Submitted
By
Rob Cogswell (President, NSJC
- 2006/2007)
Information from www.intercotire.com
Conditions
Affecting Tire Mileage
There
is always a trade off between performance and tread
life in the development of tires. To gain one thing
you have to trade off something else. Some brag
when they get 15000 miles out of a set of tires
because they understand how hard they drive while
others complain if they get 40000 to 50000 miles.
The next paragraph explains many things someone
can do to improve mileage regardless of how they
drive, the type of tire, or the power of their truck.
Tread
life is related to many factors some of which are;
the air pressure maintained, how often air pressure
is checked, rotation, driving habits, road surfaces,
road conditions, suspension setup, caster and tow
alignment, shock absorbers, etc. There is a relationship
between the size tire used and the weight of the
truck. The larger the tire and the lighter the truck
the better the mileage tends to be and the smaller
the tire and the heavier the truck the less the
tread life will be. One of the worst mistakes made
is neglect of air pressure. Air pressure should
be checked at least once a month with an accurate
gauge and checked when the tires are cool. Two things
support the weight of a vehicle...the actual air
pressure and the volume of air in a tire. When a
big tire is used the larger volume of air will support
the vehicle with very little air pressure but that
does not mean the tire is stable. The tire will
look like it is under inflated because it does not
appear to be low but this low air pressure makes
the tire unstable which causes squirming of the
tread. It is OK to run tires low off road but the
pressure should be bought back up as soon as you
put the vehicle back on the highway.
The
best way to figure out what pressure to run in your
tires is to take a tire chalk and put a straight
line across the tread. Drive a short distance and
check the line. If it has wore off in the center
of the tire then you have too much air. If it is
wore off on the edges of the tread then you do not
have enough pressure. The line should wear evenly
across the entire surface of the tread.
It
is recommended that tires used on four-wheel drive
vehicles be rotated front to rear and rear to front
about every four to five thousand miles...don't
criss-cross.
As
you can see from the above there is no way of knowing
what tread life a given set of tires will have.
The compounds used in the tread rubber also vary
but is constant with each type of tire manufactured.
Manufacturers try to achieve a balance in tread
compounds that will provide good highway traction
and overall performance while not sacrificing too
much in tread life.
Causes
of Tire Wear
ROAD
SURFACES
Tires
provide traction because the weight of the vehicle
forces the tread rubber into the road's small surface
irregularities. Unfortunately, that same interaction
causes wear as the surface tears microscopic pieces
of rubber from the tread as it slips into and out
of contact with the road. And just as coarser sandpaper
removes material faster, coarser road textures causes
more tire wear. The following mileage estimate is
typical of what a 40000 mile rated tire driven on
smother asphalt would deliver if driven exclusively
on other road surfaces.
|
Surface
Type
|
|
Miles |
|
Smooth
Asphalt
|
|
40000 |
|
Coarse
Asphalt
|
|
36000 |
|
Concrete
|
|
28000 |
|
Extra
Coarse Asphalt
|
|
24000 |
|
Country
Road
|
|
20000 |
|
Crushed
Stone
|
|
8000 |
WHEEL
ALIGNMENT
Wheel
alignment controls the distribution of vehicle load
across the tire's footprint. Camber is the tires
tilt inward (negative) or outward (positive) when
viewed from the front and has significant impact
on handling and shoulder wear. Caster is the angle
of steering axis tilt when viewed from the side
and while it does not affect wear, it does affect
the vehicle's handling and tendency to track straight
down the road. Toe is the direction that the tires
are aimed when viewed from above. Toe-in is when
the tires are pointed towards each other; toe-out
is when the tires are pointed away from each other.
Toe greatly affects tire wear since a setting just
1/16 of an inch off will make the tires want to
travel sideways about 150 feet every mile.
DRIVING
STYLE
The way you drive your vehicle - passively or aggressively
- can greatly impact wear.
Tire
Maintenance
The
things you can do to assure getting the long life
built into the Super Swamper TSL tires are:
1.
Keep your truck in good mechanical condition and
do not run with worn out shocks.
2.
Maintain the proper air pressure for your driving
and load conditions. Check the air pressure once
a month with a GOOD accurate air gauge. Two things
support the weight of a vehicle...the volume of
air and....the airs pressure. With big tires the
large volume of air in the tire will SUPPORT the
vehicle but the tire will not be stable...the tire
will look OK but when running on the highway at
high speed it will wiggle and squirm which the tire
is not designed for and it will not deliver near
the mileage it could with the proper inflation pressure.
This is not important when running off road because
you don't have the speed plus the ground is softer
than the tire but when you get back on the highway
you should bring the pressure back up again to whatever
pressure you have determined is best for the loads
carried and your driving habits. Big tires will
almost always LOOK like they have enough air when
actually they may not for highway driving. So don't
just kick your tires...check them. If you want to
see what we mean try letting the air pressure down
to 12 pounds or so...you probably won't be able
to notice anything abnormal unless yours is a very
heavy truck...but the tire, at 12 PSI is seriously
under inflated for highway service.
3.
Rotate the tires front to back and back to front
about every four to six thousand miles...do not
criss-cross...keep the tires on the same side of
the vehicle. This practice will help to even out
the uneven wear that often shows up on the front
tires' outer lugs from hard cornering or a lot of
in-town driving.
