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Thread: Full time 4x4 vs Part time - On road performance

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    DakDakDakDak Crais's Avatar
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    Full time 4x4 vs Part time - On road performance

    Im trying to get my head around if there is an actual advantage to having a vehicle with full time 4wd (for example, a Landcruiser wagon) as opposed to a rear wheel drive car.

    My understanding is that most of these "permanent 4x4" vehicles have a completely open centre diff until locked (talking more about offroad vehicles rather than sports cars which probably have a centre vicsous diff etc) and therefore still can only drive one wheel at any given time.

    Can someone please explain the advantage here? is it because under certain circumtances it will maybe drive a front wheel and promote understeer?

    Take the mistubishi super-select 4x4 system for example. It's claim to fame is that you can drive in both 2x4 or 4x4 on the tarmac (with the diff unlocked only). Without that diff locked, and therefore completely open, how is the 4x4 mode any better on road than the 2x4 mode?
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    Most have some form of LSD centre diff (when unlocked) so as to avoid a tendency to spin a single wheel. Some also have a non 50/50 torque split (by having a planetry centre diff) to bias more torque to the rear wheels as they will typically have more weight on them under acceleration.

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    Registered User Stix Zadinia's Avatar
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    nobody beats me in a traffic light GP in the wet, except for maybe some WRX's and lancers
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    Registered User irsa76's Avatar
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    Permanent 4wd offers better stability and grip icompared to normal 2wd, in all conditions but especially in low grip situations. The slight reduction in fuel economy is worth running a permanent 4wd imho.

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    Registered User irsa76's Avatar
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    Quote Originally Posted by elfturbomax View Post
    Most have some form of LSD centre diff (when unlocked) so as to avoid a tendency to spin a single wheel. Some also have a non 50/50 torque split (by having a planetry centre diff) to bias more torque to the rear wheels as they will typically have more weight on them under acceleration.
    Most, all, early systems used normal open center diffs. No torque bias or limited slip action.

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    Quote Originally Posted by irsa76 View Post
    Most, all, early systems used normal open center diffs. No torque bias or limited slip action.
    I assumed the OP was talking about vehicles from this century. Certainly, Range Rovers and Discovery Series I & II, etc had open centre 50/50 diffs, along with Lada Nivas and IIRC 80 and early 100 Series Land Cruisers. Early Super Select Pajeros would have been torque bias without LSD and I think a lot of Jeep transfer cases would also have been planetary with torque bias.

    Having variously had a Disco I, Prado 120 and Pajero Gen 4, either of the last two were preferable to the open centre in the Disco in the wet.

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    Quote Originally Posted by Crais View Post
    and therefore still can only drive one wheel at any given time.
    What?

    The Range Rover (and therefore Discovery) was designed as a full time 4wd partly because the cheesy diffs couldn't handle the power of the V8 all on their own.

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    DakDakDakDak Crais's Avatar
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    yeah but without a fancy diff, wont the drive still only go to the wheel with least resistance? my understanding was that true four wheel drive isnt engaged unless the centre diff is locked
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    Need to be a bit careful on fast dirt roads with constant 4wd and open centre diff. If you unload the inside front tyre when cornering the cars tend to spin pretty easily.

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    Registered User Gats's Avatar
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    If you're pushing that hard to introduce understeer and shit in a 4wd you're a muppet in the wrong car and should be driving an Evo or WRX.

    Have never had a single issue driving part time 4wd on dirt, snow or rain.

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    Quote Originally Posted by Crais View Post
    yeah but without a fancy diff, wont the drive still only go to the wheel with least resistance? my understanding was that true four wheel drive isnt engaged unless the centre diff is locked
    Differentials will provide the same amount of torque to each wheel. If one has no traction it will take very little torque to turn it. Therefore the other wheel on the same diff will see no torque.
    The centre diff will do the same thing. So it is possible to have one wheel with low traction bringing the show to a halt.

    That is not the same as your comment "and therefore still can only drive one wheel at any given time".

    They will drive all four wheels with th same amount of torque at all times. Unless you have diff locks in the centre (as early Rangies and Discos do) and in the axles (which early Rangies and Discos don't as standard).

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    Quote Originally Posted by Shep View Post
    Need to be a bit careful on fast dirt roads with constant 4wd and open centre diff. If you unload the inside front tyre when cornering the cars tend to spin pretty easily.
    This guy knows. Had many people at work in prados who were amazed at the difference when you turn on the centre diff lock while high speed dirt running.
    BeamTina

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    Quote Originally Posted by Gats View Post
    If you're pushing that hard to introduce understeer and shit in a 4wd you're a muppet in the wrong car and should be driving an Evo or WRX.

    Have never had a single issue driving part time 4wd on dirt, snow or rain.
    It's the same as putting ya foot on the clutch around a corner.

    Not a muppet just don't live in a city and often drive 1000km of dirt in a day. Also part time 4wd is very different to a constant 4wd. I own 3 4wds and two are part time and one constant.

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    DakDakDakDak Crais's Avatar
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    Quote Originally Posted by BigJonWB View Post
    Differentials will provide the same amount of torque to each wheel. If one has no traction it will take very little torque to turn it. Therefore the other wheel on the same diff will see no torque.
    The centre diff will do the same thing. So it is possible to have one wheel with low traction bringing the show to a halt.

    That is not the same as your comment "and therefore still can only drive one wheel at any given time".

    They will drive all four wheels with th same amount of torque at all times. Unless you have diff locks in the centre (as early Rangies and Discos do) and in the axles (which early Rangies and Discos don't as standard).
    Thankyou. this was the explanation i was looking for.

    It wasnt about "brah i want a sikkunt racecar that I can also take camping", it was me trying to clear up something i didnt understand about how diffs work.
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    Quote Originally Posted by Scrad
    You are like some sort of car rapist, Sticking things where they don't belong, and often don't fit nicely

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    Registered User Milkman Don's Avatar
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    Full time 4x4 vs Part time - On road performance

    Quote Originally Posted by Crais View Post

    Take the mistubishi super-select 4x4 system for example. It's claim to fame is that you can drive in both 2x4 or 4x4 on the tarmac (with the diff unlocked only). Without that diff locked, and therefore completely open, how is the 4x4 mode any better on road than the 2x4 mode?
    My experience with my triton has been obvious dry/wet road, dirt, mud & snow. It’s just a torsen centre diff so as you mention can be used on roads no problem, from memory it’s a 60/40 split. Lock the centre diff and it does what the rest do splits the power front and rear. Get on the newtriton forum and there is a decent following of the ‘4wd when roads wet’ brigade.

    The confusion you have is Mitsubishi run what they call ‘MATT’ (Mitsubishi all terrain technology) which used the traction and stability control to assist with the 4wd system. It is a funny thing to get used to off road if you are used to conventional ‘needs lockers mayyyyyte’ but once get the gist of it is really a good thing. Have changed my off-road driving to suit and touch wood haven’t been stuck.

    So they run an open rear diff and will brake a wheel that loses traction and will deviate this constantly until you get full traction again, you can’t actually disable traction control fully at all. It’s as I said funny to get the hang of but I’ve crawled through a lot of things with traction/asc on that typically would of been lock the diffs and keep the wheel speed & rpms up, this you just drive through.
    Last edited by Milkman Don; 05-01-18 at 09:49 PM.
    Quote Originally Posted by Faux Forg View Post
    I agree with Rdyno

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    Registered User irsa76's Avatar
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    I've never run a conventional part time 4wd in 4wd on wet roads, I know there used to be a school of thought that you could but I was never comfortable with that. More so since I had to sort a Subaru Brumby transfercase that some twit had driven on the road in 4wd with. My own Subaru was an auto which on some varients would engage 4wd when you braked hard or turned the wipers on, they're single range part time 4wd with an electro-magnetic clutch. I did try that in 4wd on wet roads and found it would have snap oversteer, turned out to have a tight LSD in the rear. My Rav had the locking center "diff", but it would only lock upto 60km/h then unlock. Which was a PITA because the Rav was abit unstable on dirt roads.

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    Registered User Milkman Don's Avatar
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    I’ve done it once maybe, hadn’t rained for weeks and the roads were quite slippery, only advantage was through gears 1/2/3, the little 2.5td isn’t F40ish by any means but a wet road and driving to keep with traffic on slippery wet roads it will spin the wheels .coming on boost.

    I wouldn’t do it normally turns the thing into a slug and chews more fuel. I have done it to blast off a set of lights to make it in front of someone slow
    Quote Originally Posted by Faux Forg View Post
    I agree with Rdyno

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