This is the concluding part of A Beginner’s Guide To Tyre Testing by Dr.S.N.Chakravarty.

Dynamic Tests

The dynamic tests are performed on the large test drum. Most of the dynamic tests are restricted to pure slip conditions. On the large drum different test stands can be mounted.

Shearography is a contact less, image producing measurement procedure which is very suitable for non destructive testing of lightweight structures. Compared to testing methods like Ultrasonic testing, X-Ray measurement or the Eddy Current method, Shearography has the advantages of full field measurement and requires accessibility from only one side of the specimen.

Shearography is an interferometric measurement technique. The shearing optics is completely integrated into a compact measurement head. Shearography is in principle insensitive against whole body deformations which results in a high resistance against disturbing factors. With the CCD-camera technique no photo laboratory is required.

This makes it possible to practically use shearography as a means of non-destructive testing in InService-Inspection.

The progress and development in computer technique and digital image processing have further improved the practical suitability of shearography. The development of the spatial phase shift with which for each object state only one video image has to be recorded leads to a simpler measurement set-up and a faster image acquisition.

This results in improved robustness and longer lifetime of the measuring system since a mechanical phase shift module is no longer required. The actual measurement process is faster and, thus, less sensitive for disturbances.

1. Shearography in the production line for quality control
2. Shearography in Service as a mobile test system in field measurements

Here’s a 2.11 min video of Tire Testing with the STEINBICHLER INTACT 30 System.

Video of Tire Testing – Shearography

Measurement Principle

The shearography is an interferometric method with which it is possible to detect component deformation respectively the gradient of the deformation created by mechanical strain. The process is very sensitive due to the interferometric measurement principle: deformations in the range of few micrometers can be detected.

The measurement sample is illuminated by laser light. With a CCD-camera, the object is supervised. For the reproduction of the object on the CCD target of the camera (image plane), the so-called shear optics is applied. It reproduces the object on the CCD target twice: this is called image shearing. A point of the object is shown on two positions in the image plane. In other words: two neighboring object points are displayed one over the other in the image plane.

When the specimen deforms under strain, the intensity of the laser light reflected by the object changes. Superposition of the picture of two neighboring object points on the CCD target makes it possible to determine the difference of the deformation of all object points. Evaluating the intensity measured by the CCD target in each image point does this. Thus, with shearography the gradient of the deformation of the measured object in shearing direction can be measured.

L- Ray

The science behind Holography, Shearography and L-Ray’s “Differometry” is that we are building a topographical map of the inside of the tire. Essentially, we determine very accurately the distance from the camera lenses to the surface of the tire. We take the first image with no vacuum.  When a vacuum is pulled, any separations or defects inside the plies of the tire will allow the rubber to move creating a bubble.  When the moving rubber moves closer to the camera lens, the computer can detect the difference in the distance from the “at rest” condition and the vacuum condition.

It is not recommended to install a special foundation for the machine for two reasons.

First is that the machine is sitting on floor mounts that have a rubber insert that will provide some level of isolation. Second, the upper machine plate that holds the lower dome and the camera assembly is mounted on four airbags with an automatic leveling  system.

This airbag system provides the best possible isolation from factory vibrations.  However, if the machine were to be put in an environment where it is next to very significant pounding (like a forge press or something like that), the machine would most likely be affected by this severe vibration.

Most tire or retread factories do not have this type of situation so we generally don’t worry about it. The most common defect detected by the machine is a separation between plies of the tire rubber.  This is when two of the plies come apart due to excess heat, contamination during manufacturing, improper materials, etc.

Video L-Ray Tire Testing

Rolling Resistance

The question often arises whether a small cross-section tyre has lower rolling resistance than a larger one. The answer, as often, is yes and no, because unseen factors come into play.

Rolling resistance of a tyre arises almost entirely from flexural rubber losses in the tyre and tube. Rubber, especially with carbon black, as is commonly used in tyres, is a high loss material. On the other hand rubber without carbon black although having lower losses, wears rapidly and has miserable traction when wet.

