Primary And Secondary And Final Drive Ratio

[90GTVert]

This is an excerpt from my 90cc swap thread that I though would be useful here....







Now you have a choice to make. You should have both the gearset from the scooter and the ATV at your disposal at this point. Either set should be compatible with the ATV transmission case.



Both sets of gears are likely to be different final drive ratios, so you should choose the set that will be most likely to help you accomplish your goals with the engine swap. In my case, the 90cc gears would be more useful for top speed and the 50cc gears would provide superior acceleration.

90cc Gears
Primary = 15/47 = 3.133333333333333:1
Secondary = 11/31 = 2.818181818181818:1
Final Drive Ratio = 8.830303030303029:1

50cc Gears
Primary = 13/48 = 3.692307692307692:1
Secondary = 12/36 = 3:1
Final Drive Ratio = 11.07692307692308:1

If you aren't sure what all that means, let me explain it briefly and how you can find your final drive gear ratios. I believe a lot of 50cc and 90cc rations will be the same as these, but it never hurts to check.

There are 2 gearsets in the transmission that form one final drive ratio. They are the priomary and secondary gears. Primary gears are the gear on the end of the shaft drive by the clutch and the gear that it drives. Secondary gears are the gear on the axle and the gear that drives it off of the primary gears.



To determine your primary, secondary, and ultimately the final drive ratio, begin by counting the teeth on each gear and writing it down. t is often helpful to mark the tooth that you start counting on with a dot from a marker so you don't forget exactly where you started. If you have any doubt that you miscounted, do it over again to be sure.

You sohuld now have two numbers for your primary gears and two numbers for your secondary gears. Using my 90cc gears and an example :

Primary : 15 / 47
Secondary : 11 / 31

Now we can use division to determine the ratios of each gear set. Divide the number of teeth the gear being driven has by the number of teeth of the gear driving it. Again using my 90cc gears as an example :

Primary : 47 / 15 = 3.133333333333333:1
Secondary : 31 / 11 = 2.818181818181818:1

Now you know both your primary and secondary gear ratios, so you just need to combine them to determine the final drive ratio. Multiply the primary and secondary ratios to find the final drive ratio. Using my 90cc gears as an example again :

3.133333333333333(Primary) x 2.818181818181818(Secondary) = 8.830303030303029:1(Final)

You've determined the final drive ratios of each gearset, but what do those numbers mean? The gear ratio expresses how many revolutions of input are needed to output one revolution. In the case of the final drive ratio, the number represents the quantity of revolutions of the clutch bell turning the driveshaft that are needed to rotate the tire one full revolution. For example, my 90cc gears have an 8.830303030303029:1 final drive ratio. So my clutch bell revolves 8.830303030303029 times in order to rotate my rear tire just once.

The more times the clutch bell has to rotate to spin the rear tire once, the greater the potential for acceleration. The less revolutions the clutch bell needs to make to rotate the rear tire once, the greater the potential for top speed.

Numerically Higher Gear Ratios = Greater Acceleration/Higher Cruise RPM
Numerically Lower Gear Ratios = Greater Top Speed/Lower Cruise RPM

Determining exactly what speed you can achieve with these gear ratios is a little tricky, but the numbers should give you some idea of what you want to try. I went with the scooter gears (11.08:1) on this build and so far I've seen 58MPH at as much as 8,800RPM. Acceleration is great.

[90GTVert]

From a post trying to help someone figure out what gears they wanted....



To begin with, figure out your current gearing. You said it's 13/52 primary and 13/44 secondary in another thread. Divide each set of numbers to get the primary and secondary ratios. So 44 divided by 13 is 3.38 or 3.38:1. 52 divided by 13 is 4 or 4:1 gearing.

Now you know what your primary and secondary ratios are. To get your final drive ratio, multiply the primary by the secondary. In this case, 3.38 x 4, which equals 13.52:1. That's your final drive ratio.

If you are trying to figure out a percentage change in gearing, just multiply the final drive ratio by that percentage. Say you want 15% upgear. 13.52 x 0.15 = 2.03. So your gear ratio needs to go up or down by 2.03 to make a 15% change. You want to upgear, so 13.52 - 2.03 = 11.49:1. By the way, just to be clear about the :1. That's how gearing is stated, because the input must turn x amount of times to turn the output once.

Once you know that, you just do the math on each gear set you are interested in and see how they'd work out. I'll use the last set you posted, 15/42 secondaries, as an example. Your primary gearing remains at 4:1. Your secondary gearing is 42 divided by 15, so 2.8:1. Primary x secondary is 4 x 2.8, which comes out to 11.2:1. Pretty close to the 15% goal of 11.49:1. Actually about 17%.


The idea would be that your RPM would decrease by the amount you change gearing. 10,000RPM at 45MPH would drop 17% lower, or 8,300RPM... in theory. In reality, you may need to adjust the CVT a bit to get revs where you want.


One other way to try and guess at gearing is more math.

MPH x Gear Ratio x 336 / Tire Diameter = RPM

Let's just say it's a 20" diameter tire to make things simple.

50MPH x 13.52 gearing x 336 / 20" tall tire = 11,357RPM

50MPH x 11.2 gearing x 336 / 20" tall tire = 9,408RPM


Again, that's all assuming you have a fixed gear transmission to get the exact RPM changes. CVT throws things off a bit, but you can sorta get an idea of how much things will change at least. Just more numbers to play with.


BTW, if you need to know your tire diameter, you can find it by the size. Say it's a 130/70/12.

((First number x second number (as percentage)) x 2) + (third number x 25.4).

You are multiplying the last number by 25.4 because the first number is mm and the last is inches. Inches x 25.4 = mm.

((130 x 0.70) x 2) + (12 x 25.4) = 486.8

486.8 is the diameter in mm. To get inches, divide by 25.4.

486.8 / 25.4 = 19.17" tire diameter.


Knowing all of that, you should be set to play around and figure out what you want if I haven't screwed it all up from being tired.


I should also note I have a couple of calculators so you can just plug in tire and gear numbers at www.49ccscoot.com/calculators.html

[90GTVert]

If you don't want to go through all of the disassembly and counting as shown above to figure out your final drive ratio, you can use an easier method. The accuracy is best with disassembly and counting, and that way you know primary and secondary ratios which makes choosing aftermarket gears easier. This method is good for getting an idea of what you're working with though.


www.youtube.com/watch?v=4SBiNmeeqhY