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Discussion Starter #1
Say I wanna check the diffy ratio. My 260Z diffy is believed to be a stock 260Z R180 diffy with a gear ratio of 3.364.

1.) If I put the car in neutral, and jack up one rear wheel, then turn the tire while watching the driveshaft, how many turns of the wheel will it take for the driveshaft to make one complete revolution.

2.) What is the number of teeth on pinion gear and on the ring gear? Could a lower number of teeth been used to achieve the same ratio? Why or why not?

3.) Bonus! Explain why there is factor of two involved in question #1 in terms of the two diffy side gears and the pinion mate gears. Would changing the tooth ratio of the side and pinion mate gears change this factor?
 

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50:50

You take the tooth count of the pinion gear and divide it by the tooth count of the ring gear and that gives you your ratio: You take the tooth count of the crown gear and divide it by the tooth count of the pinion gear and that gives you your ratio.
ANSWER: If the crown gear has 37 teeth and the pinion has 11 the ratio is 37divided by 11 which is 3.364 so the ratio is 3.364:1 1 being the crown gear and3.364 being the pinion. So for 1 complete revolution of the wheel the pinion gear has to turn 3.364 revolutions, .....I think....I missed math that day!
 

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Re: 50:50

1) 2 * (1/3.364) = 0.5945 Turns at the wheel for the driveshaft to make one turn. But that's pretty useless since the measure would be difficult to determine. More useful would be to turn the driveshaft until the tire makes one revolution. Or, (1/2) * 3.364 = 1.682 turns at the driveshaft to make the tire turn once.

2) "39/11", you can not lower the number of teeth and still acheve the same ratios, this is a simple problem of fractions. By lowering the pinion to 10 you would need a 34 ring to even come close, that's 3.4:1 instead of 3.364:1 (As a side note, due to this same problem many 4-wheel drive vehicles actually have different ratios front and rear, a common example is a 4.10 front and 4.11 rear ... this comes about because of the available ratios for each axle, most ratios are matched.)

3) Since one output shaft of the differential is not moving then the other will spin twice as fast, this is the same principle that planatary gear boxes use (or gear-splitters in truck lingo). The tooth count of the side gears and spider gears would make no difference since both side gears HAVE to be the same and the spiders only act as idlers ... therefore the "ratio" from this point of view will always be 2:1.

ConorP
 

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Just pull the rear cover, read the numbers off the ring gear, and do Graham's math. Any effort to determine ratios or whether a LSD is in the diff without pulling the cover is really a waste of time.

With the cover on a welded diff acts just like a LSD. And I dare anyone to determine whether a diff has a 3.70 vs a 3.90 or a 4.11 vs a 4.38 just by spinning the input shaft and counting stub axle rotations.

- John
 

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Agreed, I looked at this from the trivial point of view just to answer the question. Checking the stampings is way faster and acurate.

ConorP
 

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Discussion Starter #7
Gold stars to ConorP & John Coffey...

Black stinky rotten egg to Al 260Z, your favorite foot-in-mouth Z professor.

John gets a gold star for the Gordian Knot solution: The **** with mental gymnastics - cut out all the bullshit and check the nos. on the gearset. (easier even than counting teeth).

ConorP is right on on all counts, especially in pointing out that you want to make multiple revolutions of the wheel for a single revolution of the driveshaft. (Shame on me!)

As I posed the question, I could not for the life of me remember whether to multiply the ration by two or by one-half with one wheel blocked. (One-half is correct.) Nor could I visualize the diffy action that causes this. I've been playing with this mental visualization all weekend, and just barely grasp it. See what you can do when you're not distracted by some lousy radio station blaring at you! ;-). I certainly can't explain it any better than ConorP did.
Recall that the pinion carrier shaft revolves with the ring gear. If one side gear (output shat) is blocked, then the free side gear shaft must advance twice as much. The ratio of the side to pinion gear is immaterial because the ratio of the side gears to each other is always one-to-one.

Gear ratios are interesting as they illustrate some basic characteristics of numbers. The number of teeth on a gear must always be an integer number. The ratio of two integers is a rational number. Different gear sets can give the same number ratios such as 40/10 or 20/5. (4/1 is the same, but a gear with one tooth would not have very smooth action!) Remember simplifying fractions to their lowest possible denominator? (Sorry to remind you!) Well, in the 260Z case 37/11= 3.363636363636.... (isn't that fun - the 36363636 repeats infinitely!). You could get the same ratio by doubling the numbers to 74/22, but there is no reducing this pair. Furthermore, 37 and 11 are both prime numbers - no factors at all for either number!

Later all!
:-D
Al
 

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Discussion Starter #8
Change that gold to silver for ConorP!

Conor, you wrote:

>2) "39/11"
should be 37/11

Details, details!
Oh, what the ****,
A gold star for ConorP after all !
(grading on a curve)

LOL
Al
 

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Re: Change that gold to silver for ConorP!

I'll be amazed if you read this AL260Z (since it's buried in the archives now)

Sorry about this mix up, I was plugging in my own cars ratio when doing some of the math. So for my 3.54 rear end "39/11" is correct ... but we weren't talking about my car.

At least this got some mental thinking going on ... instead of "I've checked everything and my headlights don't work."

ConorP
 
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