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Dead Duds - sorry, but this just isn't physics...
Probably not the most propitious of topics over which to stop lurking, but...
all of the talk of "it's physics, end of argument" is so much baloney, I'm
afraid. I don't want to debate the _model_ Scott has described, as that really
isn't the issue for me. Let us _assume_ that the model, in its entirety, is
logically coherent, and that the empirical information he's entered into it is
accurate. Does that mean that the model's predictions are correct? No.
A model, no matter how complex, is a very poor substitute for reality. As we
can't mathematically deal with the whole of reality in one go, models are a
very valuable tool for analysis, but only if one understands and accepts their
limitations. No model of the Torsen, no matter how elegant, can fully take into
account all of the factors which determine the progress of a car through a
turn. The model Scott advocates appears to imply the transfer back and forth of
large amounts of torque, which, within the model he uses, leads to a rapid,
violent succession of understeering and oversteering. Fine and dandy, but only
for that model, and only under specified conditions.
Dave, and others, believe (as far as I can judge from their posts, with
apologies if I misrepresent them) that, regardless of the model's internal
logic (or otherwise), other factors, beyond the scope of Scott's modeling,
leave the car safe and predictable through the same maneuvers. Can the circle
be squared? Certainly. They _observe_ that, in reality, under the circumstances
in which they have tested their cars, the model's conclusions are inaccurate.
Other's may observe that under differing circumstances, the model's conclusions
are valid, and the spiders run wild.
If the physical property suggested as a cause of spider bite was overwhelming,
then every aggressive driver would see it on a regular basis. Apparently not
so. That doesn't mean the physics are incorrect.
Take a bumblebee. Measure it, test it, apply (entirely reasonable, and usually
accurate) aerodynamic models to it. (1) Theoretical conclusion: The bumblebee
cannot fly. It weighs too much, its wings are too small and beat to slowly. (2)
Practical observation: Bumblebees fly. (3) Logical corollary (assuming we
accept the physics of the aerodynamic model): (i) other factors, beyond the
model's scope, assist flight and/or (ii) the aerodynamic model does not apply
well to small round insects with black and yellow stripes.
What Scott seems to be doing is making a series of flying logical leaps:
1) From the existence of the physical property (neither accepted nor debated
here) _within_ his model
2) To the idea that this modeled property is accurate in the real world (quite
3) To the belief that whatever element of that property remains valid under
real world conditions is significant enough to overwhelm other physical factors
mitigating the property (likewise debatable)
At the end of the day, given Scott's experiences (and others') and given his
math (and the tech papers etc), it seems likely, from the evidence presented,
that rapid transfers of torque are possible, and hence that spider bite _can_
exist under certain circumstances. This belief is aided by the physics model,
but it can only be _proven_ by the model under rigorous application of the
ceteris paribus assumption, i.e. never in the real world.
Dave's experiences (and others') and a little thought to the imponderables
which lie beyond the math and the modeling suggest that, under a very wide
range of circumstances, the condition _does_ _not_ apply.
Personally, as far as I have been able to replicate Scott's description of the
circumstances (backed up by Dave's method of going round and round the
roundabouts), it doesn't happen to my car, under any circumstances I have yet
been able to generate. To me, it's more important how the car actually drives,
rather than how, in theory, it might.
For those for whose reality involves getting bitten, that's none too cheering,
but whatever property of the Torsen (if indeed it's the guilty party) generated
their circumstances, that property reliably fails to generate the problem for
other people with other setups or at other places and points in time. Physics
holds the car to the road, physics pushes it off, but I'll accept the "it's
physics, end of argument" assertion only when I see a model that contains every
imponderable, every parameter, every circumstance. Until then, Scott's argument
will remain not "The Physics", but just "One possible physical element".
'94 S4, which no doubt just adds another datapoint to Qhip's dislike of the car
and its drivers.