TheSamba.com Forums
 
  View original topic: Syncro Chat: VC's, solid shafts, decouplers, AWD/4WD, etc. Page: Previous  1, 2, 3 ... , 45, 46, 47  Next
Sodo Sat Jan 20, 2018 9:15 pm

Jon_slider wrote: So, how can syncro front wheels power the van in a curve on dry pavement?

uh-oh, it has awakened. Lets proclaim it magic.
The same magic that wears the drive side of the front CV joints. Or just ignore it (like the fossil record) :wink: .

87vanwes Sat Jan 20, 2018 9:21 pm

Sodo wrote: haha OK maybe it WAS put to bed. :wink: It should be.

But don't spend a LOT of time adjusting axle heights. If all 4 tires are the same brand, same size they will have the same carcass circumference, and over a wide range of pressures, will have the same rolling distance. Even with varying axle heights they still roll the length of the steel-belted carcass.

With all due respect... Not true... The low tire pressure light in many cars operates off the wheel speed sensors for the abs system. In most cases the driver would never notice the difference looking at the car. If you look at a vanagon westy with proper tire inflation the average Pearson would say the rears are low.

Sodo Sat Jan 20, 2018 9:31 pm

I don't know anything more about it that a little bit I've read, and this YouTube. It sure seems intuitive that axle height would change the rolling distance. VanWes how much are you raising/lowering your axle centers?
In the video the rim drops a full inch of radius and rolling circumference doesn't change.


Waldi Sun Jan 21, 2018 4:38 am

Jon_slider wrote: Lets see if we can agree on the following 6 statements of Fact, not opinion.

1. With a Solid shaft coupled, the front and rear wheels always spin at the same RPM.

2. In a curve the front wheels of a 2wd Van (or decoupled syncro) always spin at a higher RPM than the rear wheels, because the pavement distance of the front wheel arc is longer than the pavement distance the rear wheels travel.

3. When a VC is in the hard condition, the front wheels can not spin faster than the rear wheels, and in a curve the RPM of the front wheels is going to cover less distance than the arc over the ground. Therefore the front wheels are resisting the force of the road, that is trying to make the front wheels spin at a higher RPM than the rear wheels. This is the same with a solid shaft as with a VC that is not in slip mode.

4. When a VC is in slip condition, or decoupled, the front wheels can be spun by the road at a higher RPM than the rear wheels are spun by the motor. Therefore the front wheels can not be pulling the van forward, while the VC is slipping, or decoupled.

5. A syncro has no planetary differential with separate front and rear wheel outputs.

6. A planetary center differential is capable of powering the front wheels faster than the rear wheels in a curve.

So, how can syncro front wheels power the van in a curve on dry pavement?

1. yes, but this is not what makes driving better in 99% .

2. depends on the curve. It is not the case If the rears slip more than the fronts needs to go through the curve.

3. If the VC is in hump mode, it acts like a solid shaft, and puts lot of load on the drivetrain n the tires. But the VC will decrease this load as soon the hump mode is off. The ss will not.

4. Wrong. Anyone (exept you ;) know now that the fronts pull as long the rear tire slip is bigger than the way the front tires has to turn.

5. No, because the Syncro do not need it.

6. A planetarey diff will power always the side with less resitance more than the other. Without locker it is useless on slipery conditions,
and with locker it is useless in 99% like a solid shaft.


The syncro cannot power the fronts if they have to do a longer way than the rears inclusive tire slip.
But than again, why should the fronts pull on drive pavement ?
A Syncro cannot give milk. Is a cow better than a Syncro ? ;)

raoul mitgong Sun Jan 21, 2018 9:54 am

87vanwes wrote: Sodo wrote: haha OK maybe it WAS put to bed. :wink: It should be.

But don't spend a LOT of time adjusting axle heights. If all 4 tires are the same brand, same size they will have the same carcass circumference, and over a wide range of pressures, will have the same rolling distance. Even with varying axle heights they still roll the length of the steel-belted carcass.

With all due respect... Not true... The low tire pressure light in many cars operates off the wheel speed sensors for the abs system. In most cases the driver would never notice the difference looking at the car. If you look at a vanagon westy with proper tire inflation the average Pearson would say the rears are low.

My thought is the larger contact patch of a low tire will have a different tire slip % than a full tire at speed. The encoders on the wheel could be picking up that difference (but would have to also know steering angle or every turn would set it off).
Tough to argue with the video. A 26" tire has a circumference of 81.7", a 24" tire only 75.4". Video does not show a 6.3" difference.
-d

kamzcab86 Sun Jan 21, 2018 10:17 am

87vanwes wrote: With all due respect... Not true... The low tire pressure light in many cars operates off the wheel speed sensors for the abs system.

