As the speed of a tire increases,
its components tend to grow or deform. This deformation
leads to heat generation. This is a normal effect of
the deformation of the tire's components due to centrifugal
force. The faster the tire spins, the greater its tendency
to deform. And the more it deforms, the more heat is
generated.
But for tires driven at high speeds,
controlling heat levels is critical. Engineers must
find ways to control tire growth so that heat levels
are confined to acceptable tolerances.
To overcome this problem, speed-rated
tires are designed with advanced belt configurations
engineered to resist and control deformation. This allows
the tire to be driven at higher speeds without excessive
heat buildup.
How does the speed rating relate
to handling? The fact that a tire is speed-rated indicates
very little about its handling capabilities. The speed
rating, as mentioned earlier, is almost entirely dependent
on the tire's belt structure.
Think of the speed rating as being
like a temperature grade. It tells nothing about the
tire's construction, its handling or its wearability.
It is merely a measure of the tire's integrity at high
speeds.
It depends on what you want in a
tire. If you're looking for an ultra-high performance,
Z speed-rated tire that employs expensive KEVLAR belts
and advanced performance compounding, then, of course,
you can expect to pay more than you would for a standard
passenger radial. But for an S speed-rated tire, the
cost will generally be much closer to that of a passenger
radial.
Yes. There are several reasons you
might want to switch speed ratings:
* If you never drove to the
limits of your original equipment tires.
* If you want a different type of performance from
the tire itself.
* If high speed-rated radials are just too expensive
for you.
But before you decide to buy the
least expensive tire you can find, remember to consider
the performance of the tire, not just its speed rating.
You should still try to match the tire's handling personality
to that of your car. If you decide to move to a lower
rated tire, you'll have to reduce your vehicle's maximum
speed capability. If you're not going to drive beyond
the tire's speed limits, then there is nothing wrong
with moving to a lower speed-rated tire.
According to most states' laws,
tires are legally worn out when they have worn down
to 2/32" of remaining tread depth. To help warn
drivers that their tires have reached that point, tires
sold in North America are required to have molded indicators
called "wear bars" across their tread pattern
from their outside shoulder to inside shoulder. Wear
bars are designed to visually connect the elements of
the tire's tread pattern and warn drivers when their
tires no longer meet minimum tread depth requirements.
However, as a tire wears it is important
to realize that while its dry traction and handling
will improve…its ability to perform in rain and
snow will diminish. At 2/32" of remaining tread
depth, resistance to hydroplaning in the rain at highway
speeds has been significantly reduced and traction in
heavy snow has been virtually eliminated. If rain and
wet roads are a concern, you should consider replacing
your tires when they reach approximately 4/32"
of remaining tread depth. Since water can't be compressed,
you need enough tread depth to allow it to escape through
the tire's grooves. If the water can't escape fast enough
your vehicle's tires will be forced to hydroplane (actually
float) on top of the water, loosing traction. If snow
covered roads are a concern, you should consider replacing
your tires when they reach approximately 6/32"
of remaining tread depth to maintain good mobility.
The reason that you need more tread depth in snow is
because your tires need to compress the snow in their
grooves and release it as they roll. If there isn't
enough tread depth, the "bites" of snow your
tires can take on each revolution will be so small that
your traction will be reduced. Because tread depth is
an important element for snow traction, winter tires
start with deeper tread depths than standard all-season
or summer tires. Some winter tires even have a series
of wear bars molded in their tread pattern indicating
approximately 6/32" remaining tread depth.
Since tires affect the personality
and performance of your vehicle, all four tires should
be as identical as possible or handling problems may
arise. If your tires don't match, it is possible that
one end of your vehicle won't respond as quickly or
completely as the other, making it more difficult to
control.
If your tires have a lot of remaining
tread depth, but you need to replace just one that has
been damaged by an accident, road hazard or a vandal,
you should replace it with a tire that exactly matches
the others. Select a replacement tire of the same brand,
line, size and speed rating. While there may be a less
expensive tire available, it wouldn't be a bargain this
time because it would be different than the other three
tires on your vehicle.
If two of your tires have a lot of
remaining tread depth, but you need to replace the other
two because they were damaged or have worn out, you
should replace them with a pair of tires that come as
close as possible to matching your existing tires. While
identical new tires are desirable, others of the same
size and type can also provide good results. Only consider
selecting new tires that are from the same tire category
as your existing tires. New tires should be installed
on the rear axle. While your vehicle is being serviced
ask your mechanic why one pair of tires have worn faster
than the others.
Was it caused by a lack of tire rotation,
out-of-spec wheel alignment or loose mechanical parts?
