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How to dynamically control understeer/oversteer in AWD?

posted by  garlandw on  Mon Mar 17 14:49 CST 2008
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last visit: Wed Sep 10 08:58 CST 2014
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I have read a few posts discussing under and oversteer in RWD/FWD yada, yada. I don't recall much discussion on controlling under/oversteer in an AWD as you go through a turn. I don't want to open a discussion on which is better. Rather, I would like some insight on how exactly one can effectively make corrections 'on the fly', so to speak, with AWD. For a RWD or FWD car of a given setup (sway bars and what have you), the car will have a bias. Throttle and steering input can be used in the turn to induce/reduce under/oversteer as desired (one hopes). But in AWD - let's assume a 50-50 torque split for the sake of the argument - throttle changes will not be nearly as effective since it will not cause a shift in the front/rear balance of forces.

I have never driven an AWD anywhere close to cornering limits so, tell me, how do you control oversteer, say, in AWD should you hit a patch of gravel in the rear suddenly? Just steer into the slide and get off the throttle?

Or am I way off the mark?

Bill








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    How to dynamically control understeer/oversteer in AWD?

    posted by  KlausC on  Mon Mar 17 15:23 CST 2008
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    last visit: Fri Oct 24 10:15 CST 2014
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    LOL. That is the beauty of AWD. The cornering limits are soooo much higher! But if you do loose it, it will over/under steer and the old physics takes its normal place. Recovery is much faster than either RWD or FWD, if the driver keeps his foot on the gas while braking with the left.
    In the newer AWD cars, traction control takes over and makes unusual attitude almost impossible to attain. Of course, any system is not fool proof, and wrecks will happen, but it usually takes a fool.

    I mentioned left foot brake, right throttle. Traction control only works if throttle is being applied, so if you are on ice and trying to turn but braking at the same time, the car will slide and not turn. Applying gas instead of more brakes lets the AWD system take over and let you turn.

    Klaus
    --
    The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)








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      How to dynamically control understeer/oversteer in AWD?

      posted by  garlandw on  Mon Mar 17 16:43 CST 2008
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      last visit: Wed Sep 10 08:58 CST 2014
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      Klaus,

      I don't understand how the cornering limits are soooo much higher with AWD. The limits are defined pretty much by the tire patch. If you are going into a tight turn too fast I don't see how AWD can help. If you are powering through a turn, sure; distributing the same overall force through 4 patches helps a lot.

      My question was not about absolute limits. It was about being able to add under/oversteer on the fly. Your two foot technique won't work on a car that does not have traction control. Let's keep it simple: a straight 50-50 AWD with no traction control, no ABS. Let's go further and assume a 50-50 weight distribution and a 50-50 sway bar bias. Perfectly neutral handling. So, you are going into a tight turn that should be doable except you hit the brakes to avoid a deer. That lightens the rear and it slides out. Seems to me that all one can do is countersteer, right? You get off the brake but that only helps to restore neutrality, not correct for the oversteer in progress.

      I am just focusing on attitude control, not absolute limits. AWD is the clear winner overall in the absolute.

      Bill








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        How to dynamically control understeer/oversteer in AWD?

        posted by  KlausC on  Mon Mar 17 17:18 CST 2008
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        last visit: Fri Oct 24 10:15 CST 2014
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        An AWD with 50/50 bias in weight and torque is perfectly neutral, and you will not be able to force over or under steer. My V70 will eventually oversteer by a little bit due to the suspension setup (stock).
        Avoiding an obsticle in the road with your foot off the gas will result in understeer in the V70, real world. With power on, it will oversteer. Neither conditions will be as pronounced as the FWD or RWD cars, because the wheels will not loose traction.
        In your initial hypothisis, all 4 wheels hitting loose sand (or ice), the car will slide sideways in a neutral slide.

        The big SUVs doing sudden lane changes and flipping over are because they are top heavy. Never buy a car/truck that is taller than it is wide :)

        Klaus
        --
        The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)







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