Syncro Chat: VC's, solid shafts, decouplers, AWD/4WD, etc.

[raoul mitgong]

[Syncro Jael]

An earlier post:

[Syncro Jael]

Can I also add a comment for drawing #3

In this scenario would not the front wheels would also be "instantly" engaged via the viscous couplers pre-tension?

And this post from Derek Drews testing:

VW adds another tiny sentence to this, saying that only when the engine is revving at idle and with the G-gear switched in, the viscous coupling is able to absorb all the torque to the front wheels and keep them from moving.

Test Results I cannot resist commenting here on some interesting and enthusiastic commentary from some of our list members about the usefulness or non-usefulness of the VC for offroading. For example, in the last week one list member said: "The problem with going _too_ slow with a syncro is that the VC won't do much work below 3 krpm. Then a front locker isn't of much use either." Another list member said: "...you are right on with this info. that's why the serious experienced Syncro drivers are running an aggressive VC for added off road traction and a Decoupler for normal driving. Tom's list of happy Decoupler users reads as a virtual Whose-Who in the Syncro drivers group, the ones that have them, have them, the ones that don't just don't get it!" The implication of these statements is that the VC is not a very effective tool for off-roading because it does not react quickly enough at low speeds to engage. If one is rock crawling and the VC does not really engage, then it is not an effective tool. The statements themselves struck me as in contrast to my experience in off-roading but this was merely an impression and therefore of questionable validity. Have I been deluding myself and the above statements are actually true???? This last week I decided to test these theories. I had already removed one of my rear axles, so what I did was to effectively remove the other rear axle by disconnecting my rear differential lock and then driving the van around with the rear diff lock off and on, thus testing at what speed the viscous coupling actually becomes operational. The results???? The results are that if you did not know that the diff lock was being applied and then de-applied, you would probably not notice that the rear axles were disengaged at all. One can drive out of a parking space at an engine rpm of as little as 1,000 revolutions per minute and the VC transmits enough torque to the front of the vehicle so that it drives almost normally. Since I knew that the test was being conducted, I was able to detect a slight slippage sort of like a clutch slippage at the very slowest engine rpms -- say 900 rpm or really just idle speed-- but applying almost *any* power at all would cause the sensation to disappear and the front axle to become fully operational. As a further test, I disengaged the rear axles and then asked my wife to drive the van around the neightborhood and asked her if she could tell anything was different. She could not. Accordingly, I view the tendency of people to think that they need an "aggressive" VC in order to maximize off-road ability as probably not warranted.

http://users.rcn.com/derekdrew/vanagon/viscous_couplings_vanagon_syncro.htm

EDIT: in drawing #2, A VC, coupled, within factory specifications, wouldn't it help eliminate some of the rear wheel spin during acceleration? If the VC can react "instantly", "less than a quarter turn", "0.05 rpm", to rotational differences between front and rear? And if it does this while accelerating, can it not reduce "total" rear wheel spin in a long straight drive? And does this mean that it is in fact transferring forward pulling torque through the VC and the front diff driving down the road pushing the brick through the wind and road resistance? Anyone care to take a crack at them while looking at this drawing?

I am just putting some things out there to discuss while we look at these drawings.

_________________
1987 Syncro Westfalia Hightop - NAHT
Subaru EJ25 Forged Frankenmotor, Triple Knob.
Jael = (Mountain Goat)

[djkeev]

My brain hurts just a little right now........... Too much thinking today!

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[tencentlife]

I've been watching this thread with amusement. I won't comment on the highly embarrassing slugfest aspect, except to say that I have new respect for Idaho Doug for his persistence, so much so that I vote that he change his handle to Idaho Dogged.

You guys have come so close to this one thing, but keeping missing it and I can't stand the suspense.

The differential rate of rotation across the VC (∆VC) is the product of the difference in rate between front and rear ring gears multiplied by the r/p ratio.
(my edit: this is to clarify the relationship between wheel slip and ∆VC because some contributors appear to see it as a 1:1 relationship, as in 1 turn of a slipping wheel produces 1 turn at the VC)

To wit:

[kamzcab86]

[Syncro Jael]

[djkeev]

Well Kam, my eyes glazed over when I saw it then and again now!

Once you Pi into and equation and it isn't pie I can eat? .......

