Method and apparatus for diagnosing vehicle wheel alignment

The method for diagnosing vehicle wheel alignment characteristics in which the longitudinally spaced steerable and non-steerable wheels are equipped with means for permitting the practice of displaying the alignment characteristics of the planes of rotation of the steerable wheels in relation to the geometric center line and the thrust line effect of the non-steerable wheels on the steerable wheels, in combination with the steps of adjusting the plane of rotation of the non-steerable wheels for bringing the thrust line effect thereof into substantial tracking relation with the geometric center line of the vehicle, and monitoring the effect of the adjustment imparted to the non-steerable wheels.

BACKGROUND OF THE INVENTION 
The ideal geometric configuration of a four wheel vehicle is a rectangle in 
which: the steerable wheels will run parallel with each other and are 
equidistant from the center of the connecting axle or its equivalent; in 
which the non-steerable wheels will run parallel with each other and are 
equidistant from the center of the connecting axle or its equivalent; in 
which the non-steerable wheels either track with the steerable wheels or 
are equally off set from the steerable wheel tracks; and in which the 
vehicle body has its longitudinal geometric center line coincident with 
the longitudinal center line for the steerable and non-steerable wheels. 
The practical and economic considerations involved in the production of 
wheeled vehicles take into account the complications in connection with 
manufacturing tolerances present in the various parts and the possibility 
that tolerance mis-matching can build up variations from the ideal 
geometric configuration. As a consequence of the possible mis-matching of 
tolerances in the parts making up a finished vehicle provision is made for 
mechanically adjusting wheel positions relative to the chassis or body of 
a vehicle. In some vehicles all adjustments are found in the steerable 
wheel assemblies, while in others the adjustments are provided in both the 
steerable and non-steerable wheel assemblies. Generally vehicles are 
permitted to have some deviations from the ideal conditions of wheel 
alignment and wheel to body alignment. As long as the deviations are not 
regarded as serious the vehicle is put into use. 
The many conditions attached to the original assembly of vehicles, and the 
equally many things that can occur to change wheel alignment on vehicles 
in use, make the design of apparatus to diagnose those conditions and 
changes very difficult. In the past apparatus has been provided that is 
capable of limited wheel alignment diagnosing ability. Some apparatus is 
simple to operate and some is very difficult and complicated, but in 
either type of apparatus there is little or no provision for obtaining a 
complete understanding of the interrelations of the steerable to 
non-steerable wheels, or of the relationship between wheels and vehicle 
body, or of the part that center point steering wheel position plays in 
relation to the other characteristics. 
Examples of apparatus devised for examining vehicle wheel alignment 
characteristics include Carrigan U.S. Pat. No. 2,601,262 issued June 24, 
1952, which is a light beam projection system incorporating passive light 
reflective mirrors on the steerable and non-steerable wheels and a chart 
spaced away from the vehicle to be in alignment with the light beam. Wheel 
alignment testing equipment involving placing it against machined surfaces 
on the wheels is shown in Holub U.S. Pat. No. 2,972,189 issued Feb. 21, 
1961. This equipment is directed to the capability of locating and 
measuring misalignment in the frame and front and rear housings of a 
vehicle, as well as checking wheel tracking, all with line of sight 
telescopes and mechanical components. A somewhat similar vehicle wheel 
alignment device has been disclosed in MacMillan U.S. Pat. No. 3,091,862 
issued June 4, 1963, but this is limited to use of sighting tubes and 
portable targets. 
More sophisticated apparatus for indicating wheel alignment characteristics 
has been shown in Manlove U.S. Pat. No. 3,164,910 issued Jan. 12, 1965 and 
U.S. Pat. No. 3,181,248 issued May 4, 1965 respectively. In these 
arrangements mechanical pointers are caused to move over scales for 
checking alignment characteristics. The use of light beam projecting means 
for wheel alignment checking is disclosed by Holub U.S. Pat. No. 3,337,961 
issued Aug. 27, 1967. However, the projectors illuminate scales at 
different times and so avoid simultaneous operation. Another system for 
using light beams in association with steerable wheels has been disclosed 
by Senften U.S. Pat. No. 3,782,831 issued Jan. 1, 1974 in apparatus for 
determining the angular position between a fixed and a movable body, such 
as the position of the steerable wheels relative to the axle. Electronic 
scanning devices butted against the vehicle steerable and non-steerable 
wheels has been disclosed by Hirmann U.S. Pat. No. 3,855,709 issued Dec. 
