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.

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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