[TR] OD lubrication

[Jim Muller]

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 at rcn.com
'80 Spitfire, '70 GT6+