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I keep hearing people talk about rear end gears and tossing out numbers like 4.10, 4.33, ect. or 12 bolt and 10 bolt...what does it mean!?

I ran a quick search but i wasn't sure how to search and it came back with nothing so forgive me if this has been asked before...just trying to expand my knowledge. :D
 

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The 4.11 or 3.73 number is actually the gear ratio. More correctly it is 4.11:1 or 3.73:1. It means that driveshaft turns 4.11 times for every 1 turn of the wheels.

The pinion gear is what the driveshaft attaches to. The pinion gear is the smaller of the 2 gears. The pinion gear turns the ring gear. The ring gear is the larger of the 2 gears and turns the carrier (aka differential) which turns both axles and wheels. You determine the ratio by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear.

For example:
- a ring gear with 37 teeth and a pinion gear with 9 teeth is a 4.11 ratio (37/9 = 4.11111)
- a ring gear with 41 teeth and a pinion gear with 11 teeth is a 3.73 ratio (41/11 = 3.72727)
- a ring gear with 41 teeth and a pinion gear with 12 teeth is a 3.42 ratio (37/9 = 3.41666)

However, the ring rear is matched to the pinion. So a 41 tooth ring gear designed for a 11 tooth pinion will not work with a 12 tooth pinion (and vice versa).

The 10-bolt or 12-bolt means the number of bolts attaching the ring gear to the carrier. MOST rears have the same number of bolts on the rear cover but that's not how you tell. Some 8.2" 10-bolts (in Oldsmobiles) have 12 cover bolts. They are NOT 12-bolts.

More numbers: 8.2" and 8.5" 10 bolts refer to the diameter of the ring gear. An 8.2" 10-bolt has a ring gear with a diameter measuring 8.2" and has 10 bolts attaching it to the carrier. An 8.5" 10-bolt has a ring gear with a diameter measuring 8.5" and has 10 bolts attaching it to the carrier. A 12-bolt rear has a ring gear measuring 8.875" in diameter and has 12 bolts attaching it to the carrier.

In the same way, a Ford 9" has a 9" diameter ring gear. The 8.8" Ford has a ring gear of 8.8" in diameter and has been called a 12-bolt clone because they are very similar in design. The larger the ring gear, the stronger the rear (for the most part but there are other things that make one rear stronger than another).
 

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That was a great explination. I just hate it when I spend a lot of time typing an answer for someone and by the time I hit submit someone else beat me to the punch and said the same thing I just said. Damn all that time typing for nothing and the first guy got all the glory. Luckly for NovaResource that didnt happen in this case.
 

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Now that's Nova Resourse for ya...Had to copy and paste that in my own mail......Thanks NR...
 

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Discussion Starter #6
Sooo...the higher the ratio the better? if so, is that always the case? Does your engine or transmission have any effect on your rear end?
 

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Sooo...the higher the ratio the better? if so, is that always the case? Does your engine or transmission have any effect on your rear end?
It depends on your engines power band. If your engine makes peak power at ~7000 RPM's, then you probably don't want to stay down in the lower RPM range too often. My cars power peaks ~7500, but that is untested/dyno'd yet, and I run 4.11's with 275's. It puts me going 30 MPH at around 2500 RPM in second. Needless to say, it pulls HARD in first though.

Think of rear gears as torque multipliers. It multiplies torque received from the transmission, to the differential, to the rear wheels.
 

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A high numerical gear will allow the engine to rev faster. A 4.88 or 4.56 gear is common in some drag cars, depending on their setup. That being said, you run out of engine at the end of the track or going down the highway.

With 2.73 gears, I could go 65-70 mph at about 2300 rpm. With 3.73's, I'm at about 3000 rpm or a little higher. This puts increased wear on the engine, more fuel consumption and the exhaust is louder. Bad for daily driving.

