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My SUV (Mitsubishi Outlander Elegance automatic) has the ability to switch between four-wheel and front-wheel drive at any time during operation with a little dial next to the handbrake.

I feel that I can somehow "perceive" a difference in where the drive power comes from when I switch between these two modes, but with my car's frame being a rigid body my intuition tells me that this shouldn't be possible.

Physically speaking, how could the power from the front and back wheels be transmitted through the chassis in such a way that I can actually feel that drive power is coming from the back wheels ("floating" me along the road) and not just the front wheels ("pulling" me along the road)?


Edit I specifically mean when I'm travelling at a steady, low speed in a straight line over an even surface.

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migrated from physics.stackexchange.com Jan 6 '13 at 21:58

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Well, the suspension is not rigid, right? Sounds reasonable that you feel the difference. –  Bernhard Jan 6 '13 at 19:19
    
@Bernhard: Okay but my seat is attached to a frame that is rigid, though that frame is mounted on the suspension. It's not bumps and stuff that I'm feeling differently (I'm sure I would); if I'm travelling along a flat road nice and calmly it's as if I can perceive the transmission itself. Can that still be the suspension? If so, how?! –  Lightness Races in Orbit Jan 6 '13 at 19:20
    
If you tell us the specific model of car, we might be able to give you detailed explanations about what you're feeling. –  Bob Cross Jan 7 '13 at 13:56
    
@Bob: Mitsubishi Outlander Elegance automatic –  Lightness Races in Orbit Jan 7 '13 at 13:57
    
Thanks - I put it in the text of the question. –  Bob Cross Jan 7 '13 at 16:19
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3 Answers 3

They should feel very different!

When cornering the difference is at it's greatest. Instead of the general understeer a front wheel drive car will trend to do through a corner because traction is broken relatively easily when accelerating through a corner, you will expect a four wheel drift as all four wheels will be able to break traction.

When accelerating, front wheels will lose traction as the weight shifts backwards, whereas the back wheels will grip better as they load up.

Not much difference under braking, although engine braking feels a little different as all four wheels slow you rather than just two...

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What about a steady, low speed in a straight line over an even surface? –  Lightness Races in Orbit Jan 6 '13 at 19:34
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You should not feel any difference, other than a bit of extra noise (as the transmission is drivign all 4 wheels) and slightly less slip on a slidy surface. –  Rory Alsop Jan 6 '13 at 21:31
    
aha! Interesting... –  Lightness Races in Orbit Jan 6 '13 at 21:55
    
There is always the chance you have a bad u-joint and you are feeling the vibration. –  mikes Jan 7 '13 at 1:43
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In a perfect world you shouldn't. However because the the front power train is now engaged you now have more moving parts. More moving parts = more/changed vibration and noises. I would wager what your are perceiving is just normal operational byproducts of the front power train.

Also the front drive train is now connected to the transmission which is connected to the engine. So it's possible you may feel more vibration through the steering wheel. Albeit vary slight increase.

The steering will also feel tighter. I know you are talking about a straight road... but I've never been on a perfectly flat, level, and straight road. I'm guessing might contribute as well.

TRY THIS

I just thought of something. I'm assuming your are the one driving. That might be inducing the placebo effect. So try this. Have someone else drive while you are blindfolded. Then have them start and stop switching on and of the the 4wd without letting you know. Then if you perceive the right mode then you know there really is something different about how the vehicle moves. I would also suggest turning up the stereo when switching modes. That way you can't hear anything engage or disengage. Let us know how it works out!

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That would be a good test, but there ain't no-one driving my car but me ;) –  Lightness Races in Orbit Jan 7 '13 at 12:42
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The original question is still a little light on specifics so I'm going to base the answer on the second generation Outlander which, as cited, uses the AWC electronically controlled 4WD:

When “4WD Auto” mode is selected, the Outlander's 4WD system always sends some power to the rear wheels, automatically increasing the amount under full throttle acceleration.

So selecting this mode immediately changes the torque to the driven wheels, transferring some away from the front wheels and to the back wheels. This will change the weight transfer of the vehicle under acceleration and engine braking. This will also induce greater transmission loss as driving four wheels requires more interacting moving parts than 2WD. This will be most obvious under engine braking.

Some important aspects of the original question, however:

I specifically mean when I'm travelling at a steady, low speed in a straight line over an even surface

In the real world, there is no such thing as a straight line over an even surface outside of an auto-testing facility.

but with my car's frame being a rigid body my intuition tells me that this shouldn't be possible.

Your car is not a rigid body. Nothing about a car is truly rigid, including the frame. Remember, you are riding around on a system of linear and torsion springs, the oscillations of which are all damped differently.

Now, given all of the above, it is also important to remark on one of the comments:

I seem to be able to perceive through my bottom on the chair that the power is coming from the back as well as the front

It is critical to remember that the butt dyno is well documented as the least accurate instrument in the vehicle.

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I do not agree that transferring torque from the front to the rear wheels will change the weight transfer of the vehicle under acceleration. Equal acceleration gives equal weight transfer, regardless of where the tractive force is being put to the road (weight transfer under acceleration is primarily a function of wheelbase length, CG height, spring rates, and the amount of acceleration). Of course, you may have increased potential for acceleration when the rear wheels are driven, due to increased drive traction, but that's a bit of a different story. –  mac Jan 9 '13 at 22:29
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