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Toyota 4WD systems explained

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  • gpm5gpm5 Posts: 785
    I have driven the sequoia this winter on the freeway up steep mountain passes in the snow. At 65 mph there is no slippage or fishtailing of the rear, and there are no noises at all. The only time there is ABS pump noise is when you mash the accelerator from a stop or at slow speeds when on a snow covered road.
  • wwestwwest Posts: 10,706
    100Hp engine, three open diff'ls, one wheel raised off the ground, (or on ice, mud hole, etc.)its spinning only dissipates 25%(?) of the HP yet the vehicle doesn't move, where is the other 75% being dissipated?
  • What makes the engine develop 100 horses??? if the wheel spinning one let's say ice gets 5 ft-lbs of torque, then every other wheel is limited to 5 ft-lbs of torque (assuming 3 perfect open diffs). The engine can rev and it's power will go into rotational inertia, heat, noise...and those tires.
  • gpm5gpm5 Posts: 785
    If the spinning is not stopped as in the sequoia system then all torque goes to zero or something close to it. No friction=no torque. Its like racing your engine in neutral.
  • yes

    it's like racing your engine in neutral
  • heatwave3heatwave3 Posts: 462
    wwest: I think one point you are not seeing in the differences between awd and 4wd is the dedication of torque in awd vs making torque available in a part time 4wd system. This is the principle difference between a 4wd system and an awd system.

    The awd system with limited slip diffs and visc liq diffs can be designed to provide a minimum % of torque to a wheel under all conditions whether there is ice, gravel or dry pavement. No such minimum can be assured with the open diffs and tt4 traction system of a Sequoia.

    Imagine the following scenario. Your vehicle is placed with all 4 tires on very slick ice at a standstill. In an awd vehicle, I know that in the case of the Denali 38% of the torque will be delivered to the front wheels and 62% will be delivered to the rear and because of the limited slip diff in the rear and the viscous center coupling.

    I am assured that the tires will rotate even though they might spin. The vehicle will certainly move forward as it builds momentum.

    I truly have no idea how the Sequoia will respond, however in a vehicle with open diffs the engineering would suggest the tires will not move. If they don't rotate the TT4 system has nothing to measure and no benefit is gained by braking a wheel thats not rotating. Exactly how is that a better traction system than one that guarantees power to all 4 wheels under all circumstances?
  • cliffy1cliffy1 Posts: 3,581
    Your question seems to have been lost in a haze of misinformation. In answer to your question, the Tundra is a conventional part time 4WD system that can not be used on dry pavement without damage. Believe me, I wish the Tundra came with the Active-Trac system. My lease expires in December and I am crossing my fingers that it will be available by then. Until then, the LSD rear will help traction, but not nearly to the extent that the Active-Trac will.
  • cliffy1cliffy1 Posts: 3,581
    Why do you even bother? You're embarrassing yourself. Where did you ever come up with the idea that the Sequoia comes with a viscous coupling center?
  • wwestwwest Posts: 10,706
    was 100HP, ignore it.

    I was simply, and only, addressing the issue of AWD/4WD(?) using three open diff'ls, I'm not in the least pretending that such a configuration actually exists.
  • I understand, but say an engine makes x amount of power. With no wheels slipping and 3 open diffs, each wheel gets .25*x. With one wheel slipping and only able to handle a*x, each wheel is then only receiving a*x. So the total power to the ground is now 4*a*x. This just doesn't change. The engine may be able to produce more than 4*a*x, but it won't be able to for very long. Very rapidly the engine will redline and fuel will be shut-off (if no fuel cut-off then the engine will just blow-up).
  • eagle63eagle63 Posts: 599
    "AWD is grip, slip, react, transfer and increase grip. That's why all the sophisticated sensing in the world will not replace having the grip in the first place. "

    -That's actually what I like about part-time 4wd. 50/50 torque split no matter what. no sensors, no guessing. In deep snow it can't be beat.
  • heatwave3heatwave3 Posts: 462
    cliffy: you state "Why do you even bother? You're embarrassing yourself. Where did you ever come up with the idea that the Sequoia comes with a viscous coupling center?"

    Maybe you might want to get off that pedestal of knowing everything. I certainly don't claim to know everything there is to know about 4wd systems but I've owned a variety of different 4wd vehicles and have what I consider a pretty extensive experience of 20 years operating them.

    You've claimed to know alot of information which might be well respected by many people unfamiliar with how these systems work. I've respected your views and the information you have provided, however you have been inaccurate on several occasions regarding some pretty basic info readily available on the net.

    Instead of feeling compelled to respond to me with your sighs and indignation, you might want to consider simply getting your information right instead of speculating or assuming information I share must automatically be wrong because I drive a GM.

    Now to answer your question. Please go back and reread my posts. I've never claimed that the Sequoia has a liq ctr diff. In fact I've stated clearly on various posts that the system has 3 open diffs. (which is one of the weaknesses of Toyota's approach to 4wd IMO).

    I did state that the Land Cruiser which you claimed has the exact same 4wd drivetrain as the Sequoia, in fact, is not the same as the Sequoia. I provided a source (above) which states that the ctr diff on a LC is a viscous liquid locking ctr diff. I also pointed out that the overseas models have a locking rear diff which is also lacking in the Sequoia. No where did I claim the Sequoia has a "viscous ctr coupling".
  • heatwave3heatwave3 Posts: 462
    eagle63: what you fail to understand is that the Sequoia cannot deliver "50/50 torque split no matter what. no sensors".

    You fail to understand the nature of open diffs even with traction control. There was an excellent site provided earlier which can better explain why, but basically your Sequoia will sense slipping in the front tires (for example) and potential move all the torque to the rear wheels. The system IS sensing this slippage and IS moving the torque throughout the open diff system until it finds a wheel that's not slipping. What I would like to know is how the system handles traction if all wheels are slipping as on ice. Open diffs will act as if the vehicle was placed in neutral with the engine accelerating but no power is transferred to the wheels.

    The Sequoia does not and cannot deliver 50/50 distribution of torque to both ends at all times. If you don't believe me ask cliffy since he appears to be the only information source that Sequoia owners will believe.
  • wwestwwest Posts: 10,706
    "Sequoia will sense slipping in the front wheels (for example) and potential move all the torque to the rear wheels"

    NO !!

    The Sequoia system will apply the brakes to the slipping wheel(s) (front, in your example)allowing the engine torque to remain elevated and the torque distribution will therefore remain approximately "equal", at least that is the "target" (equal rotational rate for all four wheels) for the ecu firmware.

    If all four wheels are (persistently) slipping the only solution for that is a tow chain.
  • eagle63eagle63 Posts: 599
    "eagle63: what you fail to understand is that the Sequoia cannot deliver "50/50 torque split no matter what. no sensors".
    You fail to understand the nature of open diffs even with traction control. There was an excellent site provided earlier which can better explain why, but basically your Sequoia will sense slipping in the front tires (for example) and potential move all the torque to the rear wheels."

    -I don't own a Sequoia and couldn't care less what kind of 4WD system it has. I'm talking about plain vanilla part-time 4wd.
  • heatwave3heatwave3 Posts: 462
    eagle63: Now I getcha. I thought you were referring to the Sequoia's 4wd system when you made your comments about sensors and 4wd systems.

    Yes, there is the advantage of simplicity and "forced torque" through a part-time 4wd system. The major downside however is the inability to use the system on dry pavement. You give up the advantage of the extra traction in turns and the simplicity of not having to activate 4wd should the roads suddenly turn nasty.

    That's why so many manufacturer's are trying different designs to come up with the maximum traction of 4wd combined with useability of use on all road types.
  • heatwave3heatwave3 Posts: 462
    wwest: you don't appear to understand your own drivetrain and its advantages and liabilities. Think through you post once again

    "The Sequoia system will apply the brakes to the slipping wheel(s) (front, in your example)allowing the engine torque to remain elevated and the torque distribution will therefore remain approximately "equal", at least that is the "target" (equal rotational rate for all four wheels) for the ecu firmware."

    Your own post tells you that what your saying is inaccurate. When the TT4 system determines that a front wheel for example is slipping, it automatically brakes that wheel to slow or stop its rotation. By doing so, it automatically drives the torque away from that wheel. In fact it can brake that wheel until all torque previously going to that corner has been re-distributed elsewhere. It does not, in fact, "maintain" the torque at that wheel as you state. It does exactly the opposite.

    It will continue to brake wheels until it finds a wheel or wheels that can actually use the torque without slipping. It does not do anything to maintain a 50/50 split of torque.

    If the front wheels are slipping on ice for example there will be no torque delivered to the front wheels. There is no target torque distribution the ECU is trying to maintain as the system does not measurement of torque. It fact it doesn't care what the torque is. It only measures slippage or impending slippage and then attempts to prevent it from occurring by grabbing the brake on that wheel.

    If all 4wheels are slipping, 3 open diffs will act as if the vehicle is in neutral, with the engine disengaged from the tires. In contrast, an awd vehicle will rotate the tires, possibly slipping but moving forward nevertheless, as it builds momentum the slippage will be reduced and the vehicle will move forward.
  • pschreckpschreck Posts: 524
    How can a vehicle gain momentum if all four wheels are slipping? If there is any traction to gain momentum with, then the wheels aren't truly slipping. And if a AWD vehicle has enough traction to move forward why wouldn't the full-time 4WD vehicle have enough traction to move forward?
  • wwestwwest Posts: 10,706
    There is no need for an LSD on the GS300, nor ANY lexus with TRAC. For a RWD vehicle as long as both rear wheels have a reasonably close or equal, level of traction, the open diff'l will deliver equal torque to both rear wheels.

    TRAC only comes into play if one rear wheel begins to slip and then TRAC will instantly apply, release, apply, release, etc, the brake to that wheel, using the on/off cycling of the brake to modulate the frictional coefficient of that wheel such that it's rotational rate roughly equals that of the opposite wheel.

    If this condition persists for more than a few hundred milliseconds and the operator does not react quickly and lift the throttle then the TRAC ecu will itself dethrottle the engine.

    The Sequoia, ML, and I think the new LC, AWD systems work the same way except TRAC will use the brakes on any wheel or wheel to alleviate wheelspin.

    Three open diff'ls with TRAC to insure true AWD operation on most surfaces, dry, slippery, snow, ice, etc.

    The Sequoia only locks the center diff'l in L4 and with the transmission shifter in "low", otherwise it uses TRAC to apportion torque in AWD mode if slippery conditions are encountered.
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