An Improved Non-Intrusive Automotive Suspension Testing Apparatus with Means to Determine the Condition of the Dampers.

TESTER EQUIPMENT DESCRIPTION

The testing procedures are: the operator adjusts the tire pressure to within 5% of manufacturer recommended tire pressure [ 1], and then positions the tires of the vehicle on the suspension tester platform. The vehicle transmission should be shifted into neutral for the test and the brakes may be applied if necessary. The suspension tester can be set up to start automatically or manually and can be operated at the console or by remote control. In automatic mode the test will start when a vehicle is driven onto the platform. Each wheel is tested independently by a vertical sinusoidal oscillation of the suspension tester platform with a constant amplitude of 3 mm (0.1181 in). The suspension tester contains load cells to measure the vertical force of the tire on the moving platform and a sensor to measure plate position. The operating frequency range is from 25 Hertz down to 0 Hertz. After a test is completed, the following information is available on the monitor screen:

• Adhesion vs. frequency
• Wheel hop frequency
• Minimum adhesion at wheel hop frequency
• Static weight at each wheel
• Adhesion balance from side-to-side
• Phase angle balance from side-to-side
• Ride stiffness
• Phase angle vs. frequency
• Ride harshness isolation
• Minimum phase angle
• Tire stiffness
• Conclusions

After a test is completed the following is part of the additional information available for automatic printout.

• Front Suspension Results
• Rear Suspension Results
• Suspension Adhesion Suspension Results
• Suspension Damping
• Customer Identification
• Summary
• Warnings and Conclusions

INTERPRETATION OF MEASUREMENTS

Vehicle performance measurements are affected by the following parameters:

• Suspension damper Suspension type
• Sprung mass Vehicle elevation
• Unsprung mass Vehicle dynamics
• Suspension spring Repeatability of tester
• Tire characteristics Vehicle position on tester

The following evaluation studies have been done analytically and experimentally on the quarter-vehicle simulator and on various vehicles.

EFFECT OF SUSPENSION DAMPER VALUE (C₁) - Both the phase angle curve and the adhesion curve are greatly affected by the damping value. With low damping values, the adhesion and phase angle curves both show high peaks and low valleys in the region between the sprung mass resonant frequency and wheel hop frequency. With an adequate damping value at the unsprung mass resonant frequency, both minimum adhesion and phase angle increase. Adequate damping values have higher minimum phase angles with a gradual slope near wheel hop frequency, a high minimum adhesion, a high minimum phase angle, and a higher ride stiffness.

Figure 8a: Analytical Effect of Suspension Damping on Phase Angle.
C₁=.18 (1), .88 (5), 1.75 (10), 3.5(20), 7 (40) kN sec/m (lb.sec/in.);
M₁= 234 kg (515 lbs), M₂= 43 kg (95 lbs), K₁=56 kN/m (320 lb/in),
K₂=182 kN/m (1040 lb/in), C₂=0 kN sec/m (lb.sec/in.)

Figure 8b:Analytical Effect of Suspension Damping on Adhesion.
C₁ = .18 (1), .88 (5), 1.75 (10), 3.5(20), 7 (40) kN sec/m (lb.sec/in.);
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95 lbs), K₁ = 56 kN/m (320 lb/in),
K₂ = 182 kN/m (1040 lb/in), C₂ = 0 kN sec/m (lb.sec/in.)

The dampers of a vehicle are necessary to control vibrations at the resonant frequency of the sprung and unsprung masses.

Critical damping for a system is the minimum amount of damping required to prevent the displacement of the mass from passing the equilibrium position after an initial displacement. The damping ratio or damping factor, z, is the ratio of the damping present in the system to the critical damping and can be obtained from the phase angle and adhesion graphs. Passenger cars suspension damping ratio (z₁) usually falls between 0.2 and 0.4 for the sprung masses [7]. The damping ratios usually vary with frequency and amplitude. The approximate values of the critical damping for the sprung and unsprung masses can be found using equations 14 and 15, and should not exceed z_1,2= 0.5.

