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

IdahoDoug · Posted: Mon Mar 30, 2015 11:53 pm Post subject:

Syncro Jael,

Good point above and illustrates something very important. Here's your question in quotes:

"Couldn't the same be said for a solid coupler?

If the front diff is turning exactly the same speed as the rear transaxle there would be practically no torque transfer to the front end either. There would be some minuscule gap in the splines that needed to be closed up with some of that torque before it would transfer. I know that drivelines twist also, using up some torque to get the other components rotating."

Yes - absolutely correct. Driving in a straight line with a solid shaft replacing the VC the solid shaft is also not applying torque to the front wheels. Since folks are beginning to get into the crux of the discussion, I should say this assumes equal tire diameter, including tiny mfr variances, pressure, downforce from vehicle weight, etc. These minor things get ahead of the story, so it would be best to address them later.

Back to our solid shaft Syncro. I love Syncro Jael's comment about the splines and a small gap in the splines, etc. It makes a perfect way to envision this. Lets say the male splines of the front of the shaft do have a gap to the female end in the front diff - some play where it mates. If you are driving along in a straight line, the male splines are not putting pressure against the female splines. Why not? Because the front tires are being spun against the pavement and rotating the exact speed of the rears, and the front diff and female splines are already rotating. They are not powered by the solid shaft - they are powered by the front tires. Get it?

Now compare this with something like my LandCruiser, which is in fact a full time 4WD setup. It has a center differential, which has an input shaft driven from the engine, and TWO solid output shafts - one for the rear axle and one for the front axle. When it is being driven down the same road straight ahead, if both of its solid shafts had play in the male and female splines, their would be no gaps. Both shafts would be getting torque as we drove along because that is what a differential is designed to do - allow for different output shaft speeds while delivering power to both of them at all times. The differential is specifically designed to do that. Yes, the LandCruiser's front tires are also rolling along the ground like the Syncros and being driven by the passing ground in the same way. But, when I apply engine power, torque flows from the engine, to the center diff, and powers both f/r driveshafts all the way down to the contact patches and pushes the LandCruiser faster using all four contact patches. Turn the LandCruiser, and the front driveshaft speeds up, the center differential does its impressive job of continuing to apply equal torque to the front driveshaft and the slower turning rear driveshaft throughout the curve. Here's a key point: Because this system can power two axles at different speeds, it would immediately twist an axle with some play in the splines and mash the splines back together - keeping those splines under pressure. In contrast to the Syncro, it can do this specifically because the two output shafts can spin independently of one another in speed.

Now back to the solid shaft Syncro. The engine is powering the rear axle differential directly from the transmission - no driveshaft. There is a driveshaft heading forward to power the front axle. But importantly, this driveshaft cannot turn either faster, or slower than the rear axle differential because they are directly connected. So if when you began rolling in the Syncro there was play in the front shaft splines when you began - that play will remain there with no gear pressure on the front differential. And thus no torque to the front. If the front shaft was powered from a center differential like the LandCruiser, the front shaft would immediately accelerate a few degrees and mash the splines together. But with the Syncro setup, it cannot. It simply cannot.

Things get worse from there for the solid shaft Syncro driven straight on the street. Enter a curve and the front wheels must speed up relative to the rears. But there is no differential - only solid connections!! So instantly you have enormous drivetrain binding as the fronts attempt to force the rears to rotate at the same speed.

So, to answer your question, Syncro Jael if you had a solid shaft Syncro driving in a straight line it would still not apply driving torque to the front differential if there were play in the splines. Unless you stopped, jacked up the front end and applied torque to the front wheels to take the slack out, then lowered them keeping that spline pressure. Then yes, you would drive away with the front differential getting driving torque. Until you turned and then instantly the front differential is binding the heck out of the rear differential an vice versa.

It all comes down to the lack of a center differential to constantly apply torque to the f/r axles no matter the relative axle speeds.

And yes I had a great drive across WA state this evening. Love the Seattle/Vancouver area!! Wow, lucky to have it only 5 hours away....
_________________
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

Waldi · Posted: Tue Mar 31, 2015 6:12 am Post subject:

WestyBob · Posted: Tue Mar 31, 2015 9:23 am Post subject:

raoul mitgong · Posted: Tue Mar 31, 2015 9:50 am Post subject:

I think I have a new question.

