System for Testing a Tire to Avoid a Torque Steer Effect

The present invention relates generally to tire testing devices and more particularly to a system for testing pneumatic tires in order to avoid a torque steer effect when the tires are used with an automotive vehicle. Torque steer generally refers to an undesirable tendancy of an automotive vehicle to move or turn laterally when torque, or thrust, is applied to its driven wheels. Torque is applied, for example, when the vehicle accelerates or decelerates.

The torque steer effect is particularly acute in cars that are steered and driven by the same wheels. "Front wheel drive" vehicles are normally steered and driven by the same wheels and accordingly are influenced by the torque steer effect. In recent years, the number of such cars produced has increased significantly. Overcoming the torque steer effect has thus become increasingly important.

In attempting to reduce the effect of torque steer, Applicants realized that torque steer could be caused by a variety of additive factors, such as plysteer or the design of the vehicle itself. They also recognized that the radius of a tire could play an important part in avoiding undesired torque steer characteristics.

The radius of a static, inflated tire changes as a load, or weight is placed on it. Applicants have noted, however, that knowledge regarding the static, loaded radius of a tire is of only limited importance in attempting to eliminate the torque steer effect.

Applicants have ascertained, however, that the radius of a tire also changes as it rotates, and particularly as it rotates under a load. This latter type of radius--the radius of a tire as it revolves under a load--is referred to as the "effective rolling" radius. It can be thought of as the radius of an imaginary, perfectly round circle that has the same circumference as the tire when the tire is revolving under a load. After substantial effort, Applicants discovered that if the "effective rolling" radius of each steerable, driving wheel is kept within a close tolerance, the torque steer effect can be significantly reduced.

Just as the radius of a tire changes as it begins to revolve under a load, the rolling circumference of a tire also, of course, changes in proportion to how the rolling radius changes. The circumference of the tire revolving under a load is referred to as the "effective rolling" circumference.

The effective rolling radius of a revolving, loaded tire cannot be accurately ascertained simply by determining the distance from the center of the tire to road surface. This is partly due to the deformation that the upper portion of the tire undergoes when it revolves under a load. The upper portion of the tire, which is not in contact with the road, tends to bulge out. As a result, the tire has a larger effective rolling circumference than is suggested by the distance between the center of the tire and the roadway. Thus, the measurement of such a distance cannot accurately indicate the tire's effective rolling circumference or effective rolling radius.

According to one feature of the present invention, a tire is inflated, a load is applied to the tire as it is revolved, and the effective rolling radius of the tire is determined. According to another feature of the present invention, the effective rolling circumference of a tire is measured to determine the tire's effective rolling radius. According to yet another feature of the invention, an apparatus carries out the methods described above.

A further feature of the invention involves a process for manufacturing tires in which tires are tested for satisfactory effective rolling radii prior to their shipment to customers. In still another feature of the invention involving the manufacture of vehicles, the effective rolling radii of tires are checked before the tires are mounted on a vehicle.

It is thus an object of the present invention to substantially reduce the torque steer effect. Other objects of the invention are:

(1) to easily and quickly determine the effective rolling radius of a tire;

(2) to conveniently measure the effective rolling circumference of a tire;

(3) to allow tires to be more closely matched before they are mounted on a vehicle and thus reduce customer complaints about the torque steer effect; and

(4) to use a small number components to determine the effective rolling parameters of a tire so that the invention may be readily and inexpensively manufactured.

These and other objects, advantages, and features of the present invention will hereafter appear in connection with the accompanying drawing, wherein like numbers refer to like parts throughout and wherein:

FIG. 1 is a perspective view of an automotive vehicle having two front tires that have been tested in accordance with a preferred form of the present invention;

FIG. 2 is a schematic diagram illustrating, in exaggerated detail, the differences between an unloaded radius, a loaded radius, and an effective radius, in order to explain the operation of a preferred form of the present invention;

FIG. 3 is a block diagram showing the steps in manufacturing a tire according to a preferred form of the present invention;

FIG. 4 is a partial, front elevational view of a preferred form of the present invention, showing two chucks in a spaced relation;

FIG. 5 is a partial, front elevational view of a preferred form of the present invention, showing two chucks in a closed relation and a cross section of a tire;

FIG. 6 is a schematic diagram illustrating the interaction of a tire and a rolling load drum in a preferred form of the present invention;

FIG. 7 is a graph illustrating the interaction between a first detector, a second detector, and a processor in a preferred form of the present invention;

FIG. 8 is a short flowchart showing an overview of the program under which the microprocessor of the present invention operates; and

FIG. 9 is a detailed flowchart illustrating the program under which the microprocessor of the present invention operates.

