There is NO spider lash in a properly tuned posi.
That is the whole point.
I guess you need to actually get your hands dirty and "feel" it.

 

It’s my understanding that the biggest reason for the spring pack is economics. To tune the differential as GTR or Tom Watt do takes quite a lot of time, far more than feasible for mass manufacturing. The spring pack design requires ‘snow flake’ clutches to address the problem of chatter. This design is prone to breaking. GTR installs solid clutches which are more durable but will likely chatter if the spring pack is installed. I know GTR has been tuning differentials for many years, has a large backlog of jobs waiting. If the tuning didn’t work I’d think many wouldn’t be lining up to have this done as part of the rebuild.

My differential has been built this way. I’ll find out soon how it works out.

 

I would like to remind participants that (I believe) the purpose for this section ("Tech/Performance") of this forum is intended to provide technical information on the subject of the mechanicals, both the actual machinery and the technique of servicing such, that for the enthusiast seeking aid relevant to the hobby. And of course, if someone might actually have a contrasting position to my statements made of the technical function, that of the subject topic, the device, please bring such forth, as this sort of discourse is often how the best conclusions are established and possibly/hopefully we all learn something, and the forum has meaning.

But on the other hand, the Forum does not benefit from personalities exhibiting emotional responses not presenting relevant conversation on the actual topic, but rather chose the desperate attempt of substantiate an opposing position with derogatory statements not of the subject at hand.

Scott.

P.S. B.T.W. I have been getting my hands dirty in this business for decades, daily, and I'm quite familiar with "Corvette" diffs along with a cornucopia of other examples for many other applications and from many other manufactures; so please, lets' not start down the road of the narcissistic behavioral comparisons of who's bigger!

 

Well, you were the one who unzipped their fly by starting your response with “I know this will cause trouble…” so while I do agree with the need to keep it civil, your “attitude” is clearly on display so you shouldn’t be surprised with some of the responses.

 

Ya'll need to simmer down in here. Stay on topic. Knock off the personal shots.


Thanks, I appreciate the self control.

Elmer

 

In proceeding with thoughts for discussion on this subject of "Springs or No Springs"; in the effort to acquire the "preload" on the clutch packs, this through the process of "crowding" the gear sets, this supposedly justifying the elimination of the preload spring hardware, what other effects are we creating that were not of the manufactures original intent and therefore not considered in the engineering of the components at hand?

The O.E.M. utilized the springs to apply a preload value to the clutch packs opposite each other forcing the gear sets tooth contact from each other thereby creating a clearance sum (measured lash) this providing for lubricant intrusion and a drive load relationship that was applied to the appropriate "drive-face" flanks of the tooth profile, as intended.

With the gear sets "crowded" (never a good idea!) this creates an additional collision force as the gears' teeth swedge together, this rather than a simple ramping up and down of loading on the crowned thrust faces, and imparts a greater force to deflect the teeth, this particularly increasing the stresses at the base of the teeth where they are most often prone to initiate fractures even when properly set up; and then hopefully the top edge of the teeth do not actually impact the root of the opposing gear teeth! But all of which, if only, would lend to far greater wear rates of everything involved and then we have to consider that under "normal" applications particulate contamination, of this type material, of the fluid is probably the greatest offender to the life span of the roller bearings!

Then for consideration, the springs which apply a preload value are a "flexible" load tension device, therefore as compared to the gear crowding process, they maintain a more constant preload value (and apparently a greater sum) on the clutch packs as the unit exhibits wear in service, that's a plus right! And of course, the gear crowding process is a situation were we have a culmination of "solid", supposedly rigid componentry that we are attempting to create a resultant "preload" (spring) value from on the assembly as hole, this is better understood as an "interference" fitment and in such instances something has to give, bend, compress, the sum of all, right? Are any of these items in this train of componentry intended for this?

Yes, please do watch the video! Notice how with the attempt of the acquired "preload" (measured ?) in the resistance of the differential in rotation of the side gears and thereby feeding though the gear sets, there appears to be a "cogging" motion effect (as would be expected), what is flexing to allow the passage in the interference fitment?

And by the way, this idea of crowding the differential gear seats isn't new, it's been practiced "for-ever", and for different reasons.

Scott.