24, 1974 wherein the scanning devices measure chassis geometry from the 
wheel position. 
Alignment equipment limited to front wheel toe is shown in Butler U.S. Pat. 
No. 3,865,492 issued Feb. 11, 1975. This type of equipment has severe 
limitation as to what information can be obtained about the alignment 
characteristics of vehicle wheels and body. The prior art includes the 
electronic run-out compensation means of Senften U.S. Pat. No. 3,892,042 
issued July 1, 1975 which is concerned with getting wheel alignment 
checking means properly oriented with the plane of wheel rotation. More 
recently, electronic alignment apparatus for indicating front wheel toe 
has been disclosed by Rishoud, et al U.S. Pat. No. 3,963,352 issued June 
15, 1976, or by Florer et al in U.S. Pat. No. 4,095,902 issued June 20, 
1978, or by Lill in U.S. Pat. No. 4,097,157 issued June 27, 1978. 
The most recent apparatus is shown in Hunter patent application, Ser. No. 
942,302, filed Sept. 14, 1978, wherein apparatus is provided which has the 
ability to diagnose alignment characteristics. The application of 
Hollandsworth and Grubbs, Ser. No. 967,072, filed Dec. 6, 1978, now U.S. 
Pat. No. 4,239,389 issued Dec. 16, 1980, is directed to the electronics 
involved in alignment apparatus of the type disclosed in the Hunter 
application. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention is concerned with apparatus for use in diagnosing vehicle 
wheel alignment problems, and is particularly directed to the method for 
diagnosing vehicle wheel alignment characteristics. 
The important object of this invention is to provide a method for 
diagnosing vehicle wheel alignment conditions by the steps of fixing the 
angular position of the steerable wheels to provide a basis for 
determining if the non-steerable wheels are aligned as desired with 
respect to the geometric center line of the vehicle. 
An equally important object is to provide simple mechanical and electrical 
means in an organization with displays so wheel alignment problems can be 
diagnosed by application of the foregoing method. 
A further object is to provide electronic diagnostic means in association 
with active and passive sensors mounted on the vehicle wheels, such that a 
given angular position of one set of wheels can be relied upon for 
diagnosing alignment characteristics of the other set of wheels in 
preparation for effecting adjustments of the latter wheels. 
Still a further important object of the present invention is directed to 
diagnosing vehicle wheel alignment in which the steerable and 
non-steerable wheels are longitudinally spaced by means for displaying the 
alignment characteristics of the planes of rotation of the steerable 
wheels in relation to the geometric center line and thrust line effect 
thereon of the non-steerable wheels, and means for monitoring the effect 
of adjustments to the non-steerable wheels when it is found necessary to 
bring the plane of rotation of the non-steerable wheels into substantial 
tracking relation with the geometric center line of the vehicle. 
Further objects of the present invention are to simplify the construction 
of the essential apparatus, to increase the accuracy of results in the use 
of the apparatus, to devise apparatus that may be used with a variety of 
wheel mounting arrangements, and to be able to check out the critical 
wheel alignment parameters quickly, accurately and with instruments having 
unique characteristics.

DETAILED DESCRIPTION OF THE INVENTION 
A presently preferred arrangement of apparatus is shown schematically in 
FIG. 1, and the associated electronic provisions are seen in the block 
diagrams of FIGS. 2 and 3. In these views, the vehicle wheel arrangement 
has steerable left and right wheels 10 and 11 respectively, and 
non-steerable left and right wheels 12 and 13 respectively. These wheels 
ideally should be mounted on the vehicle chassis or frame so the 
non-steerable wheels have the thrust line parallel to the geometric center 
line 14. In actuality the ideal conditions of alignment are never quite 
obtained in the original construction of the vehicle, and after a period 
of use the wheel alignment can deviate more from the ideal because of hard 
usage or by reason of accidents or dropping a wheel into a chuck hole. 
The steerable wheels 10 and 11 are each equipped with active sensor means 
16 and 17 respectively. Each sensor is operable to project radiant energy 
beams transversely as indicated by the broken line 18, and to project 
longitudinal radiant energy beams indicated by the respective broken lines 
19 and 20. The projected beams 18 and 19 and 20 are sensed by sensors in 
the means 16 and 17, and by suitable cables 21 and 22 the signals are 
transmitted into electronic circuits disposed in a suitable console 23. 