Once the problem has been found, it can be corrected
before it damages your new tires. Keep in mind that
your ultimate goal is that all of your tires always
wear out at the same time so they can be replaced as
a set.
If all of your tires are wearing
out together, you have the greatest flexibility in tire
selection. If you were happy with the original tires,
simply replace them. If you want longer treadwear, a
smoother ride or more handling, there are probably tires
that will help you accomplish that.
A tire's first requirement is that
it must be able to carry the weight of your vehicle.
No matter how good a tire you select, if its capabilities
are "overworked" just carrying the load, it
will have little reserve capacity to help your vehicle
respond to quick emergency. So when you are in the selection
process, make certain that your new tire's size is designed
to carry the weight of your vehicle! Don't undersize.
The other size consideration is overall tire diameter.
Since many of the functions of today's vehicles are
highly computerized, maintaining accurate speed data
going into the computer assures accurate instructions
coming out. And an important part of the speed equation
is your tire's overall tire diameter. For cars and vans,
staying within a ±3% diameter change is desirable.
Pick-ups and sport utility vehicles (SUVs) are usually
engineered to handle up to a 15% oversize tire. Most
tire dimensions can be calculated. See the section below
on "How do I calculate tire dimensions?"
While at first a ±3% diameter
increase or reduction in tire diameter may sound very
limiting, in most cases it allows approximately a ±3/4"
diameter change. Additionally to help with the selection
of substitute sizes, a system called "Plus Sizing"
was developed. Use Plus Sizing to take into account
the diameters of the available tires and the wheels,
and then helps select the appropriate tire width that
ensures adequate load capacity. Maintaining the tire's
overall diameter helps maintain accurate speed data
going into the computer.
Example: 185/60R14 85H or
185/60HR14
The first number is the width of
the tire in millimeters, measured from sidewall to sidewall.
To convert to inches, divide by 25.4 in the example
above, the width is 185mm or 7.28".
The second number is the aspect ratio.
This is a ratio of sidewall height to width. In the
example above, the tire is 7.28" wide, multiply
that by the aspect ratio to find the height of one sidewall.
In this case, 185x0.60 =111mm or 7.28"x0.60=4.36".
The last number is the diameter of
the wheel in inches. To figure the outside diameter
of a tire, take the sidewall height and multiply by
2,(remember that the diameter is made up of 2 sidewalls,
the one above the wheel, and the one below the wheel)
and add the diameter of the wheel to get your answer.
185mm x .60 = 111mm x 2 = 222mm + 355.6mm (14")
= 577.6mm or 22.74"
Do you drive your car only in sunshine,
or also through rain and snow? Do you drive your light
truck on the road, off the road, or are you the one
responsible for clearing the land to build the roads?
To successfully meet each of these driving conditions
requires a different type of tire. Ask yourself these
questions to determine which performance category you
should choose from: If you use more than one set of
tires and wheels (for example, summer tires in summer
and snow tires in winter), you can select tires that
exactly meet your diverse needs. If you use one set
of tires for every season you may get good performance
under many conditions, but you will compromise your
vehicle's performance when the conditions are at their
worst. So the important thing to do is to select your
tires so that they match the worst driving condition
you expect to encounter. When you're stuck in the snow
or in the mud because your tires don't have the appropriate
capabilities, you'll curse their limited performance
in your worst driving condition...and you'll quickly
forget how smooth and quiet they were at other times!
If you only drive around your neighborhood and a "long
trip"is one that's just down to the corner convenience
mart, almost any tire will do. But if you drive your
vehicle on congested city streets and expressways during
rush hour you will be better served by more responsive
tires.
If you drive extensively on the interstates
you will want quiet, smooth riding, long wearing tires.
Or if you like to drive quickly on twisting roads or
through the mountains you will want good handling tires.
And if you drive on the track or in autocross events,
you will want the best competition tires available.
If your worst driving conditions and your typical conditions
are similar, one set of tires will be all you need.
If you live at the edge of the snowbelt and infrequently
get snow you may want to select an all-season tire.
If your SUV is used as the family's station wagon and
driven on the road all of the time, overly aggressive
light truck tires aren't for you (unless you really
like the "look").
If your worst driving condition occurs
frequently (you drive through snow all winter) and is
dissimilar to your typical driving condition (you commute
to work on the expressway during the week and spend
your weekends at the beach), you may want to consider
selectingtwo sets of tires for your vehicle. Each set
will be designed to master the specific conditions without
compromising your driving satisfaction at the extremes.
While purchasing two sets of tires may appear expensive,
the set you're not using won't wear while you are using
the other set, and combined they'll provide longer total
wear than either set could individually!
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