[Healeys] Tire Recommendations? Yokohama

[Dave & M]

Hi Terry,

There is little doubt that the tire aging process starts as soon as
they are manufactured & is "the start of a clock".  Tread hardening, & 
tread
separation are the most common problems associated with aging. ---
This hardening is actually a slower extension of the original
vulcanizing process which is used to harden the soft green rubber to
it's intended hardness when it is manufactured. It never stops.

How long it takes to harden a tire to unsatisfactory traction or
safety levels, depends on temperature, amount of ozone exposure, &
amount of ultraviolet exposure, both during storage & during use.  As
Ed said, tires age even while on the shelf.

On the tread separation/safety issue, several European car
manufacturers recommend a tire age limit of around six years, & Ford &
Chrysler have recently adopted a maximum tire age recommendation of
around six years.

I  had to temporarily reinstall my 10 year old tires which measure 80
on a "Shore A Scale" durometer. Another  set of  relatively new
Michelins, at the time,  measured 60 on the same scale. The loss of
traction when cornering & on wet roads was very obvious with the
harder tires.

To put the Shore A Scale in perspective, my thumb nail measures 97, the
underside of my thumb measures 50. Some very soft "race only" tires
can have a "Shore A" hardness of 30 to 40.

In summary, I believe that measuring the tires changes in hardness
over time is a good way to determine if the tire has aged to
unacceptable levels where tread separation may become a problem. If
you do not monitor tread hardness changes, the six year rule is a safe
compromise.  An increase of 25% hardness from original would likely be a
cause for serious concern.

Regards,
Dave Russell


TERRY COLL wrote:
> I'm still wrestling with this 5 or 10 year tire limitation.  What exactly
> happens to the tires and what exactly causes it?  Does it apply if the tires
> have been stored inside and not exposed to sunlight?
> 
> Terry Coll  '64 BJ8