The circuits are connected to displays in which the display 24 visualizes 
the alignment of the left steerable wheel 16, the display 25 visualizes 
the alignment of the right steerable wheel and the display 26 visualizes 
the total alignment conditions of the non-steerable wheels 12 and 13. 
The active sensors 16 and 17 are mounted on the respective steerable wheels 
10 and 11 by adapters 27 which are adjustable to compensate for wheel 
run-out, all as described in the prior patent of Senften U.S. Pat. No. 
3,891,042, granted July 1, 1975. By these adapters the sensors 16 and 17 
can be properly related to the plane of rotation of the wheel on which 
each is mounted. In cooperation with active sensors 16 and 17, the 
non-steerable wheels are provided with passive units 28 which support 
reflective means in predetermined relation to the plane of rotation of the 
wheels 12 and 13. Each passive unit 28 is constructed in accordance with 
the disclosure thereof in the copending application of Hunter, Ser. No. 
942,302, filed Sept. 14, 1978. These units embody flat mirrors 29 set to 
be perpendicular to the plane of wheel rotation, and retro reflectors 30 
mounted on mirror blanking shields 31 so as to be quickly moved between 
inoperative positions exposing the mirrors and operative positions 
blanking out the mirrors. 
The practical operation of the apparatus seen in FIG. 1 is put to use with 
the circuit arrangement seen in FIGS. 2 and 3. It is understood that each 
sensor 16 and 17 develops two signals, one by means 16A and 17A for the 
transverse or cross-looking beam, and one by means 16B and 17B for the 
longitudinal beam. These signals are brought into the console electronics 
by cables 21 and 22 (See FIG. 1). The cable 21 contains leads which 
transmit from means 16A, 16B and 16C the transverse beam signal and the 
longitudinal beam signal. In like manner, the cable 22 contains leads 
which transmit from means 17A, 17B and 17C transverse and longitudinal 
signals. The construction of the sensors 16 and 17 is shown in the prior 
patent of Senften U.S. Pat. No. 4,126,943, issued Nov. 28, 1978, and that 
construction is incorporated here by reference. 
When the sensors 16 and 17 are activated from the console 23, beams 18, 19 
and 20 are generated, and these beams are scanned by photosensitive means 
16A, 16B and 17A and 17B in the sensors. The sensor signals are the 
analogs of the ramp voltages generated by means 16C and 17C at the instant 
of beam activation of the sensors for one direction of response of the 
ramp generators which are potentiometers. As seen in FIG. 2, the leads 21 
and 22 are connected into left and right transverse and longitudinal angle 
computers 21A and 22A respectively with suitable cross-over leads 21B and 
22B from the opposite sensor. The computers develop signals in leads L, 
L', R and R'. If the retro reflectors 30 are in position so the shields 31 
blank the mirrors 29, the signals from the sensors 16 and 17 are analogs 
of the left and right toe for the steerable wheels 10 and 11 relative to 
the geometric center line 14. When the shields 31 are moved to expose the 
mirrors, the signals from sensors 16 and 17 are analogs of the left and 
right toe for the steerable wheels 10 and 11 relative to the thrust line 
developed by the non-steerable wheels 12 and 13, and represented by the 
arrow 32. 
Turning now to FIG. 3 the sensor signals are shown transmitted on leads L, 
R, L' and R'. The signals are connected into sub-circuit amplifier 34 for 
the left steerable wheel 10, into sub-circuit amplifier 35 for the right 
steerable wheel 11, and into sub-circuit amplifier 36 for the total rear 
toe developed from the alignment of the non-steerable wheels 12 and 13. 
These signals are processed in the indicated polarity and scaled with the 
indicated multiplier. The output from the amplifier 34 is connected 
through compensation circuit 34A so as to be visualized at the left 
display 24. In like manner, the output from the amplifier 35 is connected 
through compensation circuit 35A and is visualized at the right display 
25. The output from the amplifier 36 is connected to display 26 where the 
total alignment of the non-steerable wheels 12 and 13 is visualized. 
Returning to FIG. 1, there is depicted a vehicle wheel alignment in need of 
correction since the non-steerable wheels 12 and 13 produce a thrust line 
along the direction represented by arrow 32, and the steerable wheels 10 
and 11 are directed to the right relative to the geometric center line 
represented by the line 14. In order to discover the alignment problems 
and be able to make adjustments to align the steerable and non-steerable 
wheels relative to the geometric center line 14, certain procedures need 
to be followed. 