(14)

(15)

The damping value of automotive dampers varies with frequency, amplitude, and direction. Dampers can be divided into the compression (jounce) stage and the rebound (extension) stage. Rebound damping is usually one to six times greater than compression damping and allows the damper to dissipate energy stored in the spring and minimizes body motion velocity. Table 3 shows that the damping values in compression have a greater effect on adhesion and phase angle than damping values in rebound.

Table 3: Experimental Effect of Rebound and Compression Damping Values.
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs),
K₁ = 56 kN/m (320 lb/in), K₂ = 182 kN/m (1040 lb/in),
C₂ = 0 kN sec/m (lb.sec/in.)

Conventional hydraulic dampers have little effect on the suspension system performance at frequencies of 20 Hertz and above. The damping present in an automotive suspension is made up of the viscous (hydraulic) damping and friction damping. At frequencies above 20 Hertz the suspension's mounting bushings, tires, suspension spring, and friction damping of the suspension system become very important in limiting the motion, vibration, and noise of the suspension system (also referred to as ride harshness isolation). The damping value required in the suspension system decreases as frequency increases from the wheel hop frequency.

The force transmitted to the sprung mass that is obtained from going over a bump is reduced as the compression damping value of the shock absorber is increased because the velocity of the unsprung mass is lower. Conventional multi-stage dampers have a higher damping value at lower velocities and a lower damping value at higher velocities. The modified damper has a smaller damping value in compression at low velocities and a larger damping value at higher velocities [ ].

ADJUSTING MULTI-RATE DAMPERS - The adjustable multi-rate dampers tested have three different damping settings: regular, firm, and extra firm. The effect of changing the settings can be seen in Table 4.

Table 4a: Damper Setting Affecting both Phase Angle and Minimum Adhesion of a 1981 Chevrolet Luv.

Table 4b: Damper Setting having more of an Effect on Phase Angle of a 1976 Buick Estate Wagon.

Table 4 shows that minimum adhesion and phase angle may or may not always improve in the firmer damper setting because the damper could have higher damping values only at lower velocities and has lower damping values at higher velocities.

DAMPER MOUNTING - Improper mounting can sometimes be detected by the suspension tester by comparing two wheels on each side of the axle. In Table 5, the top nuts on both stud-mounted front shock absorbers were not properly tightened, allowing some movement in the upper mount. These new after-market dampers failed the phase angle criteria but did not fail the adhesion criteria. After proper installation the same dampers passed both criteria.

Table 5: Effects of Improper Mounting on a '76 Buick Estate Wagon.

The suspension tester cannot detect all types of improper mounting. The strut-rod rattled as it was run on the suspension tester and it was found that the top nut was not properly tightened, thus allowing very small movement in the mount. In Table 6, the left side was initially mounted improperly and the right side was not changed.

Table 6: Effect of Improper Mounting on an '85 Toyota Camry

REPLACEMENT OF DAMPERS - New after-market dampers were installed on vehicles that failed the suspension tester criterion. Table 7 shows the average minimum phase angle and adhesion before and after replacement.

Table 7: Changes in Suspension Characteristics due to Replacement of Dampers

EFFECT OF SPRUNG MASS (M₁) - The sprung mass has a greater effect on adhesion, ride stiffness, and ride harshness isolation. It has a small effect on phase angle. Different side-to-side weight distributions also effect the side-to-side measurement of adhesion, ride stiffness, and ride harshness isolation. Figure 10 shows the effect of the sprung mass on both phase angle and adhesion.

Figure 10a: Analytical Effect of Sprung Mass on Phase Angle:
M₁ = 234(515), 279(615), 324)715), 370(815), 415(915), 460(1015) kg (lbs);
M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in), K₂ = 182 kN/m (1040 lb/in),
C₁ = 1.75 kN sec/m (10 lb.sec/in.)

Figure 10b: Analytical Effect of Sprung Mass on Adhesion:
M₁ = 234(515), 279(615), 324)715), 370(815), 415(915), 460(1015) kg (lbs);
M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in), K₂ = 182 kN/m (1040 lb/in),
C₁ = 1.75 kN sec/m (10 lb.sec/in.)

EFFECT OF UNSPRUNG MASS (M₂) - A higher unsprung mass increases the inertia of the suspension system which reduces the minimum phase angle and wheel hop resonant frequency, but increases the ride stiffness and amount of energy absorbed by the tire, which increases the ride harshness isolation. The unsprung mass has a greater effect on the wheel hop frequency and minimum phase angle, a moderate effect on ride stiffness and ride harshness isolation, and a small effect on adhesion. A higher unsprung mass causes a lower minimum phase angle between the sprung and unsprung masses resonant frequency. A higher damping value is required for a larger mass. The effects of the unsprung mass can be seen in Figures 11a, 11b.

Figure 11a: Analytical Effect of Unsprung Mass on Phase Angle:
M₂ = 43 kg (95 lbs), 48 kg (105 lbs), 52 kg (115 lbs), 61 kg (135 lbs);
M₁=234 kg (515 lbs), K₁=56 kN/m (320 lb/in), K₂=182 kN/m (1040 lb/in),
C₁=1.75 kN sec/m (10 lb.sec/in.)

Figure 11b: Analytical Effect of Unsprung Mass on Adhesion:
M₂ = 43 kg (95 lbs), 48 kg (105 lbs), 52 kg (115 lbs), 61 kg (135 lbs);
M₁ = 234 kg (515 lbs), K₁ =56 kN/m (320 lb/in), K₂ =182 kN/m (1040 lb/in),
C₁=1.75 kN sec/m (10 lb.sec/in.)

SUSPENSION SPRING EFFECT (K₁) - The suspension spring stiffness has a greater effect on ride stiffness, adhesion and phase angle from the sprung mass resonant frequency to the wheel hop frequency, a moderate effect on minimum phase angle and minimum adhesion, and a small effect on ride harshness isolation. Figure 12 illustrates the effect of unsprung mass on both phase angle and adhesion. Variable rate springs can have a spring rate lower than that of a conventional linear springs for the same vehicle under normal load conditions. This results in the normally loaded vehicle with variable rate springs having the same or lower adhesion than the normally loaded vehicle equipped with conventional linear springs but improves the ride quality. Under higher load conditions, the vehicle with variable rate springs will demonstrate higher performance than the comparably loaded vehicle with conventional linear springs.

Figure 12a: Analytical Effect of Suspension Spring Constant on Phase Angle:
K₁ = 42 (240), 56 (320), 84 (480), 105 (600) kN/m (lb/in);
M₁=234 kg (515 lbs), M₂=43 kg (95lbs), K₂=182 kN/m (1040 lb/in),
C₁=1.75 kN sec/m (10 lb.sec/in.)

Figure 12b: Analytical Effect of Suspension Spring Constant on Adhesion:
K₁ = 42 (240), 56 (320), 84 (480), 105 (600) kN/m (lb/in);
M₁=234 kg (515 lbs), M₂=43 kg (95lbs), K₂=182 kN/m (1040 lb/in),
C₁=1.75 kN sec/m (10 lb.sec/in.)

TIRE EFFECT (K₂ AND C₂) - All of the suspension tester information is acquired through the tires. Therefore, to accurately assess the condition of the dampers, it is important to adjust the tires to the recommended pressure. This pressure is usually between 28 and 35 p.s.i. for passenger vehicles. It is important that wheels on the same axle have equal tire pressures. Figure 13 shows the effect of tire pressure on the phase angle and adhesion.

Figure 13a: Effect of Tire Inflation Pressure on Phase Angle.

Figure 13b: Effect of Tire Inflation Pressure on Adhesion.

As shown in Figure 13b, the relationship of tire pressure to adhesion is an inverse relationship in the proper range of inflation and load on the tire.

The true tire response to a bump is a complicated, non-linear problem that varies with vehicle speed. Usually, increasing the width of a tire and/or decreasing the aspect ratio increases the tire spring constant for the same tire pressure. The effective tire spring constant decreases with increasing vehicle velocity. Along with amplitude and frequency of the displacement, the tire construction and sprung mass can also change the spring constant of the tire by more than 10%. During the suspension test, the tire spring constant will be slightly larger than the spring constant of the rolling tire.

The rolling tire, in Figure 14a, shows that the stationary wheel will have a spring constant up to 22% higher than that of a rolling wheel. However, the temperature of a rolling tire will increase, consequently increasing both the tire pressure and the tire spring constant.