Imagine if you will, you are driving along with a VC, perfectly straight, and you have ever so slightly bigger front tires. Starting with zero torque transfer/binding. As you drive, the front tires will slowly start to impart a torque on the VC as the slop is removed from the system, trying to slow the other side of the VC. Once the VC reaches the 30-50nM "spec" across the two shafts, does it slip and reset to a lower torque (like a sine wave) over and over again or does it reach an equilibrium and hang out there?

Tire differences will accumulate over each revolution. Bigger up front will put the entire van in tension (dragging the rear tires) and smaller up front will put the van into compression (pushing the front tires). Assuming that one of these two states will exist (however negligible) due to unavoidable tire differences, is one preferable over the other for traction? Are the mirror image versions (dragging/pushing) equal in efficiency to each other (mpg)?
-d

PS. I understand what the system was designed to do. I am more interested in what it is actually doing.
For example, I make flat surfaces for a small niche industry. Geometrically speaking, my stuff isn't flat. Due to raw materials, manufacturing limitations, cost, tolerances, weight and diminishing returns as we approach perfection, we have a flatness tolerance of .015".
_________________
84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
86 Tintop Syncro to Westy project (Tom Servo)
91 Westy (Only the top 12 inches of this van (a burn victim))

WestyBob · Posted: Tue Mar 31, 2015 10:20 am Post subject:

raoul mitgong · Posted: Tue Mar 31, 2015 10:56 am Post subject:

Figuring out the torque at the rear wheels for a 2.1l, (using 3200 rpm, 117 ft lb, @ 71 hp, and 26" wheels), I figured at 10mph, tires at 129 rpm, there is 2895 ft-lb of torque (t = hp x 5252/rpm) available to the wheels. At 65mph, there is 445 ft-lb of torque available to the wheels (due to gearing).

Not an automotive engineer so not 100% sure I didn't make an error above (I left out trans efficiency also).

Using Dylan's 50nM figure, this is about 1% to the front wheels at 10mph and 8% at 65mph of "available" torque before slipping (I think).

No conclusion here, just more data.

-d
_________________
84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
86 Tintop Syncro to Westy project (Tom Servo)
91 Westy (Only the top 12 inches of this van (a burn victim))

IdahoDoug · Posted: Tue Mar 31, 2015 6:31 pm Post subject:

Jeepers. Raul, did you read my post(s)? Where to you get torque going to the front wheels when they are already spinning faster than the rears in normal everyday driving? I just got done explaining the strongly related issue which is even if the VC were a solid shaft (MUCH more able to instantly transfer torque to the front tires than a mere VC) it would still not do so. It still wouldn't - even a solid shaft!! And a solid shaft has much greater "residual torque" than a squishy VC - it is 100% transfer with zero residual - the ultimate torque transfer device!

I must repeat if only to hear the keys clicking here in Idaho. The front wheels travel FARTHER in a typical mile than the rears - all by themselves. On Syncros, 2WD Vanagons, Teslas, city buses, even motorcycles - its an irrefutable fact of physics. The entire point was the rears are not applying torque to the fronts because the fronts are already spinning faster due to a typical assortment of curves, corners, and parking. This idea of "residual torque" flowing to the front wheels is pure absolute fantasy already but I thought if I pointed out that the front diff and wheels are already spinning faster it would be an easy conclusion to realize the rear shaft is spinning slower and thus there is no way it is in fact applying spinning torque to the fronts. At all. Zero. The only time - ONLY TIME - the rears ever spin faster then the fronts is if the rear wheels are spinning against whatever surface the Syncro is driving on. In turn, that is the ONLY TIME the VC can transfer torque to the front - when rear shaft speed exceeds front shaft speed and tries to speed up the front shaft using the viscous coupling. At no other time can the VC apply torque to the fronts. It. Is. Impossible.

Those who feel their Syncro is more stable in windy conditions on the road. Those who feel that their Syncro is more stable than a 2WD in the rain, etc. Pure placebo effect.
_________________
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

Jon_slider · Posted: Tue Mar 31, 2015 9:24 pm Post subject:

http://www.van-cafe.com/home/van/page_109_1248/rebuilt-viscous-coupler-coupling.html
"until the load becomes too great for the vc. The vc then will slip saving internal tranny parts the extra stress and load"

"Sport vc [55Nm] does not need to be paired with a decoupler however, it takes more power to slip up the vc with its added torque load and is not recommended for dry pavement / daily driving."

