The easiest way to measure backspace is
to lay
the wheel face down onto the ground so the backside of the wheel is
facing up. Take a straight edge and lay it diagonally across the
inboard flange of the wheel. Take a tape measure and measure
the distance from where the straight edge contacts the inboard flange to
the hub mounting pad of the wheel. This
measurement is backspace. The above photo shows three wheels with
2",3", & 4" backspace.
Measuring Wheel Offset
To calculate offset you'll need the following measurements:
Wheel
backspace
Wheel
Width
Wheel
Center line (outboard flange to inboard flange measurement /
2)
Subtract:
Wheel
center line from Wheel backspace to get offset.
If backspace is less than the wheel centerline the offset is
negative
If backspace is greater than the wheel centerline the offset
is positive
Tip:
To
convert from inches to mm multiply by 25.4
To
convert from mm to inches divide by 25.4
Backspace to Offset Conversion
Chart
The table on the right is q quick reference for finding offset, pick
the rim width and follow the row over to the backspace of your wheel.
Fitting a wheel and tire package is
different
for each vehicle, but by following these guidelines your chances for
success will be much greater. In most cases you'll have to use the
physical dimensions of the current wheel/tire package to determine the
dimensions of the new wheel/tire package.
Items which are potential trouble spots:
Tie
Rod Ends
A-arms
Brake
Calipers
Shocks and Shock Mounts
Inner
& Outer Fenders (esp. front tires turned to lock)
In the drawing on the left, we've made
two
measurements
Front
Side Clearance
Back
Side Clearance
These measurements when used with:
Tire
Section Width
Tire
Diameter
Rim Width
Rim
Backspace
Help determine if wheel/tire clearance is adequate for the new
wheel/tire package you've selected
Suggestion:
Start your search for new wheels by picking the tires first. Get
the tire manufacturer's rim width recommendations and physical
dimensions for the tires you want. Pay close attention to Section
Width and Measured Rims specs., these are important numbers to be
used when selecting rims and determining vehicle fitment.
Ensuring proper caliper clearance inside
the
wheel is important. The following chart should enable you to have
the dimensions required by most wheel manufactures.
A.
Caliper Overhang Distance
Used to determine if wheel dish is adequate (in some cases a
spacer is required for clearance)
B. Diameter of Hub Center
Required if wheels are hub
centric
C. Wheel Stud Diameter
Required along with bolt
circle
D. Height of Hub Center
E. Length of Lug and Thread Type (Fine or Coarse)
Required to determine if longer studs are
necessary
F. Distance from CL of Hub to Caliper
Used with A to determine if a spacer is required for proper
fitment
G. Width of Caliper
Used with F to determine if wheel ID is adequate to clear
rotor/caliper package
H. Diameter of Hub Mounting Face
Used to determine if hub is adequate to support
wheel/spacer
Typical Lug Nut Torque Specifications
Lug
Size
Ft/Lbs
Torque
7/16"
55-65
1/2"
75-85
9/16"
95-115
5/8"
135-145
12mm
72-80
14mm
85-95
IMPORTANT
NOTICE:
As with all types of wheels retorque lug nuts after the first 25 miles
& at 100 mile intervals until lug torque is maintained.
Note: Always refer to
Owner's
Manual for proper factory specifications that take precedence over the
listed recommendations.
Wheel Terminology
Bolt pattern or lug pattern or bolt circle
is determined by the number of bolt holes and the bolt circle diameter.
Hub Diameter or center bore is the hole at the center of the
wheel.
Rear spacing or back spacing is the distance from the
backside of the wheel mounting pad to the outside of the rim flange.
Offset: The distance from the centerline of the wheel to the
mounting surface of the wheel.
Negative offset: When the back of the bolt pad is closer to
the inside of the wheel; when mounting surface is inboard of the rim centerline.
Positive offset: When the back of the bolt pad is closer to
the street side of the wheel; when the mounting surface is outboard of the rim centerline.
Bead-Loc A
device which captures the tire bead between it's flanges, usually
secured by bolts to keep tire bead from dismounting. Usually used
in dirt circle track or off road applications where low tire pressures
are used and hitting ruts or other vehicles are common.
Left: An example of a Bead-Loc wheel
Two & Three piece modular wheels
require
periodic maintenance. You'll want to work out your own maintenance
schedule, but here's an example of what the manufacturer recommends.
Each
Season disassemble, thoroughly inspect, clean, re-seal, and
re-torque each wheel:
Replace wheel bolts each season
Wheel
Bolt Torque:
1/4" bolts 15 ft/lbs. or 180
in./lbs
5/16" bolts 20
ft/lbs.
After
thoroughly cleaning all mating surfaces with an appropriate
cleaner, add a thin skim coat of silicone sealant to these surfaces,
assemble wheel and torque bolts to recommended
torque
Install a new valve stem
Add a
thick coat of silicone sealant to the drop center area of
the wheel and let it cure for 24 hours before initial
use
Below is an example of cracking which can occur on
wheels
which don't support the back rim half with the center. This wheel
happens to be a Dura-lite wheel.
So your tires keep going flat, before you blame those leaky slicks,
check your wheels for leaks.
Inflate the tire/wheel combination to 40psi
Spray
a solution of soapy water onto the wheel
Mark
areas where bubbles appear with a tire crayon
If
leaks in the wheel are found follow the maintenance procedure
above to reseal the wheel
The most common cause for leaking modular wheels is; the tire
changing person has stuck their tire spoon into the silicone seal and
damaged it during a tire mount.
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