[TR] OD lubrication

[Jim Muller]

On 14 Aug 2008 at 10:38, Randall wrote:

> in your earlier post, you said:
> " An IRS would transfer all that dynamic 
> force to the axles by way of the diff gearing."

Ah, I see where you are going...

When the OD shifts, the engine speed must somehow match the wheels' 
roadspeed.  Either the engine or the car or both, but mostly the 
engine, must change speed quickly.  Something in the drivetrain must 
flex or torque or bend or somehow deform to allow a displacement 
mismatch between engine and wheels before those speeds match.  The 
more total deformation is possible the more time the system has for 
the engine and wheels' speeds to match.  Were there no deformation at 
all the engine and wheels would have to match instantaneously, which 
in principle would mean infinite stresses for an infinitely short 
time.  That's a good way to break bearings or gear teeth, etc.

So the question is, where does that displacement come from?  I 
suppose some could come from the springs in the clutch disk, but not 
much!  Some could come from tire deformation.  I suppose halfshafts 
could torque up.  By far the biggest source would seem to be the 
solid axle suspension allowing the diff to rotate w.r.t. the frame.  
Admittedly there aren't many other places.


> [the suspension is] big, and tough, compared to the diff mounts

Ah, that a different factorization of the problem.   In the model I 
presented the diff mounts would be part of the frame.  Yes, they 
would experience the internal stress of the closed system between 
engine, frame, and driveshaft.  If they are a weak or soft link in 
the physical structure they could presummably break from repeated 
flexing.

The point I was trying to make originally is that if you let the 
suspension holding the diff provide the required deformation, there 
is a tradeoff, i.e. that it torques the body by impulsively loading 
one rear wheel and unloading the other.  Your point is that this is 
preferable to flexing the diff mounts.

I confess I've never experienced a diff mount failure, don't know if 
Spitfires do in general.  Given how the diff is mounted, I guess it 
isn't likely.  Then again, the Spitfire's moving drivetrain parts are 
probably lighter than a TR's too.

Interesting discussion, Randall.  Thanks for driving your point.

"The difference between theory and practice is bigger in practice 
than it is in theory."  Sorry, I tend to fall on the theory side 
until practice says otherwise.  Breaking frame members sure qualifies 
as practice!


-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+