You also run out of top speed. A car at WOT with "steep" high numeric gears (4.88 ie) will tend to push the motor really hard at the end of the track. It might hit 7800 or 8000 rpm with a fast engine. If your motor doesn't make power up there, you essentially hit a wall. Your valves start floating and the engine wants to come apart. The idea is to get a gear that will allow you to accelerate the fastest without pushing the engine beyond its operating range. On the street, it's better to compromise towards to 3.42/3.73 gear sets to make highway driving livable.

Kev
 

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A high numerical gear will allow the engine to rev faster. A 4.88 or 4.56 gear is common in some drag cars, depending on their setup. That being said, you run out of engine at the end of the track or going down the highway.

With 2.73 gears, I could go 65-70 mph at about 2300 rpm. With 3.73's, I'm at about 3000 rpm or a little higher. This puts increased wear on the engine, more fuel consumption and the exhaust is louder. Bad for daily driving.

You also run out of top speed. A car at WOT with "steep" high numeric gears (4.88 ie) will tend to push the motor really hard at the end of the track. It might hit 7800 or 8000 rpm with a fast engine. If your motor doesn't make power up there, you essentially hit a wall. Your valves start floating and the engine wants to come apart. The idea is to get a gear that will allow you to accelerate the fastest without pushing the engine beyond its operating range. On the street, it's better to compromise towards to 3.42/3.73 gear sets to make highway driving livable.

Kev
Right..If your rear end is a posi..( both Rear Tires turning at equal torque...say....4.56...You'll feel as tho you need a 5th..6th gear to bring the rev down....With a lower gear ratio....3.73...you can drive comfortable on the highway in 4th gear not having that feeling of needing another gear....your engine won't rev so high....But you still have plenty of torque applied to both rear tires getting off the line...just not as fast...
 

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Right..If your rear end is a posi..( both Rear Tires turning at equal torque...say....4.56...You'll feel as tho you need a 5th..6th gear to bring the rev down....With a lower gear ratio....3.73...you can drive comfortable on the highway in 4th gear not having that feeling of needing another gear....your engine won't rev so high....But you still have plenty of torque applied to both rear tires getting off the line...just not as fast...
man ....i picked up a whole lot of information from this thread.thanks!!!!:yes:
 

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Sooo...the higher the ratio the better? if so, is that always the case? Does your engine or transmission have any effect on your rear end?
Sorta. Think of rear gears like transmission gears. A car accelerates faster in 1st gear than it would starting out in 3rd gear. First gear in a typical TH350 is 2.52:1 while 2nd gear is 1.52:1 and third is 1:1. However, if you only used 1st gear your top speed woudl be much lower. This is why race cars have low (numerically high) rear gears so they can accelerate quickly. They are no fun to drive low gear on the highway because the engine is revving higher to make the same vehicle speed.

The trans doesn't have an effect on the rear end but it can help out. For example, overdrive transmissions. Most transmissions have a 1:1 final gear ratio. Overdrive transmission have gear rations less than 1:1. The 700-R4 has a final gear (4th) of 0.70:1. Meaning the driveshaft is spinning FASTER than the engine speed. Overdrive is just any gear ration less than 1:1.
 

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Discussion Starter #12
Sorta. Think of rear gears like transmission gears. A car accelerates faster in 1st gear than it would starting out in 3rd gear. First gear in a typical TH350 is 2.52:1 while 2nd gear is 1.52:1 and third is 1:1. However, if you only used 1st gear your top speed woudl be much lower. This is why race cars have low (numerically high) rear gears so they can accelerate quickly. They are no fun to drive low gear on the highway because the engine is revving higher to make the same vehicle speed.

The trans doesn't have an effect on the rear end but it can help out. For example, overdrive transmissions. Most transmissions have a 1:1 final gear ratio. Overdrive transmission have gear rations less than 1:1. The 700-R4 has a final gear (4th) of 0.70:1. Meaning the driveshaft is spinning FASTER than the engine speed. Overdrive is just any gear ration less than 1:1.

