Final drive ratio same as diff ratio?
 

Hi
I note in Subaru tables such Final Drive ratios as 4.444, 4.111 and 3.900. Is this the same as the 'Diff ratio' (ie: pinion:crown wheel ratio)? I ask because one source I have quotes:

"You may have heard terms like rear axle ratio or final drive ratio. These refer to the gear ratio in the differential. If the final drive ratio is 4.10, then the ring gear has 4.10 times as many teeth as the input pinion gear."

Yet I read elsewhere that the Final Drive ratio includes the transmission ratio and maybe even wheel diameter.

On balance, seems to me that if table quotes FD=4.111 then that is the diff ratio. Correct or not?

Sorry - still trying to learn.
Thanks

On balance, seems to me that if table quotes FD=4.111 then that is the diff ratio. Correct or not?

Correct.

Bollocks.

Except that..................
European/UK (as opposed to Japanese)Subaru have a rear diff ratio of 3.545 and a front diff ratio of 3.9 but the rear diff is driven through the centre differential which has a drop gear of 1.1:1 which converts the rear 3.545 diff ratio to a 3.899 final drive ratio. To all intents and purposes 3.9 front and rear.

Jap boxes have a centre diff drop gear of 1.0:1 so diff ratios of 4.44,4.11 and 3.9 are also final drive ratios.
I am not sure why Subaru went to the trouble of complicating things with a 1.1:1 drop gear???:cuckoo:????????????/

Thanks Harvey for your confirmation. Yep I was aware of 1.1 centre diff change (none in mine in New Zealand!). Cheers

Harvey from what i have read it was to reduce the prop speed for comfort.

Cheers Arch but I am sceptical. Why does a 10% drop in prop rpm make a difference to comfort?

Prob some tefal head in Japan decided that a reduced RPM would reduce vibration hence reduce noise which as you know from prop aircraft etc increasing rpm of a poorly balanced mass will increase the noise/vibration and generally reduce the comfort. Would you feel the difference by the 10% reduction in propshaft rpm, I doubt it.

Cheers Arch. Aircraft props are 1.5 to 2 metre diameter so noise is a factor but on a prop shaft which even with flange is only 76mm diameter I can't see that vibration will have anything to do with it. I suspect that speeds in the UK or Europe may be higher than those achieved in Japan so for some reason they wanted to reduce rotating speed but I honestly can't think of why and hopefully somebody with more knowledge will come along and enlighten us.

Aircraft prop, fortunately isnt strapped 6 inches below our backsides lol But would definately be interesting to know the reasoning.

the 10% speed difference is to keep the center diff constantly spinning, so the bushings will be lubricated.

also, the transfer case can not be 1:1, or else the gear will wear prematurely.

If the tooth on one gear is always hitting the same tooth on the other gear, excessive wear will appear on the "most out-of-spec" tooth. If a tooth is designed to hit every tooth on the other gear, the break-in process will be a lot quicker

Anthony think you may need to think again the cars equiped with a 3.9 ratio rear diff have 1:1 drop gears.

Aren't those cars come without a center diff?
Just the both ends of the output shaft is used.

some thing like this?
http://www.subaru4you.co.uk/images/p...ngerWRX500.jpg

No they have a centre diff

Oh dear ;)

Some people do get very confused don't they?:lol: