Thread: Explain gearing, posi traction etc...

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Can someone explain in simple terms or direct me to good SIMPLE READABLE info about rear end gearing and what posi and non posi is? How do I decide what to put in my car...how do I know what's compatible with engine, trans etc...
Jeri

I'll try : Posi trac is when both wheels put power to the ground .
non posi : would be where only one wheel puts power to the ground .
"How do I decide what to put in my car?" your choice .... what are you going to do with the car ?
"how do I know what's compatible with engine, trans etc?" Again what are you going to do with it ? what trans auto/manual? what you thinking of running under the hood ?

If your going to run 1/4 miles then build it for that ,Daily driver ? build it for that ....... So what direction you wanting to go ?
Tech gives a good explanation of gears some where in a thread but I couldn't find it . .... " I'll keep looking "

hope that gets ya started ....
T L

I bought a beat up 78 T/A... I also have a Pontiac 455 engine that I want to have rebuilt. I want a manual transmission. I want the car to be comfortable for street driving, however, I also want it to have enough get up an go that I don't have to worry about my kids v6 volkswagen leaving me as if i'm standing still...(yeah, that happens with the 301 turbo). I don't plan to do any racing...but I do want something impressive under the hood anyway...Is that vain?

The posi thing makes sense to me...that's simple enough...why wouldn't you want both wheels putting power to the ground? Also, what do the gearing ratio's mean...how do you choose what's right for your car or needs?

The higher the number, say 4.11, the lower the ratio. For the street I'd go posi and somewhere around 3.00. The posi will help to curtail the fish-tailing on wet/loose pavement and is a benefit if you become stuck in snow as long as the rear wheels aren't buried. Economically a higher gear ratio is usually better, but low end performance suffers.

Jeri, the rear end gear you use will also depend on the transmission you use. A 4 speed trans will run at a ratio of 1 to 1, from the input to the output in 4th gear. 5 speed and 6 speed transmissions will have 1 or 2 gears that run at less than 1 to 1 (overdrive).

When you combine the gear ratios for the rear end and transmission, you want to try to have a happy harmony. If you run a 3.00 final drive (rear gear) and a double overdrive 6 speed trans, odds are the engine may be turning a idle (or less), even on a 65 MPH highway. At the same time, running a 4.56 final drive with a non-overdrive transmission, you could be spinning your engine at over 5000 RPM at 55 MPH.

Just a rough guess, but if you go with a 5 speed or 6 speed transmission, I'd look at a 3.42, 3.55 or 3.73 final drive. If you plan on a 4 speed, I'd go between a 3.00 to a 3.25 rear gear.

Jeri, the numbers used to identify the differential gear ratio are an indication of the number of times the crankshaft of the motor turns as opposed to how many times the drive axles in the differential turn. Using Ken's examples, with a 4.11:1 ring and pinion, the crankshaft of the motor is turning four point one one times for each revolution of the drive axles, so that at 3,000 crankshaft rpm's for instance, the drive axles are turning 729.92 times (3,000 divided by 4.11). With this gear, at this crank rpm's and with a tire that measures 88 inches circumference, the vehicle would be traveling at a speed of 60.82 mph. (3,000 divided by 4.11 times 88 divided by 1056). (1056 is the constant in the formula). I used an 88 inch tire because it is 28 inches tall and that is a fairly common size. (28 times pi (3.14159)= 87.96 inches rounded off to 88).

Now, let's look at the work applied to the drive axles with this combination. If a 4-stroke motor fires each cylinder every other crankshaft revolution and you have 8 cylinders, then for each revolution of the drive axles, you'd have the engine crankshaft turning 4.11 times and delivering 16.44 power pulses to the drive axles. (8 times 4.11 divided by 2). We have to divide by 2 because, remember, the crankshaft only makes 1/2 power pulse for each 1 revolution of the crank on any given cylinder, so with one crankshaft revolution of an 8 cylinder motor, you have only 4 cylinders firing. The other 4 will fire on the second revolution of the crank.

Now, with the aforementioned 3.00 gear, you can see that we're going to slow the motor down for the same 60 mph speed of the car. 8 times 3 divided by 2 = 12 power pulses delivered to the drive axles for each revolution of the crankshaft.

So, it's easy to see that if you're using less power pulses to go the same speed, you'll use less fuel. The flip side of it is that you're not delivering as many power pulses to the drive axles, so you won't have as much power to move the car.

This is why you'll see drag strip cars using a very high numerical gear. They have to deliver the maximum number of power pulses to the drive axles in a given distance (1,320 feet). There's more to it, of course. The other thing to consider is that the motor makes more horsepower the higher it is revved (up to a point), so they use a gear that will allow the motor to reach the maximum engine revs right at the finish line.

In your case, with a front-motor, rear-drive car, the optimum choice for a gear will be somewhere around 3.70:1. That will give you reasonable mileage and reasonable acceleration. Everything we do to our cars if they must serve double duty (street and high-performance) is a trade-off.

Everything I've talked about here assumes a 1:1 ratio in the transmission. You want a manual transmission, so you could have a 5 or 6 speed transmission with the 5th and or 6th gear being overdrive, so that you would slow the motor down for mileage in the top gears and still have a low gear ratio in 1st, 2nd, 3rd and 4th to provide acceleration.

As far as the posi, most cars weren't produced with posi because of the additional cost of producing the parts. Grandma doesn't need posi when she's going to the grocery. There are several aftermarket units available which will bolt in and provide you with 2-wheel traction, Auburn www.auburngear.com and Detroit Locker just to name two. Here's a link to a supplier and some very good explantions of available units.
http://jawsgear.net/store/catalog/lockers.php?cPath=31
You can also equalize traction pretty well by replacing the conventional right rear shock only with an air shock, pumping the pressure up until you get the same amount of weight on both rear tires under acceleration.

You see, Jeri, on a front-motor, rear-drive car, the right rear gets light under accereration due to the way the chassis twists. That's why you'll see the right rear tire worn out on a kid's car with an open (non-posi) differential. When you apply more pressure to the tire on the right rear with an air shock, you equalize the pressure on both rear tires and both of them hook equally.

I'll add more to this as time allows, have to run right now

Originally posted by Swifster
Jeri, the rear end gear you use will also depend on the transmission you use. A 4 speed trans will run at a ratio of 1 to 1, from the input to the output in 4th gear. 5 speed and 6 speed transmissions will have 1 or 2 gears that run at less than 1 to 1 (overdrive).

When you combine the gear ratios for the rear end and transmission, you want to try to have a happy harmony. If you run a 3.00 final drive (rear gear) and a double overdrive 6 speed trans, odds are the engine may be turning a idle (or less), even on a 65 MPH highway. At the same time, running a 4.56 final drive with a non-overdrive transmission, you could be spinning your engine at over 5000 RPM at 55 MPH.

Just a rough guess, but if you go with a 5 speed or 6 speed transmission, I'd look at a 3.42, 3.55 or 3.73 final drive. If you plan on a 4 speed, I'd go between a 3.00 to a 3.25 rear gear.

Tom, That's exactly what I was getting at...I know that they all work together, but I don't really understand all the details of HOW they work together...I was thinking of either rebuilding an original Pontiac 4 speed tranny's because of the "nostalgia" factor...but if I understood what the difference between the old 4 speed and a newer 5 or 6 speed was...I could make a decision based on something other than a feeling. It sounds somewhat complicated. Here's a link to a pontiac gear ratio explanation that I found...is this accurate? http://www.wallaceracing.com/reargear.htm

The truck driver just dropped off the latest and last rustbucket...(for now) lol...it's a 78... lots of "surface rust"...i'm sure i'm going to need to replace most of the panels...they're pretty pitted...but the rest of it doesn't look so horrible...well, except for the engine in the trunk lol...

There are locker rear ends like the Detroit locker which completely locks up both rear wheels and unlocks when it senses you turning a corner. They are great for drag racing but annoying (noisy) and potentially dangerous if a novice is at the wheel. Hitting a wet spot under power can turn you around. Then there is limited slip like the Detroit True-Track (which I have) which does not completely lockup but gives a posi-traction-like delivery of traction when it senses tire slip. The GM positraction and the Ford Track-Loc rear ends use discs to provide traction to both rear wheels while the True-Track uses worm gears. Each has its advantages and disadvantages. Choosing the one that is right for you depends on your vehicle and driving habits.

another couple of comments on 'posi'

A lot of people whom talk about 'posi' don't actually know what is what. ( not the guys above, they were on the $$ )

Just a couple of notes, ( some are rehash-ing the above )

a true pozi means the two axles are 'positively' locked together. This is great for the track, but irritating and downright undesirable on the road. I understand they audibally 'CLUNK' anytime you take a turn. Dunno, never had a true pozi...

limited slip: several types of limited slip exist... some use 'cones' others use clutch disc stacks. This type will keep the axles locked to a preset torque load, after which they will 'slip', allowing the wheels to turn at different speeds. When driving in a straight line, as on a hiway, the lube tends to sling out of the discs, and they 'lock'. So, the next time you take a turn the rear end will audibally CLUNK as the discs break free. Sometimes, they will clunk repeatedly through the duration of a long, shallow turn. It also FEELS like a flat tire while doing this. You have to put special additive in the gear lube, but it only helps some. Better for the hiway, not as good for the track. It is a good compromise for the street.

now for the 'however':
pozi and limited slip wear the tires ( to the inside ) reduce gas milage ( on the street ) and increase the potential for the rear end to 'fishtail', PARTICULARLY on wet road.

'open' rear differential, is what the automotive mfg's prefer for stock application. It allows the tires to wear better, and gives overall performance improvement to the daily driver.

I put a clutch type limited slip under my 78, and the only time it was useful was the few times I pulled off to the shoulder, and one tire was in the mud. I'd certainly have been stuck without the limited slip. Overall, I hated it. It wore out my Hoosiers quickly, and the car fishtailed WORSE after I installed it, and I had the misconception that it would help prevent the fishtailing.

I have since swapped it out to an open diff, and am much happier.

Read up on the Detroit True-Trac, a modified Torsen-style third member. I like it in my '32, and it acts like an open rear end most of the time. It is only when I lay into it that it engages both rear wheels. I would not recommend it for a heavy car that is drag raced. But for the street in a light car, it is ideal. You can find out more about it at Randy's Ring and Pinion (choose "Worm Gear Differentials): http://www.ring-pinion.com/content/m...lt.asp?pid=164

I will add a couple more things that will help explain. cars have differentials so they can go around corners better. GOing around a 90 degree corner, the inside wheel of a vehicle travels a lot less distance than the outside wheel. If the wheels were connected with a solid axle and tried to get through the corner at the same speed, one or both of the tires would have to slip to allow for the difference in distance. The differential allows the wheels to turn at different speeds. This is a good thing, except when one of the tires loses traction. When that happens the differential allows the spinning tire to take all or most of the power and the other tire just sits there. ( a one tire fire!) To remedy this, several different designs were developed, some use several clutches, others use gear combinations and others still use a ratcheting system. They are known by many names including positraction, Trac Loc, Detroit Locker, limited slip, True Trac and sure grip. These systems allow one tire to turn faster than the other, but they apply resistance to the event to reduce the chance for one wheel to spin wildly while the other barely moves. AS Techie said, since Granny does not often spin her tires, well most anyway, she does not need to spend the extra money for the traction device. You on the other hand love to smoke 'em so it is worth the extra coast for you.

Now the gear ratios. They work like a lever. The lower the gear, the longer the lever. If you need to move a heavy weight easily, you use a longer lever. It makes moving it easier, but you have to move the end of the lever further. A strong person may be able to move the load with a shorter lever. In the case of a car, if you want to make it easier to move, you use a lower gear (numerically higher). The trade off is, you will need to turn the motor more times to move the same distance. A big motor like your Pontiac 455 has lots of torque that come in at a fairly low RPM, so like the strong person, it may not need too low a great to get moving. Also a big motor has a lot of rotating mass that makes very high RPM dangerous, for the motors longevity at least. But that is a whole new topic. Bottom line is a posi traction with 3.23-3.55 gear ratio and 4 speed should snap your neck real good with that big Poncho pushing it. If you decide to go with an overdrive transmission, then you can go to an even lower gear maybe 3.42-3.90 and still have low RPM for freeway cruising. Hope that helps

Pat

As usual, you guys amaze me with the wealth of information you provide... Forgive me for this two line response...i'll need some time to "digest" this thread...i'll be back Feel free to add additional thoughts...i'm lovin it !!!! Here's a pic of my rusty birdies in the roost...

One more for good measure...

Wow! You make me jelaous with all that room for 3 cars! And look at that nice clean floor too! But where are all the tools? Looks like you have a fun winter ahead.

Pat