Referring to FIG. 1, an exemplary automotive vehicle 11 having front wheel drive is shown. The automotive vehicle 11 includes a drive train assembly 13, a steering assembly 15, a set of front, steerable wheels 17, 19 and associated tires 21, 23, and a set of rear nonsteerable wheels 25, 27 and associated tires 29, 31.

The drive train assembly 13 includes a power plant 33 and two shafts 35, 37, each of which is interconnected with one of the two front wheels 17, 19 (FIG. 1). The two front wheels 17, 19 are thus rotatably driven and propel the vehicle 11.

The steering assembly 15 includes a steering wheel 39. The steering wheel 39 is connected to the two front wheels 17, 19 (FIG. 1).

Since the vehicle 11 has front wheel drive, the effect of torque steer is particularly noticeable. Thus, if the power plant 33 provides torque, or thrust, to the two front wheels 17, 19 (and the radii of the two front tires 21, 23 are not properly "matched"), the vehicle 11 will tend to move or turn laterally, even though the steering wheel 39 has not been turned. Usually, under such circumstances, the vehicle 11 tends to turn toward the tire having the smaller radius.

Applicants have discovered that if the two front tires 21, 23 are "matched" within certain tolerances, the effect of torque steer is substantially reduced. The "matching" process involves the determination of a particular type of radius of the tires 21, 23.

The profile of a fully inflated tire, standing free and unassociated with any vehicle, is defined by a dotted, circular line 41 in FIG. 2a. The radius of the dotted, circular line 41 is marked as R.sub.N.

After being inflated, tires are usually mounted on a vehicle and thereafter support part of the vehicle's weight. When a tire 43 (FIG. 2a) is thus put under a "load" (not shown) and rests on a flat roadway 45, a bottom portion 47 of the tire 43 deforms, contracting to adjoin the roadway 45. The distance from a center 49 of the tire 43 to the roadway 45 is called the "load radius" and marked as R.sub.L in FIG. 2a.

After putting the load on the tire 43, the silhouette of the tire 41 is no longer round, and the circumference has decreased. The bottom portion 47 of the tire 43 is shorter than it would be if the tire 43 were in an unloaded condition; the loaded tire 43 must roll more times than an unloaded (and therefore undeformed) tire in order to cover the same distance.

An imaginary, perfectly round, circle 51 may be constructed which has the same circumference as the loaded tire 43. The radius of this circle, marked as R.sub.E in FIG. 2b, is the effective radius of the loaded tire 43. Thus, the loaded tire 43 and the circle 51 would roll the same number of times to cover the same distance.

Still referring to FIG. 2, the effective radius of the tire 43 is a function, inter alia, of the air pressure within the tire 43 and how badly the tire 43 is deformed at the surface of the roadway 45. Applicants have discovered that the effective radius is also changed when the tire 43 is rotated. The effective radius of the tire 43 as it rotates is referred to herein as an "effective rolling radius."

Applicants have also discovered that if the effective rolling radii of the tires 21, 23 associated with the two steerable driving wheels 17, 19 are kept within 0.060 inch (and preferably within 0.040 inch) of each other, the torque steer effect on the vehicle 11 is substantially reduced (FIG. 1). Although other factors, such as a suspension system 53 of the vehicle 11, may affect torque steer, keeping the effective rolling radii "matched" has a dramatic effect on reducing torque steer characteristics. "Matching" tires is easily accomplished during the manufacture of the tires.

Referring to FIG. 3, six basic steps of manufacturing a vehicle tire are shown in blocks M1-M6. In step M1, rubber compounds are mixed and some tire fabric is coated with the compounds. In addition, various component parts of the tire, such as tread and belts, are fabricated and cut to approximate sizes. In step M2, the prepared components are assembled together on a mandrel. In step M3, the assembled components are cured, thereby solidifying the component parts into a unified whole.

In step M4 (FIG. 3), the tire's characteristics are measured. This step is particularly important for tires to be used with front wheel drive cars. Thus, in this step, tires having effective rolling radii outside of a particular tolerance may be noted. Such a tolerance might equal, for example, the desired effective rolling radius of the tire, plus or minus 0.020 inch.

In step M5, tires which have particular characteristics, if any, are indicated. Two methods of indication are: (1) physically marking certain tires with appropriate indicia, or (2) separating the tires into different categories. Tires that are segregated because of a larger or small effective rolling radii may still be used if they are properly "matched." Thus, for example, two tires may each have an effective rolling radius that exceeds an arbritrary tolerance interval by the same amount. The two tires may still be used on a vehicle, so long as they are used together on either the front or rear of the same vehicle. In step M6, tires are prepared for shipment to a customer.

