Request for Question Clarification by
aht-ga
on
19 Mar 2004 11:17 PST
The information on the Tire Rack website is targeted towards 'regular'
drivers, and road-racing competition. For competition purposes, often
there is a different tire pressure used in each of the four tires.
This is done for performance tuning purposes. As an example,
oval-track racing is very popular in North America (personally, I'm
not a big fan of cars going round and round a big oval for several
hundred laps, I prefer road races). When setting up a car for an oval
track, where the weight of the car will be borne mostly by the tires
on one side of the car, a left-right pressure differential will help
control how the car behaves while rolling, while a front-back
differential will determine how the car trades off between steering
and rear-wheel traction. If you think of the tire as an extension of
the suspension at each corner of the car, higher air pressure will
result in a stiffer suspension in that corner, lower air pressure will
result in a softer suspension in that corner.
For the Alpina, which is tuned for even-more-spirited driving than the
'regular' Z8, the higher air pressure up front, in concert with the
changes to the suspension system, is intended to provide better
steering response at the cost of some ride comfort (it is stiffer,
after all). Unless you also change out your suspension components,
though, you will not necessarily benefit from mirroring the Alpina's
recommended tire pressures.
Your best course of action, is to start out with cold inflation
pressures of 35 psi on all four corners. Break in the tires with a
little trip into the country, finding yourself a nice set of twisty
corners to see how the car rolls into turns.
Then, after the tires have cooled overnight, raise the front pressures
up about 5-10%, say to 38 psi. Only raise the front, as raising the
rear beyond 35 will most likely cost you traction (more on this
later). Then, tackle the same route again. If you perceive a
difference, stick with it. If not, then just lower it back to 35 psi
as the better compromise between performance on the road, and ride
comfort.
You were also wondering about my comment regarding the fact that it is
the *compressed* air inside the tire that holds up the car. There is
an important difference between saying "compressed air" and just
"air". Air, as a gas, is highly compressible. The pressure at which
the air is compressed reflects how much pressure that air exerts on
everything around it. The higher the pressure, the harder it is to
squish the tire, meaning that the tire will find its shape (for a
given pressure), and hold that shape against a higher amount of force
(such as when your car is cornering, and more of the weight transfers
to the tire). By holding its form, the tire's contact patch
experiences the least amount of deformation, maintaining your grip on
the road.
Technically, for a given vehicle weight, when you go to a larger/wider
tire with a larger/wider/longer contact patch, the tire pressure can
actually go down a bit. This is because the weight of the car is now
being transferred to the ground over a greater area (the larger
contact patches). Since 'psi' stands for 'pounds per square inch', if
your contact area goes up, the pressure required to bear the same
weight will go down. The difference is small enough to be not worth
calculating, though. As well, by maintaining the same or higher tire
pressure as was used in the smaller tire, the car benefits from a more
stable contact patch and the tires are even less susceptible to
deformation under load.
The advantage to using a tire sized for a larger wheel, especially a
low profile tire, is that there may actually be less air volume inside
the tire; that means that the pressure in the tire will increase
faster under load than with a tire that holds more volume. Again, this
helps when cornering, its like equipping your car with stiffer
variable rate springs. That said, you do want your tires to retain the
ability to absorb the small bumps and dips in the road surface, or
else you will lose traction every time the tire loses contact with the
road. So, you cannot increase the pressure too high. That's why
top-fuel dragsters (another uniquely North American phenomenon) run
low-pressure tires on the rear; losing traction in one of those beasts
can be career/life ending.
So, in order to help me understand how I can best help you, can you
please clarify what you would see as an acceptable answer to your
question? The exact tire pressures will be a uniquely personal thing
for your specific situation, best determined (as mentioned above) by
starting at 35 psi, then road-testing, followed by repeating the
road-test the next day with the front tires at a higher pressure, then
determining whether you like it.
Regards,
aht-ga
Google Answers Researcher
