Calculating Gear Ratios

[robertdjung]

From Kaptain Krunch:

basically you calculate a gear ratio by dividing your driven sprocket or pulley size by the drive pulley or sprocket.

(((the driven sprocket or pulley is the sprocket/pulley that is not connected to your power source, either on a jackshaft, axle, or input to transmission, the drive is the one that is directly on the engine shaft, or other source of power)))

So for example, you have a 12tooth engine sprocket and 72tooth sprocket on your jackshaft, 72/12= 6 thats a 6:1 ratio meaning your engine will spin 6 times for every one time your jackshaft spins. Say you have a 10" pulley on your jackshaft, that runs to a 30" pulley on your axle, 30/10=3 thats a 3:1 ratio. Now to figure out what the overall ratio is, you simply multiply, 3:1x6:1= 9:1 ratio from engine to axle.

A lower gear ratio will give you more torque, but less speed, where as a higher gear ratio is the exact opposite. A 9:1 ratio will give you much higher torque, pulling power, and takeoff but a much lower top speed.


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Thanks, Kaptain!

[02mx-5]

WOW. Thank you. That makes it so much clearer. I just made a thread asking about this.

[Rickracer]

Also, if you are using a motorcycle engine, there is also a primary ratio (the ratio between the crankshaft and the clutch shaft) that has to be figured in. On my RM 250 motor, it's 3:1, so you must multiply the final drive ratio x the primary ratio to get the overall gear ratio. For example, right now my final drive ( front and rear sprockets) is 13/23 which works out to about 1.7:1, then I have to multiply that by 3, and 1.7 x 3 = 5.1, so that is my overall gear ratio. :cool:

[1992suburban]

You got it rick!!!

[Kaptain Krunch]

Robert, i wrote that in a hurry and i see a few mistakes

From Kaptain Krunch:

basically you calculate a gear ratio by dividing your driven sprocket or pulley size by the drive pulley or sprocket.

(((the driven sprocket or pulley is the sprocket/pulley that is not connected to your power source, either on a jackshaft, axle, or input to transmission, the drive is the one that is directly on the engine shaft, or other source of power)))

So for example, you have a 12tooth engine sprocket and 72tooth sprocket on your jackshaft, 72/12= 6 thats a 6:1 ratio meaning your engine will spin 6 times for every one time your jackshaft spins. Say you have a 10" pulley on your jackshaft, that runs to a 30" pulley on your axle, 30/10=3 thats a 3:1 ratio. Now to figure out what the overall ratio is, you simply multiply, 3:1x6:1= 9:1 ratio from engine to axle.

A lower gear ratio will give you more torque, but less speed, where as a higher gear ratio is the exact opposite. A 9:1 ratio will give you much higher torque, pulling power, and takeoff but a much lower top speed.

I put the corrections in bold, it would be great if you could change those, Thanks!

[robertdjung]

corrected, thx :thumbsup:

[anderkart]

Here's a couple Jackshaft ratio calculator links:
http://www.compgoparts.com/TechnicalResources/JackshaftRatioCalculator.asp
http://www.fasttrackraceway.com/calc/index.php

Here's 3 more gearing calculators for karts/Minibikes that dont have a Jackshaft:
http://www.diygokarts.com/speed-calculator.html
http://www.gokartracing.org/Calculate.htm
http://www.sadik.net/gokart/calc.htm

[jamfz]

I would like to post something that I couldn't find anywhere else. I had to make calls to find out what I needed to do. I was building a mini chopper same set up as a go kart.
I started out with 12 tooth to 72 tooth and had a 6 ratio it was not enough to pull me with a 16" wheel on back and a 3.5 briggs. It took off really slow but had good top end. I put in a jack shaft with 18 tooth and 10 tooth. I ended up with a 10.8 ratio and now it takes off good and still goes 20mph. I never found a good explanation for how much to increase the ratio to get power. From my experience with a 6 i had to drag my feet to take off and now with a 10.8 I hit the gas and go. Also it will burn the tire if I take some of my weight off the seat. Finally that jackshaft calculator is right on for ratios and I went 2.5 mph faster than it said I would but I don't have a governor. Hope This Helps Someone. MY RATIO= 12/18=1.5 72/10=7.2 then 7.2x1.5=10.8 good torque with a 16" wheel.

[bajagokart]

i have a 16.2:1 ratio on low end and 6:1 on high end with my torque converter lol

[toystory_4wd]

Also, if you are using a motorcycle engine, there is also a primary ratio (the ratio between the crankshaft and the clutch shaft) that has to be figured in.

Yeah- Just figured that out. I've been planning an offroader using an XV250 engine, and was having fits trying to calculate a decent rear sprocket size for my purpose. My calcs were WAY off published specs for the bike, and I couldn't figure out why. Then, on a spec sheet somewhere, I noticed TWO reduction ratios listed, primary and secondary. The secondary was easy- the ratio between the tranny output sprocket, and wheel sprocket, but the PRIMARY ratio screwed me up. Didn't match any of the tranny gear ratios, nor an average. So I started to wonder if the crankshaft was NOT a direct 1:1 to the tranny. So- I shifted the tranny into 4th gear (1:1) and put a socket wrench on the crankshaft. I marked both the socket and the sprocket. Rolling the engine over by hand, I confirmed that the engine was turning 3+ revs for every turn of the tranny, this matched the published primary ratio of 3.13:1. This is why my numbers hadn't been jiving, I was operating under the assumption that a bike driveline was like a car's- for every turn of the engine crank, the tranny's input shaft would crank once. NOT SO! Lesson learned! :idea2: