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Re: [T3] In need of TIRES

On 25 Jun 2005 at 9:08, Everett Barnes wrote:

> > Rubber does not "dry" out, and I really don't know what the term "dry rot" 
> > means when applied to tires. Yes, they get oxidized on the surface, but 
> > the only surface that really matters is the tread, where wear exposes 
> > relatively fresh material all the time.

> It doesn't dry out?  What is the definition for window rubber that is so dry 
> that it is rock hard?

To "dry out" requires that some sort of solvent must evaporate and get lost. 
There is no such solvent in tire rubber. Plastics like vinyl contain 
plasticizers which ARE solvents which slowly evaporate over time, causing them 
to shrink and become hard. I'm not sure about our window seals. They may 
contain plasticizers, but they probably don't.

> I've had tires that had great tread but the sidewalls were cracked from age. 
> The cracks were not that visible.  They cornered poorly and the car was 
> affected by cross winds due to sidewall flex.  On one car, the rear tires 
> came apart and the bead separated from the rim at 65 mph.
> Sidewall cracking is bad.

At least on radial tires, sidewall flex is expected and is part of the radial 
design. This extra flexibility is due more to the cord design than anything 
related to the rubber. The rubber in a tire has nothing to do with the strength 
of its casing, but I certainly agree that rubber that is cracked down to the 
cord leaves the cord vulnerable to a number of insults and can lead to failure 
of the casing.

Was the tire that came apart on you a bias ply tire? Bias ply tires have stiff 
sidewalls, but in those the stiffness is due to the fact that they generally 
have more sidewall cord plies than radial tires. The rubber that spaces the 
cord plies apart plays a part in this stiffness, but only to the extent that 
this rubber layer serves as a spacer. It's not the strength of the rubber that 

If you could strip all the sidewall rubber off a tire, without damaging the 
cord underneath, this tire would drive just as well as the same carcass with 
sidewall rubber applied. The sidewall rubber is there only to protect the cord 
and has no other function. In this respect, a tire is just like a rubber brake 
line or fuel hose. The outer jacket is only there to protect the cord 
underneath, and the German fuel hose just omits the outer jacket for 
applications where external abrasion is not likely.

On tires with steel belts, the rubber serves to keep water away from the steel. 
Rusted belts is one of the modes of failure for steel belted tires.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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