WTF??? If the tire turns once and the drive shaft turns three times then the ratio is 3:00 to 1. If I'm missing something here I apoligize, but how does the other tire have any thing to do with figuring the ratio? However, when I did mine I counted the teeth on the ring gear and on the pinion gear, takes out all the guesswork.
I'll be the bearer of bad news, in that you are missing something. An "open" differential has a lot to do with the other tire being part of the equation. Let's use your example of a 3 to 1 rear gear ratio, and lets look at it from the perspective of the driveshaft/pinion. When you're going in a straight line, three complete revolutions of the driveshaft will turn BOTH wheels one revolution. If you could steer sharply enough in a circle so that the "inside" rear wheel did not rotate at all, then three turns of the driveshaft would result in the "outside" wheel rotating two complete revolutions. When one wheel is locked, ANY differential will basically multiply the rotation of the other wheel by a factor of two. Note that I said ANY differential, meaning that even a posi-type differential will do this, it just takes some force to overcome the friction of the clutches in the posi unit.
I'm glad I apoligized in advance, I guess you're never to old to learn something new. I'm gonna put that therory to the test with my roadster which I know has a 3:91 Mopar 8 3/4 rear end. I feel confident that your explanation will hold true but I'm retired and always like to prove things to myself, thanks for the tutorial. Next time I'll be able to reply with confidence.
DAMNIT!!! that one just made me spit coffee all over the keyboard. Boy is the wife gonna be pissed with sticky keys....
The numbers of teeth on the spider gears are immaterial. As long as it's not a locking diff, two turns of one wheel (with the other locked) equals one turn of both wheels. The pinion turn count equals the diff ratio. The OP did everything exactly right. The axle is 2.29 or thereabout. That's a very tall axle fopr a rod, and will not really be suited to use with an overdrive. Bigger rear tires will make it worse. It may be worth looking for something in the 3.3 - 3.7 range. Mart. Oh yes, I am a transmission engineer. EDIT: I may be an engineer but that doesn't stop me replying without reading the reply's on page 2!
Oh yeah, here's a video I put on youtube showing the exact method on an early ford rear. http://youtu.be/bkvp3Fq21ZM That's a genuine (UK) Birmingham accent. think Ozzie Osborne but a bit less slurred. Mart.
jmho 2.43 ratio. a little more than 2.25, but not as much as 2.50. but i'll keep reading and learning
The correct way has been told/written numerous times and some people still won't believe it so why keep beating it to death
These are so entertaining to read. The turn the wheel one turn and double the driveshaft revs instead of turning the wheel 2 turns is priceless.
dang, I missed this one when I was on a trip this weekend... The 80s GMs that did not have OD usually had really low number ratios. 2.29 is the answer.
I guess all I have to say is SORRY!!!!!!!. Didn't mean for this to turn into an opinion pissin match. Really, I have what I need to go ahead. I am SURE i have done the whole thing correctly BUT will be pulling the cover anyway just to be sure, not a big deal. Thanks to all Jim
If you turned one wheel two revolutions, and you count 2 and quarter turns of the driveshaft, and it's a 1980s GM rearend, then you have 2.29 gears. It's not rocket surgery.
I agree pulling the cover is the most accurate way to do it. I have seen many ring gears stamped with the number of teeth on them. www.randysringandpinion.com has several caculators you can use, goto the RPM calc and punch in the hieght of your tire and speed and how many rpms (tach) you are running and you can quickly figure out what gear you have by ajusting the ratio to match your rpms at a given speed. Example 50mph with a 25in tall tire at 2500 rpm will put you real close to a 3.73 gear with a 3 speed auto trans. MrC.
Playing around with Randy's Rin & Pinion RPM Calculator, for smiles and giggles, I figured some optimal tire sizes and speeds for an approximate highway cruising RPM of 2000. 29" tire - 109MPH 28" tire - 104MPH 27" tire - 101MPH 21" tire - 79MPH 17.5" tire - 65MPH With a 350 turbo, you would have to run a 25.5 tire to hit 2000 RPM at around 65MPH. (conference calls are awesome for figuring this kind of stuff!)
Yeah, 2.29s with an overdrive is definitely not the hot setup. The good news is that if the rear's out of an '86 midsize car, it's a 7.5 and S-10 gears will work. Lots of 3.42 and 3.73 ring&pinions waiting for you to take them home for cheap.
Posi units don't have clutches, Limited slips do..... And you really shouldn't use the word ANY, I bet I can take a open and make both wheels turn the same speed going around a corner, all I need is a welder.
Well then, I guess all of those vendors that sell positraction clutch replacement packs are going to have some old inventory, aren't they. You do realize there are two different types of GM positraction differentials, correct? There is an Eaton style which uses clutches, and the Auburn cone style, which does not have clutches. Further, there are a couple different versions of the clutch style, one that is spring-loaded (Eaton), and another like was used in the early 9.3 inch Olds/Pontiac rears, which uses pins and ramps to put pressure on the clutch packs when one wheel looses traction.
Hey Ebb, Ya might wanna do some more research.. Right off of Eatons website. "The Eaton Posi limited-slip differential prevents wheel slip before it can get started." Aubrun Cone, again off of their site.. For 48 years Auburn Gear Inc. has been the leader in performance limited slip differentials. Our unique cone style differential provides more bias torque as compared to any other limited slip brand. The difference in a posi(locker) and a limited slip is a Posi (locker) can't be overpowered. Any differential that uses clutches, cones etc.. is a limited slip and can be overpowered. A posi is like a Detroit locker, tru Trac or any other tooth style locking differential. And yes I do realize the differences in differentials as it is just part of what I do for a living.
I though Positraction was a Chevrolet trademark name for a limited slip differential....???? You can make up your own definitions for words, but it gets difficult to communicate with others if your definition isn't the same as everyone's.
You said: And then this: And you also added: Eaton, the manufacturer, states that their "Posi" is a limited slip, not a locker. They do make both types, but they aren't calling their locker a Posi. You are.
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