Teach me about rear-ends.

As I continue to learn about working on vehicles and have now tackled most everything short of pulling an engine, I still want to learn about rear-ends for the sake of future modifications. Perhaps someone - this sounds like 5-90's specialty - can explain to me rear ends. What I am specifically interested in is the numbering, the issue of splines, as well as the differences between the different rear-ends I hear thrown around here. Let the education begin.
thanks
BSD

pick up a service manual, 4wheeling mag or search around the internet (and here) its kind of a broad topic.

I learned a lot of what i know that way, when i had my suburban and my bronco i bought a service manual for eache and read them cover to cover. And i've worked on enough jeeps and been around enough jeepers to gather a lot of know how. if i typed out all that i've learned about rearends we would be here awhile

on pirate there is tons' of stuff in the tec section

Now thats a rear end.

uke: uke: uke: uke: -----

Well, my specialty is more along the lines of automotive electrics and engine work, but I'll take a stab at it...

A differential is an assembly that allows two wheels on the ends of what would normally be a solid axle to turn at different speeds (like in a turn.) This is accomplished through the use of "spider" gears (between the carrier and the axle shafts) and "side" gears (into which the axle shafts are splined) coupling the ring & pinion assembly to the shaft. I don't know a good way to explain it - so experiment next time you have the cover off and the wheels jacked up so they'll spin freely. Block the front wheels, and put the transmission in neutral so you may turn all three elements by hand (left wheel, right wheel, and input/pinion gear) and watch what happens to the differential gears.

A "open" differential will allow a wheel that breaks traction to have ALL the spin - that's why you get stuck.

A "limited slip" (also "Posi" or "LSD") is a differential with clutch packs to engage the side gears. As I understand it, when a wheel "breaks free" and begins to spin faster, the clutches disengage on the "free" side, and engage on the "tight" side, and that allows the wheel with traction to spin.

An "automatic locking" differential is similar to an LSD, but uses a mechanical locking device (usually a pawl clutch of some variety) to accomplish the coupling.

A "selective locking" differential is either an "open" or LSD, with the user-controlled option (typically powered by compressed air, but now also electric) to "lock" the two axle shaft together. This forces them to turn at the same rate, and overrides the differential entirely.

Lastly, a "spool" or "Lincoln locker" (welded spider gears) locks the two axle shafts together at all times - by definition, it ain't a differential, since the two shafts can't spin at different speeds.

Some companies use model numbers for nomenclature (like Dana,) some use the number of cover bolts (popularised by GM,) and some use the ring gear outside diameter (ChryCo and Ford come to mind.) Some companies - particularly heavy-duty builders - use the weight capacity of the axle (like Rockwell and Eaton.)

I don't have the figures handy, but as the Dana model number gets larger (30/35/41/44/50/60/70/80,) the axle gets heavier, and the ring gear gets larger. Typically, a larger ring gear (outside diameter - the differential case has to fit inside) will make for a stronger differential - since the contact band will be larger and it allows for a larger case (which is typically cast.)

ChryCo axles - the XJ got the 8.25" stock, but some put a 9.25" under there, and there is also a 10.25" for heavy-duty pickups - put the ring gear size right out where it can be see. Ditto Ford - 8.8" and 9" come readily to mind.

You'll also hear talk of GM 10-bolt, 12-bolt, and 14-bolt axles. This refers to the numbrer of bolts that hold the cover down - but I've heard tell that it (coincidentally) matches the number of bolts holding the ring gear to the differential case. I have not had an opportunity to check this out - but again, more is better (because it's a larger housing.) The 14-bolt is typically found under 3/4-ton or larger pickups, the 10-bolt and 12-bolt may also be found under RWD passenger cars, 1/2-ton trucks, and S-10s.

Then, there are the "big boys" - if the rating isn't given, it can usually be inferred from context. For example, when someone here talks about "Rockwells," they're usually referring to a Rockwell 2-1/2-ton axle, like that found under the M35A2 6x6 2-1/2-ton truck. Rockwell also makes a 5-ton, and some real monsters for use under prime movers (you know them as semi-trucks.) This refers to drive axles - the ones under the trailer don't drive anything.

For splines, there is typically a given "spline count" for a shaft diameter - the shaft diameter determines (largely) the smallest useful spline that may be put on it. "Spline count" is simple - mark one spline, and count around the axle shaft.