The procedures to be described are given by way of example and with the 
understanding that they will illustrate methods of using the present 
apparatus. With the circuit arrangement seen in FIG. 3 it is possible to 
eliminate the electronic memory circuit provisions seen in the copending 
application of Hollandsworth and Grubbs, Ser. No. 967,072, filed Dec. 6, 
1978, now U.S. Pat. No. 4,239,389 issued Dec. 16, 1980, or similar memory 
circuits proposed by others. Now the present apparatus employs the 
positioning of a steerable wheel in a known set attitude with respect to 
the geometric center line by appropriate presentation of the passive units 
28 at the non-steerable wheels 12 and 13. The procedure is practiced by 
mounting the active sensors 16 and 17 on steerable wheels 10 and 11, and 
passive sensors 28 on the non-steerable wheels 12 and 13. The passive 
sensors 28 are set with the retro reflectors 30 operative and the mirrors 
29 blanked out by shields 31. In this setting, the mechanic turns the 
steering wheel (not shown) until the left steerable wheel 10 is positioned 
with the left display pointer 24A at substantially zero. The display 24 
monitors the wheel motion because the beam projected longitudinally is 
reflected by the retro reflectors 30 which establish a geometric center 
line based upon wheels location. This operation positions the left 
steerable wheel substantially parallel with the geometric center line 14 
(FIG. 1), and it is left in that angular position. The mechanic next moves 
the retro reflectors 30 out of the way so the mirrors 29 of the passive 
units 28 are exposed to the longitudinal beams 19 and 20. Display 24 is 
observed because it will now respond to show the non-steerable wheels 
thrust line effect on the left steerable wheel 10. That thrust line will 
be as indicated at line 32, and is due to the toe angle of the 
non-steerable wheels relative to the geometric center line 14. 
The next step the mechanic takes is to mentally remember or by notation 
write down the reading on display 24 and whether it is to the left or 
right of the initially displayed zero position of pointer 24A. It is also 
feasible to equip the displays with settable pointers to assist memory. 
This responsive movement at display 24 is the amount and direction of the 
thrust line for the wheel 12, and an essentially equal amount and opposite 
direction is present in the companion wheel 13. At the same time, the 
mechanic observes display 26 which is the total non-steerable wheel toe. 
The reading at display 26 is compared with the specification for the 
vehicle being examined. Since the non-steerable wheels 12 and 13 are 
generally movable together, the correction to be made at the individual 
wheels 12 and 13 is one-half the amount observed at display 26. For 
example, if the amount at display 26 registers in the out direction and 
the vehicle specification calls for "zero" toe, the reading at display 26 
is divided by two and the non-steerable wheels need to be adjusted to the 
one-half value per wheel 12 and 13, but in the opposite direction 
indicated by display 26. However, it is necessary to combine the previous 
notation of readings at display 24 with the reading at display 26. The 
combining is done algebraically by adding directions if both read "in" and 
subtracting directions if they are opposite, one being "in" and the other 
being "out". It is possible to have a non-steerable wheel alignment 
condition where one of the wheels needs to be corrected more than the 
other, and the movement is opposite. 
In making the non-steerable wheel adjustments, the mechanic may begin with 
the left non-steerable wheel 12 and needs to watch display 26 so it moves 
the determined amount and in the proper direction. The same observation is 
made as the mechanic adjusts the right non-steerable wheel 13, but now the 
amount and direction of movement of the display 26, is made from the 
position of the pointer 26A at the conclusion of the adjustment of the 
left or opposite non-steerable wheel. 
An alternate method employs the positioning of a steerable wheel, as wheel 
10 for example, in a known set attitude with respect to the thrust line by 
appropriate use or presentation of the passive units 28 at the 
non-steerable wheels 12 and 13. All sensors carried by the wheels 10 and 
11 are mounted as above disclosed, but sensor units 28 are first presented 
or set with the mirrors exposed. In this setting, the mechanic brings are 
left steerable wheel 10 into a positioned with the display pointer 24A at 
substantially zero. The display 24 monitors the motion of the rotational 
plane of wheel 10 relative to the thrust line of the non-steerable wheels. 
