Syncro “AWD” Capabilty

[Waldi]

Do you have a VC test bench in USA at all ?

[IdahoDoug]

Hellenic,

So did you miss the part on the Porsche 993 system where they had to "make the rear tires slightly smaller in order to transfer torque to the front wheels all the time". Doing that achieved only a "5-15%" transfer. So applying logic to that statement, if the tires are the same size on the rear, it exposes a weakness of using a VC to transfer torque from the slower rotating rears exactly as I stated.

If the rears "slip all the time" then Porsche would not have had to do that. So do a simple power to weight ratio calculation on the Porsche 993 vs the Vanagon Syncro and you will see that with the Porsche's massive power the engineers were not seeing the rear tires slip all the time to activate their VC to the fronts. So they did something to fix that, (which was using different size tires).

So if they had trouble getting rear wheel slip with a lot of power and less weight on the drive wheels, do you think there's a teensy, weensy little possibility that the heavier, incredibly low powered 95hp Vanagon has an even greater problem transferring torque to the front tires?

Doug
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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]

[hellenic vanagon]

[raoul mitgong]

Waldi wrote:

[Waldi]

[gears]

[raoul mitgong]

[hellenic vanagon]

[hellenic vanagon]

[IdahoDoug]

Hellenistic - So, Porsche went to the trouble with all that rear wheel power/wheelspin to specify tires that are 1% smaller in diameter (and naturally that % increases with tire wear to make the system engage more over time) to achieve front tire torque for a road system? And you believe the Vanagon WITHOUT that power and without that tire diameter difference transfers torque to the front tires all the time?

Waldi - I am quite familiar with contact patch dynamics, having studied it. There is no traction without torque applied. There is no torque applied or traction generated without slippage. However, its worth pointing out that a tire achieves that "slippage" during the phase of the front of the contact patch becoming the rear of the contact patch as the tire rolls and the rubber stretches and tread "squirms" which is a stretching/rebounding effect. It is largely the tension through the contact patch from front to rear that achieves this, and the VC doesn't "see" that. The idea that the tires only achieve this tractive effort by the axle spinning against the ground is false, and we should discuss how that "traction" happens. There is some of that (which the VC would "feel") but not much in ordinary driving around levels of torque. To change the ratio of traction achieved through tension into outright wheelspin you have to apply enough power to put the tire into "breakaway" mode through the entire contact patch. I have studied high speed photography of tire contact patches under engine torque and braking torque, driven the cars over the glass plates in the track, etc. There is tension and stretching and rebounding through the contact patch in 3 general phases. This is what makes the rubber tire such an amazing engineered product actually. It's a great conversation and a cool topic to discuss.

General:

My main point here is that if Porsche saw a VC with equal size tires in a powerful rear engined, RWD car as needing the "help" of smaller rear tires to be effective for a road system, it should send a message to the folks that think the VC is "always sending torque to the front tires" on a Vanagon on the road. If we did further research on the Porsche VC, I suspect we would also find that Porsche designed it to be much more sensitive than VW in order to raise its sensitivity to transfer torque appropriate to a road oriented performance enhancing system. Likely different fluid viscosity, likely different plate shape, number and orientation, and likely other factors. I'd also be curious if they built in some type of "one way" declutch feature so it does not cause the front tire braking of binding, etc? I cannot imagine Porsche engineers would tolerate parking binding and the negative aspects of other binding while driving.

We can split hairs on tire size and other things but the Vanagon was designed with a slipping VC clutch that is largely inactive until enough rear wheel spin happens to engage it. It was designed as a simple, low cost item to engage the front wheels on low traction surfaces with significant wheelspin - snow, offroad, etc. It was not designed, to provide constant power to the fronts on the road and enhance handling and performance with all 4 contact patches pulling. That type of performance is achieved with a center differential vehicle like a Quattro or in modern times with electronics. Or, in our example above with the Porsche VC system - likely with a completely different VC response characteristic.

Because the Vanagon VC system was designed as a low cost way of achieving power to the front wheels on a van to achieve some rough terrain and slippery surface mobility, there are some negatives in the other end of the spectrum such as road performance. Among them are undesirable front wheel braking on curves (very undesirable on ice when turning), lower fuel economy, binding while parking and a shorter life than a mechanical differential.

I think that's what Jon and I have been saying for some time. Its a great system but I feel the "benefits" have been exaggerated through misunderstanding of the forces at work, and the operation of the VC itself. Speaking for myself, I would enjoy seeing the discussion focus on data and facts and less on "The Syncro VC is perfect in every way and makes the Syncro outperform all other systems under all conditions and even though it was designed for X, it also does Y."

Doug
_________________
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

[hellenic vanagon]

[hellenic vanagon]

[raoul mitgong]

My steering wheel at 400 degrees makes for a very tight turn. A 0.5 g turn at a full steering wheel rotation? Doughnuts?
EDIT: So a 4wd/AWD (not sure which in that graph) is creating more g's than a rear wheel drive at the same steering wheel rotation. Is this the same vehicle just coupled/uncoupled?
-d
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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))

[Waldi]

[IdahoDoug]

R - Yeah I was wondering about that as well. We used either steering wheel angle or tire/wheel angle. I'm also wondering as mentioned earlier about the vehicle, the surface, and a bunch of other stuff and what the graph points are as it says at the bottom something about the rear wheels. Hopefully we'll figure it out - no worries. Still lots to talk about.

Doug
_________________
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

[Merian]

this thread is a good advertisement for electric wheel hub motors...
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[hellenic vanagon]

[IdahoDoug]

I looked through the Peschke white SAE paper you linked and could not find that graph. No worries, just curious about the background and don't want to derail the discussion....
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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]