Suspension variables influencing static vehicle wheel alignment measurements.
This paper is part of a bigger research effort that aims to capture the influences of static wheel alignment measurement accuracy for road going vehicles. Vehicle alignments can and often are the bottleneck in automotive and truck assembly lines and a greater understanding of the issues are very valuable. The alignment equipment in this research has been tuned and adjusted to minimize external variables and the team of authors have 300+ vehicle measurements. Of the many things that influence the accuracy and repeatability of vehicle suspension alignment measurement and adjustment, the measurement procedures can be the most significant. This includes but is not limited to alignment machine setup and vehicle tire pressures.
CITATION: Patel, H., Casino, M., Noakes, D., Kauffman, N. et al., "Suspension Variables Influencing Static Vehicle Wheel Alignment Measurements," SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):2016.
Vehicle wheel alignments are very time consuming in the automobile assembly process [1, 2, 3, 4, 5] and are critical to optimum vehicle performance yet the accuracy of its measurement is not very well characterized [6, 7, 8, 9, 10]. This research effort includes hundreds of passenger vehicle wheel alignment measurements in the Motorsports Research Building at the University of North Carolina at Charlotte. The design of experiments works to capture the various influential factors. Many things that influence the accuracy and repeatability of the vehicle suspension alignment measurement were evaluated. This includes but is not limited to adjustment configuration, suspension design, static alignment settings, and bushing stiffness.
As with earlier work by this research team,  measurements were made with a Hunter Pro-Align with DSP700 wheel sensors and all vehicles were reviewed and any with suspension damage were rejected prior to measurement. The collection of vehicles measured included the category of small front wheel drive, full size rear drive, seven passenger SUV, sports car, and race car. Since a correlation was made between the softness of the suspension and variability, two very softly sprung vehicles were included
Accuracy of the equipment is not a big part of this study since after a strong attempt to measure it, we were not able to achieve measurements that could sense any reasonable measurement errors. The equipment resolves to 0.01[degrees] and our calibration equipment was only accurate to 0.05[degrees]. Repeatability was evaluated by performing repeated measurements on the same day, a week apart, and over several months. Typically, ten repeats were made during each session and the standard deviation was compared.
The majority of the previous variability in suspension adjustment and measurement was determined to be from the stiffness (or lack thereof) of the suspension bushings . This was found to be the case even for static suspension settings far from the norm. The NASCAR[c] race car with its 6+[degrees] LF camber had some of the lowest variation in the whole test. Further evaluation would reveal strong measurement errors without increased variability from incorrect tire pressure and inaccurate platform levelness.
TEST MEASUREMENT DETAILS
For this research, a full vehicle alignment measurement included camber and toe-in for the left front (LF), RF, LR, and RR wheels. Caster sweeps were also performed, (typically a +17[degrees] to -17[degrees] steer sweep for both front tires). Due to the time restrictions in a shop environment, wheel alignment technicians may be under pressure to shortcut certain alignment protocols. Some shortcuts may be trivial and some are very sensitive. Variables studied include such things as correctly compensating the alignment heads for angular correction, tire pressure errors, and levelness errors in the measurement platform inducing wedge (a corner or cross weight preload). Influence on the suspension parameters camber, caster, toe, cross camber and cross caster variation are presented.
A total of 304 full vehicle measurements were completed. Although some more specialized measurements were not run ten times, most of the vehicle measurements were repeated ten times and the primary assessment approach was to compare the values and standard deviation of these measurements. If the measurement values are extraordinary, (the NASCAR 6[degrees] LF camber) that information is presented as well.
The test protocol was repeated for all tests except for tests of the influence of protocol variables.
TEST VEHICLES AND CONDITION
The vehicle was lifted off the wheel stand bearing plates between each alignment measurement.
1. To avoid wedge effects, the top of the bearing plates were leveled with an LS Starrett brand 98Z-12 spirit level , which has a resolution of 80-90 arc seconds or a levelness of 0.6 mm (0.025") on a 1.4m (55") track.