This operation positions the left steerable wheel substantially parallel 
with the thrust line 32 (FIG. 1) and it is left in that position. The 
mechanic observes display 26 which is the total toe of the non-steerable 
wheels 12 and 13. The reading at display 26 is compared with the 
specification for the vehicle being examined, and one-half of the 
difference is remembered or written down for later combination to 
calculate rear alignment condition. With left steerable wheel 10 retained 
at its prior set position, the mechanic next covers mirrors 29 and exposes 
retro reflectors 30 of the passive sensors 28. Display 24 is observed as 
its response now indicates the non-steerable wheels thrust line effect. 
That thrust line will have some finite direction, such as indicated at 
line 32, and is due to the toe angles and directions of rolling movement 
of the non-steerable wheels relative to the geometric center line 14. The 
observed thrust line amount and one-half of the remembered total toe 
variance from specifications are combined as described in the previous 
procedure. 
If non-steerable wheel adjustments are to be made, according to foregoing 
method, the mechanic continues with utilization of steerable wheel 10 as 
an adjustment aid, and relies upon the display pre-setting. The mechanic, 
having combined prior measurement results, now exposes mirrors 29 to 
re-establish displays 24 and 25 with respect to non-steerable wheels 
thrust line. In this setting, the mechanic turns the steering wheel (not 
shown) until the left steerable wheel 10 is positioned with the left 
display pointer 24A at substantially one-half the calculated left 
non-steerable wheel adjustment required. This operation positions the left 
steerable wheel as a reference line as the left non-steerable wheel is 
appropriately adjusted. The mechanic proceeds to correct the left 
non-steerable wheel plane of rotation until display pointer 24A 
substantially returns to zero. Non-steerable wheels alignment is then 
completed by correcting the right non-steerable wheel, with mirrors 29 
still exposed, until total non-steerable wheel total toe angle observed on 
display 26 moves to the desired or specified value. 
The result of these step by step manipulations is that the non-steerable 
wheels 12 and 13 are brought into proper relation to the geometric center 
line 14 of the vehicle. The practice of these procedures is primarily to 
enable a mechanic to determine the alignment of the non-steerable wheels 
of a vehicle, and to correct the same if it is possible to do so. If such 
wheels are not adjustable, then these procedures will at least allow the 
mechanic to find the thrust line of the non-steerable wheels and then 
adjust the positions of the steerable wheels 10 and 11 to compensate for 
that condition when the thrust line does not match the geometric center 
line. If these two lines are not badly out of coincidence or matching 
relation, the adjustment of the steerable wheels is deemed to be 
satisfactory for that vehicle. 
A modified procedure for determining vehicle wheel alignment problems may 
be practiced by using modified circuit and console apparatus. In this 
modification, the console 23 and its circuitry of FIG. 3 is changed to a 
console 40 (FIG. 4) housing the electronics seen in the block diagram of 
FIGS. 2 and 5, and including a selector switch SW accessible on the 
console panel along with displays 24 and 25, as in FIG. 1, but now having 
a display 41 which develops a different function from that for display 26. 
The displays 24 and 25 in console 40 are connected to the amplifier unit 
34 and 35 when the switch SW is moved to the position F (FIGS. 4 and 5). 
The signal information supplied to these amplifiers 34 and 35 through 
leads L, R, L' and R' is the same as described with reference to FIGS. 2 
and 3. Thus, in the setting of switch SW to select all contacts F, the 
left and right steerable wheel displays 24 and 25 are activated. 
When switch SW (see FIG. 5) is moved to the contact positions indicated at 
R, a different set of wheel alignment parameters is presented at the 
console 40 for visual observation at displays 24 and 25. Now information 
about the left non-steerable wheel 12 is developed in the amplifier 43 by 
signal from the sensor 16B in the sensor 16 and through a branch lead from 
lead L', by feed back by lead 43A from the wheel run-out compensation 
circuit means 34A, and by a second feed back at lead 42. The information 
about the right non-steerable wheel 13 is developed at amplifier 44 and 
its output 44A is connected through switch SW to the run-out compensation 
circuit means 35A and feed back is obtained by lead 44B from compensator 
35A and by feed back through lead 44C from run-out compensator 35A. In 
addition the amplifier 44 obtains alignment status signals from leads L, 
R, L' and R' as shown, and a feed back at lead 42A. In addition, alignment 
information concerning non-steerable wheel thrust is developed at 
amplifier 45 through the lead 45A from run-out compensator 34A and through 
lead 45B from run-out compensator 35A. The output of amplifier 45 is 
conducted by lead 46 and switch SW to lead 42B and is visualized at 
display 41. As shown, the signals fed into each amplifier 34, 35, 43, 44 
and 45 have the indicated polarity and are scaled by appropriate 
multipliers, with proper run-out compensation at circuit means 34A and 
35A. 
