Aluminium wheels-slow air leak.

[john clarke]

You are absolutely right--gave it a bit of thought over lunch and came up
with the same answer.  Condensation is a much more likely avenue especially
in cold climates like mine--the difference in heat capacity between rubber
and aluminium is quite significant (don't ask what they are, Phys Chem was a
long time ago).  We agree on the source of the moisture as well--either
relative humidity, or accumulated water in the compressor tank,
whatever---air pumped into a tire, unless it's filtered, is going to have
water vapour in it.  Not only that, since the atmosphere contains carbon
dioxide, the moisture is going to be slightly acidic as well.

As for the centrifugal force being tangential to the radial motion---great,
if you don't plan on turning :-)

John

PS  One of my tires was down to 22psi lunchtime--trying to figure this out
has angered the Audi Gods :-(

>From: "j rice" <ricematthews3 at hotmail.com>
>To: johnclarke303 at hotmail.com, bradym at sympatico.ca
>Subject: Re: Aluminium wheels-slow air leak.
>Date: Wed, 13 Mar 2002 17:56:31 +0000
>
>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
>contact with the rim isn't part of the equation.  It is possible that some
>moisture could get trapped in the tight crease between the tire and rim,
>however I would propose another cause of the corrosion......I would bet
>that any corrosion is merely the result of humidity (micro droplets) in the
>air that was used to fill the tire.  This humidity probably becomes greater
>as any standing moisture vaporizes when the tire heats up to operating
>temperatures.  I'm sure vapor pressures play into it, but thermodynamics
>wasn't really my specialty.  This is all my theory and is also the most I
>have thought about the subject as I have never experienced this problem on
>any car I have owned.  On a side note, many older British cars came through
>with "tube type" tires.  Reportedly, quality control on their rims wasn't
>the best and rim leaks were common.
>-Mark Rice
>
>**********************************************************
>Message: 8
>From: "john clarke" <johnclarke303 at hotmail.com>
>To: bradym at sympatico.ca
>Cc: quattro at audifans.com
>Subject: Re: Aluminium wheels-slow air leak.
>Date: Wed, 13 Mar 2002 12:34:14 -0330
>
>I meant as you are driving, the centrifugal force will fling the condensed
>moisture to the wheel rim---gravity will suck it to the bottom lip of the
>rim when you are parked, there probably is only small droplets so there
>isn't enough mass to overcome the surface tension and/or static attraction
>to the lip to make it fall over the edge into the tire.  Even if it does,
>the wheel surface will have been wetted enought to cause
>corrosion--actually, come to think of it, the join between the tire and the
>wheel forms a nice little groove for moisture to collect in.  This with the
>static charge from rubber rubbing on asphalt, the aluminium in contact with
>the steel hub, produces a weak electrochemical cell which will hasten the
>corrosion.
>
>Just my theory--this morning is the most thought I've ever given to it.
>
>John
>
>
>>From: "Brady Moffatt" <bradym at sympatico.ca>
>>To: "john clarke" <johnclarke303 at hotmail.com>, <quattro at audifans.com>
>>Subject: Re: Aluminium wheels-slow air leak.
>>Date: Wed, 13 Mar 2002 10:31:49 -0500
>>
>>Umm, isn't that force called gravity, and wouldn't the water pool in the
>>bottom of the tire when the car is parked.
>>
>>Cheers,
>>Brady Moffatt
>>Montreal, Quebec, Canada
>>86 4ksq, 282,000km
>>72 Datsun 240Z, 180,000 miles
>>
>>----- Original Message -----
>>From: "john clarke" <johnclarke303 at hotmail.com>
>>
>> > I guess I should have added that it's the moisture in the compressed
>>air
>> > that causes the wheels to corrode--the centrifugal force means any
>>condensed
>> > water will migrate to the wheel rim and pool there when the car is
>>parked.
>>
>>
>




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