I'll take another look after dinner and a few Gin & Tonics! THAT should clear my head!

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[tencentlife]

[tencentlife]

Since this is such a tough crowd, I know someone will challenge that the ∆VC is a product of the r/p ratio, so, as I said just above, here's your test, just do it in your head:

Lift the front end of your imaginary car. Lock the two front wheels together, to eliminate the effects of the front differential, so one turn of either wheel will cause the ring gear to also make one full rotation. Connect the front and rear pinion gears by a driveshaft with an open, frictionless coupling (like a dead VC). The rear wheels are planted on the ground and don't move at all so their differential's pinion gear also doesn't move.

Now turn either front wheel one full rotation. How many times has the front diff's pinion gear turned? The same number as the r/p ratio, of course.

Now put the same vehicle on the highway rolling straight ahead at any speed you like, lock the rear imaginary differential same as the front. Induce some rate of slip to, say, the rear axle, and let that continue, at whatever rate of speed you want, it doesn't matter, until the rear axle has turned one full revolution more than the front axle while the car speeds down the road. How many more times has the rear pinion gear spun than the front? The same as the first model, of course, the r/p ratio number.

A test like this helps you isolate the movement of any driveline component relative to any other from the absolute speed of those same components. If you can't grasp this you won't be able to grasp the drivetrain dynamics while the car is moving.
_________________
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Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[Syncro Jael]

[tencentlife]

SJ, I'm not here to debate or answer questions, I don't have time nor do I intend to get dragged off my intended path, and I never watched your vid, but I've read your own explorations and you are actually onto something important to this discussion that I am going to describe as I go along, so if you bear with me I think the answer will become clear for you, or really it will just confirm what you're already thinking. Darren's model of the dual wheel on the roller is asking the same question, it has to do with a very basic aspect of physics. It's going to take a while because I have a lot of work to do, but it'll be here bit by bit. Meanwhile, I gotta go slam some heads on a waterboxer!
_________________
Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[IdahoDoug]

10c,

Cheater! I was gonna play that one in a bit. Been off lugging extra Vanagon bits to the dump 340lbs minus a couple batteries.

Yes, one rear wheel spin revolution gives what - 4.86 turns at the VC on a stocker? Another cool factor you'll find while rolling along at 60mph or so is that the front wheel contact patches have some 12-15 (? from memory) hp of braking torque from rolling resistance, etc. So the level of torque delivered from the VC would have to exceed this to actually be said to be "driving" the front wheels, or to have them actually "pulling" the Syncro along with engine power. If they received exactly the 12-15, the contact patch is perfectly neutral. More power and it becomes a pulling, contributing contact patch like an Audi Quattro or other vehicle with a center differential (which has steel gears generating constant torque to all 4).
_________________
1987 2WD Wolfsburg Vanagon Weekender "Mango", two fully locked 80 Series LandCruisers. 2017 Subaru Outback boxer. 1990 Audi 90 Quattro 20V with rear locking differential, 1990 burgundy parts Vanagon. 1984 Porsche 944, 1988 Toyota Supra 5 speed targa, 2002 BMW 325iX, 1982 Toyota Sunrader

[raoul mitgong]

[tencentlife]

OK, I have to back up and omit these calcs, they're wrong:

[tencentlife]

So, moving on: Let's relate the ∆VC as rpm to the VC bench-test graph posted on p.36 that measured torque transfer at ∆VC rpms. That graph seems to comport well with similar bench test results I've see elsewhere, so I'm taking it as credible.

But 6% is, I think, an totally implausible wheel slip rate for a T3 even under full power, but instead let's model the 3% rear-wheel tractive slip rate that's been thrown around.

At Darren's 30mph wheelspeed figure of 388, the ∆VC as rpm would be 388 x .03 x 4.86 = 56.57rpm (which jibes perfectly with his 226rpm figure based on 6% at 60mph, as ¼ of that amount).

On that graph, that would deliver a potential torque transfer somewhere bewteen about 120 Nm at 82ºC to 185 Nm at 30ºC, or about 90 to 140 ft.lb. These values are while the VC is in merely viscous transfer, well short of hump state.

