Message text written by "Jim Muller"
>As DM mentioned, there is some chassis flex which would transfer some
weight from one diagonal to the other. But that wouldn't be nearly
as much as the solid axle effect. With a solid axle, when you put a
torque on the diff, the axle and therefore the wheels counter it.
You are literally torquing the axle assembly on a longitudinal axis.
The reason it doesn't apply to IRS is that the diff is supported by
the chassis, so if you put a torque on the diff, the chassis counters
it. There is no downforce or upforce on the wheels as a result of
driveshaft torque, only what the chassis provides via the springs.
<
My whole point is that the torque applied to the drive shaft does not come
from the hand of God, it comes from the engine. If you remember Newton's
third law any torque applied to the drive shaft is countered by an equal
and opposite force applied to the differential - and the engine (OK the
engine/transmission combination). It either all loops back on itself or it
results in acceleration (Newton's second law).
In the case of a solid axle car the torque from the drive shaft will be
opposed by a torque from the rear axle which manifests itself as a transfer
of downward force on the rear wheels. Meanwhile, the source of the torque
is exerting an equal and opposite torque upon the engine mounts which
transfers it to the car chassis and ultimately to the front wheels and
results in a tranfer of downward force on the front wheels. And the ground
itself closes the loop. If it didn't close something would start spinning
inside the car. Ask anyone who has broken an engine mount about that.
In the case of the IRS model the torque from the driveshaft is opposed by
the differential mounts to the car chassis. This force is equal and
opposite of that from the engine/transmission combination. In this case
the chassis short circuits the path through the ground and there is no
inherent transfer of downward force on the drive wheels as a result.
(There is a reactionary force from the angular acceleration of the rotating
engine parts but this phenomenom exists in both scenarios)
In the other axis, that of the rear axle, the torque applied to the rear
wheels is opposed by the suspension which will try to lift the nose of the
car. This is countered by gravity. Since this is an open system the net
result is a force vector in the direction of the front of the car which
results in car accelerating in a forward direction. And this is the
ultimate goal of this whole exercize.
Basic mechanics, summing up the various force vectors.
Dave Massey
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