I'm not gunna hijack this into another 8.8 vs 9 thread, if the OP asks I'll get into it. Otherwise it's not like it isn't other places in this forum, look up other threads including mine, or start your own

 

Better? Well, IMO, exactly this:

The 9" is a different type design. The 8.8 "borrows" it's basic design premise from the old, original Dana Spicer design, dating way back (1940s, maybe?). The Dana axles were made in many sizes, the biggest being reserved for big pick-ups.

9" Ford has it's pinion gear mounted in a separate support part from the actual center-section. This means that the pinion gear may be removed from the center section without disturbing the axles or ring gear. This is NOT the case for the 8.8 (or Dana designs). Why is an easily removed pinion important? Much easier replacement of pinion parts: bearings, seal, pinion bearing preload may be set-up on the bench, then pinion and it's retainer assembly returned to the center section: it's held there by 5 grade Eight 3/8"-16 bolts, sealed by a large "O"-ring. Real beauty of this design is that the pinion gear itself has THREE bearings, instead of TWO, thus the gear is "straddled" between 2 bearings, instead of being "hung-out" in space cantilever-style, with bearings supporting the gear on only one side of it's teeth. The straddle-mount idea is strong, but the bearing on the pinion's small end must necessarily be rather small, as it interferes with the ring gear; there isn't a whole lot of room in there for the bearing.

On the other hand, the two tapered roller bearings on the 9" design pinion are VERY close together; this gives up strength inherently. Generally, the pinion in the other design axles, commonly called "Integral design", are longer, and have wider spacing between the bearings. The 9" is strong, no doubt about that. So is the 8.8 Ford. Each have weak points. 9" axles are kept in place only by the outer wheel bearings. 8.8 has the axles secured from sliding out of the differential by "C" clips in grooves at the inner axle (splined ) end. GM axles used this concept for years. Later 9" axles (my '79 Ranchero, for example), used the axle's surface as the wheel bearing's (outer) inner race: a destroyed bearing thus ruined, usually, the axle, too. All earlier 9" used standard ball bearings for outer bearing use: they were removable and replaceable without incurring axle shaft damage. 8.8 "rides" the outer bearing directly on the axle shaft (ugh!). So did many many GM axles for many years.

If still more info is desired, please ask. I can go into more specific 9" useage, 4-pinion differential types, Traction-Loks, etc. Happy to share! impish

 

I believe you began this discussion by suggesting an axle swap that the OP never inquired about, then voicing your opinion on the quality of the great 8.8

 

Your points are well-taken, and relevant. However, I have to take issue with the pinion diameter claim: the pinion diameter is actually dependent on the gear ratio. There are ratios where the pinion diameter is SO LARGE, it just barely passes through the 5" opening in the center section. In those gearsets where the pinion is large, the thickness of the ring gear is diminished.

The third member deflection question was addressed pretty thoroughly by Ford over the years, with various changes beinmg made to gussetting and strengthening features. The Nodular Iron carrier, identified by a big "N" on the front, was one of the factory's best. Today, aftermarket carriers of various materials are being made competitively. impish

 

Pinion gear sizes change in relation to gear ratios, but the pinion shaft diameter never does

 

All this pinion stuff is irrelevant as far as I see it. Sure the pinion is slightly weaker in the 9". But that in no way makes up for all the huge shortcommings of the 8.8. Your only going to break the pinion of a 9" if you can put enough torque thru it. The 8.8 will fail just from regular use. If you can break a 9" pinion the answer isn't an 8.8 it's a D60, 10.25, etc.

 

This is true! The shortness of the 9" pinion is a shortcoming, no pun. For that reason, a larger rear pinion bearing was used by the factory in certain selected applications, high-performance, taxi-cabs, police. Most aftermarket 9" offerings use this bigger bearing concept. This does not change the pinion shaft strength directly, however. It "stiffens" the pinion assembly to lessen bending moments in the shaft itself.