2. The front and rear of the vehicle were then jounced to settle the suspension.
3. Digital 'compensation' was performed on the DSP700 heads to correct for wheel bearing to mounting flange misalignment prior to each test.
4. All alignment measurements included Caster sweeps.
5. The vehicle was jounced again prior to the recording of final readings.
All of the vehicles measured in this research have no visible or evident wear. Bushings look like new, no measurable defection could be produced in ball joints, and no damage or scrapes are visible on any suspension members. Measurements were made several cars including a small front drive car (figure. 1), a full-sized rear drive car (figure. 2), an SUV (figure. 3), two Luxury Sedans (figures 4 and 6), a sports car (figure. 5), and a NASCAR race car. There were also a few special tests done on a lowered FWD coupe, a second sports car, and a sporty RWD Sedan (see table 1).
Although much of the review is based on previous results, additional observations required more intensive study on a particular vehicle.
Test vehicle V2 (figure 2) was used in this way and may be the most wheel aligned vehicle in North America. At our latest test, we had measured the wheel alignment on this vehicle over 160 times.
In addition, our previous research provided a correlation between softness of the suspension and wheel alignment repeatability. Because of this, we added two vehicles chosen specifically due to their soft suspension reputation. These were the 1991 Cadillac DeVille and 2001 Mercury Marquis. Although very old for our testing, both vehicles appeared to have been very well maintained and suspension components and bushings were in surprisingly good shape.
ALIGNMENT EQUIPMENT DETAILS
Suspension repeatability was assessed by performing complete alignment measurements repeatedly and using the standard deviation of the measurements as a reference. To assess stability of the measurement, we did this across weeks and months on the same vehicle.
All of the testing was performed on a Hunter ProAlign (Figures 2 and 4) with DSP700 wheel sensors (Figure. 3) wheel alignment machine.
Alignment Equipment Calibration and Accuracy
We found the accuracy and repeatability of the alignment equipment to be more accurate than any of the systems we used to measure it. The measurement resolution of the alignment heads is 0.01[degrees] and it is difficult to find a sensor with the usual ten times higher accuracy. As a result, we measured the zero values with a 300mm L.S. Starrett spirit level  and the angle change with a digital angle indicator . The dilemma is that the spirit level has an accuracy of 0.024[degrees] and the digital gage only has a 0.05[degrees] accuracy. In no measurement were we able to achieve measurements that could sense any reasonable measurement errors. Our certainty of readings was therefore only 0.05[degrees]; however, we reported the instrument 0.01[degrees] resolution in the findings.
Aluminum Calibration Frame
Hunter did make calibration tools in the past; however, these seem to have been discontinued (per our sales rep). In an attempt to achieve this, we designed our own.
To eliminate vehicle influences and reduce variability an aluminum frame was constructed (figure. 7). This allowed mounting of the DSP700 heads without the tire wheel clamp. This was used in support of our reverse head measurements. The results are reported below in the section titled "Aluminum Alignment Head Support Frame".
Measurement Surface Levelness
The measurement of one variable influence was the levelness of the bearing plates (non-planar distances). That is, what is the effect of a bearing plate that is higher or lower than the horizontal plane of the other plates. Again, we made all of the bearing plates level using the L. S. Starrett spirit level  described above.
A simple example of this is that if the vehicle is driven onto ramps that have the left ramp one degree higher than the right ramp, then all of the left side camber angles will be one degree more negative and all of the right side camber angles will be one degree more positive.
ALIGNMENT VARIABLES FOR THE FULL SIZED RWD CAR
Since so much data was gathered on vehicle V2 (Mercedes), it was repeatedly used for special alignment machine tests. In the end, the alignment of this one vehicle was measured over 160 times. This vehicle was used for:
* Operator influence (six operators)
* Wedge influence (floor misalignment)
* Date and time influence (over fourteen months)
* Tire pressure influence (four conditions)
* Front toe preload (lateral force up to 5kg)
ALIGNMENT EQUIPMENT VARIABLES
The V2 sedan was used to evaluate the equipment. We used the statistical data from our control tests and compared them to the variable test condition results. Most of these measurements were done by the same operator in the car (Co-author M. Casino).