Figure 14a: Rolling vs. Still Tire Spring Constant [ ].

Figure 14b: Tire Spring Constant vs. Tire Pressure.

Figure 14c: Tire Spring Constant vs. Frequency [ ].

Figure 14d: Tire Spring Constant vs. Load for Different Tire Sizes and Pressures.

The tire spring constant is greatly effected by the inflation pressure. A higher tire spring constant allows more of the road disturbance to be transferred to the sprung mass and increases the acceleration of the unsprung mass at wheel hop frequency. A higher tire pressure increases road noise and causes more disturbances to be transferred to the sprung mass. A change of one p.s.i. of tire pressure can change the minimum adhesion by 0.5% to 2.2%. From testing over 100 different vehicles, an increase of one p.s.i of tire pressure decreased adhesion by an average of 1.2%. Figure's 15a and 15b show the effects of the tire spring constant on phase angle and adhesion.

Figure 15a: Analytical Effect of Tire Spring Constant on Phase Angle:
K₂ = 153 (875), 182 (1040), 273 (1560) kN/m (lb/in);
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in),
C₁ = 1.75 kN sec/m (10 lb.sec/in.)

Figure 15b: Analytical Effect of Tire Spring Constant on Adhesion:
K₂ = 153 (875), 182 (1040), 273 (1560) kN/m (lb/in);
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in),
C₁ = 1.75 kN sec/m (10 lb.sec/in.)

The tire spring constant has a greater effect on adhesion, phase angle, wheel hop frequency and ride harshness isolation, and a small effect on ride stiffness. The inherent damping of the tire is negligible compared to the suspension damping. Figure's 16a and 16b show the analytical curves simulating various tire damping values.

Figure 16a: Analytical Effect of Tire Damping on Phase Angle:
C₂ = 0, .53 (3), .88 (5) kN sec/m (lb.sec/in.);
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁= 56 kN/m (320 lb/in),
K₂ = 182kN/m (1040 lb/in), C₁ = 1.75 kN sec/m (10 lb.sec/in.)

Figure 16b: Analytical Effect of Tire Damping on Adhesion:
C₂ = 0, .175 (1), .35 (2), .53 (3), .7 (4), .88 (5) kN sec/m (lb.sec/in.);
M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁= 56 kN/m (320 lb/in),
K₂ = 182kN/m (1040 lb/in), C₁ = 1.75 kN sec/m (10 lb.sec/in.)

SOLID-AXLE RESONANCE - A non-independent suspension system has two resonance frequencies for the unsprung mass. Tramp is the wheel hop frequency of the wheel in which a pair of wheels hop in opposite phase and parallel hop is the resonance frequency of the wheel in which a pair of wheels hop in phase. Tramp frequency is higher than parallel hop frequency.

Figure 17: Adhesion Curve for a Solid-Axle.

The solid-axle phenomenon as seen in Figure 17 has a sharp spike in the adhesion curve that is dependent on the damping value. Vehicles with anti-roll bars may or may not show similar results. The solid-axle in the following figure was tested without dampers, with one new damper, and with two new dampers. As demonstrated in Figure 18, the phase angle is sensitive to damping.

Figure 18: Solid Axle Effect Due to Removal of Dampers on a 1984 Oldsmobile Custom Cruiser.

VEHICLE TOLERANCES - Vehicle tolerances simply means that vehicles of the same make, model, year, and specifications will show some deviation in suspension tester results. These deviations occur due to variations in dampers, springs, tires, and friction in the suspension. Spring and damper characteristics can vary as much as +/- 10% due to manufacturing tolerances.

VEHICLE ATTITUDE EFFECTS - A 4% vehicle pitch attitude difference from the front to rear axle caused up to 2% deviation in minimum adhesion. The variations caused by vehicle attitude are dependent on the type of vehicle.

VEHICLE DYNAMIC EFFECTS - The suspension tester is a tool for the rapid diagnosis of the suspension system. It does not attempt to account for the effects of vehicle motion. An example of this would be aerodynamics. The aerodynamic lift (or down force) characteristics of a vehicle affect the adhesion when the vehicle is moving. A wheel that is out of balance or wheel with radial runout will affect the minimum adhesion and ride harshness isolation.

