-oriented programming so I remember starting out and producing code that never really worked correctly because I had to keep adding to it. One of my early projects had pages and pages of spaghetti code that I was able to replace w/ half a page of concise logic - I just needed a couple years experience to be able to see that I was duplicating a lot of code for no reason...

I like my method because you can monkey with it and it'll give you "bananas-per-hour@rpm" if you measure the wheel using "bananas" rather than inches/cm/feet/meters/yards/miles/km/etc.

 

[TopCat]

i use this extremely simple applet from back in the TZI scooter world days. http://scooters.tziworld.com/progs/ratiocalc172.zip
it is setup for 1 stage, 2 satge, friction drive and even geared hubs :-D

Thanks for posting this E-racer.
At last a 2 stage jackshaft reduction calculator that maybe even a maths dummy like me can get the hang of

Regards
Tom