Control Test #1, 50.2% Wedge
In figures 8 and 9, the first five cases were the 'control' with a driver in the car and everything else at normal conditions. We considered this "no wedge" as we did not displace a wheel up or down and had all tire pressures at the factory vehicle set point. In fact, the cross weight or wedge is defined as the LR and RF weights over the total vehicle weight and it wasn't a perfect 50% but rather 50.2% Any wedge at this point is a result of vehicle suspension variability; however, as compared to our other measurements, it was considered small.
Wedge by Inserting a 3/4" Plate under the LR Tire
In figures 8 and 9, test #6 was the measured effect of wedge by inserting a 19mm (0.75") plate between the bearing plate and tire at the left rear of the vehicle. This was done on scales so that the wheel load and wedge would be measured, (again, wedge is the LR + RF wheel loads / vehicle weight).
Figure 8 shows the effect on the front camber and figure 9 shows the result on rear camber. In both of these figures, cases 1, 2, 3, 4, and 5 are previous runs with standard conditions and #6 is with the 19mm block under the LR tire. Although this is semi-intuitive, (lean the car a degree, all of the cambers change a degree) it does show the impact of not having a flat platform onto which the alignment is measured. That is, the changes shown in figures 8 and 9 are the result of just one wheel being non-level. Furthermore, with that one wheel platform high, the right side camber changes 0.3[degrees] positive and the left side changes 0.4[degrees] more negative. For accurate camber and toe measurements, it is important the bearing plates are level and coplanar.
Tire Pressure Influence
Several influences were measured but the most significant was the inflation pressure of the vehicle tires. This influence is similar to wedge in that as the tire is deflated, the vehicle drops on that corner and the suspension responds similarly to a vertical offset in the floor. However, although the results are similar, the source is very different and so both need to be monitored (see conclusions).
Figures 10 and 11 show the effect of inflation pressure on front Camber angles. Cases 1 to 5 were all standard tests on different dates. On test case 6, we increased wedge to 56.3% by increasing the LR and RF tires to 3.4 bar (50 psi) and dropped the LF and RR to 1.0 bar (15 psi). On test case 7, we reduced wedge to 45.7% by increasing the LF and RR tires to 3.4 bar (50 psi) and dropped LR and RF to 1.0 bar (15 psi).
The average variation in front camber measurements is 0.06[degrees] for each of the standard test cases. For case 3 (56.3% wedge) and case 4 (45.7% wedge) the error rose to 0.41[degrees] and for the rear cambers, the average camber error was only 0.11[degrees]. For accurate measurements, it is important that wedge not be introduced by two low tire pressures.
One Tire Pressure Error Impact
The most interesting phenomena was the impact of only changing one tire pressure. On test case 5, we increased the LF, LR, and RR tires to 3.4 bar (50 psi) and dropped only the RF to 1.0 bar (15 psi) which resulted in a reduced wedge of 47.6%. On test case 6, we increased the RF, LR, and RR tires to 3.4 bar (50 psi) and dropped only the LF to 1.0 bar (15 psi) which resulted in an increased wedge to 53.6%. In these single tire pressure error cases, the average error in front camber angles rose to an outstanding 0.66[degrees], and the average error in rear camber angles rose to 0.37[degrees]. These were both higher than either of the two previous cases.
This means that a single tire pressure error can have a stronger influence on the alignment results than even two diagonal tires being underinflated. This may be only valid for a front tire pressure error and only influence the front tire camber angles. The average toe angle also showed remarkable changes but some toe adjustments were made during the previous months and the confidence is less.
The use of Specific Car Manufacturer Procedures and the "Front Wheel Toe Preload Test"
Mercedes has a special test request whereby a lateral front tire preload is induced to toe out the front tires as when driving. It was calculated that the force would be around 1 kg. As described in our previous study, we induced a load of 0 to 5 kg laterally outward against the front tires and measured the front wheel toe change . No toe influence was measured in this test; however, as we stated before, this protocol may become more important on a heavily worn vehicle.
