The Hayseed's two-bits worth:
Aero drag and horsepower produced by a gas aspirated engine vary exactly the
same with change of air density. At zero density aero drag is zero and HP
is zero. rolling resistance is not affected by air density, so the cars
(such as streamliners) that have the greater part of their resistance
brought on by rolling resistance will suffer the worst when air density is
lowest. If aero drag is lower and HP is lower, rolling resistance is still
high, so a streamliner benefits from high barometer and low temperature.
Obviously the engines with oxygen-bearing fuels will not be affected as
much.
Dave D is quite right when he said that high speeds produce lots of
rolling resistance. The SAE paper describing the goldenrod calculated
higher rolling resistance than aero resistance at 400 MPH. All this stuff
depends on what kind of car you run. When I run a fendered street roadster
the aero drag is so monstous that rolling resistance is comparatively small,
so in goes the ballast.
As to dry vs. wet salt: do we know for sure that speeds are slower in the
morning? If they are, could it be because the running gear (rear end, wheel
bearings, trans, etc.) are not yet warmed up as much as when you run in the
afternoon heat? It does seem to be a fact that the salt is wetter in the
heat of the day and capillary moisture comes to the surface and brings some
salt with it. Sit still some day and watch the ground and you can see it.
When you get as old as I am you sit a lot.
Dave the hayseed
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