Just so that i understand what your saying here, when you say 2.52:1, is that saying the tranny is spinning 2 1/2 times to the driveshaft once or vice versa?

you actually answered another question in this statement that i had about overdrive. I knew the gear ratio's in overdrive were similar to how you described them but i didnt fully understand how it worked. Just to put things in context for me, my truck is a manuel transmission with a 5th gear, ive always understood it to be overdrive, does that sound right?


This is great info guys thanks, i wont feel like such a dunce when i hear ppl jabber on about it now:D
 

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Just so that i understand what your saying here, when you say 2.52:1, is that saying the tranny is spinning 2 1/2 times to the driveshaft once or vice versa?
Correct. the input shaft of the transmission will spin 2.52 times to 1 rotation of the shaft. consider this, that a lower first gear can have the same effect as lowering your axle ratio. the first gear in a TH350 is 2.52:1, and in a 700r-4 is 3.06:1. so, the ratio of the transmission input to tire rotation is:

TH350=2.52*3.73=9.3996
700r-4=3.06*3.73=11.4138

this means that the 700r-4 has a lower input (engine speed) to output(wheel speed) ratio at launch than a TH350. so in addition to being able to run lower gears and still drive on the highway (because of the overdrive), you also get more torque multiplication off the line.
 

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Just so that i understand what your saying here, when you say 2.52:1, is that saying the tranny is spinning 2 1/2 times to the driveshaft once or vice versa?

you actually answered another question in this statement that i had about :D
Just to be clear, the engine is spinning 2.52 times for each turn of the drive shaft. If you have a 3.73 rear ratio, then the engine is spinning 9.399 times for each rotation of the tire. 2.52 * 3.73 = 9.399
 

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Sooo...the higher the ratio the better? if so, is that always the case? Does your engine or transmission have any effect on your rear end?
The transmission only effects the rear gear ratio in a gear other than a 1 to 1 ratio. Most manual Overall gear ratio changes is lower in first gear than it will in drive. With my Richmond 5 speed first gear was 3.20 and I ran a 4.88 rear gear. So you mulitply 3.20 X 4.88 = 15.62 final drive ratio. In fifth gear a 1 to 1 ratio put the overall gear back at 4.88. With an overdrive it works the opposite and with a lower gear in the rear and an overdrive it will raise the final drive ratio by whatever % the overdrive is. I'm not sure how to figure the overdrive final ratio.
 

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Tall gears ( 4.11:1) are slow. You use slow gears to go quick.

Short gears ( 2.41:1 ) are fast. Fast gears are slow 'out the gate'.

I had a 2.41:1 on my 78 firebird. It was awesome on the hiway. I could dump it into first gear at 45 MPH and grab a car length instantly. Could leave it in second till near 90! Cruising at 60, I could dump it into second, and pass someone easy.

And I really gotta make a comment on the afore mentioned pozi comment..

Whether or not the rear end is positive traction, either locker, spool, clutch type... has no effect whatsoever on gear ratio. It only effects a tire which is slipping. At speed, a pozi rear feels just like an 'open' rear with exception to the fact that the 'open' rear does not 'pop' when going around turns.

It really surprised me that so many replies didn't catch that..

A little explanation for the dude whom posted the original question: and more that you never knew, and didn't want to learn:

An 'open' rear end has nothing to stop one wheel from spinning. If you loose traction on one side, that wheel will spin, and the other will not. All power will be effectively transferred to the spinning wheel. Not good for getting off the line.

Drag racers prefer a 'pozi' rear end. There are many designs. The detroit locker POSITIVELY LOCKS ( pozi ) the axles together under load. one side cannot slip. There are variations of the locker design. Most engage with a noticable CLUNK. Some can be engaged manually, most function automatically. Most are not street friendly.

Then there are limited slip.. these are often referred to ( incorrectly ) as pozi rears. Limited slip is also varied in design, but mostly they use either cones or clutches to create friction between the axles. They CAN turn independently, but must overcome the friction of the clutches/cones to do so.