Alignment Stability over Weeks and Months
Also in summary and with additional data from our earlier work,  our summary is that wheel alignment is a stable thing. On the Mercedes E320 and Porsche 911, wheel alignment measurements changed very little over the fourteen months. We typically recorded no more than 10% change in Camber (0.1[degrees] change from around -1.1[degrees]) for both the E320 and 911.
Aluminum Alignment Head Support Frame
To complete this study, we also need to mention our earlier work with the fixture developed for repeatability measurements of the DSP700 sensors (figure. 7). This fixture provided rigid drilled and reamed mounting holes for the sensors without a vehicle present. The average measured standard deviation for the test frame for both camber and caster are at the resolution of the DSP700 heads.
The results of reverse head measurements revealed that the RF measurement head always read 0.04[degrees] higher than the LR head whereas the LF head always measured 0.02[degrees] higher than the RR head. Curiously, when the same Camber comparison test was repeated on the four door sedan, the swapped heads had similar differences but with different heads.
The reverse head results showed that variance is related more to fixturing than sensors, and that the number changes are small.
Suspension Stiffness Influence
In contrast to our earlier results, additional data updated our observation about repeatability of measurements on stiffy sprung vehicles, . Our earlier observation was that the stiffer the suspension, the more repeatable the alignment would be.
To test this more thoroughly, we located two vehicles with reputations for very softly sprung suspensions.
Soft Suspension Vehicle V5
Since we had determined earlier that suspension stiffness had a strong impact, we located a 2001 Mercury Grand Marquis for this test (see Figure 4). The front suspension is a double wishbone style mounted to a sub-frame. The Rear suspension is a live beam axle with trailing arms and a Watts linkage for lateral control.
The results did not support the softness impact on the measurements. The average standard deviation in camber for this vehicle was only 0.016[degrees] (about the mean of our vehicle measurements). Obviously, a beam axle car should not have much rear camber change; however, this vehicle has a front sub-frame and it is suggested that the suspension bushings may not be as soft as previously thought. That is, the softness may be between the sub-frame and the chassis. This may support why this vehicle does not follow the previously measured influence of suspension softness on low repeatability measurements.
Soft Suspension Vehicle Two (V4)
The second soft suspension vehicle for this test was a 1991 Cadillac Coupe DeVille (see figure 6). This vehicle has a strut suspension both in front and at the (air bag) rear suspension. The average camber standard deviation measured by two operators was 0.030[degrees] or about 25% higher than the mean of all values measured.
The result is that although the earlier results suggested that for a modern vehicle, the repeatability of suspension alignment is not strongly affected by the softness of the suspension.
THE USE OF SPECIFIC CAR MANUFACTURER PROCEDURES
Most manufacturers have specific procedures details and we followed them where practical. Typically these include things like weighing down the car with weights in the front seats or trunk. Our approach was to insert drivers and passengers to fulfill this.
As weight will load and defect the suspension, so will the level of fuel in the gas tank. Although not a car manufacturer procedure, we started implementing a full tank protocol on our last vehicle tests.
Front Wheel Toe Preload Test
One very unique protocol request is by Mercedes. In this special request, a lateral front tire preload is induced to toe out the front tires as when driving. It was calculated that the force would be around 1 kg. As described in our previous study, we induced a load of 0 to 5 kg laterally outward against the front tires and measured the front wheel toe change . No toe influence was measured in this test; however, as we stated before, this protocol may become more important on a heavily worn vehicle.
A WORD OF WARNING
One phenomena arose during the testing that would only show up in testing and not in normal use. We found that the rear suspension of a vehicle would ratchet forward when the vehicle was raised and then lowered. The impact was that if the transmission was left in park, the car would inch forward 6 mm (0.25") each time the car was raised and lowered. After several repeated measurements, we found that cars would be up against the front bumpers at the rear wheels. These bumpers are placed at the rear wheels to prevent a car from rolling off the bearing plates.
