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