A portion of measuring step, M5, may be carried out by a measuring device 55, such as the one shown in FIGS. 4 and 5. By using the measuring device 55, the effective rolling radius of a vehicle tire 57 may be determined. One may accordingly mark or segregate tires that have effective rolling radii outside of a predetermined range. If only tires having an effective rolling radius within a particular range are shipped, the torque steer effect will be substantially reduced when the vehicle 11 is driven, no matter how the tires are paired together by the customer.

As shown in FIGS. 4 and 5, the measuring device 55 includes upper and lower frames 59, 61, a load assembly 63, and a counter 65. An upper chuck 67 is rotatably mounted on the upper frame 59. The upper chuck 67 includes bead seats 69 and a port 71. The bead seats 69 are suitable for tightly holding an upper side 73 of the tire 57.

The port 71 is interconnected with a supply of compressed air, a valve, and a pressure transducer (not shown). When the tire 57 (FIG. 5) is correctly placed in the measuring device 55, the valve opens and allows air to travel from the supply of compressed air, through the port 71, to an interior 75 of the tire 57. When the pressure transducer senses that the air pressure in the interior 75 has reached a predetermined level, it sends an electrical signal to the valve. The valve, in turn, closes and prevents additional air from entering the tire 57.

The lower frame 61 of the measuring device 55 supports a vertical spindle 77, substantially inside of a sleeve 79, and a lower chuck 81 (FIGS. 4 and 5). The spindle 77 is interconnected to an electric motor (not shown) and may rotate as well as move in a vertical direction. The lower chuck 81 is mounted on top of the spindle 77 and has bead seats 83 for holding a lower side 85 of the tire 57. The lower chuck 81 also is axially movable from an open, retracted position shown in FIG. 4 to a closed, extended position shown in FIG. 5.

During the operation of the measuring device 55, the upper and lower chucks 67, 81 are initially in the open position shown in FIG. 4, and the tire 57 is inserted therebetween. The lower chuck 81 then moves up toward the upper chuck 67, and the bead seats 69, 83 hold the sides 73, 85 of the tire 57 (FIG. 5). The tire 57 is then inflated via the port 71 to a pressure between 20 and 40 p.s.i. Applicants have found the pressure of 38 p.s.i. to work well. Thereafter, the lower chuck 81 is driven by the electric motor, and the upper and lower chuck 67, 81 rotate at a speed of approximately 60 r.p.m.

The load assembly 63 in the measuring device 55 includes a rolling load drum 87 and a pressure assembly 89 (FIG. 4). In one form of the preferred embodiment, the rolling load drum 87 is a cylindrical roller made of aluminum; it has a diameter of 33.625 inches and is at least as wide as the tire 57. Although a roller of different dimensions will function, it is important to the operation of the measuring device 55 that the diameter be known precisely.

In one form of the preferred embodiment shown in FIG. 4, the pressure assembly 89 includes a mechanical screw jack 91 and a gauge 93. The jack 91 revolves and pressures the rolling load drum 87 against the revolving tire 57, which, in turn, causes the rolling load drum 87 to rotate. It is preferable that the outside of the rolling load drum 87 have a high coefficient of friction so that it will not slip upon coming in contact with the revolving tire 57. In a preferred embodiment, the rolling drum 87 is coated on the outside with a fine, tungsten-carbide grit.

The rolling load drum 87 is pressed against the tire 57 with a force equal to approximately 85% of load rating of the tire 57, as set out by the U.S. Tire Rim Association. When this force is reached, the gauge 93 so indicates, and the jack 91 stops driving the rolling load drum 87 any further against the tire 57. Thus, the speed of the lower chuck 81 and the pressure of the rolling load drum 87 roughly simulate the conditions that the tire 57 would be subjected to if used with the vehicle 11.

As shown in FIG. 5, when pressed against the tire 57, the rolling load drum 87 places the tire 57 under a load. The rolling load drum 87 thereby deforms the tire 57 in a manner similar to the deformation that would take place if the tire 57 were used to support a moving vehicle. Thus, by measuring the effective rolling circumference of the tire 57 under such conditions, the effective rolling radius of the tire 57 may be determined.

The counter 65 in the measuring device 55 makes this determination. As shown in more detail in FIG. 6, the counter 65 includes a light source 95, a photosensitive detector 97, a revolution detector 99, leads 101, 103, and a processor 105.

The light source 95 is situated near the perimeter of the rolling load drum 87, which has been perforated with a series of 720 evenly-spaced holes 107 (FIG. 6), one hole for each 1/2 degree of rotation of the drum. As the rolling load drum 87 revolves, light from the light source 95 is intermittently blocked by the rolling load drum 87 and then allowed to pass by one of the holes 107. Thus, 720 pulses of light are created for each complete revolution of the rolling load drum 87.