A higher spline count for a given model of axle will usually indicate a larger shaft - which means that a greater torsional load is required to fail it (sudden traction on large tyres, for instance.) Typically, you will note this with ChryCo 8.25/9.25 axles, Ford 9", and Dana 44-up (there are typically two sizes - which also translates to two possible side gears, and two possible wheel bearings. Don't go by lug pattern - that usually means nothing.) Since splines are 'always engaged,' unlike gear teeth, more splines usually means a stronger coupling (unlike gear teeth - where you want fatter, and preferably helically-cut - teeth.)

This concludes the nickel tour on axles (and is probably worth what you paid for it.) Any questions? Like I said, this is out of specialty for me, so I honestly posted this in the hopes of being corrected somewhat - and learning something myself...

5-90

Thanks 5-90, knew I could count on you. One question I have, how does one figure out the gearing? By that, I read of people changing the gears in their rear-ends to adjust power. I.E. getting bigger tires or wanting to rock crawl. From what I read, it appears that the lower the number, the greater torque? However, I am unsure of the numbering system. Also, it appears that putting in lower numbered gears in your rear end reduces the effective top speed of you vehicle since your engine or driveshaft is turning more for less turns at the wheel?
Thanks
BSD

(Sence 5-90 answered his question quite well now we can start the smart a$$ comments)
On that note riverfever is also a gureu on rear ends.

(now back to a more serious note)
4.88's would be 4.88 turns of the pinion to one turn of the axle, 4.56's would be 4.56 turns of the pinion to one turn of the axle, and so on. Also to expand on 5-90's spline count V/S gear teeth the higher (deeper) you go in gear ratio the weaker the ring and pinion will be due to having more but smaller teeth.

scoobyxj said:

4.88's would be 4.88 turns of the pinion to one turn of the axle, 4.56's would be 4.56 turns of the pinion to one turn of the axle, and so on. Also to expand on 5-90's spline count V/S gear teeth the higher (deeper) you go in gear ratio the weaker the ring and pinion will be due to having more but smaller teeth.

Click to expand...

Actually it is more accurate to compare pinion revolutions to ring gear revolutions rather than axle revolutions. Because what you said is only true if the vehicle is going straight (no differentiation in the differential). So a gear ratio of 4.56 to 1 would be 4.56 revolutions of the pinion for every 1 revolution of the ring gear and that would be the same as one revolution of the axle shafts and the tires while going in a straight line. Then both axle shafts and tires would be turning 100% of the speed of the ring gear. But say you are going around a corner. The wheel and axle shaft on the outside of the turn are turning faster than the tire and axle shaft on the inside. The outside axle and tire might be going 150% of ring gear speed and the inner tire and axle would be going 50% of ring gear speed. It has to add to 200% and the numbers would be different depending on what angle the turn was. By the way I consider myself a rear end (the automotive kind) guru so if you have and specific questions I might be able to answer just ask, but it would take forever to just start from scratch and try to explain everything about it.

Any gearset that isn't 1:1 is a torque multiplier - you multiply the input torque by the gear ratio to get the output torque.

Say you have 100 pound-feet going into a gearset...
At 3.07:1, you get 307 pound-feet out
At 3.54:1, you'll get 354 pound-feet
At 4.88:1, you'll get 488 pound-feet

Granted, that's an oversimplification, but it's close enough for government work.

When people change their gears for "power" - they're not making any more power at the source, they're just multiplying it more. So, changing from 3.54:1 to 4.88:1 won't increase the power going INTO the gearset - just the power coming OUT of the gearset (see above example.) To increase power generated takes engine modifications.

Also, since an engine make power most effeciently at it's peak torque output/volumetric efficency, often gears are selected for "highway cruise" - but that takes a little more math, and has to account for a few more factors. Cruise gear in the transmission (either 1:1 or overdrive,) tyre size, cruising speed, crankshaft RPM for peak VE/torque output, and such must all be accounted for - then you can get a cruise gear. I'd have to dig up my notes to give you a primer on that (but I can tell you that most manufacturers are going about it the wrong way - they're gearing to get the lowest cruising RPM possible. I'm waiting for an "off-idle cruise" - down around 1000 rpm - for an engine with peak VE up around 3800-4000 rpm. As an example. I quit using overdrive in my 88, and picked up 2-3MPG for short hops. That tells me that the 3.07 gears that came from the factory are too low - and they should be around 4.10 - as I recall - for peak freeway cruise.)

