Fig.1: Tire with random pattern (white spray) and synchronization marker

Fig.2: Sketch of the set-up (Aramis HR) + high performance flash

Fig.3: Measurement system and testing machine

Fig.4: Synchronization using an external trigger

Fig.5: z-Displacement

Fig.6: Major strain

Task for the optical field measurement technique: The global determination of displacements and strains on the tire sidewall in relation to different speeds.
The 3D-grating method is used because it enables the geometrically complete deformation analysis of the observed surface. Apart from the 3D-coordinates of the surface under any given load condition, and thus their absolute 3D-displacements, this method also gives the plane strain tensor of the object's surface.

Experimental set-up and the measurement: With the used roller-type test stand the tire is pressed against a drive roller with a pre selected pressure and camber angle. The rolling speed of the tire is controlled by the speed of the drive roller's rotation. The measurement system has been set up so, that the complete tire is in the measurement volume.
The extremely short exposure time necessary for a successful measurement at the required high speeds was realized using a special flash. This ensures that the movement of the tire during the integration time is smaller than the resolution of the system.
As a reference state, the tire is recorded while motionless and without pressure. Once the pressure is applied to the tire, the sought after deformation states are recorded at different speeds. The synchronization of the image recording and the flash were achieved using a trigger signal sent by the test stand.

Experimental data:

Diameter of the tire: approx. 650 mm
Testing speeds: 0...300 kph
Measurement volume: the whole tire, side view
Grating pattern: stochastic pattern sprayed onto the tire
Flash data: 2.5 Joule in 600 ns

Experimental results: The measured values are the 3D-coordinates of homologous points under any given load condition. These are used to determine the global 3d-displacements and strains. As an example, the results of the displacement in z-direction and the major strain are documented below.

Z-Displacement: The image of the Z-displacement at 0 kph clearly shows the tire's static bulging of up to 8 mm, due to the wheel load in the region of the tire contact patch. With increasing speed of up to 200 kph the shape and size of the bulge in the direction of the rotation remain almost constant and is overlaid only by a speed-dependent negative offset (necking of the sidewall due to the centrifugal force). At speeds over 200 kph additional dynamic effects in the form of a standing wave (in relation to the contact point), whose amplitude decreases in the direction of the turn, can be detected. With increasing speed the amplitude, wavelength and the length of this effect increase in the direction of the turn, until it covers the entire circumference at 290 kph.

Major Strain: The detected major strains have their maximum near the contact point. At speeds of over 200 kph dynamic influences are detectable, and, just like with the z-displacement, the distribution of measured values is wave-like in the direction of turn. Here too, the amplitude, wavelength and the length of the effect increase with the speed, until it covers the entire circumference at 195 mph (290 kph).

Source: GOM mbH