Besides the tread, the tube of an inflated tyre is so firmly pressed against the casing that it, in effect, becomes an internal tread. The tread and the tube together absorb the majority of the energy lost in the rolling tyre while the inter-cord binder (usually rubber) comes in far behind. Tread scuffing on the road is even less significant.

Patterned treads measurably increase rolling resistance over slicks, because the rubber bulges and deforms into tread voids when pressed against the road. This effect, tread squirm, is mostly absent with smooth tyres because it cannot be bulge laterally by road contact because rubber, although elastic, is incompressible.

Small cross-section tyres experience more deformation than a large cross-section tyre and therefore, should have greater rolling resistance, but they generally do not, because large and small cross-section tyres are not identical in other respects. Large tyres nearly always have thicker tread and often use heavier tubes, besides having thicker casings. For these reasons, smaller tyre usually have lower rolling resistance rather than from the smaller contact patch to which it is often attributed.

Resilometer or drum tests have been developed to evaluate separation resistance, bead durability, fatigue resistance, flat spotting, heat generation, bruise resistance, standing wave characteristics, growth rolling resistance and weathering resistance.

The best method to determine the integrity of a new tyre construction is to examine its performance when the tyre is subjected to actual road tests. Vehicle testing of tyres is conducted on industry proving ground of highway, extensive graveled roads and zigzag stretch. Proving track testing is utilised to evaluate general durability, high-speed performance, tread wear under varying service conditions, traction, skid resistance, gravel durability, fuel economy, cracking resistance, cut resistance and retreadibility.  Often tyre testing involves instrument fitted  vehicles  to evaluate ride, response, harshness, cornering forces, handling, stability and general vibration analysis.

Several new testing techniques are of interest. A glass plate facility permits a road view of the tyre. As a tyre rolls over the glass, a high-speed camera photographs the action of the tyre tread.This permits traction and tread wear behaviour analysis.

Another new development utilises telemetry to continuously monitor the internal forces of a tyre being tested on a vehicle. This is a powerful tool which can permit analysis of dynamic tyre behaviour.

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Continuing from Tyre Testing Part 1 of Dr.S.N.Chakravarty‘s, here is Part 2 of the three-part series.

High Speed Test

High-Speed Uniformity (HSU) is defined as a measurement starting at a speed of over 50 km per hour and increasing to 250 km per hour or more. The essential characteristic of a test system used for this purpose is a high natural frequency of its mechanics as well as its measuring system.

The test process for HSU is performed at a variable speed over a wide range. The measured load variations acting on the axle are ‘action forces‘ and ‘reaction forces‘.

Here is a 1.20 min video of a HSU Testing Machine from Kokusai USA Inc.

Video of A High Speed Uniformity Testing Machine

All these HSU values are extremely small at lows speeds – some times too small to consider.

• Unbalance (centrifugal) force is an example of action forces and is also recognizable with an unloaded rotating wheel or on an unbalance measuring machine.  Action forces are supported by the axle of the test wheel.
• Reaction forces are, for example, changes in the dynamic displacement forces caused by the inertia of material thickness variations.  They are generated when the tyre contact are in the pass-over zone from unloaded radius and vice versa.

Forces generated by dynamic actions mainly include Radial Force Variation (RFV), and in particular Tangential Force Variation (TFV). TFV is almost completely due to inertial displacements at higher speed.

In summary, with the HSU machine fitted with a measuring hub, the above mentioned load act together on the axle of the measuring hub, much as on the vehicle, where they act through the vehicle axle, causing problems due to vibrations.

Slip

A tyre when braking, the moment of the resulting force in the contact surface, made up of the braking force and the wheel load, is balanced by the brake torque around the wheel center. As a result, the normal force acts just in front of the wheel center.

Within the contact area a shear stress arises that increases until the adhesion limit is reached, after which it decreases broadly proportionally with the locally occurring normal stress.

The tyre is the least understood component of a car.

