Date: Fri, 21 Oct 94 14:25:53 CDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: "J. Walker" <JWALKER@ua1vm.ua.edu>
Subject: some stuff about tires ...
Trailer Life, November 1994, page 60.
Technical:
An RV'ers Guide to TIRES
Part I
by Rich Johnson
Behind every tire dealer's shop, you'll find a huge pile of old dead
tires just waiting for the recylcing truck to haul them away. These
came off cars and trucks, trailers and motorhomes. If they could talk,
these tires would tell stories about the places they have traveled and,
all too often, the abuse and neglect they have suffered.
Tire reliability and longevity are directly related to two key factors:
application and maintenance. By selecting the proper tires for your
vehicle and specific driving needs, and by providing proper maintenance,
you can greatly reduce the likelihood of tire problems. A failure in
either of these areas can quickly bring you to a halt, interrupting the
enjoyment of a vacation, adding a lot of extra expense to the trip
and/or just being a nuisance. It behooves us to understand how to
protect our tires and reduce wear.
USEFUL LIFE
How long can tires be expected to last? There is no concrete answer.
A lot depends upon initial tire quality, driving habits and frequency,
maintenance and unusual circumstances that result in unintentional
abuse. Rubber compounds used in modern tires are formulated for
extended wear, but if the tire is out of balance or the wheels are out
of alignment, tires still wear out quickly.
Some tires, such as those especially made for trailers, are made with
compounds that are more resistant to sunlight and ozone, which cause
cracking. However, at least one tire manufacturer recommends that
tires be replaced at intervals no greater than seven years (Part 2 of
this article will cover ozone damage in detail.)
Tread wear also offers an indication of when it's time to go shopping.
Small blocks between the tread ribs of some tires are known as tread-
wear indicators. The Tire and Rim Association recommends that tires
on vehicles with a gross vehicle weight rating (gvwr) greater than
10,000 pounds be replaced when less than 4/32 inch of tread depth
remains (not including tread-wear indicators). On vehicles with less
than 10,000 pounds gross weight, front tires should be replaced when
worn down to the tread-wear indicators or when 2/32 inch or less of
tread remains. A tread-depth gauge (available at auto-parts stores)
can be used to measure exactly how much tread remains.
Tires vary widely in design as well as materials. We usually think of
them as made of rubber, but natural rubber is a realtively minor
ingredient. The basic building blocks of the rubber compound are
polymers, combinations of natural rubber and two types of synthetic
rubber. Another building block is carbon black, a reinforcing compound
that provides strength. Then there are oils, used as softeners to
provide traction. A high-performance tire has more oils, helping it
stick to the road, but will wear out faster. Sulfur is used as a
cross-linking agent to keep other components together. And anti-
oxidants and antiozodants are used for protection.
TYPES OF TIRES
Most tires in use today on passenger cars are designated with the
letter P in front of a set of numbers (for example, P225/75R15). This
system replaced the alpha-numeric system (for example, GR78-15).
P-metric system identification offers this information: P225/75R15 LRC.
This is what it means:
P = passenger-car tire
225 = cross-section width in millimeters
75 = aspect ratio in percent
R = radial-ply design
15 = bead-seat diameter in inches <rim size>
LRC = Load Range C
The aspect ratio is the proportion of a tire's cross section height
(from the tread to the bead seat) compared to the cross-section width
(side to side). The height is 75 percent of the width of a tire that
has an aspect ratio of 75.
Molded into all tire sidewalls are load-rating numbers. P-metric tires
are rated as either STANDARD LOAD or EXTRA LOAD. Standard load tires
are limited to the weight that can be carried with a maximum inflation
pressure of 35 psi. Extra load tires are limited to the amount of
weight that can be carried with a maximum inflation pressure of 41 psi.
Extra load tires are designated with "XL" on the sidewall, while a
standard load tire has no special designations. The maximum inflation
pressures and maximum load ratings listed on P-metric tires should not
be exceeded.
Metric and standard methods of identification are in use for light-
truck tires, but the load/inflation identification molded into the
sidewall is different from P-metric car tires. This is a common
metric size for light-truck tires: LT235/75R16 LRD. That means:
LT = light truck
235 = section width in millimeters
75 = aspect ratio
R = radial-ply design
16 = bead-seat diameter in inches
LRD = Load Range D
This tire may be offered in various load ranges, e.g. C, D, E and F,
as indicated in accompanying load-inflation tables.
The metric identification replaced standard identification, but it is
still in use; for example, 750R16LT, which means:
7.50 = section width in inches
R = radial-ply design
16 = bead-seat diameter in inches
LT = light truck
These tires also are available in various load ranges, as indicated in
the tables opposite.
While on the subject of tire sizes, it's important to note that tires
and rims are avilable in both 16-inch and 16.5 inch sizes. NEVER try
to mount a tire on a rim that does not match exactly in size.
Yet another tire is the flotation type, which is a special category
under the truck-tire classification. It is distinguished by a set of
numbers beginning with the tire's overall diameter in inches; for
example, 31x11.50R15LT, which means:
31 = overall diameter in inches
11.50 = section width in inches
R = radial
15 = bead-seat diameter in inches
LT = light truck
It's important to note that load and inflation identification molded
on light-truck tire sidewalls is different from those of car tires,
in that the psi number molded on the sidewall of an LT tire is the
MINIMUM PRESSURE FOR THE MAXIMUM LOAD. This is in contrast with
car-tire pressure and load-rating numbers, both of which are MAXIMUM
ratings.