The photosensitive detector 97 is positioned to receive the pulses and generate an electrical signal each time that it receives a pulse of light. The number of the pulses that the photosensitive detector 97 generates over a certain time period is a first speed signal.

The first speed signal (or "rolling load pulses") has a value that represents the rotational speed of the rolling load drum 87. The first speed signal is transmitted via the lead 101 to the processor 105.

The revolution detector 99 (FIG. 6) is mounted on the spindle 77 (FIGS. 4 and 5) and provides an electrical signal representing the rotational speed of the tire 57. In one form of the preferred embodiment, the revolution detector 99 includes a pulse tachometer 109 (FIG. 6). The tachometer 109 provides 360 electrical pulses for each complete revolution of the spindle 77.

These "tire pulses" are a second speed signal that has a value which represents the rotational speed of the tire 57. The second speed signal is transmitted via the lead 103 to the processor 105.

Pulse doubler circuitry 111 creates an individual pulse for each leading and trailing edge of a pulse generated by the tachometer 109 (FIG. 6). Thus, 720 pulses are generated for each rotation of the tire 57.

The processor 105 receives the first and second speed signals and compares their values in order to determine the effective rolling radius of the tire 57. In one form of the preferred embodiment, the processor 105 includes a Comptrol Inc. "Intelligent Meter," Model No. IMC 85-910. The processor 105 also includes a microprocessor 113 (FIG. 6) and a built-in, alpha-numeric display 115.

The microprocessor 113 (FIG. 6) has an internal clock (not shown). After the tire 57 and rolling load drum 87 are both rotating at full speed, the processor 105 begins counting with its internal clock. As shown in FIG. 7, this time is described as T.sub.0. At the next leading edge of a series of "tire pulses" (TP) the microprocessor 113 notes the time that has elapsed (T.sub.b1) and thereafter begins to accumulate the total number of "tire pulses" (N.sub.b). After the time, T.sub.b1, the microprocessor 113 also notes the time (T.sub.a1) that elapses until the leading edge of the next "rolling load pulse" (RP). The microprocessor 113 then begins to accumulate the total number of "rolling load pulses" (N.sub.a).

After 2,800 "tire pulses" have been received (and the tire 57 has revolved four times) the microprocessor 113 stores the corresponding time (T.sub.b2) and stops counting "tire pulses." The microprocessor 113 then stores the time (T.sub.a2) corresponding to the next leading edge of the "rolling load pulse" which occurs after the time, T.sub.b2.

The microprocessor 113 then subtracts T.sub.b1 from T.sub.b2, in order to determine a time, T.sub.b, and substracts T.sub.a1 from T.sub.a2, in order to determine a time, T.sub.a. The pulses produced by the photosensitive detector 97 and the pulse doubler circuitry 111 may be out of phase. Thus, the ratio of the number of pulses received from each (N.sub.a /N.sub.b) must be corrected by the ratio of the total time duration (T.sub.b /T.sub.a) in order to provide a more accurate ratio of the number of revolutions of the rolling load drum 87 to the revolutions of the tire 57.

According to a preferred embodiment, which requires a low number of tire revolutions (thereby saving time), the microprocessor 113 carries out the following algorithm: ##EQU1## where N.sub.a =Number of "Rolling Load Pulses" counted;

T.sub.a =Time period for N.sub.a ;

N.sub.b =Number of "Tire Pulses" counted;

T.sub.b =Time period for N.sub.b ; and

R.sub.1 =Radius of the Rolling Load Drum.

The effective rolling radius is thus determined by comparing how many times the circular, rolling load drum 87 had to revolve in order to roll the same distance as the tire 57, which revolved under a load. Thus, the effective rolling circumference is compared with the known circumference of the rolling load drum 87 and then multiplied by the known radius of the rolling load drum 87.

The pulses produced by the photosensitive detector 97 and pulse doubler circuitry 111 may be out of phase and of unequal duration. These factors may introduce an error in the measurement of the effective rolling radius. In order to reduce such measurement error, the tire 57 is allowed to rotate four times and produce 2880 tire pulses.

A number of different microprocessors will operate in the present invention. A preferred form of the present invention, however, uses an Intel Semiconductor Microprocessor, Model No. 8085A.

As shown in FIG. 6, the display 115 in the processor 105 includes a keyboard 117 that an operator of the measuring device 55 may activate. In response, the microprocessor 113 will calculate, and the display 115 will show, either (1) the effective rolling radius of the tire, (2) the effective rolling diameter of the tire; or (3) the ratio (ERR/R) between the effective rolling radius of the tire and the radius of the rolling load drum. In addition, an operator may activate the keyboard 117 to program, or record, a new value in the microprocessor 113 for the radius of the rolling load drum.