A good match between tyre and car is also generally determined subjectively, the test driver assessing handling based on concepts such as steering feel, controllability, feeling of safety and straight-ahead stability.

There is very little standardization in these concepts, which means that vehicles can be ranked, but comparison of assessments made by different organizations is nearly impossible.

An alternative approach is the objective tests, or open-loop test, in which a vehicle is fitted with instruments is given a clearly defined input such as a certain prescribed variation in the steering angle, for which vehicle behaviour is characterized by measured readings such as reaction times, the lag of certain variables in relation to the steering input, etc.

An advantage of this approach is that these maneuvers can be imitated in simulation models, enabling the effect of design changes on vehicle behaviour to be accessed at an early stage.

To test the tyre inside a laboratory, the tyre should roll over a road surface. One cannot build a test circuit in our laboratory, therefore have three road surface simulators:

1. Small test drum on which the tyre test trailer can be positioned.
2. Large test drum on which the dynamic test rigs can be mounted.
3. Flat plank tyre tester on which the tyre is rolling over a short flat road surface.

Quasi-Stationary Tests

The stationary slip characteristics are measured with the Delft Tyre Test Trailer. These experiments are carried out on the road or in the laboratory on the small test drum. Both pure slip as well as combined slip conditions can be measured.

Transient Tests

The flat plank machine is able to measure the quasi-static and the transient-tyre behavior. It is especially suitable for measuring ply steer, conicity and relaxation lengths. The tyre dynamics can not be measured with this test stand because the velocity of the tyre is very low.

The following characteristics can be measured:

• Tyre response due to longitudinal slip (constant or step change)
• Tyre response due to lateral slip (constant or step change)
• Tyre response due to road camber variations
• The tyre stiffness characteristics (3 rotations and 3 displacements)

(….to be concluded in Part 3)

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In an earlier post on tyre machinery industry, I expressed that it’s the rapid pace of change faced by tyre producers that is driving the tyre machinery manufacturers to invest more in R&D, evolve rapidly in technology and offer customized automated solutions for maximum output to their customers.

For tyre manufacturers the consequence of increasing the automation to eliminate human errors will be, that stringent testing on 100 per cent of tyre production will be a binding procedure in future. When Dr.S.N.Chakravarty sent me this informative piece on Tyre Testing, it felt apt to be shared with you for a beginning on this topic and to exchange notes.

Here is Part 1 of his three-part series.

As in all fields of technology, methods of measuring performance is essential for development. Major areas of tyre testing system are:

1. Laboratory testing of compounds, cords, bead etc.
2. Indoor (Resilometer) endurance testing
3. Outdoor or field testing.

After designing and manufacturing, a tyre is then subject to rigorous indoor (endurance) and field test evaluation. (Laboratory testing will be dealt later).

Performance evaluation of a tyre is carried out by mainly

1. Plunger strength
2. Wheel endurance or Resilometer test and
3. Wheel high speed performance tests.

Plunger strength indicates carcass load bearing capability and safety factor whereas endurance and high speed wheel tests give idea about tyre performance.

Endurance test is carried out by freely rotating the tyre on a smooth surface metal wheel or drum at a fixed speed; increasing load step-wise to certain kilometer limit or till failure.

In high speed test, tyre is rotated on drum or wheel at a step wise increasing speed at constant load, holding for a fixed period at every speed step, till failure occurs or to a minimum speed limit.

Resilometers are now being electronically programmed to act as simulators. Further, observations or resilometer testing is being done  more  efficiently by means of closed circuit TV. Stroboscopic system and high speed cameras capable of producing high resolution pictures are also used for proper observation.

Different specifications for evaluation of tyres are available in various National and Government as well as International standards. Widely known specification is from Department of Transport (DOT), USA.

Another important test that is carried out with tyre is the measurement of heat development by determining the temperature developed in the tyre during resilometer test. A new technique to assist in the indoor evaluation of tyre is a system which utilises infrared to obtain thermal history of the tyre during test.