 

OK
Let's discuss it, but let's just call it clutch pack pre-load, and examine the full range.
Diffs vary from an open diff, to "tuned", to springs in 200#, 400#, 800# increments, and then there are spools.
Let's agree that the open and the tuned are the smoothest.
The heavier spring packs cause an increasing amount of popping and and even hopping noises. I could even feel that while racing.
The spool obviously has the best traction in a straight line, but forces one tire to slide during turns. A lot of hopping at low speeds.
I do not think the preload in a Tom's tuned posi is very stiff. I can rotate it with my hands, but barely. But at a 6" radius.
I believe the standard posi springs (IIRC that is the 400#?) require two strong arms to rotate one tire, and have about an 80-90# torque spec to rotate a tire.
So maybe the tuned posi would take 40 ft-lbs to rotate a tire??
Only the tuned posi and the spool has no "slack" in spider gear engagement, which reduces breakage.
The spiders take the full brunt of the engines TQ, at all times, and if they have to move .130" first, and then impact under full power, that is a lot of shock load.
All of the clutch pack pre-load variations should lock-up the same once there is high engine torque on the pinion gear.
So the difference in clutch pack pre-load occurs when the torque provided by the pre-load is greater than the load from the pinion gear.
Engine TQ can approach 5000 ft-lbs at the tires. If 50% of that turns into side-force / clutch pack loading, that can get very high.
So the initial pre-load of 80# at the tire, is very low, vs the 2500# max, less than 4% of max.
It is only going to take 8# of engine TQ to make 80# at the tires due to the gear ratios.
The pre-load supplied by the spiders would surpass the spring pack pre-load after that.
So with a tuned posi you gain: (1) zero slack in spider gears (2) smooth turning at all times and (3) less clutch pack pre-load at very light throttle.

Does anyone see a driving situation at light throttle where acting a little more like an open diff is a problem?
Has anyone ever had to go to the 800# spring packs? And what did that help?

 

Agreed, so what we are simply saying is that as we attempt to increase the resistance of the differential action we tend to increase the loading in the tire to pavement relationship and this often leads to unappreciated suspension reactions, increased tire wear, even to the point of skidding.........Yep.

Agreed, as your 80-90 lbs of torque in resistance of tire rotation respective to the other being stationary is typical of the value utilized by the O.E.M.'s in these types of vehicles and is based of the limit of the tires' coefficient of friction to the pavement before one does start to create the some of previously outlined unappreciated reactions.
And yes, as has been stated previously, the "tuned" springless effort will generally yield less as the process of attempting to create a side-loading value by crowding the gear sets has it's limits!


Agreed, "slack" or particularly "excessive" lash is a bad thing; but a sum of clearance as intended in the original engineering is also is a "required" thing; as the bevel gear set is just a gear set, and most of the same general rules for proper function still apply. Just as for the ring & pinion set, which is a hypoid spiral bevel gear set, and in order to acquire proper service performance, proper lash sums are required in order to acquire the proper gear to gear relationships (often observed as a "pattern"), lubrication (also as a coolant), considerations for movements, even heat expansion, as particularly with gear sets anything else just leads to something less; and an interference fitment with the greater additional impact loading is a far greater detriment than that of the "correct" clearance specification and any shock loading created here. Right!

But perhaps (?) your .130" clearance sum might just have been an indication that your example could maybe have benefited from a "tune-up", back to something akin to "as-new" specifications, maybe?


I'm not a mechanical engineer by title or trade, but I do understand enough to know that the calculations and considerations involved in order to establish accurate numbers that could be quoted here would not be possible as we would not be privy to all that would required and a singular result would only be of a singular instance in the rather dynamic event we are trying to express; so I'll pass on the numbers game.

But, also I don't think one would find that the measured rotational torque sum at the pinion shaft equates directly into a rotational load effort at the pinion to side gear relationship, as don't forget, the operation of an indirect gear set in the load path requires effort and parasitic losses are inevitably experienced, not to mention we'll need to apply enough torque to break a tire lose before any of this matters, so if it's easier for the vehicle to just roll forward then that's what happens; if that were not so then how would the car with an "open" differential and any power ever move?

And yes, as stated previously the resultant thrust loading, a byproduct of the bevel gear relationship in operation, is understood to at some point provide a sum greater than that of the preload springs; but, remember that this system was not necessarily engineered for dragstrip or other racing endeavors but rather as for instances of traction loss due to environmental surroundings and in these instances often the sum of engine torque required to spin a tire might not be so great! And in the myriad of possibilities here this gets back to my previous statements that the system is engineered as a hand-off, initially the preload springs provide a resistance in the differential function, but not so much as to unnerve the driver in low speed and load maneuvering, and this which in itself raises the threshold before one tire might break-loose, this increasing the possible torque input being applied to the system from the engine and then if all works as planned the hand-off to the increased loading from the bevel gears takes over.