An alternate arrangement of electronics circuitry is seen in the block 
diagram of FIG. 6. As compared to the circuit of FIG. 5, the FIG. 6 
circuit assumes unlimited access to needed signals to instrument 
individual non-steerable toe measurement and display. When switch SW is 
moved to the contact positions R, information about the left non-steerable 
wheel 12 is developed in the amplifier 43 by signal from the sensor 16B in 
the sensor 16 and by feed back by lead 43A from the compensation means 
34A. The amplifier 44 obtains alignment status signals from leads L, R, L' 
and R' as shown. The information about the right non-steerable wheel 13 is 
developed at amplifier 44 and its output 44A is connected through switch 
SW to display 25. In addition, alignment information concerning 
non-steerable wheel thrust is developed at amplifier 45 through the lead 
45A from amplifier 43 and through lead 45B from amplifier 44. The output 
of amplifier 45 is conducted by lead 46 and switch SW, and is visualized 
at display 41. As shown, the signals fed into each amplifier 43, 44, and 
45 have the indicated polarity and are scaled by the multiplier. 
With the modified apparatus as set forth above, a mechanic is able to 
determine alignment conditions for vehicle wheels in the following manner. 
It is first necessary to position the switch SW at the contact position F 
(FIG. 4) and to expose the retro reflectors 30 to the longitudinal beams 
19 and 20. Next the mechanic turns the vehicle left steerable wheel 10 
into a position such that the zero position is visualized on the display 
24. This wheel 10 remains in that position. The foregoing steps are 
followed by the steps of exposing the mirrors 29 at units 28 and moving 
switch SW to the contact position R. This causes the displays 24 and 25 to 
move to indicate the respective left and right non-steerable wheel toe 
relation to the geometric center line 14. If these wheels deviate from the 
desired toe alignment, the mechanic is able to adjust each wheel and 
observe the change at the displays 24 and 25 until the desired toe is 
displayed. It is, of course, understood that non-steerable wheel 
adjustment is not possible unless provision is made for that purpose. If 
the wheel toe visualized at displays 24 and 25 is not dangerously out of a 
desired range, it will be possible for the mechanic to adjust the 
alignment of the steerable wheels 10 and 11 so as to take into account the 
toe alignment condition determined by application of the above modified 
apparatus. 
SUMMARY 
The foregoing disclosure is directed to a unique way of diagnosing vehicle 
wheel alignment by utilizing a given position of one of the steerable 
wheels to establish a basis in the electronic circuits for determining 
certain relationships of the wheels to each other and to the relationship 
with the geometric center line for the vehicle. For example, the unique 
method embodies displaying the alignment characteristics of the planes of 
rotation of the steerable wheels in relation to the geometric center line 
and thrust line effect on the steerable wheels of the non-steerable wheels 
so that a mechanic can be guided in adjusting the non-steerable wheels to 
a desired alignment position by observing the effect of such adjustments 
as they are made. The means herein disclosed is able to display the 
alignment characteristics of the plane of rotation of the steerable wheels 
in respect of the geometric center line, and with respect to the effect of 
the thrust line of the non-steerable wheels on the steerable wheels in a 
desired sequence. 
It has been pointed out that the apparatus will enable a mechanic to 
perform alignment diagnosing, after mounting the sensors and reflective 
means on the respective wheels, by placing one of the steerable wheels in 
a set position where the plane of rotation of that wheel is substantially 
parallel to the geometric center line. This is done by observing the 
appropriate display until it is zeroed. Thereafter that wheel is left in 
the zero position during the remainder of the diagnosis and adjustments. 
This procedure is made possible by the provision of reflective means such 
as retro reflectors which establish location of the non-steerable wheels 
with respect to the steerable wheels, and such as mirrors which establish 
alignment information about the respective wheels. 
The scope of the present disclosure is set forth in the above specification 
which includes variations now known, and which may suggest other 
variations of utility.