On 14 Aug 2008 at 8:11, Randall wrote:
> Seems to me that both differentials have a torque reaction that
> must be absorbed by the diff mounting; which in the case of the
> solid axle is the axle tubes and rear springs.
That's exactly the difference. The torque on the IRS diff is
countered by the mounts to the frame. So it is a "closed" system,
engine to frame and engine to driveshaft, driveshaft to diff, diff to
frame. The system itself is under internal stress that may flex the
frame slightly, but none of it is transferred directly to the tires,
i.e. to the ground.
With a solid axle the torque on the diff is countered by suspension
to ground, limited of course by the traction of the wheel being
lifted. It's a classic difference between an IRS muscle car and a
solid axle one where one wheel is loaded and one unloaded. Compare
it to the dynamic effect of the flywheel mass torquing the engine
over on its mounts when you rev the engine in neutral. That's a
dynamic effect proportional to the rate of change of rpm. In
contrast, the solid exle effect is static, present even at a constant
rpm. During hard acceleration the rpms will of course change unless
you're going up a steep hill or at such high speed that drag prevents
you from going faster, but the static effect is still there due to
the engine torque. If you did this on a dynamometer you'd see the
car body roll over to one side.
What the suspension involvement means for drivetrain stress is that
more displacement is available to accommodate a change in forces as
the engine is deccelerated from the shift.
'80 Spitfire, '70 GT6+
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