But, with the lesser initial preload provided otherwise ("tuned" springless) the separation in the hand-off becomes greater, and the potential for one tire to get away is greater, and then as stated previously after that...............it's all over!

So, (1) "zero slack", sounds good but as stated previously crashing the gear sets is not the proper as engineered execution and does incur bad functional results, even if your not aware of such. At least not yet!
(2) & (3) If this is your goal, just reduce the spring load!

Scott.

 

So let me get this straight...you are not an engineer...you admit you do not understand the math....but yet you proclaim that this is not proper engineering....and there are bad results....that we are not aware of...

So what are these bad results?

 

I'm sorry, but I just don't think I'm going to be the person, based on your understanding and communication efforts presented here, whom would be able to set you straight. Not to mention this type of rhetoric doesn't provide useful substance for the Forum.

Good luck,
Scott.

 

If Mr. PBF is correct about this.......ALL the years Tom was building high quality differentials, with tuned posi's.....AND ALL the years Gary Ramadei has been building high quality differentials, with tuned posi's.....BOTH with well known reputations for the best builders in the industry...were BOTH providing differentials that were improperly built. And all those recipients of those bad builds have not only been silent, .....but instead praise both of these guys for the differentials they built. Whats up with that? But now,......the truth is out. They were all duped.

I am not about to engage in any conversation with the long winded technical rationale that is being posted, but I have great confidence in the decades of builds these two guys above have produced,....AND further supported by Leigh's comments, who also has years of racing and old car knowledge, as well as a very rational scientific mind, and has proven it over years on this forum. That is good enough for me, without me spending hours and days studying differential designs to become an expert.

Its fine to have some discussion ....given this is a forum,....but for those reading this and trying to decide right from wrong about C3 Corvette differentials.....I suggest you be careful who you choose to believe. That's my contribution to this thread.

For me,....I will take the decades of experience from two seperate differential builders, with nothing but documented recommendations as being the best over many years.....versus anyone else, and certainly some guy on a forum nobody knows professing these two guys have been all wrong.

As for the lecture on the forum and allowing discuss and conversation......nobody is stopping it. In fact....here we are doing it. But it is also true when posts are made with completely opposite opinions,....we all have to pick one. I know I have. In fact, I have a differential built by Gary, going into my77......with a tuned posi......should I sell it or get it rebuilt properly? I sure don't want to suffer the consequences of an improperly built differential.

 

And before we go down the road of injecting personalities and comparisons between individuals, that's not really the (my) point, nor truly relevant! As I previously stated this discussion should be confined to the "device" and exploring its' function and intent, as this is the "Corvette Forum". Right ! Where individuals come to participate in the hobby of Corvette ownership; as for the rest, all I can say is that "the chips fall where they may", as this isn't someones' personal Face-Book page created for the purpose of touting their own personal escapades............ at least not on the "Tech" page anyway. Right!

Also realize that I'm not soliciting for any business, jobs, favors, trades, or anything of benefit, other than just sharing information that 'might' prove beneficial others in the hobby. If what you read you might disagree with, that's fine, ignore my statements if it makes you feel better, or you can logically engage in the conversation with your own thoughts, observations, conclusions or further inquiry (not that I might have an answer, but perhaps someone else will), that might be relevent and in this manor the Forum works as it should; heck even I might even learn something! .

Otherwise, we're just back to the ol' "who's bigger" routine...........Sad.

Scott.

 

This keeps going and going. I think the vast majority of us with any amount of experience with these old cars know what's what here!
Myself, I wouldn't even begin to question Gary's opinions on how these should be built. Particularly in a performance application.
In fact, I would be asking him questions and taking notes! And following his advice to the T.
Still hasn't answered my one question.
Where can I purchase a quality diff yoke?
I actually need one!

 

The pointing out of the decades long reputations of two very well known C3 Corvette differential rebuilders is absolutely relevant, as it goes toward their success with building differential and "tuning" positraction units without spring packs. That success with that method of building these differentials over this long period of time speaks for itself.....so all the technical ideas and theories you are presenting would basically have to prove this long term success was all wrong......and there is zero evidence of that being presented. So, its not about personalities, its about the technical subject proven by time and experience....just as you are suggesting.

This is my last post on this thread. Not interested in debating it further.

 

I’m going to go out on a limb here and determine that the differing opinions are not going to be changing one another any time soon.

I also think the repeated retreating to issues of personality and civility makes this thread liable to end up in a conflict situation pretty soon, so I’m going to call it now.

If the OP or the participants want to start another thread and resume the debate, if you believe there would be any value in it, then start another thread when the temperature lessens a bit. Thanks.