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Torque vectoring:why does the inside front wheel also have same torque

  1. Mar 4, 2014 #1
    My question relates to torque vectoring during a turn(high speed). Wikipedia says torque vectoring is mainly done on the rear wheels during a turn. The front wheels receive the same amount of torque though. Why do the front wheels receive the same amount of torque during a turn?

    Say my engine produces 500Nm torque. My centre differential distributes it 60% to the rear(=300Nm) and 40% to the front(=200Nm). Say also,my front and rear axle differentials multiply the torque by 3 times.

    So,if I am turning left,my rear right wheel gets a torque of 900Nm after torque vectoring(the rear left wheel is at 0Nm). But,according to the videos and wikipedia,the front wheels are each at 300Nm. Why? [I think this promotes understeer].

    Please have a look at this diagram from Nissan Juke taking a left turn,the front wheels are each at 25%:http://www.nissan.co.za/en/web/models/JUKE/Performance/images/1365681727649049510.jpg
     
  2. jcsd
  3. Mar 4, 2014 #2

    cjl

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    In the absence of any special techniques or fancy differentials (and most AWD cars do not have a limited slip diff on the front wheels), an open differential will always provide a 50-50 torque split. As for why this is done? Because it is simple, and in most cases, a fancy front diff on an all wheel drive car provides little benefit. On a front-wheel drive car, the answer is quite different - many high performance FWD cars do have specialized differentials that carefully control the torque going to each wheel in a turn.
     
  4. Mar 4, 2014 #3
    One other point is that having unequal torque on the front wheels is unpleasant to drive because it causes torque steer.
     
  5. Mar 14, 2014 #4

    jack action

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    Science Advisor
    Gold Member

    Here is my understanding.

    There are only 2 types of differential: open or limited-slip. Both divide power to each wheel.

    The open diff transfer the power from one wheel to another by varying the wheel rpm (increasing one, reducing the other). So, as said previously, the torque output is always 50/50, even if the power isn't the same.

    This is the best setup for typical driving, as wheel rpm is different from left to right when taking a turn. Not adjusting the rpm would mean losses as some dragging would be involved while taking a turn. As an added bonus, the open diff is the most economical one to build.

    But when you have different traction characteristics from left to right, the one having the lowest coefficient of friction may start to slip and hence waste power (to the point of losing it all, if the other wheel has stopped rotating).

    The only way to solve that problem is to lock the right and left axles together. The best way is to do it by some friction mechanism, thus the limited-slip diff. The friction may comes from clutches, cones, worm gears (Torsen) or even the brake system and it may be preset or computer controlled; still it has the same effect in the end and that friction power represents a loss.

    In the extreme case of having a completely locked diff, one can see that both wheels have the same rpm, so the only option for transferring power from one wheel to another is to transfer torque. If one wheel is not touching the ground, then it is easy to understand that the full power goes to the wheel that touches the ground even if both wheel are rotating. This is obviously the best setup for a limited traction situation, preferably in a straight path.

    Now to address the present problem. AWD like the ones the OP talks about are usually designed to be mostly FWD, sending power to the rear wheels only when traction is weak. First of all, we can note that having a torque difference on front-wheel-steered FWD vehicles may induce torque-steer.

    So, the front diff should be an open diff (50/50) since it is mostly used for everyday, no-traction-problem, situations. When you loose traction, then you switch to AWD. And because you know your in a low traction situation, making the rear diff a limited-slip one makes sense, since it is always used in low-traction situations.

    As for the understeer problem stated in the OP, sending the power to the rear wheels in a low traction situation is done exactly for that: reducing understeer.

    http://www.autos.ca/auto-tech/auto-tech-limited-slip-vs-torque-vectoring/
    http://www.popularmechanics.com/cars/news/4225886
    http://torque-vectoring.belisso.com/
     
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