Jeff McNeal wrote:
> soon. Well, 15 pounds of pressure in the tire! I went around the car,
> filling each one up to the suggested 36 pound maximum and took the car for
> another spin around the block. Not only is the scraping gone (duh!), I
> almost felt like I had power steering! The ride quality and handling both
> were dramatically improved. Just a reminder not to overlook the obvious
> things -- sometimes they can make a HUGE difference in comfort, handling and
> safety.
Others have spoken on this matter, and most are correct, given the
conditions they cite. Handling is improved with higher pressures for
very hard driving--this is true. It is also true that tire wear and
overall life is maximized by adjusting the pressure to the loading and
driving style.
In general, radial tires are a bit more forgiving of higher and lower
inflations. The flexure of the sidewalls tends to keep the tire patch
more consistent over a wider range of inflation when compared to a
bias-ply tire.
That said, no tire will produce maximum performance (tire life vs. wear)
at either extreme of inflation. It's a simple matter of optimums.
Pumping up the tire to the absolute maximums when the car is driven
gently and the loads are light will eventually cause wear in the center
of the tread. Nor will the tire perform adequately at standard pressures
when the loads are high, and cornering forces are extreme. Tire
pressures are variable for a reason. The maximum pressure rating for the
tire is designed to provide the driver with a safe limit for inflation
when the tire is heavily loaded and driving demands are high, nothing
more, nothing less.
Nevertheless, radial tires can survive for a very long time with
considerable abuse regarding tire pressures. My favorite anecdote in
this regard was an old fellow arriving in the shop with a two-year-old
Tercel, which he used for business, and had accumulated many, many miles
in that couple of years. He complained, "the ride's hard, and those
tires are just plain junk... they haven't lasted nearly long enough." A
look at the odometer revealed that the car had gone some 78,000 miles in
that time. The tires were bald. I put a tire pressure gauge on each
corner, and discovered that the inflation was, within a few psi, about
90 psi on every tire. Hmmm. Did he pump them up to that rate himself?
Nope. The answer lay in how the cars were shipped from Japan.
Toyota owned its own fleet of ships for transporting cars. They were,
and are, a veritable design marvel, made to carry cars to the US in
maximum capacity, then transport foodstuffs from the US back to Japan.
The decks of the ships were hydraulically adjustable for this reason. To
maximize the number of cars which the ship could carry, the lower deck
was filled with cars, the cars were chained down, and then the tires
were pumped up to ninety psi to lock the car in place. Then, the next
deck was lowered hydraulically to within an inch or two of the roofs of
those cars and filled with the next layer.
The tires of each car were supposed to be relieved of that pressure when
they arrived, and this fellow just happened to get a car that someone
had forgotten to relieve the pressure. The ride was hard, because the
tires were inflated to four times normal pressure. But, still, they wore
reasonably well, although they were desperately thin in the center. We
said to this fellow, "umm, almost 80,000 miles is pretty good mileage,
overall. What do think about a new set of tires?" And, he reluctantly
agreed and went away happy.
As for your tire pressure question, obviously, 15 psi is too low for
decent performance. I would suggest setting the pressures to that of the
factory recommendations, to within 2 psi all around. Drive the car and
assess the performance. If it's still not quite what you expect, raise
the pressures by 2 psi all around, and drive the car again, in the same
way. Chances are the performance will pretty close to what you expected.
If not, go up another 2 psi and try again. Simply raising the pressure
to the maximum allowable may not give you the best wear, although it
will certainly raise your maximum cornering ability. If you drive the
car more or less normally most of the time, you'll get the best
combination of wear and cornering ability at some figure close to the
manufacturer's rating. Remember, too, that very high pressures will
limit tire performance in wet and icy conditions.
Cheers.
--
Michael D. Porter
Roswell, NM
mailto: mporter@zianet.com
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
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