The truck tire LOAD RATING should not be exceeded, but the inflation
pressure can be increased by 10 psi beyond the number on the sidewall,
according to the Tire and Rim Association (TRA), to which all the major
tire manufacturers belong. The addition 10 psi helps keep the tire
cool while carrying its rated maximum load at highway speeds. It's
important to note that the additional 10 psi DOES NOT increase the
carrying capacity of the tire.
Still another category is SPECIAL TRAILER TIRES, identified on their
sidewalls with the letters ST. They are usually P-metric type, but
are heavy-duty versions available in B and C load ranges, and differ
in that they are compounded for extra resistance to ozone and sunlight
deterioration. It is recognized by tire manufacturers that trailer
tires don't travel as many miles per year as passenger-car or light-
truck tires, so they aren't likely to wear out as quickly.
ST tires are preferred when mileage is limited, and checking or
cracking usually determines replacement need rather than tread wear.
Aside from that, P-metric or LT tires can be used on trailers,
providing the load range is adequate for the weight and the wheels are
suitable.
Radial-ply tires, such as Goodyear's Marathon Radial, have been
successfully used on trailers for many years even though questions are
still raised about the possibility that radials contribute to trailer
sway. Radials provide excellent performance on trailers.
Load and pressure ratings for ST tires should be used per se, as
indicated on the tire sidewall; the TRA recommends reducing a
passenger-car tire's rating by 9 percent when used in trailer service
to help ensure against overloading.
WEIGHT VS DURABILITY
It is not beyond the realm of possibility that tire failures can
sometimes result from a manufacturing defect, but the usual causes are
overloading and/or underinflation and road-hazard abuse. Little can
be done to predict or prevent failure due to manufacturing defect, but
a lot can be done to keep the failure from being YOUR fault. If you
have a tire failure you believed can be traced to a manufacturing
defect, return it to the dealer for a close inspection to determine the
cause.
It does little good for us to give a lot of tender love and care to a
tire that is overloaded. Unfortunately, in the world of RV's,
overload is all too common. Studies by A'Weigh We Go, a private firm
that weighs RVs at rallies, determined that as many as 70 percent
exceed the rating of at least one suspension component. A Michelin
study found that nearly 60 percent of motorhomes had at least one tire
overloaded.
It would be a comfort if we could assume that all RVs have ample tire
capacity for the load they are reasonably expected to carry, but such
is not the case. In order to offer as many features as possible,
manufacturers often do not allow enough payload capacity, which is the
amount of weight available for passengers, supplies and fluids. Even
when capacity is adequate, some owners add way too much weight. All
vehicles have weight limits; the owner must know what the limit is
and live within it, or compromise safety.
To make sure original or replacement tires will happily carry the load,
it is necessary to know the weight the tires are carrying. To do this,
take the vehilce to a commercial truck scale and have it weighed. This
should be done when the vehicle is totally loaded for travel, complete
with fuel, water, propane, supplies, passengers, and whatever else you
plan to carry along on a trip.
Weight the vehicle axle by axle and side by side. Although most
platform scales are state-certified, a margin of error still exists.
Compare all the axle weights and the side weights with a gross weight
reading to determine the margin of error. Errors will result if the
vehicle is not level on the scale. If a scale has unlevel aprons,
position the vehicle so the wheels that are not being weighed are only
an inch or so off the platform, to permit level vehicle attitude to be
accomplished.
If the RV rig is a tow vehicle and a trailer, position the trailer axles
on the scale with the tow vehicle off the scale for readings on the
weight carried by the trailer tires. Divide the total weight by the
number of tires. This will give only an approximate weight per tire
because it assumes equal weight distribution.
The best method is to use individual wheel scales, for more accurate
readings. Unfortunately, such scales are not easy to find.
Make sure the tires are rated to carry at least as much weight as their
ratings indicate and preferably about 10 percent more for a safety
margin.
------------------------------------------------------
The next installment of this two-part article will cover inflation,
road hazards, visual methods of determining causes of tire damage
and how to know if sidewall cracking has progressed far enough to
justify replacement of the tire.
---------------------------------------------------------------
Tire and Rim Association Ratings
Tire Load Limits (lbs) at Various Minimum Cold-Inflation Pressures (PSI)
Tire Size PSI> 35 40 45 50 55 60 65 70
LT215/85*16 1495 1640 1785 1940(C) 2050 2180 2335(D) 2430
LT225/75R16 1500 1650 1790 1940(C) 2060 2190 2335(D) 2440
LT235/85*16 1700 1870 2030 2205(C) 2335 2485 2623(D) 2765
LT245/75*16 1700 1865 2030 2205(C) 2335 2480 2623(D) 2765
7.50*16 LT 1620 1770 1930 2060(C) 2190 2310 2440(D) 2560
* Indicates position where R (radial-ply) or B (bias-ply) designation
will appear ... i.e., LT215/85R16.
# PSI shown is for Radial Ply tires. Diagonal (Bias) Ply tires should
use 5 psi LESS than the pressures shown.
Letters in parentheses (C) indicate load ranges C through G.
-----------------------------------------------------------------------
joel's comments:
i didn't type the whole Load Limit table, as the tire sizes and weights
went way off the scale for vw buses, even (i think) the winnebago-vw
new camper. and i left off all the Dual-wheel pressures and loads.
i also don't understand what the heck he meant by that very last
sentence in the artcle (about the tires being rated to carry their
rating).