Besides these, Hydro Burst test (to determine bursting pressure), Rolling resistance of tyre and Static and Dynamic balancing tests are often carried out with the tyre.

For development as well as quality check Holographic Interferometry and Shearographic studies and X-ray analysis for casing flaw is carried out with tyres.

For Passenger tyre uniformity test is an important criteria.

The tyre industry is continuously developing indoor tyre tests to duplicate, at an accelerated pace, the type of performance encountered in service.

Testing machines are becoming increasingly important in the tyre industry. Government and international test institute and certifying bodies use testing machines for R&D and to develop standards. Car manufacturers and the aftermarket require not only technical specifications, but also data from conducted tests according to specified procedures.

Why test ?

Another important factor in tyre development is the ecological aspect. Lower rolling resistance means lower fuel consumption and consequently a reduction in emissions. It is also important to reduce noise pollution. Research vehicles and tests have shown that tyre noise generation is the main source of vehicle noise.

The laboratory test machines can also measure important parameters such as durability (endurance) at high speed; tyre characteristics / force and moment; rolling resistance; noise levels; tyre stiffness; foot-printing, including dynamic; and wheel fatigue.

Economic efficiency of equipment is as important for testing machines as it is in manufacturing.

Endurance Tester

The endurance tester has been designed to determine the service life of tyres. The test procedures of this machine are in compliance with DOT, SAE and ECE & BIS standards.

Image: Poling Group

The machine can be configured in several ways; one or two car tyre stations, one or two truck tyre stations, or a combination of both.  A number of options can be included, for example slip, camber, deflection, temperature / circumference / rolling resistance measurement, pressure regulation or foot-printing.

Noise Testing

The traffic noise is an increasingly important issue for tyre and highway industries, with demand from many quarters for quieter products.  A necessary property of modern tyres is a low noise level in all situations. As mentioned earlier, tyre noise generation is a major source of vehicle noise.

Two main methods are currently used for direct measurement of tyre / road noise on trafficked roads – statistical pass-by and close-proximity. Of these, the statistical pass-by (SPB) method is used most frequently because of it’s simplicity and the representative-ness of the results. Essentially the method consists of measurement of individual pass-by noise levels of a large number of vehicle traveling through a fixed test area.  This is the internationally accepted ISO method and has been adopted by many countries.

The frame and drive train of the noise tester are situated in a pit and the bearing on the road-wheel only develops low running noise, thereby creating the optimum conditions for measuring  tyre generated noise.

A trailer has been constructed that allows acoustical dynamic or static tyre testing under the wide variation of parameters – slip angle, camber, torque, speed, wheel load, tyre size, rim size and rim material – on real road surfaces. Generally the acoustical near field of the tested wheel is measure using four microphones.

Force and Moment Tester

The force and moment tester meets all requirements for tyre characteristics, such as dynamic altering of the slip angle, camber angle adjustment and hub drive. All moments and forces are measured, making this equipment suitable for testing tyres as well as for use in their development.

Image: MTS Systems Corporation

A five component measuring hub records all moments and forces in detail.  The industrial PC in the control cabinet stores the data. This sophisticated measuring system is ideal for the development of high performance tyres.

Universal Tyre Testing Machine

The universal tyre testing machine can test passenger and truck tyres for four distinct operational characteristic: load deflection, bead unseating, plunger energy, and footprint. Additional tests include tread contact area ration (image scanning by CCD camera) and the dynamic behavior of the tread pattern under load.

Machines to investigate the rolling resistance of various tyres are also part of the range, as are multi-station machines used to test and investigate solid rubber roller compounds.

Standard tyre uniformity is very different from high-speed uniformity, so different machines are needed to do the testing.

The tyre and automobile industries have been successfully using test machines for measuring tyre uniformity for many years. Tests performed with these machines, which include measurements of force and geometry, have now become a standard in quality assurance as well as development, and are completely accepted in these fields. They are therefore often called standard tyre uniformity (STU) tests.

(….to be contd in Part 2)

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