When appropriately requested by the keyboard 117, the display 115 will also show the pulse rate of the first or second speed signals or will show a constant that represents the radius of the rolling load drum 87. If the operator has requested a test to be performed and the microprocessor 113 senses that the test device is malfunctioning, an error code will be displayed so as to help the operator locate the trouble.

The measuring device 55 also has the feature of being able to receive pulses from an external trigger (not shown) rather than the photosensitive detector 97 or revolution detector 99 (FIG. 6). In this way, the performance of the measuring device 55 may be checked when pulses of a known, test frequency are applied.

The microprocessor 113 stores its program on two 2K work PROM chips (not shown). In addition, a 1K word CMOS RAM with a battery backup (not shown) is used for variable storage. The microprocessor 113 is connected, via an asynchronous serial interface, to a 1000L computer (not shown).

The 1000L computer scans the keyboard 117 at a rate of 32 Hz. When the keyboard 117 is activated, the 1000L computer receives an appropriate signal and transmits a data request to the microprocessor 113. The microprocessor then determines a correct response and transmits it to the 1000L computer. The 1000L computer, in turn, ensures that a proper message is shown on the display 115. The display 115 is updated, or re-brightened, at a rate of 4 Hz.

In order to obtain appropriate data, perform the necessary calculations, and display the requested results, the microprocessor 113 steps through a sequence of program commands. An overview of this program is shown in FIG. 8; a more detailed flowchart for the program is shown in FIG. 9.

As shown in FIG. 8, the system is initialized and put in a condition to run in step D1. Next, in step D2, the microprocessor 113 makes the major inquiry of whether it is receiving data such as the first and second speed signals.

If the microprocessor 113 is not receiving such data, it inquires in step D3 whether it is receiving commands from the operator via the 1000L computer. If instructions are inputed, such as "determine and display an effective rolling radius," the microprocessor 113 recognizes this and readies itself to perform the calculations in steps D4-D5. Otherwise, the microprocessor 113 determines, in steps D6-D7, whether the operator has requested the performance of a test or indicated an intention to enter a new value for the diameter of the rolling load drum 87. A test of the device's operations is performed at step D8. A new value for the diameter of the rolling load drum 87 is entered in steps D9-D10, but only if the operator first inputs a proper "code."

When the microprocessor 113 does receive information, it determines the type of data in step D11 and institutes the "5.5 and 6.5 interrupt" subroutines in step D12. The subroutines count, respectively, the first and second speed signals; they only become operable when the microprocessor 113 arrives at steps D11-D12. If the count of the first and second speed signals seems incorrect, the microprocessor 113 will initiate the display of an error message in steps D13-D14. Otherwise, the computation asked for (effective rolling radius, effective rolling circumference, etc.) is performed and the results are transmitted to the 1000L computer in steps D15-D16.

Initialization

As shown in the more detailed schematic of FIG. 9A, an input is received from the 1000L computer in step S101. This indicates that the microprocessor 113 is to begin performing. The microprocessor 113 thus insures that it is in the "run" mode--i.e., that it is ready to receive measurement inputs and perform calculations.

The microprocessor 113 holds a constant, C, in memory that represents the diameter of the rolling load drum 87. In step S102, the microprocessor 113 insures that C has been properly "remembered" and, if so, sets an appropriate "flag," or "internal notation," in step S103.

Data Inquiry

In step S104, the microprocessor 113 inquires whether its Universal Asynchronous Receiver Transmitter (UART) is receiving measurement data through normal means. If not, the microprocessor 113 asks, in step S105, if measurement data is being received by the UART from an external trigger--i.e., whether it is about to receive test signals from an external test device. If so, a light (not shown) turns on in step S106, and the microprocessor 113 begins to act as though it were receiving "real" measurement data from the photosensitive detector 97 and revolution detector 99.

Command Inquiry

If the external trigger has not been set, the microprocessor 113 asks, in step S107, whether the operator has pushed a button in the keyboard 117 and thus whether a keyboard "flag" has been set. If not, it begins repeating the inquiries at step S104 again. If the operator is inputing data, however, the microprocessor 113 notes this, eliminates the keyboard "flag" in step S108, and begins to ask what key has been pushed.

If an operator indicates that he wishes to display the ratio of (i) the circumference of the tire 57 to (ii) the circumference of the rolling load drum 87, the microprocessor 113 notes this in step S109 and sets itself to the "ratio mode" in step S110. If the key indicating that the operator wants to know the value of the effective rolling circumference of the tire 57 is depressed, the microprocessor 113 notes this at step S111 and, at step S112, determines whether the C valid flag was set at step S103. If the flag has not been set, the sequence of data inquiries begins again at step S104. Otherwise, the microprocessor 113 sets the mode to Ratio x C at step S113 before going back to step S104 and inquiring whether measurement data is being received. If neither the Ratio or R x C key was pushed, the microprocessor 113 asks, at step S114, if the effective rolling radius key was depressed. If so, in steps S115-S116, a check for the valid C flag is made, and the mode is set to Ratio x 1/2 C.

