Final Drive Ratio Formula

[pura vida]

Can someone please give me the formula to calculate the final drive ratio? I thought applying the percentage difference between the bottom gear and drivers to the quick drive gearing ratio would give me the final drive ratio but that does not seem to be working. I don't seem to be smart enough to get my math to come out to the same as published. Any help would be greatly appreciated. Thanks

PV

 

[Wood]

http://www.snowest.com/forum/showthread.php?t=394361&highlight=Gearing

 

[pura vida]

Yup, seen that, not what I'm looking for. Great info, but can I please get the formula on how those numbers are computer? None of those numbers or combos represent what I have and what I want to go to. Thanks.

PV

 

[pura vida]

The way I'm computing it I come up with a final drive ratio of 2.100 for a stock quick drive axys.

QD=43/22=1.954

Diameter of bottom gear - 0.49396"
Diameter of 7T 2.86 drivers - 0.53105"
Bottom gear/drivers = 1.075
Multiple the two ratios -1.954*1.075=2.100

Published ratio is 2.23. WTH am I doing wrong?? This isn't that hard...

PV

 

[LoudHandle]

The way I'm computing it I come up with a final drive ratio of 2.100 for a stock quick drive axys.

QD=43/22=1.954

Diameter of bottom gear - 0.49396"
Diameter of 7T 2.86 drivers - 0.53105"
Bottom gear/drivers = 1.075
Multiple the two ratios -1.954*1.075=2.100

Published ratio is 2.23. WTH am I doing wrong?? This isn't that hard...

PV

Bottom gear does not matter (you already have that accounted for with the actual ratio)
Diameter of driver does not come into play either (the circumference difference is what matters here.

8 Tooth @2.86" =22.88"
7 Tooth @2.86" =20.02"

22.88/20.02=1.14286

1.954*1.14286=2.23315



OR MH's way as detailed in his post; either way works once you have your head wrapped around what to base your calc on.

20.02/22.88=0.875 or 87.5 percent (he rounded to 0.88 or 88%)
1.954/0.875=2.2331
1.95/0.88=2.2205 (which leads to a slight difference)

 

[mountainhorse]

2.23 is not the actual ratio...but a 'claimed...comparative" ratio... compared to what everyone is used to with ProRide and Dragon sleds.

22/43 gearing... is a ratio of 1.95:1 as you note.

When you want to compare to what we are used to with 8-tooth, 2.86" pitch drivers... you will then divide that by .92 for a 7-tooth, 3" pitch ... or .88 for a 7-tooth, 2.86" pitch.

All but the 3"-lug tracks use the 2.86" pitch... the 3"-lug tracks use the 3" pitch.


SO... your 1.95:1 gearing and 7-T, 2.86"-P drivers will give a "comparative" ratio of 1.95÷.88=2.22:1

With 1.95 and 7-T, 3"-P drivers... will give a "comparative" ratio of 1.95÷.92=2.12

Numbers I've provided are rounded to 2 decimal places and therefore are 'approximate'.




.

 

[pura vida]

Ahhhh! Gotcha. Thank you much. I misunderstood what the numbers represented. The 2.23 is just the effective difference by changing the drivers. Makes a little more sense now. I'll dig into the numbers after work tonight to see if I can figure out what I'm trying to come up with. Shouldn't be an issue now.

So, was my 2.1:1 calculation for a final drive ration on a stock Axys (with a QD) correct then? Or I'm I still missing something fundamental? Thanks.

PV

 

[pura vida]

Still feel like I'm missing something b/c as the driver gets bigger the math makes the gear ratio lower, but larger drivers should equate to a higher, not lower ratio (all things equal, your track will spin faster). Maybe I need to divide rather than multiply... Huh, I used to be smart enough to get this right. I must be officially old.

 

[mountainhorse]

Hey, we're all getting old!!

Maybe I need to divide rather than multiply...

Yes...Divide the ACTUAL ratio (bottom/top tooth count)... by the correction factor for comparison (.92 for 3"-pitch 7-tooth, .88 for 2.86"-pitch 7-tooth)



.

 

[mountainhorse]

TTT with relevant quote from previous post for Justin's inquiry.

2.23 is not the actual ratio...but a 'claimed...comparative" ratio... compared to what everyone is used to with ProRide and Dragon sleds.

22/43 gearing... is a ratio of 1.95:1 as you note.

When you want to compare to what we are used to with 8-tooth, 2.86" pitch drivers... you will then divide that by .92 for a 7-tooth, 3" pitch ... or .88 for a 7-tooth, 2.86" pitch.

All but the 3"-lug tracks use the 2.86" pitch... the 3"-lug tracks use the 3" pitch.


SO... your 1.95:1 gearing and 7-T, 2.86"-P drivers will give a "comparative" ratio of 1.95÷.88=2.22:1

With 1.95 and 7-T, 3"-P drivers... will give a "comparative" ratio of 1.95÷.92=2.12

Numbers I've provided are rounded to 2 decimal places and therefore are 'approximate'.




.

 

[WolfieSKS]

This is a pretty slick link, for calculating various gearing options. I'm looking to gear my SKS up a bit without losing the ability to chug through deep snow. Comes with a 20/40, and I'm thinking of a 21 40 or 21 39 combo.

http://www.turboaddictionparts.com/catalog/snowmobilecalculator.html