Hi Andrew,
Factory tire compounds vary considerably in the beginning, & also harden
a lot with age. The harder the tread the less traction. When tires are
made, the vulcanizing process cures, hardens, the soft rubber. This
process continues at a slower rate throughout the life of the tire due
to heat & exposure to ultraviolet light & ozone. Tires that sit on a
dealers shelf for years, that are stored in a hot location, exposed to
sunlight, near ozone generating devices, (anything that makes sparks)
may have hardened to less than half of their useful life before ever
being installed on a car.
It's not hard to tell if a tire is age hardened or just plain was made
that way. You use a tire durometer. A device that measures the tread
hardness on the "Shore A" scale. You can see one here. Durometers are
found in the tool boxes of most serious racers.
http://www.hrpworld.com/index.cfm?form_prod_id=815,363_3955&action=product
For reference, a fairly sticky street tire may measure somewhere around
60. After five years of exposure to heat, UV light from the sun, &
atmospheric ozone, it may measure 85 or 90. This represents a large loss
of traction. Tires designed for high mileage as a primary consideration
may measure 85 or 90 from the start. Sticky, competition only, tires
could run from 30 to 50.
Another effect of age hardening is that the tire may become brittle &
experience tread separation. Thus the "safety" discussiouns about
maximum tire life limits.
When you go to buy tires, check the durometer readings & look for
something in the range of 60.
Regards,
Dave Russell
Andrew K wrote:
> Hi Michael,
>
> That is an excellent point. The Avons I bought for the other car must
> have been fresh because the rubber is very soft and the adhesion to
> the road is spectacular. It breaks away very smoothly and
> predictably when you exceed it's limits in a corner. I have been
> looking at XAS at Coker or Longstone (UK) and perhaps I need to check
> on the manufacture date for the stock that they carry? Old stock
> could explain some peoples experiences with either tire.
>
> Magnus indicated that his Avons were much harder than his XAS, but I
> don't know if he meant the rubber or the ride. IF the XAS is at
> least as soft as the Avons I purchased, I imagine they will be a
> fantastic tire, as they appear to be a much more advanced design and
> during the sixties and early seventies were chosen by many
> manufacturers for their top performance cars. IF however they have
> been sitting for five years, they may be hard and all bets are off.
>
> This is all conjecture at this point of course. If only there was a
> place you could go and spend the day trying different tires on your
> car. Or wouldn't it be cool if someone tried them all and tested
> them the way one of the car mags would, for adhesion, stability etc.
>
> Not likely to happen though, so I will surely be basing my decision
> on anecdotal evidence and conjecture. Ahhh, but I have made
> decisions based on less than that!
>
> -- Andrew
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