If the force was high enough against the bumpers, the rear suspension would generate tremendous friction and the rear camber would not be achieved because the tires were not supported completely by the bearing plates. In repeated measurements, it is important to assure that the rear tires are not bound up on the rear bearing plate bumpers.
Several things do not seem to make a difference in wheel alignment accuracy. The stiffness of bushings was measured to have an influence in prior testing; however, more data shown in Table 2 for vehicles V4 and V5 did not support this assumption. Another result of interest is that the standard deviation does not appear to be related to the static alignment angles. For example, in table 3, the static left front camber angle for vehicle V7 (the NASCAR Cup car) is a large 6.61[degrees] but its standard deviation is a small 0.019[degrees]. Similarly, the left front camber on vehicle V10 (the Honda S2000 CR Club Racer) is - 3.19[degrees], but its standard deviation is 0.046[degrees]
We performed over 160 full wheel alignment measurements and found that the measurement in not strongly affected by many of the variables tested. These included, equipment accuracy, suspension preload, and operator. However, the largest effects on wheel alignment accuracy that can be expected to arise in a plant or wheel alignment shop are caused by levelness of the platform and errors in tire pressure. For those doing wheel alignment studies, there is also the concern of ratcheting suspensions inching a car forward into the bumpers.
1. For accurate camber and toe measurements, it is important the bearing plates are level and coplanar.
2. For accurate measurements, it is important that wedge not be introduced by two low tire pressures.
3. A single tire pressure error can have a stronger influence on the alignment results than even two diagonal tires being underinflated.
4. We measured no toe-in influence using a front lateral force pressure test up to 5kg.
5. We found that wheel alignment measurements changed very little over the fourteen months.
6. The reverse head results showed that variance is related more to fixturing than sensors, and that the number changes are small.
7. The repeatability of suspension alignment is not strongly affected by the softness of the suspension.
8. Caution must be used with respect to the ratcheting effect on parked tires during repeated alignments of one vehicle.
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The corresponding author is:
Dr. Peter Tkacik firstname.lastname@example.org
The University of North Carolina at Charlotte
9201 University City Blvd.
Charlotte, NC 28223
LR - Left Rear
RR - Right Rear
LF - Left Front
RF - Right Front
SUV - Sport Utility Vehicle
FWD - Front Wheel Drive
RWD - Rear Wheel Drive
SD - Standard Deviation
Harsh Patel, Michael Casino, David Noakes, Nicholas Kauffman, Daniel Rohwedder, Jugal Popat, Aneesh Nabar, and Peter Thomas Tkacik
University of North Carolina
