After disengage 4wd on my wagon, can I drive it?

MrLewayneMrLewayne Wagonist
in Engine & Drivetrain
I understand that you can tow it when this is completed, however I was wondering if you could disengage the 4wd and drive it as if it were a 2wd?

Comments

  • CharbCharb Administrator
    Yes.
  • MrLewayneMrLewayne Wagonist
    Excellent. So just loosen the one bolt, and turn the center bolt counter clockwise to 9ft lbs?

    Thank you so much. Just looking to increase my gas mileage. I appreciate it.
  • The_HeadThe_Head familEE
    Let us know how it works out for you. There has been some debate on if this will actually do anything for mileage.
  • devmanxzdevmanxz Band Wagon
    Yea, i was wondering this myself. I dont know if it will work though.
  • MrLewayneMrLewayne Wagonist
    The_Head wrote:
    Let us know how it works out for you. There has been some debate on if this will actually do anything for mileage.

    will do! :D
  • MrLewayneMrLewayne Wagonist
    I'm going to finish this tank of gas out (about 3-4 more gallons) and then disengage the 4wd and see what my average mpg is. Will post results!
  • klumklum Senior Wagonist
    hurry up im replacing my transmission soon
  • JakerJaker familEE
    I'm 99% sure that the difference will not be measurable. Even though you've dissengaged the driveshaft from the transmission, you have not dissengaged it from the back wheels. The back half of the driveshaft will continue to turn with the back wheels. They're mechanically attached to each other. All you're doing is preventing the front half of the driveshaft transferring power to the viscous coupling, or in the case of towing your wagon, you're stopping the back wheels from turning the transmission.
  • oxbloodoxblood Band Wagon
    Does anyone have any numbers on actual gas mileage difference before and after?
  • bam-bambam-bam Council Member
    I'll try it with mine sometime soon. I feel that there's some pretty serious misinformation on this site about how the viscous coupler works. When engaged, you're pulling all 4 wheels, all the time.

    I (think I) felt a difference in acceleration between when I had the driveline out and in.
  • JakerJaker familEE
    I'm not 100% sure what you're hinting at Bam, but why else would they have that lever there if not to dissengage the driveshaft from the transmission when towing with the front wheels off the ground?
  • bam-bambam-bam Council Member
    Oh, You're right about disengaging the lever. You know the internals of that mechanism better than anyone (Probably better than you care to right about now:) ). I may have confused the issue when I said I had the driveline out. I did that to tow it home as a precaution because I was ignorant of how the Viscous coupler functioned. Maybe I still am, but my reading elsewhere doesn't agree with the "engages if the front wheels spin" that I see repeated on this forum.
  • superhatchsuperhatch Moderator
    bam-bam wrote:
    my reading elsewhere doesn't agree with the "engages if the front wheels spin" that I see repeated on this forum.

    Not to hijack, but what reading? Everything I've read says that's how a VC works. Some examples (incoming wall of text):

    Syncro.org (VW site)
    When the front and rear wheels turn at different RPMs (at a rate greater than 6%), the shear force raises the temperature and viscosity of the silicon liquid inside the VC. The silicon becomes solid like and engages plates inside the VC with the result that power is transmitted from the center drive shaft (the engine) to the front differential (the front wheels).When the front and rear wheels begin to rotate at relatively the same RPMs again, the liquid deactivates," becomes less viscous and more liquid like, and as a result the front wheels/diff. disengage, and the van is powered again only or primarily by rear wheel drive.

    How stuff works
    Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels -- the wheels that are not slipping.

    When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.

    A simple experiment with an egg will help explain the behavior of the viscous coupling. If you set an egg on the kitchen table, the shell and the yolk are both stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up. To prove that the yolk is spinning, once you have the egg spinning quickly stop it and then let go -- the egg will start to spin again (unless it is hard boiled). In this experiment, we used the friction between the shell and the yolk to apply force to the yolk, speeding it up. When we stopped the shell, that friction -- between the still-moving yolk and the shell -- applied force to the shell, causing it to speed up. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.