Tire
Balancing
Tire
out-of-balance is usually in the tread area (A),
see illustration, since the tread is the heaviest
part of the tire and the part farthest away from
the center of the axle. In the illustration, (A)
represents the heavy spot; in the example let's
say it is 5 ounces. (B) is the area of the rim where
weights are usually applied. (C) is the center of
the axle, (D) is the diameter of the rim, (E) is
the diameter of the tire and here we are using a
34 inch diameter tire for the example. Area (A)
is 17 inches away from the center of the axle (a
34 inch diameter tire divided by 2 equals 17 inches)
and area (B) is 7-1/2 inches away from the center
of the axle (a 15 inch rim divided by 2 equals 7-1/2inches).
In
this illustration the 5 ounces of imbalance is far
out from the center of the axle (17 inches) and
an attempt to balance the assembly will require
putting weights on the edge of the rim which is
much closer to the center of the axle (7-1/2 inches).
To balance the tire in this example by placing weights
at point (B) to correct the 5 ounces of out-of-balance
at point (A) will require considerably more than
five ounces of weight to bring the tire into balance.
These figures will change depending on the number
of ounces of weight a tire is out of balance and
the diameter of the tire in relation to the diameter
of the rim it is mounted on. In this particular
example the amount of weight required to bring this
5-ounce out of balance into balance with the tire
mounted on a 15 inch diameter rim would be 10.6
ounces. The 'taller' a tire is and the 'shorter'
the rim is the more weight it takes to bring the
assembly into balance. Even more weight is required
when using the stick-on type of weights applied
to the drop center part of the wheel because the
weight is being applied even closer to the axle.
This is sort of like a 200 pound kid trying to seesaw
with a 100 pound kid...it just does not work unless
the heavier kid moves closer to the center of the
board.
The
best way to overcome this situation is if you were
to determine the heavy spot of the tire (in the
illustration it is 5 ounces) before it was mounted
and place a 5 ounce internal 'balance patch' or
'dough compound' directly opposite the 5 ounce heavy
spot by gluing the 'balance patch' to the inside
of the tire which would be placing the 'balance
patch' (weight) very close to the 17 inch radius
from the center axle point it would require only
about 5 ounces of 'balance patch' to properly balance
the tire. After this would be done and the tire
mounted on the wheel the entire tire and wheel assembly
could be put on a balancing machine to correct for
any imbalance in the wheel itself or some minor
dynamic imbalance that may be present. None of these
procedures will give satisfactory results if the
rim is warped or has too much run-out to begin with.
We hope this information helps you in understanding
some of the factors involved in proper balancing
and that a tire is never 'out of balance' as much
as the number of ounces of weight required to 'balance'
it using conventional balancing equipment.
Balance
Effects
There
are two major factors that influence the amount
of weight it takes to balance an LT tire as compared
to a passenger tire. These two factors are outside
diameter (OD) and tread mass.
First,
the outside diameter or the “height" of
an LT tire is much greater than that of a typical
car tire. The average car tire is around 25"
in height or OD. Compare that to the light truck
tires that can go as tall as 44". For the sake
of this example we'll say the average is about 37
inches. Using this for comparison, the height or
diameter alone of the LT tire puts it roughly 1.5
times higher than the car tire. This of course converts
directly into MORE balance weight (remember the
seesaw effect discussed earlier) when all else is
equal.
Unfortunately
that is not where the differences end. The LT tire
also has much more material around its outside diameter
than the typical car tire. The amount of materials
varies greatly by size, but it is safe to say here
that the typical LT tire has 4 times more mass (material)
than your everyday car tire. Think of the deeper,
wider tread, the wider and thicker steel belts as
well as the nylon caps and all the other larger
pieces required to make the LT tire. Remember also
that the larger diameter means longer lengths of
all those components too. All those larger pieces
combine to make it 4 times heavier.
We
described earlier how the weight at the tread surface
is a product of the tread's distance from the center.
Using the diameter and mass difference (1.5 x 4)
combine for a 6 to 1 disadvantage for the average
LT tire. Put it another way: if the radial car tire
takes 2 ounces of weight to balance, the LT tire
would take 12 ounces if all else were equal.
Dynamic
Balance
The
discussion up to now have really only discussed
balancing tires on a static or bubble balancer.
The most common type of tire balancing used today
is dynamic or spin balancing. The easiest way to
imagine how this works is to think of the tire as
if it were split right down the middle of the tread.
Imagine having two saucer shaped pieces. The dynamic
or spin balance machine balances each of those halves
or “saucers" separately. That is why the
typical spin balanced tire will have different weights
at different locations on the inside and outside
of the wheel. The static or bubble balance is a
combination of both saucers, dynamic separates the
two pieces.
Spin
balancing is regarded as the best way to balance
a tire and wheel assembly. There is however some
practical limitations here as all tire shops and
garages do not have balancing equipment that can
handle the larger light truck tires. If the equipment
is not available to spin balance the larger tires,
then the next best thing is to use the static method
as described before. This will not give you the
optimum balance job but is acceptable. Remember
if you use tape-on weights it will take more weight
the closer the weights are to the axle and you will
not get the full benefit of dynamic balance if you
do not have balance weights on both sides of the
rim.
Regardless
of the balancing method you choose always keep in
mind that if the wheel is out of round or warped
the ride will still be rough.
The
balance and ride quality of a tire is often affected
by the run-out and quality of the wheel...the wheel
and the tire are an assembly and work together as
such. When the balance weight seems high it is often
helpful to simply break down the tire from the wheel
and turn it 180 degrees. The balance numbers will
usually come down a bit. Use the valve stem as a
reference point.
The
ride quality of a tire and wheel assembly is dependent
upon many factors only some of which have been discussed
here. The options we have discussed are intended
to give you greater understanding of how some of
these factors work and what can be done to achieve
the best possible ride.