If none of the "key down" questions were yet answered affirmatively, the microprocessor 113 asks itself, in steps S117-S118, whether the A or B test keys have been pushed. If the A test has been chosen, an A test flag is set in step S119. The microprocessor 113 is then taken from the run mode to the test mode, and the "5.5 interrupt" is unmasked, or allowed to operate. Thus, the microprocessor 113 will devote its resources to performing the steps in the "5.5 interrupt" subroutine unless a command having a higher priority is requested. The "5.5 interrupt" subroutine is shown in FIG. 9D and is discussed in a later section.

Similarly, in step S120, if the B test has been requested, a B test flag is set, the microprocessor 113 goes into the test rather than run mode, and the 6.5 interrupt subroutine is enabled. The 6.5 interrupt then takes priority over any lower priority subroutine, such as the 5.5 interrupt. The 6.5 interrupt subroutine is shown in FIG. 9E and is also discussed in a later section.

After the A or B test has been performed, the microprocessor 113 determines, in steps S121 and S122, whether the run key has been pushed or the UART is receiving other data. If so, the test flag is cleared in step S123. The run flag is then set and the 5.5 and 6.5 interrupt subroutines are cleared. Otherwise, if the A or B test keys are still depressed, the microprocessor 113 goes back, in steps S124-S125, to re-perform the appropriate test.

If neither the A test or B test button has been depressed, the microprocessor 113 inquires in step S126 whether the operator has indicated that a new constant is to be entered into memory. If so, the system is readied to receive a new constant in memory. This procedure is illustrated in FIG. 9G and discussed in a later section.

Determination of Data Type

When the UART does start to receive data, it is noted at step S201. See FIG. 9B. Any "flag" representing that the external trigger is functioning or that the UART is receiving data is then cleared. In steps S202-S209, the microprocessor 113 determines what type of data it is receiving. If the data is of a first type, entitled "DC1," the microprocessor 113 sets the mode to "Ratio" in step S203. If the data is of the second type, entitled "DC2," the microprocessor 113 checks to see that the valid C flag has been set and then sets the mode to Ratio x C. Similarly, if a third type of data, "DC3," is received, a check is made for a valid C flag and, if appropriate, the mode is set to Ratio x 1/2 C.

In the determination of whether the data is of the type DC2 or DC3, if a valid C has not been set (in step S103), the microprocessor 113 checks, in step S210, to see whether the UART is presently capable of transmitting a message. If so, the transmitter buffer, or memory, is loaded with a NAK ("not acknowledged") signal in step S211, and this message is sent to the 1000L computer. The 1000L ensures that an error message denoting the lack of a valid C is displayed.

Counting

If it is determined that a calculation should be made by the microprocessor 113, the subroutine shown in FIG. 9C is used. At step S301 the timer is set to its maximum number; in one form of the preferred embodiment, the timer uses a hexidecimal number system and "3FFFH" represents the largest storable number. Also in step S301, the "T counter," which measures units of time, is set to zero. The operation of the T counter is shown in FIG. 9F and explained in a later section. Finally in step S301, the A and B flags are "set to first" and the 5.5 and 6.5 interrupts are "unmasked," or allowed to operate.

In step S302, the microprocessor 113 asks if the 5.5 interrupt has been masked yet. If not, it asks, at step S303, whether sufficient time has elapsed for the T counter to reach 50. If not, the microprocessor 113 goes back to step 301. Otherwise, in steps S303-S309, the microprocessor 113 determines why the 5.5 interrupt has not been masked, even though the T counter would have had sufficient time to reach 50 under normal conditions. (As will be shown later, if the 5.5 interrupt has been masked, the 6.5 interrupt is necessarily in a similar state.)

If the A flag still equals one, it means that no rolling load pulses are being received from the photosensitive detector 97. The microprocessor 113 determines this and sets an appropriate "fault 1" message flag. If the B flag still is one, it means that the tire pulses are not being received from the tachometer 109, and the "fault 2" flag is set. If the T counter equals 1200, it has not been decremented properly--i.e., the timer is malfunctioning. An appropriate "fault 3" flag is then set.

If any of the three fault flags have been set, the transmitter buffer is loaded with an ACK ("acknowledged") signal and the correct error code at step S310. The message is then sent to 1000L computer for presentation on the display 117.