To be more specific, since I have one of these modern vehicles with the annoying TPMS:

"The tire pressure monitoring system looks at the ABS wheel speed sensors and uses that information to track the rotational speed of the tires. Any time a tire has lower pressure, it will roll at a different number of revolutions per mile than tires that have the correct tire pressure. If one tire’s rotational speed seems off compared to the rest, the TPMS will alert the driver that one of the tires has low pressure."

The YouTuber did not measure the difference in rolling speed in his experiment.

crazyvwvanman Sun Jan 21, 2018 10:20 am

I did a similar test of tire pressure effects a while back........
Even with an extreme difference in pressures and axle heights there was a pretty negligible difference in tire rotations.


Date: Wed, 31 Jan 2001 14:06:45 -0800
To: [email protected]
Subject: Tire Height, revs per mile, pressure effects

I did a test to see the effects of tire pressure on tire revs per mile and by extension effective gearing.
Also, some have suggested that in a pinch Syncro owners could adjust for a mismatched tire by matching the tires static radius through pressure adjustments and thereby reduce the stress on the VC.

Test done with 82 Westy 2WD, new 215/75/15 Michelin LTX M+S, 35 psi at start.
Level smooth paved street, sunny, dry, 70F, 5 mph est. speed max

Using a plumb line at the center of each left side wheel, mark the tire and the ground with yellow marker.
Drive straight ahead, counting 10 revs of the tire below the driver.
Mark the ground location where the tire mark lines up for each tire.
Measure the distance from the starting marks to the finish marks for each tire. 834.5 inches

Return to the starting position and let most of the air out of the driver's tire.
Measure from the ground to the center of the wheel for each tire. left rear = 12 7/8", left front 11 5/8"
Repeat the above test and mark the ground after 10 revs of each tire.
Measure the distance from start to finish for each tire. left rear = 834.5 inches, left front = 831 inches

Measured revs per mile at 35 psi = 759
Measured revs per mile at ~7 psi = 762
Measured Difference = 3 revs per mile, 00.4%


Mark

gears Sun Jan 21, 2018 10:53 am

That's great info, Mark .. thank you. (Although I have to wonder what the difference is at actual highway speeds. Ever watched top fuel car tires during their burnout?)

Jon_slider Sun Jan 21, 2018 10:54 am

Waldi wrote:
The syncro cannot power the fronts if they have to do a longer way than the rears inclusive tire slip

thank you Waldi :-)

kamzcab86 wrote: Any time a tire has lower pressure, it will roll at a different number of [more] revolutions per mile

crazyvwvanman wrote: Measured revs per mile at 35 psi = 759
Measured revs per mile at ~7 psi = 762
Measured Difference = 3 revs per mile [more], 00.4%

80% Less pressure (28psi less pressure), produces 0.0039 more rotation.

Now lets assume that 0.4% more rear spin, causes the VC to become hard, thereby causing the front wheels to spin at the same speed as the rear. Remember that the motor can not spin front tires connected to a hard VC any faster than the rear wheel speed. The wheel rotation speeds will match. That is what synchronized means, equal rpm.

Now enter a curve while the VC is in the Hard condition, at which point the front wheels and rear wheels will still be spinning at the same speed, but the ground distance travelled under the front wheels will be longer than under the rear.

Therefore the motor has to push the front wheels, sliding, over the ground. Sliding is not pulling. Sliding is braking. Or Breaking :-)

Or, the VC stops being hard, in the curve the VC begins to slip, due to the force of the road being greater than the ability of the VC to lock up. IF the VC slips, then the front wheels can spin faster than the rear, due to the road spinning the front wheels, not due to the motor spinning the front wheels. Therefore again, the front wheels can not be pulling the van forward. In fact, the VC acts as a drag on the front wheels being spun by the pavement.

From this I conclude that coupling a VC on dry pavement is creating a braking force, similar to the braking force a Solid Shaft creates, albeit the VC force will be less, when the VC is in slip mode.

So, I agree with Waldi, even with a VC, decoupling on dry pavement will reduce the binding load on the transaxle.

Waldi Sun Jan 21, 2018 12:41 pm

"From this I conclude that coupling a VC on dry pavement is creating a braking force, similar to the braking force a Solid Shaft creates, albeit the VC force will be less, when the VC is in slip mode."