The most spectacular 9" failure I have witnessed occurred when my buddy's 1963-1/2 427 equipped Galaxie CRACKED the entire center section in two! This I would not have believed, had I not seen the broken carrier! The crack ran vertically from top to bottom, suggesting broken gear teeth initiated such extreme loading between the ring & pinion, that the "spread" force cracked the housing. This was before the "N" housing became available. I have not seen a pinion shaft broken, but don't deny the possibility.

As an aside, if the assumption is made that use of a removable pinion carrier inherently weakens the center section design, note that some of the largest heavy-duty truck differentials use just that.

Further thought-provoke: typical axle shafts, as long as they are, and scarcely any larger in diameter than the typical pinion shaft, wind up like long steel rubber bands, and experience many times higher TORQUE than the pinion gear itself (pinion torque times gear ratio). Many 1967 Corvettes equipped with the tri-power 427 option, destroyed the half-shaft U-joints in the axles, as that set-up with typical 4.11s or 4.56s stressed the U-joints 4 or 5 times beyond design torque.

Ah, those good old days! impish

 

I would wonder what kind of "regular use" of an 8.8 will result in assured rear axle failure? 8.8s have been very successfully used in several millions of vehicles to date. Had there been significant numbers of failures in "normal use", especially if failure occurred under warranty, wouldn't the use of them have been dropped?

Just a thought. impish

 

The "regular use" failings come from the lack of durablity and servicablity of the wear surfaces namely the axle/cross pin grinding and using the axle as the axle bearing race. Also from the large use of clutch type posi differentals in which the wear of the clutches leads to an enlarging of the C-clip, axle and cross pin clearence. This leads to a pounding and earlier failure of the C-clip and/or the cross pin.

I would in no way consider there use "very successfull", and no many failures didn't happen within the 36k warranty, but we have way more then 36k on our trucks don't we. Also in many later models the use of disc brakes masked and reduced failures due to the brakes retaining the axle.

Quite frankly as far as I'm concerend anyone that claims not to have seen an 8.8 fail as mentioned simply hasn't been around enough 8.8 axles.

This design is one thing in a regular car with light use where it might go 400k, but as soon as any heavy loads, significant side loading, or wheel spin are introduced the life drops fast, I've seen C-clip failures from "regular use" at as little as 120k. This in an almost stock bronco with 33" tires that was simply a young womans daily driver. She could have died when her C-clip failed, nobody can say that about a 9"

Edit, and are you really gunna agrue that they would spend the bucks to use a better design when not absolutely needed?

Shoot even my brother has had a C-clip/cross pin failure, this was a chevy 12bolt but same thing. Almost totaled his Camaro when the axle and thus wheel and brakes left the car.

 

My stock 351w put enough torque through three different third members to cause pinion failures. I wouldnt consider the pinion design "slightly weaker" then the 8.8. Does the 9" have some things going for it? Of course it does, but id rather break shafts then gear sets and thats why my trail rig now has an 8.8.

 

Ya know it's not the engine it's driveline torque, that can be accomplished with gears. So what were you running and doing to break 3 9" pinions? Something had to be rather extreme to break the pinion before the axle. Was it spooled or locked? Were you popping the clutch or something? Big tires and low transmission gear ratios with tall axle ratios? High driveline angles? Is the rig really heavy? What gear ratio? And breaking the pinion before twisting a driveline or breaking a U-joint is odd to. Smells fishy to me. Are these gear sets that you set up, or were they all set up by the same person? A lack of backlash or overly tight/loose pinion bearings could much better explain a pinion failure. Or I heard you once mention ground clearance. The ground clearance issue on a 9" is how low the pinion is, is it much more likely that hitting the pinion weekend/broke it?

The 9" like most axle types are weakest under a shock load when there is enough traction to prevent tire spin. While the axles have spring to them and will twist like torsion bars without breaking, pinions and gear teeth will break under these shock loads. But the same axle under a constant strain will fail differently depending on the strain and axle. From simple torque, usually an axle right outside the splines, and usually the left.

You have a point that an 8.8 axle is much easier to replace on the trail. But that means carrying extra axles and the risk and hassle of getting out the broken end. Not to mention that when that 8.8 axle does break, it will break in a terrible position and unless you have disc brakes take your wheel and brakes with it. And a 9" with a broken pinion at least has the wheels still firmly attached and can be driven in front wheel drive.

