Centrifugal "force" was aluminiumiummm wheels

Sat Mar 16 12:41:03 EST 2002

I jumped the gun on the presence and/or significance of macro droplets of 
water in the wheel/tire--it was just the first thing that came to mind.  It 
may have a very small contribution but I also think it is too simplistic to 
use formulae for objects travelling in a straight line or spinning in this 
case as tires and wheels spin about an axis that pivots during turns and 
moves vertically as the tires go over bumps, into potholes etc.  Anyway, 
that is a mute point as it seems the most logical mechanism for trapping 
water along the lip where the tire and wheel join, is condensation.  Seems 
to me more people in cold climates have experienced significant problems 
with corrosion on the interior of wheels.  If there is any significant water 
vapour in the air inside a tire it will condense on the steel or aluminium 
first as the metal is a much better conductor of heat and will cool quicker 
than the rubber.

As for the pH, yeah, the dissovled carbonic acid in atmospheric water is 
pretty weak but if you compound the effects of time along with the somewhat 
porous nature of cast aluminium (at least surfically it provides alot of 
surface area for any reaction) it may be a contributing factor to corrosion. 
  This water will also be a weak electrolyte and when in contact with 
dissimilar metals also in contact (ie. aluminium wheel and steel brake disc 
and hub) an electrochemical cell will be created which will again accelerate 
the process.  The liquid does not have to be a good electrolyte either (like 
coolant between a aluminium head and steel block.  The building in which I 
work is a ticking time bomb, the mental midget of an engineer responsible 
for the HVAC system decided to use steel pipes with aluminium couplings.  
The building is only seven years old and we are already having couplings 
bust do to corrosion.  Im not sure if they put any type of conditioners in 
the water that may increase its' electrolitic properties.

I guess the bottom line, if you live in a cold climate, is try to use "dry" 
air in your tires if you are concerned about corrosion.

John
>
> >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 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
>
>
>
>--__--__--
>
>Message: 10
>Date: Fri, 15 Mar 2002 20:27:58 -0500
>From: Huw Powell <human747 at attbi.com>
>Reply-To: one at humanspeakers.com
>Organization: HUMAN Speakers
>To: cobram at juno.com
>Cc: quattro at audifans.com
>Subject: Re: Cooling/ radiator question
>
>
> > > Oil temp of 60 is way too low for a warmed up engine - it should get
> > > at least slightly over 100 (C).  Also, oil pressure of 1 bar should
> > > activate the oil pressure warning circuit, I think, and perhaps
> > > even shut the engine down.
> >
> > 60 is a normal to high temp, on both my V8Q's.  The V8Q has an 8-10qt.
> > oil capacity, an oil cooler with thermostat.
> > I think that normal operation in most engines would have a lower temp 
>for
> > the oil than the coolant.  The oil is there primarily to lubricate,
> > although on some engine designs it's cooling abilities are critical.
>
>First let me rekindle the fact that I am not Audi V8 savvy...
>
>but in general, as I learned on this list over the years, the oil temp
>should get over 100 C in order to boil off condensation.
>
>also, on all my, admittedly cylinder-starved (only 5! no V!) Audis, the
>oil is always hotter than the coolant when the engine is warm.
>
>Anyway, if 60 is a "normal" oil temp for an Audi V8, then my comments
>have been, as usual, spurious and of dubious value evne as
>entertainment...
>
>--
>Huw Powell
>
>http://www.humanspeakers.com/audi/
>
>http://www.humanthoughts.org/
>
>
>--__--__--
>
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>
>
>End of quattro Digest

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