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 selecting two 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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