No. It produce only "braking force" if the fronts have to go a longer way.
This will not happen on highways where the slip on the rears will be nearly always more than the way the fronts have to do more in highway curves.

A good VC will not get to hump mode on dry pavement under normal driving conditions.

Jon_slider Sun Jan 21, 2018 1:06 pm

Waldi wrote: It produce only "braking force" if the fronts have to go a longer way.

We agree.
The VC, when hardened, prevents the front wheels from spinning faster than the rear wheels. So in a curve on dry pavement, the front wheels cannot pull.

Waldi wrote: A good VC will not get to hump mode on dry pavement under normal driving conditions.

we agree again
a VC that is slipping, is not pulling the van forward in a curve on dry pavement.

we also agree that decoupling a syncro on dry pavement reduces the load on the transaxle.

thank you

raoul mitgong Sun Jan 21, 2018 3:40 pm

Good data Mark. Any chance you want to repeat it, but at 65 mph? Ha.

I don’t think we will resolve how much the fronts do pull until we get some reliable tire slip data. Looking at you crazyvwvanman.

Jon,
Imagine straight line dry decoupled at 65mph. Rear tires under torque and experiencing 0.5% slip. Rpm difference at the VC would be 18 rpm.
Now pull the knob and couple it. VC rpm difference can’t stay at 18 so it drops (0-5rpm?) and the rear slip is decreased while slip on front tires increases as the front pulls and removes some of the load at the rear. Perhaps .2 slip front and .3 slip rear?
And yes, the front torque will drop when entering a turn and cross over to negative if the curve is sharp enough.

-d

Waldi Mon Jan 22, 2018 5:37 am

Jon_slider wrote: Waldi wrote: It produce only "braking force" if the fronts have to go a longer way.

We agree.
The VC, when hardened, prevents the front wheels from spinning faster than the rear wheels. So in a curve on dry pavement, the front wheels cannot pull.

Waldi wrote: A good VC will not get to hump mode on dry pavement under normal driving conditions.

we agree again
a VC that is slipping, is not pulling the van forward in a curve on dry pavement.



thank you

Sorry, but as long you dont define "curve", i cant agree with you.
As a "slipping" VC does pull the Van if the rear tire slip (way) is longer than the fronts, even in a curve. More speed means more slip on the rears, means more pulling fronts.
Thats why you feel the binding on short curves with low speeds more than on higher speeds.

Waldi Mon Jan 22, 2018 5:52 am

raoul mitgong wrote: Good data Mark. Any chance you want to repeat it, but at 65 mph? Ha.

I don’t think we will resolve how much the fronts do pull until we get some reliable tire slip data. Looking at you crazyvwvanman.

Jon,
Imagine straight line dry decoupled at 65mph. Rear tires under torque and experiencing 0.5% slip. Rpm difference at the VC would be 18 rpm.
Now pull the knob and couple it. VC rpm difference can’t stay at 18 so it drops (0-5rpm?) and the rear slip is decreased while slip on front tires increases as the front pulls and removes some of the load at the rear. Perhaps .2 slip front and .3 slip rear?
And yes, the front torque will drop when entering a turn and cross over to negative if the curve is sharp enough.

-d

The slip depends on weight, tires, pressure, speed, street/pavement conditions, uphill-downhill, coupled-decoupled.
A lot of variables.
The only for sure, there is slip.

Waldi Mon Jan 22, 2018 7:43 am

If some of you like to google translate this may be interesting.

https://www.welt.de/motor/fahrberichte-tests/artic...ndert.html

Funny is, VW/Steyer-Puch sayed the 4-wd is saving fuel. I never believed this, not had the practical experiance.
Now Audi need to decouple the 4wd to save fuel ;)

The Syncro was able to do this maually with VC and decoupler 30 years ago.
It seems most of ppl are not more able (why?) to drive manually.

raoul mitgong Mon Jan 22, 2018 10:17 am

Waldi wrote:

The slip depends on weight, tires, pressure, speed, street/pavement conditions, uphill-downhill, coupled-decoupled.
A lot of variables.
The only for sure, there is slip.

Totally agree. I uploaded this graph a year ago and posted it to the crosswind thread. I am thinking that the curve should look more like a parabola than a tent now (I'll need some quiet time to contemplate).



Anyway, there are purposely no numbers on the chart because we don't even have a baseline for rear tire slip. More slip would shift the curve upward. Less slip would move the curve down.

