[ExI] The Magpie Whisperer

spike at rainier66.com spike at rainier66.com
Tue Jun 22 17:18:09 UTC 2021



From: spike at rainier66.com <spike at rainier66.com> 


>…2: Used tires are an oddball exception to Billw’s rule (because they are more complex than you think (even if you apply and compensate for Billw’s rule.))   spike



How much do you know about used tires?  How complex are they?  I have a theory that they are more complex than either of us think they are.


For instance… if one is a thermoplastics fan or just find this sorta thing cool for any reason (I do) one has looked at the manufacturing of tires.  If you go out and look at your car, notice those little whisker thingies on the sidewalls.  Ever wonder what those are?


Tires are made by injection molding.  Liquid rubber is forced into the mold, so those are holes in the mold made for letting the air out as the rubber comes in.  Some of the rubber goes into the vent holes.  The whisker part that stays with the tire is called a vent sprues.


If one buys used tires, vent sprues are your friends, because they are talkative.  One can tell some things about a used tire if you examine those sprues.


I won’t go into all the sprue clues (because used tires are too complex, even after we apply and compensate for Billw’s rule) but I will just mention this one cool thing.


After the liquid rubber goes into the mold, it is heated and undergoes a kind of one-way chemical reaction called vulcanization, which is analogous to cooking egg where the egg undergoes the Maillard reaction.  In the tire and the egg, you can’t melt it back into liquid with heat.


Tire goes onto car or motorcycle or RV (this last one is included for a reason.)  Over time, vulcanized rubber exposed to air and ultraviolet, the vulcanized rubber oxidizes and becomes brittle.  When the vehicle is rolling, the centrifugal force and turbulent air whip those sprues around.  After the tire is a few years old, the embrittled sprues break off.


OK cool, so no mystery, but notice the difference between the sprues on the inside of the tire vs the outside.  If the tire has remaining sprues on the inside but not the outside, then one can estimate the age of the tire if the car has been exposed to sunlight.  Reasoning: the outboard sprues are more exposed to UV which oxidizes the rubber and they break off first.  The outside of the tire might also be exposed to more turbulence, because the undercarriage of the car drags air along (that part I don’t know much about, so it is speculation.)


Take a used tire from a car, compare the sprues one side to the other, and create a ratio of some kind.  Then repeat experiment with a front tire from a motorcycle, where (over time) both sides of the tire see similar UV and turbulence.  Unless the bike is parked outdoors facing east or west consistently, the sprues should be about even, so sprue ratio should be about 1, but the ratio on the car tire should be easily noticeable and not 1.


OK, suppose you have an old ratty ride and you want to score on some cheap used tires.  If two choices have about the same amount of tread left, one would want the newer tire because the rubber is less oxidized.  A prole can tell which is the younger tire by close examination of the mold vent sprues.


There’s a point to all this, which I will reveal if anyone is still reading about something as apparently simple as a used tire (which isn’t simple at all) and asks what is the point of all this, because it has everything to do with used RVs.












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