OK. so WHY would one want the rear axles to turn independently? Well, if you are driving your car in other than a straight line, it is highly desireable. Why? because in a turn, the outside wheel is moving faster than the inside wheel. Now, if the axles can't turn independently, the outside wheel would 'hop' through the turn. If you have an open rear, as do most passenger cars, ( and my lawn mower ) your turn will feel as smooth as a straight line. BUT.. if you get in the mud ( or wet grass in the case of my mower ) one tire will inevitebly have SLIGHTLY less traction than the other. That tire will spin, and you will not move.

ENTER LIMITED SLIP.. I had a limited slip in one of my firebirds. The clutches were set to about 40 ft/lb. That means, that for the axles to spin independently, there would have to be over 40 foot/pounds of torque differential. This accomplishes several things... When I was stupid enough to put one wheel off the shoulder on my way to california, in the mud.. I was able to just drive away. Had I not installed the limited slip, then I would just spun a tire, and dug into the mud. Another thing I noticed limited slip does for you.. it wears out the inside of the rear tires. RAPIDLY. That rather sucks. And, my particular limited slip, would 'pop' when going into a turn, if I had run in a straight line for a while ( think interstate ) This is due to the gear oil getting slung out from between the clutch discs, so they get real good traction on each other. Then you go into a turn, and they 'pop' loose. Annoying. There are additives you can put in the gear oil which will help with this, but I didn't notice that they actually helped. And finally.. the real reason I put in the limited slip.. the car had a tendency for the rear to move sideways when I punched it. I thought the limited slip would eliminate this.. it actually made it worse.

One last nugget.. the 'spider' gears are what allow the axles to actually move independently of one another. An old dirt track trick ( cheap hot rodding ) was to weld the spider gears, so they couldn't move. Then, so the car wouldn't be 'hopping' through the round track, put a larger tire on the outside wheel.
 

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Tall gears ( 4.11:1) are slow. You use slow gears to go quick.

Short gears ( 2.41:1 ) are fast. Fast gears are slow 'out the gate'.

I had a 2.41:1 on my 78 firebird. It was awesome on the hiway. I could dump it into first gear at 45 MPH and grab a car length instantly. Could leave it in second till near 90! Cruising at 60, I could dump it into second, and pass someone easy.

And I really gotta make a comment on the afore mentioned pozi comment..

Whether or not the rear end is positive traction, either locker, spool, clutch type... has no effect whatsoever on gear ratio. It only effects a tire which is slipping. At speed, a pozi rear feels just like an 'open' rear with exception to the fact that the 'open' rear does not 'pop' when going around turns.

It really surprised me that so many replies didn't catch that..

A little explanation for the dude whom posted the original question: and more that you never knew, and didn't want to learn:

An 'open' rear end has nothing to stop one wheel from spinning. If you loose traction on one side, that wheel will spin, and the other will not. All power will be effectively transferred to the spinning wheel. Not good for getting off the line.

Drag racers prefer a 'pozi' rear end. There are many designs. The detroit locker POSITIVELY LOCKS ( pozi ) the axles together under load. one side cannot slip. There are variations of the locker design. Most engage with a noticable CLUNK. Some can be engaged manually, most function automatically. Most are not street friendly.

Then there are limited slip.. these are often referred to ( incorrectly ) as pozi rears. Limited slip is also varied in design, but mostly they use either cones or clutches to create friction between the axles. They CAN turn independently, but must overcome the friction of the clutches/cones to do so.

OK. so WHY would one want the rear axles to turn independently? Well, if you are driving your car in other than a straight line, it is highly desireable. Why? because in a turn, the outside wheel is moving faster than the inside wheel. Now, if the axles can't turn independently, the outside wheel would 'hop' through the turn. If you have an open rear, as do most passenger cars, ( and my lawn mower ) your turn will feel as smooth as a straight line. BUT.. if you get in the mud ( or wet grass in the case of my mower ) one tire will inevitebly have SLIGHTLY less traction than the other. That tire will spin, and you will not move.