The top/tip/center of the curve (at x=0 location) is where the steering wheel is centered and driving perfectly straight. Turn left and the forward torque drops down and to the left. Turn far enough to cross the horizontal x axis (y=0) and you are into braking territory. Turn right and the forward torque drops to the right.

All the variables Waldi listed are going to affect the shape and position of this curve.

-d

Jake de Villiers Mon Jan 22, 2018 10:40 am

gears wrote: That's great info, Mark .. thank you. (Although I have to wonder what the difference is at actual highway speeds. Ever watched top fuel car tires during their burnout?)
That's different. Those are bias ply tires designed to grow and alter the gear ratio as the car gains speed. Not to mention the 8-10PSI they ruun at or the elliptical shape they assume at 300MPH!!

Steel belted radials are much more tightly held.

From the Mickey Thompson website:

How much will a Mickey Thompson drag slick grow?

Overall diameter will affect the amount of growth that you will have. There are many variables that contribute to tire growth as well. Unless otherwise noted, Mickey Thompson drag slicks are considered a low growth tire. Mickey Thompson slicks will grow approximately 1 to 1-1/2 @ 150mph (Note: Radial slicks do not grow). Please phone us and we can help you in determining tire growth for your specific application.)

gears Mon Jan 22, 2018 9:37 pm

Jake de Villiers wrote:
That's different. Those are bias ply tires designed to grow

Of course it's different. My point is that a partially deflated street tire when running at high rpm may not remain smaller in diameter after all.

Jon_slider Wed Jan 24, 2018 1:53 pm

Facts:
1. The motor of a Syncro w VC or solid shaft, can never power the front diff faster than the rear diff.
2. In a dry curve, front wheels travel a longer distance than rear wheels.
3. To power the front diff faster than the rear diff, requires a center differential.
4. A syncro does not have a center differential.

Note in the following, the “transfer case” operates similarly to a Solid Shaft or hardened VC in a Syncro. On a dry pavement curve, a VC in slip mode is not able to power the front diff faster than the rear diff, in fact, in slip mode the VC can not power the front diff at all, because by definition, it is slipping.

https://auto.howstuffworks.com/four-wheel-drive2.htm
The transfer case on a part-time four-wheel-drive system locks the front-axle driveshaft to the rear-axle driveshaft, so the wheels are forced to spin at the same speed. This requires that the tires slip when the car goes around a turn. Part-time systems like this should only be used in low -traction situations in which it is relatively easy for the tires to slip. On dry concrete, it is not easy for the tires to slip, so the four-wheel drive should be disengaged in order to avoid jerky turns and extra wear on the tires and drivetrain.

https://auto.howstuffworks.com/four-wheel-drive3.htm
It cannot be used on-road because of the locked transfer case.

Note in the following the transfer case with open differential operates similarly to a Planetary Center Differential (which a Syncro does not have)

https://auto.howstuffworks.com/four-wheel-drive4.htm
the transfer case on the Hummer does not automatically lock the front and rear axles together. Instead, it contains a set of open-differential gears that can be locked by the driver. In open mode (not locked), the front and rear axles can move at different speeds, so the vehicle can drive on dry roads with no problem.


try this thought, see if it makes sense...
Regarding RWD tire slip. IF the rear tires of a VC Syncro slip, the VC hardens, at which point front and rear tire slip equalize. When a VC hardens in a curve on dry pavement, the front diff can not spin faster than the rear wheels, and therefore a VC can not power the front diff fast enough to cover the front wheel distance over the ground. This means the front wheels have to skid over the ground. It also means the rear wheels have to slip to match the front wheels skidding.

Sodo Wed Jan 24, 2018 2:54 pm

Jon,you've done a pretty good job working out the drivetrain but eliminating the (dynamic) connection to the roadway. It cannot be eliminated in a full discussion of the Syncro drive system

Jon_slider wrote: This means the front wheels have to skid over the ground. It also means the rear wheels have to slip to match the front wheels skidding.

There can be a curve radius, speed, power and surface adhesion where rears could be slipping and the fronts could be gripping 100%. At a tighter curve fronts would be pushed. At a lesser curve, fronts would pull. Holding everything else constant, the variable that changes the surface contact from "pushed" to "pulling" is the turn radius (steering wheel position).

It's great to see you have added tire slip into your factors. When you start to consider slippage in both fwd and rearward directions (rather than the fronts being pushed ONLY) that may be the final step you need to understand the Syncro drive system.



Powered by phpBB © 2001, 2005 phpBB Group