Now, I read the linked paper on tire slip and I don't see it supporting 3% as a slip rate except as an outlier. Maybe I missed something, but in the graphed data Fig. 2, under their highest measured tractive acceleration at ~27 m/s, there are three spikes that just reach 3%. The authors admit that that graph has a lot of noise as artifacts of mismatched sampling rates in their two instrumentation systems. The median slip under the hardest acceleration looks to be under 2%, in the lesser range it's more like 1-1.25%. Braking slip median is under 1%. The Fig. 4 scatterplot agrees with this, there are no points even close to 3%, the densest clusters are both around .75% in either direction. I'd like to figure what a T3's acceleration rate could be in m/s to compare, but it's probably safe to say it's on the low end of their tested range.

They also stated clearly that their results could not be considered conclusive because of their small number of tests. This was basically a preliminary paper issued to demonstrate the viability of a research design, these are often done to support requests for funding to conduct more comprehensive studies using that method. Once those studies are funded, and the testing scaled up, whatever weaknesses are inherent to the study design usually show themselves up, resulting in some fine-tuning of the methods to finally, hopefully, produce a dataset of sufficient breadth to derive some reliable conclusions. That doesn't discount the results of the preliminary study, but you have to view that as "soft" data, at best, and they say so themselves.

So back to the 3% slip rate. I think the bench-test result of a 90-140 ft.lb torque transfer rate at 30mph also means you can forget the idea that there could be 3% rear-wheel slip under equilibrial conditions, as seems to have been argued here, unless I misunderstand. At 30mph with a 388 wheel rpm, driveshaft rpm is 1886, engine rpm with a 1.26 3rd gear is 2376, not too far below an MV wbx's rated torque peak at 2800rpm. To see how much potential torque might be put into the VC at that speed, go ahead and take the peak torque of a MV, 117ft.lb., and multiply it by the 3rd gear ratio and you have 117 x 1.26 = 147 ft.lb. torque on the driveshaft. If a VC at that ∆VC of ~56rpm can transfer 90-140 ft.lb., that means the front driveline can basically absorb almost all the torque the engine makes if it was WOT at that speed, making it effectively the same as a solid-shaft driveline, never mind the rear axle. In reality about half that torque would absorbed by the front driveline and the balance to the rear.

I do think some tractive torque is transmitted to the front at straight-ahead equilibrial conditions, and I am going to explain why I think so later, but somehow I don't think the van has become a true 4x4 while cruising at 30mph. The actual tire slip rate has to be far less than what has been alleged.

It's late, I'm done.
_________________
Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[Syncro Jael]

Again, it seems like "slip" is in question. It doesn't take much tire slip X 4.86 to spin the VC plates and engage more torque than static.

Possibly we will never really come up with the answer without getting a Syncro on a machine and test it. I don't know of too many stock low mileage Syncro's out there. Seems like too many variables, and no solid data.

I will just have to live with my own conclusions. until the campfire at Syncrofest.

Where the heck is "Bill Nye" when you need him?
_________________
1987 Syncro Westfalia Hightop - NAHT
Subaru EJ25 Forged Frankenmotor, Triple Knob.
Jael = (Mountain Goat)

[djkeev]

[tencentlife]

Yes I'd rather see Bill fighting the good fight with the many-denialism's crowd, we can manage this one.



I'm just sort of going over the things that are rattling around in my head here, things I see as important that may have been alluded to in this thread but I haven't seen clearly stated. There's two main ones left that I want to cover. This is way more than I've written in years, and boy howdy, it's real work! Hope you're ready for this.

First one is the thing that SyncroJael (is it Ron?) asked about, and it's the same thing Darren presented a model for which Doug has taken to its own thread "Raoul's steel roller challenge", here:

http://www.thesamba.com/vw/forum/viewtopic.php?t=625438


A general mechanical principle, classed under the discipline called "Statics", dealing with the relationship of objects at rest , is that force (load) will be resisted proportionally by all the elements it contacts according to the degree to which they react to the force by "pushing back", or "absorbing force", in fact there are many terms to describe it. In thermodynamic terms it would be called "dissipating energy", I tend to look at things that way. When the reaction force becomes equal to the loading force, the elements are in equilibrium, or static. But that doesn't mean the system as a whole can't be moving, complex dynamic systems often include numerous subsystems that can go in or out of equilibrium at any given moment, or at least that's how I see it.