Table 1. List of Test vehicles Vehicle Vehicle type Model no. V1 Small FWD 2005 Mini Cooper V2 Full sized RWD 2005 Mercedes E320 CDI V3 7 passenger SUV 2007 Acura MDX V4 FWD luxury sedan 1991 Cadillac Coupe DeVille V5 RWD luxury sedan 2001 Grand Marquis V6 RWD Sports Car 1999 Porsche 911 Cabriolet V7 NASCAR race car UNC Charlotte NASCAR Sprint Cup race car V8 Lowered FWD coupe Toyota Celica V9 Sporty RWD Sedan LexusIS250 V10 RWD Sports Car Honda S2000 CR Table 2. Tire pressure effect on cross weight (wedge). Test LF, RF, LR, RR, Cross (bar/psi) (bar/psi) (bar/psi) (bar/psi) % #1 2.2/32 2.2/32 2.3/34 2.3/34 50.2% #2 2.2/32 2.2/32 2.3/34 2.3/34 52.4% #3 1.0/15 3.4/50 3.4/50 1.0/15 56.3% #4 3.4/50 1.0/15 1.0/15 3.4/50 45.7% #5 3.4/50 1.0/15 3.4/50 3.4/50 47.6% #6 1.0/15 3.4/50 3.4/50 3.4/50 53.6% Test LF, RF, LR, RR, Cross (kg/lbf) (kg/lbf) (kg/lbf) (kg/lbf) % #1 512/1129 498/1097 462/1017 436/961 50.2% #2 493/1086 518/1142 483/1063 414/913 52.4% #3 414/911 534/1177 463/1021 359/791 56.3% #4 561/1236 458/1009 416/916 477/1051 45.7% #5 522/1149 469/1033 430/947 464/1023 47.6% #6 476/1049 526/1159 495/1091 408/898 53.6% Table 3. Standard Deviation of Camber and Toe in degrees by Vehicle Pooled SD 0.030 0.022 0.034 0.036 0.025 0.026 0.021 V10 0.046 0.043 0.008 0.059 0.039 0.017 0.013 0.003 V9 0.029 0.032 0.025 0.022 0.027 0.011 0.015 0.005 V8 0.040 0.031 0.020 0.028 0.030 0.007 0.007 0.007 V7 0.019 0.009 0.019 0.009 0.014 0.017 0.023 0.008 V6 0.007 0.011 0.017 0.019 0.013 0.015 0.019 0.008 V5 0.017 0.014 0.009 0.014 0.014 0.027 0.025 0.008 V4 0.031 0.019 0.027 0.042 0.030 0.038 0.037 0.015 V3 0.005 0.005 0.005 0.007 0.006 0.031 0.027 0.007 V2 0.030 0.021 0.041 0.031 0.031 0.021 0.023 0.028 V1 0.013 0.015 0.046 0.059 0.033 0.009 0.003 0.004 L R L R Avg L R L F F R R camber F F R Camber SD SD Toe In SD Pooled SD 0.020 V10 0.007 0.010 V9 0.004 0.052 V8 0.007 0.007 V7 0.004 0.013 V6 0.010 0.013 V5 0.015 0.019 V4 0.013 0.026 V3 0.007 0.018 V2 0.026 0.024 V1 0.004 0.005 R Avg R Toe SD Table 4. Average Camber and Toe In of all test vehicles in normal control tests. fleet avg 0.10 -1.07 -1.22 -1.77 -0.99 V10 10 -3.19 -3.10 -3.00 -2.65 -2.98 V9 5 -0.59 -0.75 -0.99 -1.79 -1.03 V8 10 -0.60 -1.07 -3.21 -2.94 -1.96 V7 10 6.61 0.24 1.66 -1.52 1.74 V6 15 -0.19 -0.47 -1.32 -1.33 -0.83 V5 21 -1.42 -1.24 -0.43 -0.42 -0.88 V4 20 0.00 -0.64 0.08 -0.16 -0.18 V3 10 -0.48 -0.62 -1.19 -1.57 -0.97 V2 79 -1.13 -1.22 -1.43 -1.80 -1.40 V1 10 -1.07 -1.78 -1.57 -1.77 -1.55 Vehicle # L R L R Avg Trials F F R R Camber average (cleg) fleet avg 0.06 0.04 0.03 -0.04 0.02 V10 0.04 0.03 0.01 0.06 0.03 V9 0.05 0.03 0.01 0.12 0.05 V8 -0.07 -0.10 0.14 0.13 0.03 V7 0.29 0.27 -0.06 -0.66 -0.04 V6 -0.04 -0.05 0.08 0.16 0.04 V5 0.07 0.04 0.00 -0.05 0.02 V4 0.06 0.08 0.05 0.02 0.05 V3 -0.05 -0.06 0.14 0.10 0.03 V2 0.09 0.09 0.10 0.14 0.10 V1 0.20 0.20 0.08 0.21 0.17 Vehicle L R L R Avg F F R R Toe In Avg (deg)
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|Author:||Patel, Harsh; Casino, Michael; Noakes, David; Kauffman, Nicholas; Rohwedder, Daniel; Popat, Jugal; N|
|Publication:||SAE International Journal of Passenger Cars - Mechanical Systems|
|Date:||Jun 1, 2016|
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