ENTER LIMITED SLIP.. I had a limited slip in one of my firebirds. The clutches were set to about 40 ft/lb. That means, that for the axles to spin independently, there would have to be over 40 foot/pounds of torque differential. This accomplishes several things... When I was stupid enough to put one wheel off the shoulder on my way to california, in the mud.. I was able to just drive away. Had I not installed the limited slip, then I would just spun a tire, and dug into the mud. Another thing I noticed limited slip does for you.. it wears out the inside of the rear tires. RAPIDLY. That rather sucks. And, my particular limited slip, would 'pop' when going into a turn, if I had run in a straight line for a while ( think interstate ) This is due to the gear oil getting slung out from between the clutch discs, so they get real good traction on each other. Then you go into a turn, and they 'pop' loose. Annoying. There are additives you can put in the gear oil which will help with this, but I didn't notice that they actually helped. And finally.. the real reason I put in the limited slip.. the car had a tendency for the rear to move sideways when I punched it. I thought the limited slip would eliminate this.. it actually made it worse.

One last nugget.. the 'spider' gears are what allow the axles to actually move independently of one another. An old dirt track trick ( cheap hot rodding ) was to weld the spider gears, so they couldn't move. Then, so the car wouldn't be 'hopping' through the round track, put a larger tire on the outside wheel.
Another great post..I hope the original poster has his head on straight when he reads this thread......Get your calcuator out and read everthing twice...
 

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Drag racers prefer a 'pozi' rear end. There are many designs. The detroit locker POSITIVELY LOCKS ( pozi ) the axles together under load. one side cannot slip. There are variations of the locker design. Most engage with a noticable CLUNK. Some can be engaged manually, most function automatically. Most are not street friendly.

Then there are limited slip.. these are often referred to ( incorrectly ) as pozi rears. Limited slip is also varied in design, but mostly they use either cones or clutches to create friction between the axles. They CAN turn independently, but must overcome the friction of the clutches/cones to do so.
This part is not 100% correct. First, it's posi, not pozi. And the term does not come from positively locking differentials. The term "posi" is a shortened version of "Positraction" which was Chevrolet's name for a limited slip differential. Pontiac called it "Safe-T-Track" but both were the same thing: a Limited Slip Differential (LSD) that used clutches to limit the spinning of one tire.

Because Chevy was the most common version, their name stuck and has become a generic term for a limited slip differential. Similar to "Vaseline" (a trade name) as a common term for petroleum jelly, or "Kleenex" (another trade name) as a common term for tissues.

A positively locking differential is called a "locker" not a "posi". This is a differential that uses teeth to engage both axles together and does not allow any slip (when engaged).
 

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Discussion Starter #19
wheew...im sure i'll have more questions but i need to wrap my head around all these posts...im going to have re-read everything a couple times..lol:D
 

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I am going thogh in a $.02 here as well... and speak to the POSI issue. NovaResource is dead on with his vernacular of the orgins of the term POSI,..... Mopar called their LSD SurGrip, and AMC called theirs..TwinGrip..and Dana called theirs SafeTgrip ..... all Clutch and preload spring based units....

there is one there LSD..... a Thorsen, this unit has no clutches or a ratcheting type lugs/cogs action of the Detroit locker( locker), the Thorsen differential uses Helix cut worm style gears to to create a bind action by using a torque value to over come slippage....Eaton sells the Detroit TruTrac which is the Thorsen style, as well as the Detroit Locker....

as for gears... NovaResource did a Fine explanation.. I would add only this.....

gears with a High numerical value IE: 4:11, 4.56, 488, 5.13, 6.30 are call low gears even tho they represent a higher numerical value... and the preceding explanations By NovaResource of the use or how they function is dead on...

consider this.. as explanation as well... Heavy loads...... by raising the numerical value of the differential a heavier load can be moved with less effort...with a gear of 5.0:1 as apposed to a gear of 2.50, it effectively reduced the load strain by half.... so moving a 20 ton load with a 250 gear would require 100% effort, where as the 5.0 gear needs only half the effort to get the load moving.....

But in raising the numerical value of a differential you slow top speed or..... you are limited in top speed by the Max RPM of your engine...

Hope I have added to the confusion:D

Veno
 
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