    Autozine.org
    Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.

    Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.

    The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero.

    To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.

    It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.
  • The_HeadThe_Head familEE
    tldr

    Just let the guy log his mpg for a fuel tanks in 2WD and report back.
  • bam-bambam-bam Council Member
    bam-bam wrote:
    I was ignorant of how the Viscous coupler functioned. Maybe I still am,


    No maybe about it, On further study, I see that I'm wrong. I was confusing the fact that both input and output are generally rotating at the same speed with torque actually being transmitted through the coupling. I was thinking it was coupled all the time with the ability to slip somewhat before fully locking.

    Interesting about using slightly different ratios to keep the VC 'warmed up'. Is that the case with ours?
  • JakerJaker familEE
    Based on this photo, which is an actual count of the teeth on the gears in my transmission, and the rear diff ratio of 2.529:1 which I've also counted, we can conclude the following:

    395.jpg

    1 revolution of the front wheels, which translates to 1 revolution of the 53 tooth gear leads to 2.3043 revolutions of the 23t gear, which leads to 2.547 revolutions of the 19t gear (2.3043 x 21 / 19). That 19t gear is attached to the driveshaft, so for every revolution of the front wheels, the driveshaft turns 2.547 times. With the rear diff being 2.529, that means the rear wheels turn 99.3% the speed of the front wheels based solely on the drivetrain. Since the rear wheels are going to be turning the same speed as the fronts (unless there's some slip), we can conclude that there is no viscous coupler pre-load in the Honda Wagon system.
  • superhatchsuperhatch Moderator
    Lets continue the VC discussion here: viewtopic.php?f=7&t=9772

    Cant wait to hear your mpg results. :)

    <end>Hijack.
  • suntrana45suntrana45 Band Wagon
    Yes you can drive the car in the "two-wheel" mode. I had to disconnect mine because of some serious vibration issues (any ideas on how to troubleshoot drive train would be greatly appreciated), and have put several hundred miles on car since then without any problems. Have driven over 75 on highway and a lot of in town driving....
  • suspendedHatchsuspendedHatch Wagonist
    I was told by someone I trust (huge wagon fan long since before this site came around and currently a dyno tuner) that the rear wheels are always 10% of the torque until the system engages and increases it.

    I think I will switch mine to 2WD and gather the results until winter. I've been keeping MPG records since the day I bought the car.
  • CharbCharb Administrator
    suspendedHatch wrote:
    long since before this site came around

    Curious, how long do you think the site has been around?
  • MrWhoopeeMrWhoopee Wagonist
    It occurs to me that, when the 4wd is disengaged and the vehicle is being towed or driven, there will be a difference in the speed of rotation between the front and rear at the viscous coupler. When being towed, the output (rear) will be rotating (driven by the rotation of the rear wheels) and the input (front) will not. When being driven, the same condition occurs, no rotation of the input, rotation of the output. This would cause the viscous coupling to engage, causing rotation of ALL of the rear drive components (including those forward of the viscous coupling), even though they are not connected to the front. Following this line of logic, once the viscous coupling locks up fully, the difference in rotational speed disappears and the coupling starts to cool, when that happens the rotational difference reappears and the coupling locks again. Could driving in 2wd cause repeated engagement/disengagement and heating/cooling of the viscous coupling? Any thoughts?
  • lucskywalkerlucskywalker Wagonist
    My RT got around 26 city and 34ish highway. Disengaged I got about 23 city and still 34 highway. My thought on this is a lever does not make the car any lighter, and the front wheels slip just like a front wheel drive does. Trust me that drive shaft is heavy, and yes viscous coupler slips very little, 10% before lock up sound right to me, but that's wheel spin. I know there is math to it, but through gearing the coupler hardly moves, and certainly does not spin. Never once did I lose any traction wet or dry, only in sand and mud did I ever spin the tires...

    Put it this way,, if you had a VC style LSD with this much clutch in it you would have some serious axle issues.
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