Computation

In step S311, the measuring flag is cleared after all tire and load roll pulses have been "counted" by the microprocessor 113 (FIG. 9C). The value ascribed to the last A, or load roll, pulse is subtracted from the value of the first "A" pulse, and the result is stored in a memory file entitled "A First." Similarly, the value of the last B, or tire, pulse is subtracted from the value of the first B pulse and stored as "B First."

In step S312, the value of "B first" is multiplied by the value of the N counter. The resulting value is then divided by the quantity of A First times 2880. The result of this calculation is stored in a file entitled Ratio.

If the microprocessor 113 was put in the R x 1/2 C mode (at step S209), this is "remembered" at step S313, and the value of the Ratio file is multiplied by 1/2 C in steps S314-S315. If the mode was set at R x C (in step S206), this is determined, and the value of the Ratio file is multiplied by C in steps S315 and S316. Otherwise, the Ratio file remains unaffected.

In step S317, the value of the Ratio file is converted to the language, American Standard Code for Information Interchange (ASCII). In steps S318-S320, the microprocessor 113 determines whether the external trigger flag has been set (in step S106) and then loads the ASCII number into the transmitter buffer for transmission to the 1000L computer.

5.5. Interrupt

When the 5.5 interrupt is unmasked at step S301, the microprocessor 113 begins to step through the subroutine shown in FIG. 9D. Pulses emitted by the photosensitive detector 97 are directed to a first input of an exclusive OR gate (not shown); the output of the exclusive OR gate is sensed by the microprocessor 113 at step S401.

Initially, an "A flag" is set to "F," indicating that no pulses have yet been received since the 5.5 interrupt was unmasked. The first input to the exclusive OR gate (which originates in the photosensitive detector 97) is LOW; a second input to the exclusive OR is kept HIGH by the microprocessor 113, and the output of exclusive OR is thus kept in a HIGH state.

At the leading edge of the first pulse from the photosensitive detector 97, the exclusive OR receives two HIGH inputs, making the output of the exclusive OR LOW. The microprocessor 113 senses this, and, at step S402, adjusts the second input such that the output of the exclusive OR is in a HIGH state.

At step S403, the A flag is read. On the leading edge of the first pulse, the content of the T (time) counter is recorded, at step S404, and stored in a file entitled "A first." The A flag is then changed to an "M" (for "middle"), and the microprocessor 113 waits to receive the trailing edge of a pulse from the photosensitive detector 97. When this occurs, the microprocessor 113 progresses to step S405, where the output of the exclusive OR gate is set back to a LOW state. Thus, when the next leading edge of a pulse from the photosensitive detector 97 occurs, the microprocessor 113 puts the output of the exclusive OR back to a HIGH state (in step S402) and increments N (number of load roll pulses) by one (in step S406).

The process described above continues until the A flag is set to L (for "last") in the 6.5 interrupt subroutine. After this occurs, the microprocessor 113 moves to step S407 at the next leading edge of a pulse from the photosensitive detector 97. The value of the T counter is read and stored in a file entitled "A Last," and the 5.5 interrupt masks itself.

6.5 Interrupt

The 6.5 interrupt subroutine is shown in FIG. 9E. As with the 5.5 interrupt, the 6.5 interrupt is unmasked at step S201. The output of the tachometer 109 is connected with an exclusive OR gate. The output of this gate provides an input to the microprocessor 113. As with the A flag, the B flag is initially set at a ZERO state. In addition, a "L counter" is set to a value of 2879.

At steps S501-S503, the microprocessor 113 emits a relatively long, microsecond pulse to the 1000L computer at each leading edge of the tire pulse. This "synchronization pulse" ensures that the microprocessor 113 and 1000L computer can accurately communicate with each other. Both the leading and trailing edges cause the microprocessor 113 to progress to step S504, where the B flag is read.

If the B flag is set at F, i.e., the "first" pulse is occurring, the microprocessor 113 moves to step S505, and the values of the "T counter" is read and stored in a file entitled "B First." The B flag is then set to an "M," or "middle," state.

For all subsequent leading and trailing edges of pulses from the tachometer 109, the microprocessor 113 moves to steps S506-S507. The L counter is decremented by one for each pulse. When it finally reaches zero, the B flag is changed to "L" or "last," at step S508.

The following pulse then causes the microprocessor 113 to move to step S509. The value of the T counter is read and stored in a file entitled "B last." The A flag is also set to last, so that at the next leading pulse from the photosensitive detector 97, the 5.5 interrupt subroutine will mask itself. The 6.5 interrupt subroutine is then masked.

