Upgrades
Factory Options
Wheel Essentials
There are several critical measurements
to take into account when selecting a wheel or when replacing
your stock wheels with aftermarket wheels (wheels and rims being
used interchangeably in this article). Among these important
measurements are:
Wheel Diameter
This is the total wheel diameter when measured
from bead seat to bead seat. When replacing tires
you of course must know the wheel diameter, but
this will be stamped on the old tires. If you are
browsing through a stack of wheels that are not
stamped an easy way to determine the diameter is
to measure the maximum diameter from rim to rim
and then with a ruler measure the distance from
the rim down to the bead seat. Subtract twice the
distance which you measured down to the bead seat
from the rim to rim diameter and you will have
the correct wheel diameter. Wheel diameter is a
critical consideration when changing or altering
brake components. It is also believed by many
that 15" wheels have a superior bead design
and better bead retention than 16",
16.5" or 17" wheels.
Wheel Width
The wheel width or rim width is the distance
between the outside edges of the bead seat. Most
popular rim width for 4x4s would be 6",
7", 7.5", 8" and 10". Rims
wider than this are usually only used on trucks
with extreme overwidth tires. The tire
manufacturer will publish guidelines for what rim
width should be used with which tire. In general
7" rims will comfortably work with tires up
to 10.5" wide, 8" wide rims work for
9.5" up to 12.5" wide tires and
10" wide rims are used for 12.5" and
wider. Many rims will not be available in width
wider than 8" and rims wider than 10"
are much more difficult to find.
Rim width relative to tread width has an effect
on bead retention when aired down and on the
overall tire profile. In general a narrower wheel
(from within the manufacturers guidelines) will
result in a more rounded profile with a slightly
smaller footprint but with slightly greater
protection of the wheel. Narrower wheels also may
retain the bead marginally better when aired down
than will the wider wheels. Wider wheel provide a
flatter tire profile and greater footprint. Since
the rim is wider it will not be as protected from
the rocks as would a narrower wheel with the same
tire. The tire carcass will exert less pressure
on the wider width bead when aired down and a
wider wheel may thus be marginally more likely to
lose a bead when pressures are low. Wider wheels,
with less sidewall bulge, may reduce the rubbing
of large tires on the springs at full turn.
Backspacing
Backspacing is the distance from the inside rim
surface to the backside of the wheel mounting
surface. It determines how far a wheel (&
tire) sticks into or out of the wheel well of the
vehicle. Wheels with a lot of backspacing will
stick further in. Wheels with little back spacing
will stick further out. Sometimes when swapping
in wider axles 4x4 owners will switch to a wheel
with much more backspacing to compensate. You
should note that the backspacing measurement is
critical when considering the wheels clearance of
the suspension, braking and steering components
as well as the body. All of these must be
considered in both normal conditions and when at
full suspension travel or articulation.
Offset
Offset is the distance from the exact wheel
centerline to the inside wheel mounting surface.
Offset and Backspacing are related. A large
amount of offset can change the leverage and the
loads on axle or wheel bearings and so if
possible you should stay (within reason) close to
the stock wheel offset. In addition to changing
the load on the bearings changing the offset
significantly will change the turning radius (of
the tire) and may effect both steering response
and steering stability.
Bolt Pattern
The bolt pattern has two essential parts. The
first is simple it is the number of bolts. The
second is the diameter of the "bolt
circle". The bolt circle is an imaginary
circle which passes through the centerline of
each of the hubs wheel fasteners. On a wheel with
4, 6 or 8 bolt holes the measurement is simple:
Just measure from the center of one hole to the
center of the hole directly across from it. On a
wheel with 5 bolt holes this is not possible. The
next best thing, which will provide a "close
enough" approximation is to measure from the
center of one bolt hole to a line (or ruler) that
is drawn between the opposite edge of the two
opposing bolt holes. The bolt pattern is then
indicated by the number of bolts X the bolt
circle diameter - for example a 5x5.5" bolt
pattern is 5 bolt hole arranged around a circle
which is 5.5" in diameter. This may also be
referred to as 5 on 5.5"
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