correct gear ratio calculations?

[nathan_jones]

hello every one there is a first time for everything,

and here is the for time ive cacluated gear ratios, can you just check my calcs..ppleeaase :lol: as i have used NO technicain knowledge just my own special brand of logic

ps here in the UK we updated to centimeters (cm) and millimeters and meters fifty years ago, but not miles just to confuse our american friends

ok. the speed i want to go at is 15mph

15mph = 2414016cmph

or 402433 cmpminute

my wheel circumfrence is 1256

Therefore

402433/1256= 32 rpm required to go to 15mph. Now for the motor.

the motor i was considering using was this http://www.electricscooterparts.com/MOT-24250G.htm (as i dont think i know enough to handel a RC motor yet )

the output speed is 550rpm

thus 550/32=17

so i need to reduce the 550 to 32

ok so a 9toothsprocket

9x17=153 so i would need a sprocket with 153

0r via a 52>22 chainrings (52 to motor 22 to wheel) would get to 220rpm to 32 wheel rpm i would need a 61 sprocket

please so it aint so !!!! (if i could find the crying smiliy face i would use it!)

 

[Drunkskunk]

I'm just an American, and we can't think in all those fancy meter type terms. Us rebel Yankees can bearly get our heads around the inch, but I'll give it a shot.

Your 1230mm circumphrence wheel works out to be a 400mm diameter, or a 16 inch wheel, right?

a 16 inch wheel would travel 50.25 inches each revolution. There are 63360 inches in a mile so your wheel would need to rotate 1260 times to travel that far. Now if you want to do 15 miles in one hour, 60/15= 4, or thats equil to 1 mile in four minutes. that means you would need to move 1260 revolutions / 4 minutes, or 315 revolutions per Minute.

Since the motor is rated as a top unladen speed of 550 rpm, and its a geared motor, I would conservitivly place it's rpm under load at 70% of that RPM, with the likelyhood that it will be a little better. its a guess based on 65%to 75% typical efficancy of that type motor So 70% of 550 is 385 RPM. 385 / 315 = 1.2~ so you would need a reduction of around 20% or 1.2:1 to meet your goal of 15mph.

 

[nathan_jones]

ds thanks for the reply, im just scratchin my head ill post up tommorow once ive gone over the numbers.
ps its sunnny in wales! which is a rare occasion. so im of to enjoy the weather until it rains!!! i get back soon

 

[nathan_jones]

a 16 inch wheel would travel 50.25 inches each revolution. There are 63360 inches in a mile so your wheel would need to rotate 1260 times to travel that far. Now if you want to do 15 miles in one hour, 60/15= 4, or thats equil to 1 mile in four minutes. that means you would need to move 1260 revolutions / 4 minutes, or 315 revolutions per Minute.

yes its a sixeteen inch wheel. im not disagree with what you say, i and what to say thanks again for helping, im just trying to find where i went wrong. Do you have any ideas?

i did the same cal in using metric and got a different ratio. :?

I just copied and pasted then changed the values
a 16 inch wheel would travel 1256 (see check one) cm each revolution. There are 160934 cm in a mile (see check two) so the wheel would need to rotate 128 (see check three) times to travel that far. So this is where the differnence is. Now if you want to do 15 miles in one hour, 60/15= 4, or thats equil to 1 mile in four minutes. that means you would need to move 128 revolutions / 4 minutes, or 32 revolutions per Minute. check four

check one 50.25x25.4(inch to cm scalefactor)=1275 (close enough to 1256)
check two 1 mile = 160 934.4 centimeters http://www.google.co.uk/search?hl=en&q=cm+in+mile&btnG=Google+Search&meta=&aq=f&oq=
check three 128.132x1256=160933
check four ok so we know now that 32(rpm)x1256=40192=cm required to go 15mph
40192x60=2411520
or if we go back to check two 1609348x(by the total distance required in one hour)15=2414010

so 2411520cm (0r 15miles) is the total theoretical distance travelling on a 16" wheel which is revoling at 32rpm,

have i found a glich in the matrix? or am i just stupid?

ps i check all your calcs and they are correction but math, but our ratios should be the same (or very close)
also i found his great converter http://www.convert-me.com/bb_en/viewtopic.php?t=3956
pps drunkskunk all the information on the laden/practical gearbox speed is greatly apprciated and insitful thanks for letting me know real world applications!!!


anyway where am i going wrong?

 

[nathan_jones]

"

have i found a glich in the matrix? or am i just stupid?

please be gracious enough not to answer, i assume for the time being the latter :lol:

 

[Miles]

Hi Nathan,

At first glance, the only error seems to be a mistake with the units. I think the circumference of a 16" wheel should be about 1256 mm, which makes your speed at the wheel 320 rpm - sounds about right. I'll check it through properly after I've had something to eat Personally, I avoid using cm like the plague - transpositional errors between metres and millimetres are easier to spot...

 

[nathan_jones]

...doh. 1256mm not 1256cm. thanks miles!!!

so if i reconvert ds's info

a 16 inch wheel would travel 1256 mm each revolution. There are 1609344 mm in a mile so the wheel would need to rotate 1281 times to travel that far. Now if you want to do 15 miles in one hour, 60/15= 4, or thats equil to 1 mile in four minutes. that means you would need to move 1281 revolutions / 4 minutes, or 320 revolutions per Minute. so 315 and 320 not far off, thats better .

thanks guys!!

with only 1.2 final reduction i could use a hub gear...im drooling already!!

 

[swbluto]

You might find my ebike/escooter simulator handy.