REPEATABILITY OF SUSPENSION TESTER - A possible source of deviation of the suspension tester results is the heating up of the damper due to repeated testing, which reduces the viscosity of the fluid. Continuous testing showed a deviation of less than 4 degrees in phase angle and less than 2% in adhesion during five trials. This test was performed at about 70 degrees Fahrenheit ambient temperature. EuSAMA recommends that the shock absorber temperature during the test should be between 32 and 122 degrees Fahrenheit [1].

EFFECTS OF VEHICLE POSITIONING ON MEASUREMENTS - The vehicle must remain stationary during the test to provide accurate and repeatable results. The following parameters were checked:

• Wheel stop bars and brakes
• Vehicle steering direction
• Vehicle side-to-side positioning
• Vehicle elevation effects

WHEEL STOP BARS AND BRAKES - Stop bars were mounted on a suspension tester platform to help position the vehicle and prevent it from moving during the test. Due to a reduction of the tire contact area, stop bars increase the tire spring constant. Stop bars have the largest effect on smaller tires and can decrease the minimum phase angle and minimum adhesion by up to 30%. Stop bars should not be used on suspension tester. The best method of keeping the vehicle from rolling off the platform is by mounting the suspension tester on a flat surface. The vehicle transmission should be shifted into neutral during the test. If the vehicle still tends to roll, holding the brake causes an adhesion deviation of only 2%.

VEHICLE STEERING DIRECTION - When the vehicle is on the suspension tester it is usually steering straight ahead. The effect of the steering direction was taken into account because it changes the weight distribution between the wheels. This effect was noted to be small. Differences in measured values from lock to lock of the steering wheel resulted in an adhesion deviation of less than 3%, ride stiffness deviation less than 4.5% and phase angle by less than 4%.

VEHICLE SIDE-TO-SIDE POSITIONING - The vehicle side-to-side positioning has very little effect on the results. The differences from positioning the vehicle as far as possible to each side produced a deviation of only 1.5% from the mean. If the tire rubs against the suspension tester covers, the suspension tester will display the message, “Check wheels for side-to-side interference.” This is detected by a large variation of adhesion at the lower frequencies.

PLATFORM DISPLACEMENT AMPLITUDE OF THE SUSPENSION TESTER - The displacement amplitude of the suspension tester platform is inversely proportional to the adhesion and has no effect on the phase angle. Figure 19b shows the adhesion values decreasing as the displacement amplitude increases.

Figure 19a: Analytical Effect of Suspension Tester Platform Amplitude on Phase Angle:
A = .0015, .003, .0045 m; M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in),
K₂ = 182kN/m (1040 lb/in), C₁ = 1.75 kN sec/m (10 lb.sec/in.)

Figure 19b: Analytical Effect of Suspension Tester Platform Amplitude on Adhesion:
A = .0015, .003, .0045 m; M₁ = 234 kg (515 lbs), M₂ = 43 kg (95lbs), K₁ = 56 kN/m (320 lb/in),
K₂ = 182kN/m (1040 lb/in), C₁ = 1.75 kN sec/m (10 lb.sec/in.)

VEHICLE SUSPENSION EVALUATION - The vehicles tested should be able to be evaluated decisively without the use of tabulated values or the need for the operator to reference any vehicle specifications. Using both phase angle and adhesion data it is possible to quantify the following:

• Suspension performance
• Damper performance
• Suspension balance
• Ride stiffness
• Ride harshness isolation

The EuSAMA minimum adhesion requirements have been modified to compensate for the effect of the sprung mass. The modified minimum adhesion requirement was determined analytically and experimentally from vehicles that were tested with weight added in 100 pound (45.4 kilogram) increments. EuSAMA specifications do not account for the effect of the vehicle's weight.

Dampers with minimum phase angles less than 40 degrees, corresponding to damping ratio z₂ = 0.08 of the unsprung mass, are considered to be weak. To improve ride comfort, some dampers are designed to be soft and have very low damping values and thus will have very low phase angles. Replacing the dampers on these vehicles may or may not improve the measured adhesion or phase angle, depending on the damping characteristics of the replacement dampers. For example, firmer dampers having at least a 60 degree minimum phase angle will usually increase the adhesion and increase the ride stiffness.

