centrifugal force (was Tire Slow Leak)
Larry C Leung
l.leung at juno.com
Fri Mar 15 20:22:03 EST 2002
>Centrifugal "force" acts tangentially to the radial motion (i.e.
outward).
>Thus the condensed water droplets (macro droplets) would "fling" to the
>inside of the tire. At rest, gravity will then draw the moisture
downward
>to the inside of the tire at the lowest point. Notice that long term
technically, the macro water droplets are happily travelling along in a
"straight" line, the tire has the indignity to run into these droplets,
providing the force on them that will cause them to change direction
(i.e. turn). The force provided by the inside wall of the tire is
actually called CENTRIPETAL force, calculable by the formula:
Fc = mv/r^2 where m = the mass of the water droplet
v = the tangential velocity of the tire (for all intents and purposes,
it turns (hee, hee) out to be the velocity of the car
r = the radius to the inside of the tire (or in this case, the bead,
which
isn't where the macro drops would end up if the car was moving)
The macro drops will behave as Newtonian particles, as above, i.e. car in
motion fast enough and any droplets not captured by surface tension at
the bead/rim interface will end up on the inside tread area of the tire.
Go fast enough to exceed the surface tension force (don't have a feel for
the value) and then all of the macro drops will end up inside the tire,
though the increase of temp due to hysterisis of the sidewalls will
likely raise the temp of the air high enough above the dew point to
vaporize any water droplets. The resultant vapor will simply spread all
over the inside of the rim/tire and settle on the nearest surface below
the dewpoint of the air WITHIN the tire when the tire/rim cools
sufficiently below the captured air's dewpoint.
As for acidity, the "normal" pH range of acidity of atmospheric moisture
(clean air, not in a CO2 laden city)
is about 5.6 to 6.1, about 1000 times less acidic than orange juice or
milk, though about 10 times more
acidic than a "neutral" solution of pure water (okay, that was stupid,
pure H2O has no acidity).
It DOES therefore pay to have filtered air, b/c once the water is
trapped, it isn't coming out until there are a lot of dry air flushes of
a properly heated rim/tire combo (i.e. no macro droplets).
>Someone dropped me a line stating he has heard of places where they fill
>your tires with pure nitrogen as opposed to air---I've never heard of it
but
>it may be the answer.
Nitrogen is sometimes used in race tires to aleviate the moisture
problem, as water vapor has a greater effect of temp/pressure than dry
air (which has the virtually the same coeff of expansion as Nitrogen BTW,
just it's easier to get dry, after all N2 makes up 78% of our surface
level atmosphere anyway), and with typical slicks running at low
pressures (usually less than 20 PSIG for a formula car) the diff in even
1 PSIA can significantly affect the car's performance on the track. For
street cars, filtered air is probably adequate and a little cheaper and
easier to get than dry nitrogen, which improperly extracted and stored
STILL can take on moisture.
Finally, of interest for those whom like comparing car cornering
performance:
ac = centripetal acceleration, same variables as above = v^2/r
compare ac to g, the acceleration of gravity ~ 9.8 m/s^2 or 32 ft/s^2
ac/g = G's of acceleration
For a typical car, it can probably corner around 25 ft/s^2 (~ 8 m/s^2 in
metric)
G = 25/32 = 0.78 G's
Very few street cars can meet or exceed a constant 1 G (I know, in
Physics class, we say that 1 G is as much as you can get, but that's for
non-deformable materials, not rubber on pavement), though some (Corvette
Z06 and several Porsches) do from the factory. Meeting or exceeding 1 G
is possible in somewhat more mundane cars in transition however, though
even those cars are few in number.
HTH!
the Physics Teacher
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