locked CDL fuel economy vs. viscous

[dmc]

This is not a thread bitching about the awful fuel economy my trucks gets. More of an observation as well as a starting point for discussion because I know there are many of you that will have an opinion.

Two weekends ago I took a trip through Utah. 642.3 miles. 642 of them were on dirt roads. I prefer to travel dirt roads with the CDL engaged for 2 reasons.

1- ABS disabled. this point is not debatable for me. I'd much rather have the ABS off when travelling unfamiliar roads at a high rate of speed.

2- I don't know if this is fact or not so hopefully this board will have enough opinions to confuse me even more. I assume on a low traction surface the viscous will get hotter and result in more wear. Locking the center diff will hopefully extend the life of the viscous coupling.

So in the 14 months I've had my truck I have never got below 12 nor over 13mpg on daily driving fill ups. Even the road trips I've been on haven't dipped lower than 11 or more than 14mpg.

Well on this last trip I got below 9 on both fill ups. I would immediately look at the CDL except the roads were a bit slick and the occasional wind was a factor. Luckily the route we took allowed for 2 gas stations so we only had to carry 40 gallons between the two rigs. As I add a long range tank and start relying less and less on convenient fuel in my adventures, I'm now looking toward maximizing (be it only 13mpg) my economy.

Does anyone have other evidence to suggest the CDL would cause worse economy? Is it driving style and load (although we went fairly light this trip) having a far larger effect? Any ideas on a dirt road only trip I can take with the CDL open to add further data to my confusion?

thanks,
dmc

 

[RavenTai]

1. not debatable? hmm

2. Tire scrubbing will put more drag on the cruiser, kind of like driving around with the brakes on, that energy has to come form somewhere the fuel tank is the source.

But I am sure that the conditions and driving on a dirt road were a sizable portion of that mileage hit.

With these road conditions, would you have experience a lot of wheel spin with the CDL unlocked? This is the only time the VC would make any more heat than normal driving, if you were getting a lot of wheel spin the VC would lock up anyway ending heat generation, you can disable ABS by locking the CDL and driving over 55MPH for a few seconds, after you unlock the CDL the ABS will be disabled until you restart the engine,

 

[dmc]

1. not debatable? hmm

yep. at least for me. I don't think it's necessarily for everyone but i've spent plenty of time on dirt roads and much prefer the feel and drivability of non-abs (or disabled) trucks. A lot of this came from my Taco which had hypersensitive ABS and caused more harm than good.

2. Tire scrubbing will put more drag on the cruiser, kind of like driving around with the brakes on, that energy has to come form somewhere the fuel tank is the source.

But I am sure that the conditions and driving on a dirt road were a sizable portion of that mileage hit.

With these road conditions, would you have experience a lot of wheel spin with the CDL unlocked? This is the only time the VC would make any more heat than normal driving, if you were getting a lot of wheel spin the VC would lock up anyway ending heat generation, you can disable ABS by locking the CDL and driving over 55MPH for a few seconds, after you unlock the CDL the ABS will be disabled until you restart the engine,

You know I've had the ABS remain disabled after returning to pavement and unlocking the CDL. However it usually returns after restarting the car. I'll have to try out your suggestion. I actually rigged a swith in my taco to disable the ABS. this might be a good fix.

The roads we were driving were rather windy and covered in gravel so yes there would have been quite a bit of wheel spin in the corners while drifting. I'm going to have to spend some real time with the CDL unlocked to really tell if there is a handling difference. The 1000 miles or so we did for the Utah Cruiser Expediton were also done with the CDL locked. Usually my trips do not extend to a full tank so it's hard to judge the effect on fuel consumption.

thanks for the input.
dmc

 

[-Spike-]

1if you were getting a lot of wheel spin the VC would lock up anyway ending heat generation,

I've seen a cruiser spin one tire for several minutes without any noticeable shift in torque to other wheels (CDL inoperable, high-centered 80). Where are you getting your information that this will occur, especially with very slight axle-speed differences?

For the record and back to the topic, I don't personally believe the coupling would experience high heat during normal driving situations unless it were an extreme situation like the one I described above. I could easily see the drop in mileage attributed to the higher resistance on dirt, especially if it were wet (your reference to 'slick roads', but if you were sliding around on dry dirt it would probably have a similar effect). The only way to tell would be to drive the same road in the same conditions with the CDL unlocked.

-Spike

 

[IdahoDoug]

Driving around on windy roads you were likely inducing more tire slippage with the cdl on than without it. The center diff allows you to go on a curve without causing axle windup or slippage when it gets wound up to the point something must slip (the tires will). With the center open you go around a curve with the center diff allowing for different f/r driveshaft speeds without force buildup.

Spike,

What year 80 was it? The pre 93s in the US had no viscous. Also, if the rig you describe were a 93+ in low range the viscous is bypassed so getting no workout - just the center diffy having a workout.

