Wheel offset measure

The wheel offset measure has three straight edge rulers that are interconnected for use in measuring the offset and backspace of a vehicle wheel. A mounting rule is used to hold a rim width rule and a backspace rule and to bridge the diameter of a wheel rear flange. When in place on the rear flange the rim width rule is set to the dimension of the rim width. The backspace rule is then adjusted to touch the mounting pad of the wheel. The relative position of the centerline end of the rim width rule and the zero end of the backspace rule are used to read the offset in millimeters. The backspace dimension can be read from the backspace rule relative to the mounting rule.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to measuring instruments used to determine the 
dimensions of vehicle wheels. The new device provides a simple means to 
measure vehicle wheel offset between the wheel centerline and the mounting 
pod. 
2. Description of Related Art 
There are currently in use measuring rulers and equations for the 
determination of a vehicle wheel centerline, backspace and offset. These 
methods require the taking of one or more measurements related to wheel 
width and mounting pad location. A calculation must then be performed to 
determine the wheel offset. 
These steps are necessary to convert between the two commonly used measures 
of wheel mounting location and the back edge of the wheel rim. In common 
practice wheel backspace is usually measured in inches and offset is 
measured in millimeters. Depending on the manufacturer, one or the other 
of these measures is specified and it is often necessary to convert 
between the two measures in use of the wheels with a particular vehicle. 
The present invention provides in one measuring device the means to measure 
a wheel backspace and offset without the need to perform the offset 
calculation. By direct measure and reading of these values wheels can be 
quickly checked for accuracy and calculation error is reduced. The tool is 
particularly useful in the situation where many wheels of varying 
dimensions must be checked. 
SUMMARY OF THE INVENTION 
One object of the present invention is to allow backspace of a wheel to be 
measured and to determine offset measure based on rim width. Another 
object is to allow direct indication of offset based on setting the device 
at a known rim width and adjustment of a measuring rule to the measured 
backspace. 
In accordance with the description presented herein, other objectives of 
this invention will become apparent when the description and drawings are 
reviewed.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The wheel offset measure device has three straight edge rule elements. One 
is a mounting rule which is used to bridge across the rear flange of a 
wheel and also holds the other two rules. The rim width rule and backspace 
rule are slidably mounted on the mounting rule adjacent to each other. 
There are locking fasteners to hold the rim width rule and backspace rule 
in a fixed measurement position. There are marking lines and set points to 
measure rim width, backspace and offset on the rulers. 
Referring to FIGS. 1 through 3, the wheel offset measure (1) has a mounting 
rule (2) of sufficient length to bridge across the rear flange (3) of a 
vehicle wheel (4). At the approximate rule center (5) of the mounting rule 
(2) a slide bracket (6) is attached which slide bracket (6) has two 
locking fasteners (7) or screws. A marked line for rim width line (10) and 
backspace line (11) is placed on the mounting rule (2). 
A rim width rule (8) and backspace rule (9) are slidably mounted in the 
slide bracket (6) adjacent to each other as illustrated in FIG. 1. A 
locking fastener (7) is located such that the rim width (8) and backspace 
rule (9) may be retained in a fixed position. 
The rim width rule (8) has marked lines in inches ruled on the rim outside 
edge (12) and marked lines in millimeters ruled on the rim inside edge 
(13). The inch marks are actually one half the standard inch measure to 
allow for the wheel (4) centerline being one half the rim width (18). The 
zero point for these marked lines is the centerline end (14) of the rim 
width rule (8). Negative offset is marked on the rim width rule (8) rim 
inside edge (13) so that when the rim width rule (8) extends beyond the 
zero end (15) of the backspace rule (9), offset is read as a negative 
number on the rim width rule (8) at the measurement aligned with the zero 
end (15). 
The backspace rule (9) has marked lines in inches ruled on the backspace 
outside edge (16) and marked lines in millimeters ruled on the backspace 
inside edge (17). The zero point for these marked lines is the zero end 
(15) of the backspace rule (9). Positive offset is marked on the backspace 
rule (9) backspace inside edge (17) so that when backspace inside edge 
(17) extends beyond the centerline end (14) of the rim width rule (8), 
offset is read as a positive number on the backspace rule (9) at the 
measurement aligned with the centerline end (14). 
To measure the parameters of a vehicle wheel (4) the rim width (18) is 
determined. This dimension is most often marked on the wheel (4). If it is 
not, the measurement can be made directly using a ruler lined in inches or 
using the backspace rule (9). The rim width rule (8) is then set to the 
rim width (18) dimension at the rim width line (10) and fixed in place 
with a locking fastener (7). 
The wheel offset measure (1) is then placed on the wheel (4) by resting the 
mounting rule (2) on the rear flange (3) of the wheel (4) such that the 
rim width rule (8) centerline end (14) extends into the wheel (4) and if 
necessary through the centerbore of the wheel. The backspace rule (9) is 
then adjusted until the zero end (15) rests on the mounting pad (19). The 
backspace rule (9) is fixed at this position by a locking fastener (7). 
At this point in the use of the wheel offset measure (1) the backspace (20) 
and offset (21) may be read simultaneously. The backspace (20) is read 
directly at the backspace line (11) in inches. The offset (21) is read 
from either the rim width rule (8) or backspace rule (9) depending on 
whether the wheel (4) has positive or negative offset. The simultaneous 
measure of backspace and offset is not known to be a function of other 
measuring devices which must measure each separately. 
If the rim width rule (8) extends beyond the zero end (15), the offset (21) 
is read on the rim width rule inside edge (13) marking lines as a negative 
offset in millimeters at the point of alignment with the zero end (15). 
Such a configuration is illustrated in FIG. 3. 
If the backspace rule (9) extends beyond the centerline end (14) the offset 
(21) is read on the backspace rule (9) inside edge (17) marking lines as a 
positive offset in millimeters at the point of alignment with the 
centerline end (14). Such a configuration is illustrated in FIG. 5. 
If the rim width rule (8) and backspace rule (9) align with each other, the 
offset (21) is zero as illustrated in FIG. 4. 
In this description of use of the wheel offset measure (1), the width of 
the rear flange (3) must be taken into account. This may be done by 
providing an offset of the zero point of the backspace outside edge (16) 
marked lines or may be done with an appropriate adjustment to the reading 
from the wheel offset measure (1). 
The wheel (4) flanges are not part of the rim width (18) dimension, but the 
rear flange (3) is part of the backspace (20) dimension. To obtain the 
offset (21) dimension, the width of the rear flange (3) must be subtracted 
from the backspace (20) measurement. The formula representing this 
measurement taken is [(Backspace-1/2")-(1/2 Rim Width)].times.25.4 
mm/inch=Offset in millimeters.