This thread reminds me of a similar thread on another forum. There was a discussion going about softening up old tires by soaking them in various substances. One reader is a chemist and he wrote this (quoted here with his permission):
Please read through this post. I am a joe college degreed
chemist, and have learned a thing or two about polymers through
industry.
1) Polymer aging.
Rubber is a form of a polymer. Think very long chained molecules,
branches, and cross linking (branches connecting up again). As
polymers age, long chained molecules get broken (chain scission). The
matrix (mix of various rubber types) loses its mechanical properties
(durometer or hardness, elasticity or bounce, tensile strength or
stress/strain properties). The older tire becomes more brittle, less
flexible, and possibly more prone to crumbling. Different polymers
have different physical properties changing.
2) Adding oils to polymers. Simply put like dissolves like. Allow me
to whip this horse a bit:
Would you use oil based lube on a condom?
http://blip.tv/file/216321(pick your format, and endure the diary chatter, the demo is in the
last 2 minutes of the clip).
When you buy a new tire, common wisdom states that you should not
instantly drive aggressively. Why? Because the outer surface of the
tire is impregnated with a silicone release agent. The silicone is in
the first few mils of the tire, and it typicaly gets scrubbed off in
the first hundred miles of driving.
Petroleum based oils (and mineral oil based WD-40 is in this group,
IMO) will soften the matrix of a tire like crisco will soften up a
latex condom.
Juice up an old tire with WD-40, and what do you get? A tire
impregnated with its own grease slick, deep into the old and brittle
tread. Welcome to the retread from Hell, pray for absolutely no water
out there!
3) But it makes it more soft! Sure it does, welcome to step one of
your tires going into solution like sugar into a hot latte.
Here's an other analogy. The rubber in a new tire has complete
polymers. The cross linking, or vulcanizing, is optimized.
Everything with the new tire is fresh, much like a grain of uncooked
rice. Now, age that tire for five years. The matrix breaks down,
long chains become shorter chains, flexibility begets brittleness, and
so on. Our grain of rice is more like a rice crispy. Let's wet
things up: petroleum based oils for the rubber, and water based milk
for the rice. The organic molecules start to enter their solution as
the starches get softened up by the milk. Would you want to depend
your adhesion to the tarmac by the solid matrix, or the one that is
full of holes, and softened further by solvents?
One more thing about hand feel versus road performance:
to truly test if a juiced tire is either better than the old tire will
require a reproducable test. Hand softness is a useless test, but
tensile strength, yield to plastic deformation (bend to where you stop
snapping back to your original shape), adhesion, and wear are
important. Clearly, the only way to prove these arguments out for
sure is in a controlled environment like a mechanical testing
laboratory. This is speculation, but a dried out old tire plus juice
will yield a softened and weak bit of rubber that will wear super fast
if not fly apart from the cord.
Again, In My Opinion, juicing plastics is better left to making the
inside of a car in Arizona look more like new, than trusting your life
on top of (remember that rice crispy).
4) Brave versus crazy.
Riding a 100cc bike to TWALD has been referred to as "something he did
not know you shouldn't do". That makes me laugh. I was not crazy,
because I sorted through my ride before the trip. I consulted with
experts, and was not afraid to ask ridiculous questions. I also
studied everything I could get my hands on. I also continue to learn
and add this to the sport.
Softening up a tire that is too old to ride is crazy. The idea is
based on conjecture, and machismo. I can relate to the sense of
defiant pride when peers look upon your efforts with denial and
disbelief. But using dogma and non science as your foundation is
reckless.
5) Absolute Liberty.
I cannot save you from yourself. It is my professional and personal
opinion that juicing a tire that has past its functional age is
Russian VinMoto Roulette. However, it's your bike, it's your life.
You are ultimately responsible if you mess with this stuff.
