Datsun Roadster Forum-311s.org

I know the 1600 came with the 3.89 rear end, is there anyway to tell by picture if this is a 3.89 ?

No way to tell from the outside. There's a way by jacking up a rear wheel (I think the right wheel), then count how many revolutions the driveshaft makes as you turn the wheel one full revolution. The number of driveshaft revolutions should tell you whether it's a 3.89 or 3.70.

DId a quick search and found this:

theunz wrote:You can get a rough idea of the ratio by putting a mark on the drive shaft and then rotate a rear wheel one full revolution. Make note of how many revolutions the drive shaft makes. If the drive shaft turns 3 and 3/4 times than you have a 3:75 ratio, if it turns 4 times than its a 4:00 ratio ect. The common ratios for the roadster are
3:70
3:90
4:10
4:38
4:62
5:13
These numbers are approximate, not exact.

In practice, it’s very difficult to make an accurate assessment of a 3.9 or 3.7 ratio based only on driveshaft rotations (two revolutions of just one tire - the other needs to be fixed) … you’ll probably end up approximating something close to 3.8 and then you’ll be right back where you started

Only sure way is to remove the cover and read the stamp on the Ring gear (if it’s still there) or simply count the teeth on both the Ring & Pinion gears (R/P= Gear Ratio).

From what I remember the 1600 4 speed is 3.89 (35/9). So, unless its been changed I would say it's that. But as Kai says, only sure way is take out to check, the ring is stamped with the ratio as in the 35/9. the diff ratio marries up with the stock gearbox ratios.

The 2L came with a 3.7.
The CSP311 came with 4.11 as standard, with 3.89 as an option.

???It's very easy:

Put a white dot on the drive shaft and jack up both tires.

Back off the brake adjusters so both wheels spin freely.

Put the trans in neutral and you will turn the driveshaft.

Have helper tell you when the tire has made exactly one rotation.

Count the driveshaft rotations. The driveshaft will rotate 3.5,3.7,3.9,4.11 etc times per tire revolution.
(3.5,3.7,3.9,4.11)

39 driveshaft rotations would be 10 tire rotations for a 3.9

JT68 wrote: Wed Jun 15, 2022 9:41 am ???It's very easy:

Put a white dot on the drive shaft and one rear tire. jack that tire up.
chock the other rear tire so it can't rotate.

Have helper rotate the tire with the dot exactly 10 times.

Count the driveshaft rotations. The driveshaft will rotate 35,37,39,41 etc times.
(3.5,3.7,3.9,4.11)

Might be easy, but your calculation would be wrong.
I’m sure you meant 20 times.

Or is it 5 times?

I just confirmed that with a 3.7. 3 3/4 pinion turns is one axle turn.

JT68 wrote: Wed Jun 15, 2022 11:26 am Not sure why this is confusing? It is 10 times.

If one wheel is chocked, the free axle is locked to the ring gear so 1 turn of the ring gear is 3.9 of the pinion for a 3.9.

I just confirmed that with a 3.7. 1 axle turn is 3 3/4 pinion turns.

Ok. So, with one tire chocked, how many turns of the free spinning tire does it take to turn the ring gear one full revolution on an open diff (non-posi) like on our cars?

It’s actually not that hard to tell rotating the pinion for one careful wheel revolution. A 3.7 will be 3 3/4 drive shaft revolutions, a 3.9 will be 1/10 turn short of 4, and a 4.11 will 1/10 turn past 4 for one axle rotation.

Chances are it is a 3.9.

What about the spider gears?

JT68 wrote: Wed Jun 15, 2022 3:35 pm … So yes, 10 turns of the wheel will give you 37,39, 41.1,43.8,46.2 etc turns of the drive shaft depending on the rear …

That’s only for a posi/ limited slip diff, which I agree is 1:1
But I specifically asked about an open diff. (non-posi), which is what the OP is trying to find out, and you’ve indicated it’s the same calculation:

JT68 wrote: Wed Jun 15, 2022 9:41 am ???It's very easy:

Put a white dot on the drive shaft and one rear tire. jack that tire up.
chock the other rear tire so it can't rotate.

Have helper rotate the tire with the dot exactly 10 times.

Count the driveshaft rotations. The driveshaft will rotate 35,37,39,41 etc times.
(3.5,3.7,3.9,4.11)

And again …

JT68 wrote: Wed Jun 15, 2022 1:59 pm Like I said, with one wheel chocked, the free wheel axle is locked to the ring gear, so one tire revolution is one ring gear resolution.

It is the exactly the same with posi, there is just no need to chock the wheel. (Both off the ground)


It’s actually not that hard to tell with one careful wheel revolution. A 3.7 will be 3 3/4 drive shaft revolutions, a 3.9 will be 1/10 short of 4, and a 4.11 will 1/10 rotation past 4.

Chances are it is a 3.9.

What I am saying is that the gear ratio calculation for an open differential differs from a posi/ limited slip (the BOLDED internet quotes you’ve referenced in your follow on post) … by double actually, which is the point I’ve been trying to make from the start.

Example for an open diff 3.90:1 as per the OP’s original car/question:

If you spin the free tire 10 times (with the other wheel chocked), you’re going to see 19.5 revolutions on the yolk, not 39.
At this point you can MULTIPLY it by ‘2’ if you are sure of the positioning, but spinning the tire 20 times in total will give you exactly 39 rotations on the yolk and your tape/white-out/ marker/ or whatever you used will line up exactly at the end of the 20 tire turns eliminating the guess work - There is no dividing by ‘2’ or any other calculation required.

So, am I wrong?

Kai, is this what you are referring to(?):

"Like previously mentioned, if your car has an open differential (non-positraction), you will want to perform the check with one rear wheel on the ground. Again, mark the inside of the tire and a spot on the driveshaft with your paint marker. Now, rotate the suspended tire two revolutions and count the number of driveshaft turns. The reason to make two revolutions, is the open differential actually rotates half as many times as a posi, because of the design of the “open” gear operation. Again, if the driveshaft turned 3 3/4 revolutions for two rotations of the tire, you have a 3.73 gear. A driveshaft that rotates 2 3/4 times announces a 2.76 gear ratio." (bold italics mine)

https://www.chevyhardcore.com/news/what ... -find-out/

Jon

jhayden wrote: Wed Jun 15, 2022 5:42 pm Kai, is this what you are referring to(?):

Like previously mentioned, if your car has an open differential (non-positraction), you will want to perform the check with one rear wheel on the ground. Again, mark the inside of the tire and a spot on the driveshaft with your paint marker. Now, rotate the suspended tire two revolutions and count the number of driveshaft turns. The reason to make two revolutions, is the open differential actually rotates half as many times as a posi, because of the design of the “open” gear operation. Again, if the driveshaft turned 3 3/4 revolutions for two rotations of the tire, you have a 3.73 gear. A driveshaft that rotates 2 3/4 times announces a 2.76 gear ratio. (bold italics mine)

https://www.chevyhardcore.com/news/what ... -find-out/

Jon

Yes! Thank you.

Yes, this is correct if you rotate the tire. I'll edit my post. What I should have said is if you rotate the driveshaft it is exactly the diff ratio regardless of open or posi-diff. 3.9 pinion turns =1 tire revolution. (that is the way I always do it, rotating the pinion with free wheeling axles)

So yes, if you rotate one tire with an open diff, you would have to correct for the spider gear ratio.

If you rotate the pinion instead, it will be the actual drive ratio.