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Originally written by R&S
Racing Inc
How to Measure Bolt Patterns
How to Measure Wheel BackSpace
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Items required to measure wheel backspace:
- Tape measure
- Straight edge
- Wheel w/o tire (preferred)
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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.
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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
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Backspace to Offset Conversion
Chart
The table on the right is a quick reference for finding
offset, pick the rim width and follow the row over
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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)
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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
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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.
Determine Wheel Caliper Clearance
Ensuring proper caliper clearance
inside the wheel is important. The following chart should
enable you to have the dimensions required by most wheel
manufactures.
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- 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
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Typical Lug Nut Torque Specifications
| Lug
Size |
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Ft/Lbs Torque |
| 7/16" |
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55-65 |
| 1/2" |
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75-85 |
| 9/16" |
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95-115 |
| 5/8" |
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135-145 |
| 12mm |
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72-80 |
| 14mm |
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85-95 |
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| 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 |
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Modular Wheel Inspection and
Maintenance
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
Modular Wheel Leak Detection
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.
| To the right, 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. |
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