On 15 Aug 2008 at 17:44, Randall wrote:
> > Something in the drivetrain must
> > flex or torque or bend or somehow deform
> I disagree. That's exactly why there is a _clutch_ in the
> overdrive
Not to be pedantical or anything, but a clutch is simply one form of
deformation. It is different from that of, say, a spring in that it
absorbs energy irreversibly, converting it to heat. If the energy is
absorbed uniformly over the same displacement range, which implies
that the retarding or accelerating force is uniform through that
range, then the force it exerts on the components is only half the
maximum reached by the spring, and there is no rebound either. (This
is the same reason for using a crushable material in a bike helemt
instead of an elastic foam.)
Even so, the engine and car must match speeds over some small period
of time. The shorter the time, the higher the forces, or to put the
causality right, the higher the forces, the shorter the time. And
regardless of how quickly the OD moves into engagement, the force
history, and thus the time for the speeds to match, is determined by
time history of the friction of the clutches. If fast engagement due
to a pre-pressurized piston also means more friction, then it would
create higher forces in the drivetrain. And if something else in the
system can deform also, then the forces are reduced.
I stand by my original description. When the OD state changes, a
solid axle car will experience an inpulsive loading of one rear wheel
and a similar unloading of the other, caused by the driveshaft
torquing the diff and axle tube. In an IRS car, this does not happen
but admittedly the diff mounts absorb the same impulse. In the first
case, it will affect handling, in the second I'll grant that the diff
mounts could break, In either case, something must absorb the
displacement difference between opposite sides of the OD, and a short
time over which it occurs corresponds to higher stresses on whichever
component must absorb them.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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