Some vehicles show better adhesion without dampers than others with good dampers. In Figure 20, the rear axle characteristics of a new 1992 Volkswagen Jetta GL4 and a 1982 Ford Ranger pickup truck were compared. The new Volkswagen had known good dampers, and the shock absorbers were removed from the pickup truck. Due to the light rear wheel load and stiff tires, the Volkswagen had an extremely low adhesion. Despite the fact that the dampers were removed from the pickup truck, the adhesion is considered "fair" according to EuSAMA specifications. The phase angles, however, indicate that the damping is low (this remaining damping is caused by internal leaf spring friction) on the pickup truck and high on the Volkswagen.

Figure 20: Comparison of Volkswagen Jetta and Ford Ranger

The side-to-side weight difference of an axle is used for compensation during the side-to-side suspension adhesion balance evaluation. The balance due to the variation of the sprung mass is calculated using an equation that accounts for the sprung mass and other suspension components of each wheel. A side-to-side adhesive imbalance over 10% is considered to be marginal, and an adhesive balance over 15% is considered to fail. Excessive side-to-side imbalance indicates that the dampers have uneven wear and should be replaced as a pair.

Since all of the suspension data is acquired through the tires, it is extremely important that the tires are properly inflated to the automobile manufacturer's specifications (within 5% as specified by EuSAMA [1]). The maximum pressure for maximum load that is printed on the tire sidewall (usually 35 to 45 p.s.i. for passenger cars) should not be used for the test, nor should it be used for regular driving. The optimum tire pressure consists of a compromise between adhesion (traction), ride harshness isolation, rolling resistance, and tire life. The tire pressure can also differ for the front and the rear wheels, depending on the vehicle design and load, and should be recommended by the vehicle and tire manufacturers for different loads. A stiffer tire requires a higher damping value at the wheel hop frequency.

SUMMARY AND CONCLUSIONS

The minimum adhesion relates to the safety of a vehicle at a predefined road input and should be used for comparison. The phase angle shows the damping characteristics of the suspension system and the relative position of peak vertical acceleration of the wheel. Coupled with the adhesion information, the phase angle can be used to calculate the absolute acceleration or displacement of the wheel relative to the ground.

Ride stiffness is actually the stiffness of the vehicle ride and can be used for comparison of the suspension systems. The ideal ride stiffness is arbitrary since it varies according to individual preference. Lower ride stiffness numbers indicate a smoother, softer, less responsive ride. An indication of a very low ride stiffness may indicate weak dampers. A very high ride stiffness may indicate that the dampers are too stiff. This criterion can also be used to evaluate damper performance at lower frequencies.

The ride harshness isolation is not used for the evaluation of dampers. Rather, it can be used for evaluation of the friction in the suspension system, lose or damage suspension components, road noise isolation, tire stiffness and performance of the tire.

Some new vehicles show weak performances at wheel hop due to inadequate damping in the suspension system at these frequencies, even though they have adequate damping at lower frequencies. Some vehicles with high phase angles did not show improved adhesions after the OEM dampers were replaced with new after-market dampers. These vehicles, however, did have better damping characteristics at lower frequencies. Both sprung and unsprung mass resonant frequency should be considered during damper design. A greater damping value in rebound compared to compression should be used at the sprung mass resonant frequency to improve ride comfort. An equivalent damping value for rebound and compression should be used at the wheel hop frequency to improve vehicle handling.

The analytical results confirm the experimental results obtained on the suspension tester. From the results of our study we can see small differences between the analytical and experimental results. This difference appears due to the reduction frequency (deceleration) of the suspension system, suspension spring constant and tire stiffness, and the damping value that is a function of frequency and displacement amplitude. The magnitude of the deceleration is inversely proportional to frequency. Therefore, deceleration has a greater effect on the test results at lower frequencies.

Adhesion alone is not adequate for the conclusive evaluation of shock absorbers. The phase angle is a function of the damping value at different frequencies and should be used for evaluation of the damper performance. The “minimum adhesion” is more indicative of the performance of the vehicle.