Is a Syncro with VC Full-time All Wheel Drive?

[Jon_slider]

Moderator Note: This topic is now locked. Please continue the discussion in the original VC thread: http://www.thesamba.com/vw/forum/viewtopic.php?t=613115 .

********

An opportunity to discuss how a Viscous Coupling and other Syncro traction controls work.

A quote from Wikipedia
http://en.wikipedia.org/wiki/Locking_differential
"A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually.

When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).

By contrast, a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds). Exceptions apply to automatic lockers, discussed below.
A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.

All the above comments apply to central differentials as well as to those in each axle: full-time four-wheel-drive (more accurately as "All Wheel Drive") vehicles have three differentials, one in each axle, and a central one between the front and rear axles"
===

My interpretation of how the above information applies to a VC Syncro.

A Center Differential has two output shafts and allows the Front Differential and the Rear Differential to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) Center Differential always provides the same torque (rotational force) to the Front Differential and the Rear Differential. (Equal torque, unequal rotational speed).

By contrast, a locked Center Differential forces both the Front Differential and the Rear Differential to rotate at the same speed. Therefore, each Differential can apply as much rotational force as the traction under it will allow, and the torques on each Differential will be unequal. (Unequal torque, equal rotational speeds).

A locked Center Differential can provide a significant traction advantage over an open Center Differential, but only when the traction under each Differential differs significantly.

Full-time four-wheel-drive (more accurately "All Wheel Drive") vehicles have three differentials, Front, Center, and Rear.

A Syncro has Two Differentials. When the VC hardens it functions like a Locked Center Differential. Unequal Power is delivered to the Front and Rear Differentials at Equal Rotational Speed.

When a VC is in a hardened condition on dry pavement, there will be drivetrain binding when turning, because the differentials are forced to rotate at the same speed, even though the front wheels are traveling a longer distance than the rear wheels.

All Wheel Drive is only possible when there is an open center differential with two output shafts.
_________________
My Soapboxes: Inflation; Handling; Gearing; Decoupling; Swepco

[Waldi]

Copy my post from other thread:



Jon_slider wrote:
candyman wrote:
I would like to point out that snow is my main use for this syncro while offroading is secondary.


MsTaboo wrote:
the AWD aspects of the Syncro handling make this an outstanding vehicle


WestyBob wrote:
the coupled VC always produces some variable, measurable torque to the front.


imo the belief that a Syncro is AWD is a widely held superstition. To think a vehicle with no center differential can send power to the front wheels when they are turning faster than the rear, is magical thinking.

so the idea that a VC is somehow providing AWD in snow, without rear wheel spin, is not logical, to me.

but I dont mean to start a fight, just offering an opportunity for those who are willing, to question the AWD assumption.



The VC system AWD you can call a inteligent AWD.
It is a full AWD when you need it. And when you dont need it, it still connects front and rear with about 50nm.

In the case of snow driving a curve and the front tires spin faster, it start to be a 50nm transfer in direction front when the rear tires spin faster than the front moving.
This is the case when the resistance of deep snow or while accelarating the car on a plane road with curves need power from the engine to move the car, or even more when you go uphill with curves.

My syncro was driven mostly on a steep snow offrad way uphill and downhill with 4 chains. 5km 16 180 degrees curves as a taxi for a mountain restaurant. Some time i was driving it there.
The syncro was driven under this conditions all rear long (offcourse in summer without snow) over 10 yers and 120tsd km.
Than i got it and the gearbox (main bearing and pinion/G-gear) and vc (was loose) was broken.
When i repaired the gearbox i intalled a decoupler.
Driving this gearbox now 220tsd. Alot with a 2,5t trailer.
With chains the syncro is even understeering going uphill because there is few spining of rear wheels. And more understeering using the rear locker.
But with a good step on the accelarating pedal you could get the rear tires spinning and the understeering stops.


The negative of the locked vc system driving downhill (or using the engine breaking while you enter in a curve) with curves is, that the front tires loose binding and the car is understeering.
You can change this if possible with accelarating the car, but this is in most of this conditions not possible, because you want to become slower and not faster.
The same happens when you drive downhill with a front wheel drive. The engine slows the front tires and they loose binding to the ground.


Edit:
This are the conditions where the syncro was driven down and up over 10 years sometimes with 15 person inside.

[insyncro]

Yes, 50nM transfer minimum, no matter what, period.

[Syncro Jael]

[hans j]

Yes
_________________
1986 Canadian Syncro Westy TDI - 1989 Syncro Single Cab - 2001 Audi S4 - 1981 VW Caddy ABA - 1980 VW Caddy EV - 1973 VW T-181

[IdahoDoug]

FYI, the "55nM always torque" often cited is actually not VC function. It is literally simply the resistance of overcoming the rubber seals - not coming from the VC's torque transfer ability at all. I've seen the number at 30nM with a new VW VC, and obviously wear/use would have it go down from that point.

Since it is nothing more than seal drag, it is not much to brag about, as once the VC is actually transferring torque, this 30nM is nothing but wasted energy at about 22ft-lbs. I am not sure where the 55 figure (and now the 50 figure) come from. Any sources on that?

At any rate, there is no seal drag helping the Syncro on dry pavement. Its tire contact patches do not benefit from stabilizing driving torque on the road and in wind like an Audi Quattro, Range Rover, or other vehicles with driving torque to the tires 100% of the time. That's why Waldi correctly reports on snow that the Syncro's front tires actually bind and cause slippage and understeer until he presses on the gas. As he reports, this is worse on a downhill snowy road where you cannot go faster to cause rear wheelspin and change it.
_________________
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, 1993 Toyota LandCruiser, 2002 BMW 325iX, 1982 Toyota Sunrader

[Waldi]

[davevickery]

[BavarianWrench]

Jon, I can not believe you started this up! I say everybody make a video of their VC at work. We can get a sense of what proper is. VC's are magic, spiritual devices, and will either get you stuck, or traveling, depending on your Karmic scorecard. That is why some cheat, going with straight shaft delete kits. If we are going to question, lets ask a deep one!

[Syncro Jael]

Doug,

Since my wife has owned Audi Quattro's for years I did a little search on the Quattro system. This is what I found:

The fully automated mechanical nature of the Torsen centre differential helps prevent wheel slippage from occurring, by diverting torque instantly, without any discernible notice to the vehicle occupants, to the axle which has more grip. This method of operation can be described as proactive. Furthermore, unlike the various types of electronically operated differentials, Torsen has no requirement for electronic data, from sources such as road wheel speed sensors; it therefore has an element of "fail-safe", unlike designs such as Haldex Traction, should one of the wheel speed sensors develop a fault. In comparison, viscous coupling, and electronically controlled centre differentials that are used in other four-wheel drive systems are reactive, since they only redirect torque after wheel slippage has occurred. The advantage is felt under hard acceleration, including whilst cornering, since the torque transfer between axles is seamless, thus maintaining stable vehicle dynamics, and considerably reducing the chance of losing control of the vehicle.

http://en.wikipedia.org/wiki/Quattro_(four-wheel-drive_system)

I am sure this will bring up a whole new discussion

It is good to learn from others
_________________
1987 Syncro Westfalia Hightop - NAHT
Subaru EJ25 Forged Frankenmotor, Triple Knob.
Jael = (Mountain Goat)

[IdahoDoug]

So to finish that thought, if we ever complete a discussion of the VC without it breaking down into the wishful thinking of transfering driving torque all the time thing, it will be apparent where that figure REALLY comes from. And I'd counsel against making it larger than it really is. It is literally shaft friction on rubber seals, and a loss of 22 ft-lbs is a huge proportion of the ability to transfer torque to the front tires once this loss is overcome. You will be wishing it to be 10 if we ever get to an accurate discussion of how the VC works.
_________________
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, 1993 Toyota LandCruiser, 2002 BMW 325iX, 1982 Toyota Sunrader

[Syncro Jael]

And further reading:

Viscous coupling

This 4WD system was used only on Volkswagen branded vehicles, and was never used on any Audi cars except Audi R8 model.

The aforementioned viscous coupling 4WD system was found in the Mk2 generation of transverse-engined A2-platform vehicles, including the Volkswagen Golf Mk2 and Jetta. It was also found on the Volkswagen Type 2 (T3) (Vanagon in the US), Mk3 generation of Golf and Jetta, third generation of Volkswagen Passat B3 (which was based on a heavily revised A-platform), and the Volkswagen Eurovan.

The Vanagon system was RWD-biased, due to the vehicle being rear-wheel drive by default; the engine and transaxle were in the rear, whereas the viscous coupling was found in the front axle near the final drive. This 4WD system was known as Syncro on all vehicles.

What: Automatic four wheel drive (on demand).

A viscous coupling installed instead of a centre differential, with freewheel mechanism to disconnect the driven axle when braking.

Open rear differential (mechanical differential lock optional on Vanagon).

Open front differential (mechanical differential lock optional on Vanagon).

Normally a front-wheel drive vehicle (except Vanagon, see above). In normal driving conditions, 95% of torque is transferred to front axle. Because viscous coupling is considered to be "slow" (some time is needed for silicone fluid to warm-up and solidify), 5% of torque is transferred to rear axle at all times to "pre-tension" the viscous coupling and reduce activation time. The coupling locks when slipping occurs and up to near 50% of torque is automatically transferred to rear axle (front in Vanagon). In on-road conditions, the car will not move if one front wheel and one rear wheel lose traction.

The freewheel segment, installed inside the rear differential, lets rear wheels rotate faster than front wheels without locking the viscous coupling and preventing ABS from applying brakes to each wheel independently. Because of the freewheel, torque can be transferred to rear axle only when the vehicle is moving forward. For four-wheel drive to work when reversing, a vacuum-actuated "throttle control element" is installed on the differential case. This device locks the freewheel mechanism when in reverse gear. The freewheel mechanism unlocks when the gear-shift lever is pushed to the right, past the third gear. The freewheel is not unlocked immediately after leaving reverse gear on purpose – this is to prevent the freewheel from cycling from locked to unlocked if the car is stuck and driver is trying to "rock" the car by changing from first to reverse and back.

Disadvantages of this four-wheel drive system are related to actuation time of the viscous coupling.
1.When cornering under acceleration on a slippery surface, the rear axle is engaged with delay, causing sudden change in the car's behaviour (from understeer to oversteer).
2.When starting on a sandy surface, the front wheels can dig into the sand before all-wheel drive is engaged.
_________________
1987 Syncro Westfalia Hightop - NAHT
Subaru EJ25 Forged Frankenmotor, Triple Knob.
Jael = (Mountain Goat)

[Waldi]

[j_dirge]

When people ask me about the syncro.. I tell them that I consider it the precursor to modern AWD.
.. and if they ask further I explain:

Old school 4WD has a transfer case... maybe with LSD and/or lockers.
..if you can manually connect/disconnect front and rear, then its 4WD,
..but if you can not and rely on VCs to do the work.. then its AWD.
With a decoupler and an aggressive VC a syncro becomes more 4WDish.
Of course that explanation goes further with discussion of open differentials vs LSD and/or lockers

Not saying any one is better than the other.. just that most of the old school guys I've known thru the years consider the syncro,s.. "AWD".. simply for its lack of transfer case.
_________________
-89 GL Westy, SVX.. finally.

-57 pan f/g buggy with a 67 pancake Type 3 "S"
"Jimi Hendrix owned one. Richard Nixon did not"
-Grand Tour, Season 1, episodes 4 and 5

[Jon_slider]

[Waldi]

When the front wheels spin faster, there is no need for torque on them.
If ther is need of torque on the front wheels, there is one. It starts with about 50nm and ends on 100% engine torque.

[Waldi]

Now my spacial test for ID:

Empty VC with new seals. Torque near zero.

[insyncro]

Waldi...I love it, nice job

[Waldi]

[WestyBob]