----
Maybe I'm feeling the VC creating Drag in a turn, not traction.

Raoul's calculation suggests that when a 55Nm VC Syncro is in a turn on dry pavement at 65moh, the rear connected VC plates consume 9% of vehicle power, to shear the silicone. This is DRAG!

In a turn front wheels go faster than rears. The motor turns the rears, so for the front to turn faster, it's the road, not the motor, turning front wheels.
_________________
My Soapboxes: Inflation; Handling; Gearing; Decoupling; Swepco

IdahoDoug · Posted: Tue Mar 31, 2015 11:08 pm Post subject:

Jon,

Exactly. The last couple sentences of your post tell all. I don't know about Raoul's calculations, but yes in turns the VC is creating drag. All turns at all speeds. The engine is pushing the van forward with the rear tires, and in a curve the front tires are being spun faster by the ground since they are describing a larger circle over the same amount of time and must cover more distance. In order to spin faster than the rear tires, the output shaft to the front differential must also spin faster than the input shaft from the engine. So the plates in the VC (half connected to input shaft, half connected to output shaft) are spinning relative to each other in the thick VC fluid. During the curve, the rear input plates are trying to slow the faster output plates. This is causing a braking effect on the Syncro with the front wheels actually resisting being spun by the passing road, since they are connected directly to the output plates.

You can experience this yourself in 5 minutes. A working VC will give you a binding feeling when you turn sharply such as turning into a parking spot. Mine would make the front tires grind, and if I turned sharply into the spot at a slow roll, it would stop the van. That is the thick VC fluid stopping your van with the front tires and exactly the same thing that happens at all speeds on every curve and corner, its just that you don't notice it with the engine power on, and the momentum of a two ton van.

So to experience it yourself, go into an open parking lot where you can turn a few circles safely. Roll at 3-4mph and turn the wheel to full lock. The Syncro will not roll well and stop. That is the front and rear wheels fighting each other. If you had a solid shaft coupled, you would hear a big bang as something expensive let go. The stock VC is designed to allow this state of affairs without damage. The "sport" VC will stop the van more dramatically and is placing more force on components. Progressively stiffer VCs and finally the ultimate - solid shaft - will make this effect more and more pronounced.

So, indeed it is as Jon says. The VC allows turns, but there is a cost which is drag and heat. That is why you cannot have mismatched tire sizes. VW knows mismatched tire sizes mean CONSTANT different speeds between the front and rear axles. As if you were constantly turning with no straight travel to allow the VC to cool. That will cause damage eventually.

That "residual torque" is indeed a myth as to torque of driving the vehicle forward. It is binding - something that slows the Syncro down and generates heat. The warning on the "sport" VC is for that reason. The "sport" VC generates MORE braking effect and binding, which is harder on your trans by subjecting it to higher binding forces. That is why they say not to use these on the road where binding takes place constantly on curves and corners. It certainly is not a warning because the "sport" VC is so much better on the road, right? Its a known problem and a warning is issued to prevent uninformed consumers. A tight "sport" VC causes greater binding between the front and rear axles as the price for having a VC that reacts quicker in offroad situations. That's fine offroad where the tires can slip to relieve the pressure. But not on pavement where they cannot.

Does it make clearer how the action of the VC I have been explaining seems to fit the real world behavior of a Syncro when parking? And fit the reason "sport" VCs contain a warning - which is to avoid the forces on your entire expensive drivetrain from binding? Binding is nothing more than the axles fighting. Since the rear axle is driven directly from the engine, that means the front axle must be...................braking. See 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, 1988 Toyota Supra 5 speed targa, 2002 BMW 325iX, 1982 Toyota Sunrader

raoul mitgong · Posted: Tue Mar 31, 2015 11:35 pm Post subject:

sorry, I asked too many questions at once, slowing it down.

hans j · Posted: Wed Apr 01, 2015 8:54 am Post subject:

So I guess it all comes back to solid shafts should probably not be driven where traction is high and a VC should. For the infinitely variable road conditions I drive, I'll take the automatic AWD any day.