The gear ratio number is simply devised - divide the teeth on the ring gear by the teeth on the pinion gear, and reduce to two decimal places. The single best way to determine the gear ratio is to count teeth - the "shaft turning" talked about is a field expedient way to estimate gear ratio.

Lowering gear ratio (higher, numerically) will have the effect of reducing top speed, since your crankshaft has to turn more to turn the wheels once. With our 5000 rpm redline, I'd say that going to 5:1 gears (or higher - they're available) would make your XJ a "trailer queen" more than a get there - run - go back machine. Again, I'd have to crunch numbers to be sure. Going to a higher ratio (lower, numerically) actually can increase the top speed - because your wheels are turning closer to crankshaft speed. I used to build engines for bracket racers 'way back when, and one of the first questions I'd ask was "What's you axle going to be?" I rarely got an answer above 3.10, except for "run what you brung" racers who'd drive their rig to work during the week and race it on the week-end. I think the highest gears (lowest, numerically) gears I'd seen were down around 2.05-2.10 - that I can recall. I'm sure they can get lower - check available ratios for the Ford 9" - that's a popular racing axle.

How'm I doing so far?

5-90

scoobyxj said:

(Sence 5-90 answered his question quite well now we can start the smart a$$ comments)

Click to expand...

Brokeback Mountain is a good documentary on rearends

5-90 said:

Any gearset that isn't 1:1 is a torque multiplier...
5-90

Click to expand...

It would be more correct to say any gear ratio thats lower (higher numerically) than 1:1 is a torque multiplier.

You can also say that any gear ratio that is lower than 1:1 (higher numerically) is underdrive. An example is 2.54:1

1:1 is direct drive

And any gear ratio higher that 1:1 (lower numerically) is overdrive. An example is 0.75:1

Overdrive and direct drive do not multiply torque.

Thanks guys. So, a quasi-hypothetical here. I have a friend who just bought a new jeep; TJ (short, two door, rag top). He has a 5 speed manual. He has a heavier duty rear-end, not sure of the numbers. Has the 4.0. Sticker says 19 on highway. (I was a bit surprised with a 5 speed) O.K. up to here it is real. So if he never towed with it or did any rock crawling, he would not need more torque. Heck I doubt the thing will see mud. So could he change out the ring gear so that the pinion gear would turn less to turn the axle one revolution and in turn increase his mpg?
Another question, how does a person who has a "spool" keep from tearing up their axle? I assume that it can only turn in a gradual radius otherwise it seems like he would tear up the rear-end?

60thAniv - It's true that any gearset offering a ratio lower than 1:1 is an overdrive set (an underdrive works the other way,) but for purposes of simplicity, I elected to say they're ALL torque multipliers. Besides, it's mathematically true.

BSD - "Spools" survive turns by A) being REALLY tough, and B) the fact that the tyres will slip (that's the "chirp" you hear when turning with a spool.) Tyre "gription" isn't perfect - anyone who's "laid scratch" will understand that.

As far as your other question, I'd really need to know more.
Torque peak of the engine (I could estimate it with simulation in Dyno2000)
Current axle ratio
Transmission gear ratios (you could get me the model - I think I have most of them listed in my notebooks. I do know that fourth will be 1:1, with a five-speed - that's standard)
Tyre size

Since it's a hypothetical, the point is largely moot - but that's the sort of thing you'd need to know. However, suggest to him that he do what I did - try not using fifth for a month, and see what it does to his mileage. It won't harm anything - since the fourth gear is so "wide" anyhow. Either fourth or fifth - suggest that he aim for a freeway cruising RPM of 2800 or so.

Find a nice round rear end... not too big... not too small... but tightness is a necessity

5-90 said:

60thAniv - It's true that any gearset offering a ratio lower than 1:1 is an overdrive set (an underdrive works the other way,) but for purposes of simplicity, I elected to say they're ALL torque multipliers. Besides, it's mathematically true.

Click to expand...

Ok, Ok, I get what you are saying, but when I see the word multiply I think increase. The overdrive multiplies torque, but multiplies it by a number lower than 1 so it ends up decreasing it. I think we are on the same page now?