You can get into more detailed description as to why this is so, one major principle is Moment of Inertia in the bearing elements, their tendency to remain at rest until sufficient force is applied to move them, and on and on, but like most of these sort of things you don't usually have to delve into formulae to grasp and even usefully apply the concept. Mathematics is a brilliant descriptive language, it's the thing that engineers master to ply their trade, my beef with engineers is some of them mistake the map for the territory. But imagination is also a perfectly valid way to explore these types of phenomena, in fact I would go so far as to say that none of the formulae that science and engineering use to describe our world ever came to be without someone first modeling and exploring a question in their imagination. I picture young Einstein sitting silently in the patent office staring at an empty page while that supercomputer brain is painstakingly building an unbelievable model of a universe where time is just one of many threads bound up in the overarching power of gravity.

My working knowledge of math is limited to arithmetic and trigonometry, so my imagination is how I model and test ideas, my intuition is my most powerful tool, the pencil only comes out when it's really necessary, and never without the big white gum eraser, which I have to hide the rest of the time because my cat likes to eat them. Written explanations like these are also a method by which I explore and describe things as I work through a problem, you can see me doing just that in a lot of my posts here; I don't necessarily come to the post knowing, I'm teaching myself as I write and the reader can see the process in action, if he can stand it.

I was trying to think of a good model to illustrate dynamic equilibria that would allow a quick jump to the VC's role in the driveline, when I was out walking the dog this morning and saw a really good model in some tire tracks in a sandy wash near my house. The tracks told a story of how a truck's differential behaved as it turned a sharp corner in the loose, coarse sand. The outer track left a pretty smooth, even trough with a faint tread imprint, showing little evidence of longitudinal slip, while the inner track showed the inner wheel alternately rolling forward a bit and slipping a bit rapidly again and again, leaving small slip ridges of sand across the inner track every two or three inches until the track straightened out and the two tracks once again became roughly equal, matched imprints.

The simple conception of open differential operation that I think many people would take away from the standard explanations you can find in books and articles is that the differential only transfers torque to both axles when the car is moving straight ahead but routes all torque to the axle that presents the least resistance to torque whenever the car turns. Out at the extremes this is basically true, but between driving straight and completely losing traction on one wheel, the distribution of torque is actual a continuum, following the principles of Statics. In the classic open differential scenario, the reason a wheel that has completely lost traction receives all the torque and spins wildly while the other wheel that has traction remains still is that inside the differential the side gear attached by the axle to the sticky wheel acts a fulcrum against which the spider gears roll, transferring their torque to the opposite side gear, making the slippy wheel spin.

That fulcrum in reality has a variable ability to resist the spider gears' pressure according to the variable traction the non-slipping wheel has with the ground. In the sand tracks I saw, the inner wheel was alternately absorbing the twisting force acting against its side gear and dissipating that force through the tire tread to the ground, acting as a fulcrum for the spider gears to work against, in a momentary state of equilibrium, until the force pushing on the fulcrum exceeded the grip the tire had on the sand, at which time it would slip, releasing some of that force by displacing some sand and proportionally lessening the counterpressure its side gear presented to the spider gears' pressure. Since the driveshaft force being applied to the ring gear that pushes the spider casing round and round was fairly constant, that reduction in side gear counterpressure was instantly matched by an equal increase in counterpressure by the opposite side gear, as the portion of applied force that was no longer being resisted shifted to that opposite side, taking some of the force off the inner wheel, reducing the slip force applied to the ground, and the wheel gained enough grip to once more achieve a momentary state of equilibrium. This alternate partial shifting of force left to right, momentary equilibrium to momentary instability to momentary equilibrium, the inner wheel slipping a few degrees and stopping in what would have felt like a shuddering to the driver, continued until the driver completed the turn, gradually reduced the steering angle and the two tires' difference in traction narrowed to the point where the axle system regained a more stable state of lateral equilibrium.

What this illustrates is that force applied to multiple points will be resisted by the weaker of those points to the degree that each one can present resistance, even to the point that they give way. The balance will be borne by the stronger points of resistance to attain equilibrium. Again, equilibrium is a relative term, it doesn't mean no parts of the system are moving, it refers to the relationship of any elements that act against each other at any point in time even as subsets of a larger moving system.
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Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.

[djkeev]

10cent.....

Thank you!

I love when you answer questions and write stuff!

You've given me Thoughts to chew on for sure!

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert