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Automotive Symmetrical vs Asymmetrical AWD at high speeds

  1. Jul 26, 2017 #1
    Let's say we have two identical cars (power, weight, drag etc.), the only difference being in the torque split between the front and rear wheels. One has a 50:50 split while the other has a 40:60 split torque distribution.

    Is symmetrical AWD more efficient at acceleration during high speed driving?

    The way I see it is the 50:50 car covers ground equally at all 4 corners while the 40:60 car would have the rear wheels spin faster than the front wheels if the car was on a hoist. On the road, the 40:60 car would have both axles spin at the same rate, but 60% of the torque is sent to the rear, so does that mean the torque sent to the front is wasted energy since the rear wheels are propelling the car's front wheels faster than the engine can spin it?

    Or did I understand it incorrectly and the car's front and rear axles spin at the same rate despite having a 40:60 front:rear torque split?
     
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  3. Jul 26, 2017 #2

    Ranger Mike

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    where is the engine located? in front , rear or mid section? what is the front to rear weight %?
    the % torque would not necessarily " spin " one set of wheels more than the other on a hoist as you have not detailed exactly how the torque bias is attained.
     
  4. Jul 26, 2017 #3

    Mech_Engineer

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    I think you've misunderstood. An all-wheel-drive system with torque bias controls the torque being applied to each axle (or even each wheel) through frcitional clutches or other differentials, but the wheels will always spin at the same rate unless they're slipping due to a traction limitation.

    Think of an engine which puts out 100 N-m of torque (and assume a 1:1 final drive ratio, although this is probably not the case...):
    • In a symmetric AWD system and assuming all wheels have equal-traction, each wheel gets 25 N-m of torque (even split)
    • In an asymmetric 40:60 F/R AWD system (and still assuming all wheels have equal-traction), each front wheel gets 20 N-m of torque, and each rear wheel gets 30 N-m of torque.
    In each case, the total torque accelerating the car forward is the same even though the torque splits are different. The different torque splits have different handling characteristics, mainly in cornering or traction-limited situations.

    See here for more reading: http://www.awdwiki.com/en/torque+split+ratio/
     
  5. Jul 27, 2017 #4
    thanks for the explanation. so in conclusion, all wheels spin at the same rate despite different amount of torque is being applied to each wheel?
     
  6. Jul 27, 2017 #5

    Mech_Engineer

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    Correct, unless traction is limited causing slipping at a wheel, in which case the car's AWD system and/or traction control system may intervene and limit torque to the slipping tire.
     
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