Timer

The microprocessor 113 includes an internal, 3.072 megahertz clock (not shown). A "timer" for the microprocessor 113 operates in conjunction with the clock. As shown in FIG. 9F, the timer is given a 7.5 priority and runs continuously after the system is initialized in step S101 (FIG. 9A). If a measuring flag has been set in step S301 (FIG. 9C), the microprocessor 113 notes this at step S601 (FIG. 9F). The T counter is then incremented by 1 at step S602, and the inquiry regarding the measuring flag is repeated.

If a test flag has not been set at steps S119 or S120, the microprocessor 113 notes this at step S603 and, at step S604, checks whether the operator has input new commands at the keyboard or the UART is receiving data. Finally, it sends another pulse to "refresh" any message shown on the display before going back to step S601.

If a test flag has been set, the microprocessor 113 cycles through steps S605-S606. After the N and L counters are read, the T counter is decremented until it reaches a 0 value. At that point, the microprocessor 113 sets the T counter to 1200 and determines, in steps S607-S608, which test has been requested.

If an A, or rolling load pulse, test has been requested, the N counter, which increments from 0, is read and stored in step S609. If a B, or tire pulse, test is requested, the L counter, which decrements from 2879, is read and stored in step S610. The value of the L counter is then subtracted from 2879 to achieve a correct, understandable value for the number of tire pulses that have occurred. The values taken from the counters are then converted to Binary Coded Decimal (BCD) form in step S611 and transmitted to the 1000L computer for illustration on the display.

New Constant

The process for entering a new constant for the radius of the rolling load wheel is shown in FIG. 9G. After an operator indicates, at step S126, that a new constant is to be entered, the current C is shown on the display at step S701. The microprocessor 113 then sees if the proper code sequence (1-7-4-6) is entered in steps S702-S709. If the operator incorrectly enters the code or the UART starts receiving other data, such as from the photosensitive detector 97 or tachometer 109, the microprocessor 113 automatically aborts the attempt to program in a new C in steps S710-S713 and returns to step S104. If, however, the proper code is first entered and the UART does not receive other data, a new C flag is set at step S714. The microprocessor then accepts numeric inputs in steps S715-S716. Four digits are accepted as the radius of the load roll wheel, in inches. If the operator then pushes an enter key, the new value for C is converted to a binary number, stored in memory, and the new C flag is cleared.

If a transmit flag has not been set, the microprocessor 113 begins its data inquiry again at step S104. Otherwise, the transmit flag is cleared at step S720 and the microprocessor 113 waits for an acknowledgement that the value of the newly programmed C has been properly received.

If the microprocessor 113 does not receive an enter message in step S717, it may, instead, be receiving data through the UART. If so, the data is either acknowledged or not acknowledged by the microprocessor 113 in steps S721-S724. The microprocessor 113 will then again stand ready to receive the new value for C.

Acknowledgment of Transmission

After data has been sent by the microprocessor 113 to the 1000L computer (at steps S211, S310, S319 or S720), the UART is ready to receive an acknowledgment that the data was correctly received. Under such circumstances, a "data ready" flag is set.

As shown in FIG. 9H, if the UART does receive data, the microprocessor 113 recognizes this in step S801. The data ready flag is then cleared in step S802. In step S803, if the data received is an ACK, the microprocessor 113 goes back to step S104 and the data-type inquiry. If the data indicates that a measurement is being performed, the microprocessor 113 moves from step S804 to step S201 and begins counting. If the data is neither an ACK or measurement data, the microprocessor 113 waits one millisecond and retransmits the contents of the transmitter buffer in steps S805-S806.

Although the foregoing description of the preferred embodiment will enable a person of ordinary skill in the art to make and use the same, the following detailed assembly language listing for the microprocessor 113 is included. The assembly language listing provides detailed information concerning the programming and operation of the overall system. Additional detailed features of the system will become apparent to those skilled in the art from reviewing the program. ##SPC1## ##SPC2## ##SPC3## ##SPC4##

After the effective rolling radius of a tire has been determined in accordance with the present invention, the tire can be mounted on a first steerable, driving wheel of the vehicle 11. The effective rolling radius of a second tire can then be measured. If the torque steer effect is to be reduced, the second tire will be mounted on a second, steerable wheel of the vehicle 11 only if the effective rolling radii of the two tires are substantially the same, i.e., within 0.060 inch of each other.

Those skilled in the art will recognize that only a single form of a preferred embodiment has been disclosed and that the preferred embodiment can be modified without departing from the spirit and scope of the invention. Thus, for example, a determination of the effective rolling radius should be considered the equivalent of a determination of the effective rolling diameter or the effective rolling circumference. Similarly, the order in which the invention is carried out may be altered without significantly changing the results achieved. For example, the tire 57 may be inflated either before or during rotation and a first tire may be mounted on the vehicle 11 either before or after a second tire is tested to see if it also should be mounted on the vehicle 11.