Anyway, yes while the stock 31 spline 8.8 is stronger then most stock forms of the 9" when it comes only to the task of applying torque. The 9" is stronger and more durable in every other way with much better aftermarket support.

But this is way off topic and not what the OP asked is it.


Hmm a thought, if the torque limits of the parts(axle, pinion, etc) could be found then a gear ratio break could be determined. As the ratio increases(nummerically) the strain is concentrated more on the axle over the pinion. So at above a certain ratio the axle would break first, and below a certain ratio the pinion would break first. This of course assumes all other components are strong enough, I personally have seen many more gear tooth failures then pinion shaft failures. IMO I'd put the general occurance of failures of a the 8.8 in this order from most to least; C-clip falling out, Cross pin breaking, axle bearing failure, axle breaking, spider gear failure, gear tooth failure, and pinion bearing failure. Never seen an 8.8 break a pinion shaft, the carrier bearings fail, or the housing fail.

 

Ya know it's not the engine it's driveline torque, that can be accomplished with gears. So what were you running and doing to break 3 9" pinions?
Rocky hill climbs, off camber trails, general off roading
Something had to be rather extreme to break the pinion before the axle. Was it spooled or locked?
No
Were you popping the clutch or something?
No
Big tires and low transmission gear ratios with tall axle ratios?
35s with 4.10s, np435 and np205
High driveline angles?
Factory driveline angles
Is the rig really heavy?
Little less then 4k pounds
What gear ratio?
4.10s
And breaking the pinion before twisting a driveline or breaking a U-joint is odd to. Smells fishy to me. Are these gear sets that you set up, or were they all set up by the same person?
Factory set that came in the truck, two junkyard third member and one set up by a local shop
A lack of backlash or overly tight/loose pinion bearings could much better explain a pinion failure. Or I heard you once mention ground clearance. The ground clearance issue on a 9" is how low the pinion is, is it much more likely that hitting the pinion weekend/broke it?

The 9" like most axle types are weakest under a shock load when there is enough traction to prevent tire spin. While the axles have spring to them and will twist like torsion bars without breaking, pinions and gear teeth will break under these shock loads. But the same axle under a constant strain will fail differently depending on the strain and axle. From simple torque, usually an axle right outside the splines, and usually the left.

You have a point that an 8.8 axle is much easier to replace on the trail. But that means carrying extra axles and the risk and hassle of getting out the broken end. Not to mention that when that 8.8 axle does break, it will break in a terrible position and unless you have disc brakes take your wheel and brakes with it. And a 9" with a broken pinion at least has the wheels still firmly attached and can be driven in front wheel drive.

I would rather carry extra shafts in my tow rig so i can continue wheeling after the 45 minute repair i would need to make. True, the 8.8 may send an axle spinning out of the housing, but at such low speeds chances are you would notice a snapped shaft before it left the vehicle.

Anyway, yes while the stock 31 spline 8.8 is stronger then most stock forms of the 9" when it comes only to the task of applying torque. The 9" is stronger and more durable in every other way with much better aftermarket support.

That is all i was trying to say. In STOCK FORM the 8.8 is stronger. If i had a much larger budget for my trail rig then yes, i would have a nice 9" instead of the junkyard 8.8 thats in it now. Ive always wanted me one of those Hi9 third members in a nice fabricated housing

But this is way off topic and not what the OP asked is it.


Hmm a thought, if the torque limits of the parts(axle, pinion, etc) could be found then a gear ratio break could be determined. As the ratio increases(nummerically) the strain is concentrated more on the axle over the pinion. So at above a certain ratio the axle would break first, and below a certain ratio the pinion would break first. This of course assumes all other components are strong enough, I personally have seen many more gear tooth failures then pinion shaft failures. IMO I'd put the general occurance of failures of a the 8.8 in this order from most to least; C-clip falling out, Cross pin breaking, axle bearing failure, axle breaking, spider gear failure, gear tooth failure, and pinion bearing failure. Never seen an 8.8 break a pinion shaft, the carrier bearings fail, or the housing fail.