All I know is my VC hasn't let me down, and I'd be happy to follow anyone in a solid shaft van. Hey, I don't even have power steering and I'll still follow you where ever you want to go Wink

I have less HP and torque than many engine conversions too!
_________________
1986 Canadian Syncro Westy TDI - 1989 Syncro Single Cab - 2001 Audi S4 - 1981 VW Caddy ABA - 1980 VW Caddy EV - 1973 VW T-181

WestyBob · Posted: Wed Apr 01, 2015 8:56 am Post subject:

WestyBob · Posted: Wed Apr 01, 2015 9:10 am Post subject:

hans j · Posted: Wed Apr 01, 2015 9:32 am Post subject:

I'm ok with the drag a VC produces, it keeps it warm enough to be instant traction (in the forward direction) when I step on the accelerator pedal! And if the engineers at SDP were ok with the tire rotation differences that normal driving and not equal tire pressures produce, then so am I.

Maybe Jon can do some tire diameter math here?

My van weighs 1250 pounds on each front corner, and 1350 at each rear corner fully loaded. (photo of the scales is in my gallery)

The factory recommends (http://www.roadhaus.com/tires/Silver%20Sticker%20Info.html) 40F/48R or 36F/43.5R or 36F/40R.

Now which of these are going to give me a tire diameter that is equal front and rear so it won't cause any rotational difference while driving in a straight line?

Which of those tire pressures will not cause any binding in a driveline while driving straight with a solid shaft?
_________________
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 · Posted: Wed Apr 01, 2015 9:48 am Post subject:

Thanks Bob. As for the residual torque thing. It is actually known in the biz as breakaway torque or a similar concept of stiction. It is simply a devices initial resistance to movement. Spin a bike wheel and this is imperceptible. Spin a WBX engine with plugs out such as when adjusting valves and youll be astonished how much torque it takes t move. Spin a VC and youll also be astonished. I have one on the bench. But again your Syncro does not benefit frm this at all while driving around in terms of driving torque causing the fronts to be driven. Until the rear wheels spin on the surface you are driving on. Otherwise while you are drving around on the street this breakaway torque is being generated by the front wheels acting in braking fashion.
_________________
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

Jon_slider · Posted: Wed Apr 01, 2015 10:26 am Post subject:

raoul mitgong · Posted: Wed Apr 01, 2015 10:50 am Post subject:

Hey Hans j,
Pressure won't change the tire ~~diameter~~ (edit: circumference, not diameter). Yeah, I know, I didn't believe it when I first read it either. There is a video somewhere showing an inflated tire rolling one turn at two different pressures. Same distance.
Think of the radial as a tank track, flatten it down or make it round and it will still be the same length. The steel belts in the tire keep the circumference constant.

-d
_________________
84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
86 Tintop Syncro to Westy project (Tom Servo)
91 Westy (Only the top 12 inches of this van (a burn victim))

Jon_slider · Posted: Wed Apr 01, 2015 11:06 am Post subject:

http://www.thesamba.com/vw/forum/viewtopic.php?p=6952333#6952333

raoul mitgong · Posted: Wed Apr 01, 2015 11:30 am Post subject:

https://www.youtube.com/watch?v=_2l5bOhHNxU

Video showing no "significant" change in circumference at very different tire pressures, and on a Vanagon even.

Sure there is likely "some" stretching of those belts that could change the circumference, just not what most people imagine. I think the video speaks for itself.

-d
_________________
84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
86 Tintop Syncro to Westy project (Tom Servo)
91 Westy (Only the top 12 inches of this van (a burn victim))

raoul mitgong · Posted: Wed Apr 01, 2015 11:55 am Post subject:

More on pressure/circumference. From the SAE white paper posted a few pages back:

Page 9, "Mainly the different tread depth causes differences in the rolling circumferences; conversely, loading and air pressure are of minor significance"
_________________
84 Westy with a 2.1 (Groover)
86 Tintop Syncro (Crow)
86 Tintop Syncro to Westy project (Tom Servo)
91 Westy (Only the top 12 inches of this van (a burn victim))


Hello! Log in or Register \| Help \| Donate \| Buy Shirts	See all banner ads \| Advertise on TheSamba.com

Syncro Chat: VC's, solid shafts, decouplers, AWD/4WD, etc.
Page: Previous 1, 2, 3 ... 17, 18, 19 ... 45, 46, 47 Next Jump to:

	Forum Index -> Vanagon	All times are Mountain Standard Time/Pacific Daylight Savings Time Page: Previous 1, 2, 3 ... 17, 18, 19 ... 45, 46, 47 Next Jump to:
Page 18 of 47