DougM

 

[LandyLover]

With the CDL locked there is much more friction between the wheels and ground. Using the same principal, you mielage would certainly worsen if you had front and rear differential lockers engaged. The more freely the wheels turn (less binding), the better the mielage.

Your VC can heat up if it is giving power to different axles in a quick, sustained manner. In my opinion, driving down a forest service road, even a muddy one would not be cosnidered harsh operating conditions, and would not cause excessive wear on the VC.

To meet your conditions of improved mielage (no CDL locked) and ABS disabled would require you to play with some wires... something I wouldn't do.

Cheers...

 

[-Spike-]

Spike,

What year 80 was it? The pre 93s in the US had no viscous. Also, if the rig you describe were a 93+ in low range the viscous is bypassed so getting no workout - just the center diffy having a workout.

DougM

I believe it was an LX450. If not, it was definitely 94+. (The owner will read this and may or may not choose to reveal his involvement publicly ) It was in low range, but the CDL was not engaging due to an electrical malfunction- would the VC still be bypassed? If so, the guy had the option of going back into high range to get unstuck. That would be good to know.

-Spike

 

[RavenTai]

I've seen a cruiser spin one tire for several minutes without any noticeable shift in torque to other wheels (CDL inoperable, high-centered 80). Where are you getting your information that this will occur, especially with very slight axle-speed differences?


-Spike

I do not think it will do it with slight axle speed diffrences, but the example of one tire on one axle spinning should do so, if he was truely high centered are you sure he was not spinning a tire in the front also on the ohter side from your view?


info comes from the 96 New Car features manual and experience of other members on this board.

3. Viscous Coupling Type LSD
General
Making use of the highly viscous property of silicon oil, the viscous coupling type LSD utilizes the force
that is generated when the silicon oil is sheared to accomplish limited slip in the differential. This enables
the differential to restrain the rotational difference between the front and rear wheels, thus providing the
excellent vehicle’s driving performance over slippery terrain such as rough or snowy roads.
Construction
1) Outer and Inner Housing
Using splines, the outer housing is connected to the center differential case, and inner housing to the rear
output shaft respectively.
2) Outer and Inner Plates
The outer and inner plates are steels disks that are assembled alternately. Outer plates have teeth along their
periphery that mesh with inner splines of the outer housing. Inner plates have teeth along their inner edge
which mesh with the splines of the inner housing and slide to right and left along the inner housing. Slits
are provided on outer and inner plates to increase the resistance force of the oil.
3) Spacer Ring
Spacer rings are inserted between the outer plates to maintain the necessary clearance.
4) Silicone Oil
Silicone oil expands with heat. Therefor, silicon oil is filled approximately 80 percent of the housing.
CHASSIS —HF2AV TRANSFER 67
Fundamentals
Torque is transmitted by a resistance force which
is caused by its viscosity when silicone oil is
sheared.
1) When Plates Rotate at an Identical Speed
No viscous resistance is generated since
silicone oil between two plates rotates at the
same speed as the plates.
2) When Plates Rotate at Different Speeds
Silicone oil particles that are touching plates
tend to move at the same speed as the plates.
When the plates begin to turn at different
speeds, silicone oil particles are pulled away
from each other and a resistance force is
generated. Therefore, resistance F2 acts in the
opposite direction of rotation on plate N2 which
is rotating faster. Resistance F1, which is equal
to F2, acts in the direction of rotation on plate
N1 which is turning at a lower speed.
Forces which tend to cancel the difference in
speed occur to the plates.
Transmitted Torque Characteristics
1) During Normal Operation
The amount of viscous resistance due to
difference in rotation of the outer and inner
plates increases or decreases according to the
extent of its difference.
68 CHASSIS —HF2AV TRANSFER
2) During Humping
The viscous coupling is filled with silicone oil and air. The air is mixed in the silicone oil in the form of
bubbles when the viscous coupling is operating. The bubbles absorbed the pressure difference is generated
on both sides of the inner plate as it rotates.
As the viscous coupling continues to operate with the outer and inner plates rotating at different speeds,
the temperature inside the viscous coupling increases due to agitated resistance of the silicone oil. And the
silicone oil expands under heat.
As the silicone oil continues to expand, the bubbles in the silicone oil are caused to be decreased so that
the pressure difference at the front and back of each inner plate is no longer absorbed by the bubbles.
The inner plate, which can move in an axial direction to a distance equal to the outer diameter of the spacer
ring fitted between the outer plates, is pulled toward the side where pressure is lower. As a result, the inner
plate contacts the outer plate directly to create a larger resistance. This condition is called humping.
Since there exists no speed difference between the inner and outer plates during humping, the temperature
inside the viscous coupling decreases and reduces the internal pressure. The compressed bubbles expand
again and separate the inner plate from the outer plate. Humping, however, does not occur when the vehicle
is running in a normal condition.
CHASSIS —HF2AV TRANSFER 69
Operation
When a rotational difference is created between the front and rear wheels due to slippage, torque is
transmitted by the viscous coupling from the high–speed wheels to the low–speed wheels, thus reducing the
rotational difference. As a result, optimal torque is transmitted constantly to the wheels.
1) When the Rear Wheel Rotates Faster than the Front Wheel
When the rear output shaft rotates faster than the differential case, this difference in rotation generates
viscous torque. This torque, along with the drive torque from the transfer is distributed to both the front
and rear output shaft. As a result, the drive torque of front output shaft side, which does not have a viscous
coupling, increases by the amount of the viscous torque. The drive torque of the rear output shaft, which
is connected to the viscous coupling, decreases by the amount of viscous torque generated.
2) When the Rear Wheel Rotates Slower than the Front Wheel
When the rear output shaft rotates slower the differential case, this difference in rotation generates viscous
torque. This decreases the drive torque transmitted from the transfer to the differential and is apportioned
to both wheels. As a result, drive torque decreases in the front output shaft which has no viscous coupling.
Conversely, the viscous torque form the viscous coupling is added to the drive torque of the rear output
shaft.

