Gear ratio calc - help?

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northernmike

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OK. For my moped conversion...

Measured rear wheel circumference by rolling along basement floor in straight line. 1675mm.

Rear (415h chain drive) sprocket - 41 teeth

Front sprocket - 13 teeth

Diameter of pulley which Front sprocket is an intergral part of, which will be driven by V-belt by motor - 215mm

Pulleys I have on order for this to experiment with - 3.95. 3.55, 3.15" or 100.33, 90.17, 80.01mm.

How the heck do I figure out my actual ratios?

I'm going to be running an HXT 80-100-B on 48V (130kv) ... hoping I got close to realistic pulleys.

My brain is just not wrapping itself around the numbers today.

Help?
 
Hi northernmike,

JohnRobHolmes did a gear calculating spreadsheet here somewhere and there's aldo one on sheldon browns websight.
Not sure if either are what your looking for as your talking about a moped but im sure they would help some at least ?
i think JRH's is in open office format? i have excel only and i had to get a sun plug in to use it.
let me know if you can use it and i'll find the link.

Cheers,

D
 
With my tire: 130/80V18,, you need to calculate ACTUAL tire circumference.

for my tire above (from this site: http://www.csgnetwork.com/tiresizescalc.html )
Diameter: 26.189 inches
Circumference: 82.275 inches (or 6.85ft)

6.85ft * 500 rpm * 60min/hr / 5280ft/mile = 38mph not very fast.

if you WANT 60mph, then go backwards:
60 mph * 5280ft/mi / (60min/hr * 6.85ft ) = 770.8rpm

6000/770 = ~7.7:1 ratio to go 60mph.

here's a good excel sheet to play around with:
http://lennonrodgers.googlepages.com/vehicle_calculations.xls
 
Here's what the moped folks said.... can someone explain to me where the x / y / a / b /c s go?

Hurt brain. Help with gear ratio calc?
Author: mike (---.dyn.aci.on.ca)
Verified User: northernmike
Date Posted: 01-15-09 12:20



moby. rear tire 1675mm circumference as measured.

41t rear sprocket

13t front sprocket

215 driven pulley radius

three fixed motor pulleys to choose (electric conversion) =

100.33, 90.17, and 80.01mm

how many kph / rpm am I looking at?

OUCH! my head.

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: Julian (---.corning.com)
Verified User: carpy
Date Posted: 01-15-09 12:22
Unit conversion mike.

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: mike (---.dyn.aci.on.ca)
Verified User: northernmike
Date Posted: 01-15-09 12:25
ahhhhh…

rear tire 66 inches circumference

41t ..

13t..

driven pulley 8.46 inches diameter

motor pulleys 3.15, 3.55 3.95 inches.

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: Julian (---.corning.com)
Verified User: carpy
Date Posted: 01-15-09 12:29


597 wheel rev/km

1883 driven pulley rev/km

4047 100mm pulley rev/km

4497 90mm pulley rev/km

5059 80mm pulley rev/km

That might not be right I did it super fast I have a meeting k bye

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: Julian (---.corning.com)
Verified User: carpy
Date Posted: 01-15-09 12:30


No I mean use unit conversions, mike!

Haha

Sorry for the confusion

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: Julian (---.corning.com)
Verified User: carpy
Date Posted: 01-15-09 12:32


I can do this later in more detail if you want

Email me a reminder

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: mike (---.dyn.aci.on.ca)
Verified User: northernmike
Date Posted: 01-15-09 12:54


It looks right… Scary!

SO. With a motor that turns 130rpm/V running 48V, that’s 6240 rpm, or 374400 revolutions per hour…

/ 4047 = 92kph (57mph)

/ 4497 = 83kph (51mph)

/ 5059 = 74kph (45mph)

... theoretically …

I might have to reinstate my larger rear sprocket!

THANKS JULIAN!

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: Julian (---.corning.com)
Verified User: carpy
Date Posted: 01-15-09 16:22


Hey no problem. Did you double check those numbers?

Reply To This Message
Re: Hurt brain. Help with gear ratio calc?
Author: mike (---.dyn.aci.on.ca)
Verified User: northernmike
Date Posted: 01-15-09 16:23


No .. I don’t know how! Hence my post :)
 
your final ratio is:
41/13 = 3.1538

your primary ratios could be one of the following:

1) 215mm driven / 100.33mm powered = 2.1429
2) 215mm driven / 90.17mm powered = 2.3843
3) 215mm driven / 80.01mm powered = 2.6871

TOTAL ratios could be
1) 3.1538 * 2.1429 = 6.7582
2) 3.1538 * 2.3843 = 7.5196
3) 3.1538 * 2.6871 = 8.4745


130kv at 48V would be 6240rpm lets say realistically 5500rpm or so.
your tire circumference is 0.001675km

Your speed would be one of the following @ 5500rpm:
1) (5500rpm/6.7582) = 813.826 rear wheel rpm
813.826 * 0.001675km * 60minutes/hour = 81.78km/h

2) (5500rpm/7.5196) = 731.4219 rear wheel rpm
731.4219 * 0.001675km * 60minutes/hour = 73.51km/h

3) (5500rpm/8.4745) = 649.006 rear wheel rpm
813.826 * 0.001675km * 60minutes/hour = 65.23km/h
 
215mm driven pulley measurement is not circumference, but diameter... and so are the 3 drive pulley measurements... but, does one not use the same equation to convert them both anyway?
 
northernmike said:
215mm driven pulley measurement is not circumference, but diameter... and so are the 3 drive pulley measurements... but, does one not use the same equation to convert them both anyway?
Click to expand...

its still a ratio....

Circumference is Pi * diameter.... so if you're taking a Pi*d1 / (Pi*d2), Pi cancels out and you're left with d1/d2, the ratio of diameters.
 
At least you'll get some torque :)

for a moped it'l be just fine, just keep it below 30-35 anyway (legality issues).... and you'll have fun!

there's a guy over on visforvoltage that converted his with the same motor....

http://visforvoltage.org/blog/nasukaren

maybe follow some of his hints and tips
 
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