I believe it was an LX450. If not, it was definitely 94+. (The owner will read this and may or may not choose to reveal his involvement publicly ) It was in low range, but the CDL was not engaging due to an electrical malfunction- would the VC still be bypassed? If so, the guy had the option of going back into high range to get unstuck. That would be good to know.

-Spike

VC is taken out of the picture when the CDL locks up, if it was not locking VC is stil active.

 

[-Spike-]

...are you sure he was not spinning a tire in the front also on the ohter side from your view?

Positive, there were several people trying to spot him. I wonder what the torque rating is on the VC? It might be intended for low traction, such as ice, not no traction. Didn't seem to be doing anything, but looks can be deceiving.

-Spike

 

[RavenTai]

Positive, there were several people trying to spot him. I wonder what the torque rating is on the VC? It might be intended for low traction, such as ice, not no traction. Didn't seem to be doing anything, but looks can be deceiving.

-Spike

I do not have an explanation for that one then, it did not react as it should.

I would think that being high centered enough to be stuck that there would be little weight on either axle,

in a little/no traction situation the VC should spin both axles on its own without even locking up as the resistance to difference moderate in RPM of the unlocked VC is still fairly high,

I don’t have any torque numbers but I do have a very different situation that illustrates the action of the VC,

I drove around for a couple weeks without a front driveshaft with the CDL locked up, one day at the top of my down hill driveway I unlocked the CDL and let it roll down the hill in park, the action of the VC was quite strong it held the speed down to slow walking speed, when I got the bottom where it leveled the braking effect of the VC evaporated and the truck still rolled on the very slight incline at about the same speed as the steep part, so the more RPM difference front to rear the more the VC resists that diffrence, at very low RPM difference there is little resistance.

 

[dmc]

so i sense that the feeling is running in full time 4wd on dirt roads, unless extremely slippery, will not cause pre-mature wear on VC. yes I realize that toyota designed it for that purpose but I also realize that I put a lot more time in on dirt roads than the typical 80 owner. (the mall crowd that is not mud peeps)

Disabling the ABS with a switch is fairly simple to do. You just trick the ABS ecu into thinking the diff lock is engaged. Raven if your suggestion works I won't need it however. I realize this is a pretty moot point for 2MPG but more than anything I want extend the life of my VC as long as possible. Have you priced one out lately? I think a used T/case would be cheaper. I figured I put about 20k miles on dirt in my Taco over the 4 years I owned it and expect to do even more in my 80.
thanks,
dmc

 

[RavenTai]

Raven if your suggestion works I won't need it however.

it will work, somewhere in this board there are the exact specs for how fast you have to be going and for how far to keep the ABS deacticated, I want to say it is something like more than 50 feet at more than 50 MPH, or maybe it was 500 feet, I do not remember the exact numbers but that will get you close,

you can briefly engage the CDL at highway speeds on pavement with no side effects as long as you are moving in a straight line and will be moving in a straight line for the duration of CDL engagement. turning with the CDL locked on pavement of course would be hard on the drivetrain and casue unpredictable handling so dont do it in traffic where you might have to make an emergency lane change.

 

[RavenTai]

so i sense that the feeling is running in full time 4wd on dirt roads, unless extremely slippery, will not cause pre-mature wear on VC.

Unless you are getting a lot of wheel spin a dirt road should be the same to the VC as driving on road, the VC will get hot but it always does.

But I do not drive 1K miles on dirt roads so take that for what it is worth.