Knowing Your Tyre Spring

Devices for measuring tyre spring rate are classified depending on the following criteria:
* Time-variation of tyre applied forces:
o static measurement;
o dynamic measurement;
o both type measurement.
* Direction of applied force:
o vertical (radial);
o longitudinal;
o lateral.
* Part of device that is moving:
o tyre hub;
o contact surface.
The test rig for measuring static vertical spring rate of passenger tyre and the vertical force transducer are shown, respectively, in figure 1 and figure 2 [1]. The test rig consists of several subassemblies: wheel attachment, loading system, measuring systems for vertical displacement and force.
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Figure 1 - Set-up for measuring vertical tyre spring rate [1]
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Figure 2 - Vertical force transducer [2]
Some device specifications are:
* wheel diameter = 12' to 15';
* fixed spindle, non-rotating wheel;
* vertical movement is effected by contact patch of the force transducer;
* vertical force up to 5000N.
Operation mode: a small vertical displacement of contact patch is induced, in such a way to primarily increase, and secondarily decrease tyre deflection. For every vertical position of contact patch, tyre deflection and vertical force are measured, using, respectively, a inductive transducer and strain gauge transducer shown in figure 2. The results are plotted vertical force vs. tyre deflection (figure 2 at chapter "Truck tyre spring rate").
A machine to measure vertical stiffness and damping coefficient of agricultural tractor tyre was built at Louisiana State University [2].
Measurement type, performed on a rigid surface for non-rolling tyre are:
* static stiffness and contact area;
* dynamic stiffness and damping coefficient.
The tyre loading machine subassemblies are: frame, axle, wheel attachment, loading system, oscillatory mechanism, displacement transducer, tyre load scale, and data logging system, figure 3.
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Figure 3 - The loading machine [2]
Some machine specifications are:
* tyre axle shaft can move in the vertical direction and can rotate, allowing for measurement at various tyre angular positions;
* wheel attachment system allows for various tyre sizes;
* for static measurements, a hydraulic cylinder pinned to the top of the frame provides up to 31 kN of compression force;
* a scale with three load cells, supporting the steel contact pad, is located directly under the tyre;
* a linear displacement transducer gives the position of the axle relative to the frame;
* during dynamic experiments, a sinusoidal motion is imposed to the axle by an eccentric shaft actuated by a hydraulic motor through a chain drive; the motor rotational speed can be adjusted to impose a certain oscillation frequency to the axle.
The data from the three load cells and from the linear displacement transducer are collected by a 48-channel data logger and transferred to a portable computer.
Some aspects of static experiments
* the vertical load is increased until it just exceeds the maximum value recommended for the set inflation pressure;
* the vertical load is slowly reduced to zero.
The increasing-decreasing load cycle is repeated two to four times to reduce measurement errors and to detect any hysteresis. Also, sets of measurements are made for different inflation pressures. A schematic static experiment is shown in figure 4.
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Figure 4 - Schematic of static experiment for tyre spring rate [2]
Deflections versus load curves are plotted for various tyre angular positions to study the effect of lug position on stiffness. Measurements effected on 14.9R30 agricultural tyre shown the lug position did not appear to affect static tyre stiffness significantly.
Some aspects of dynamic experiments
* the tyre is lowered onto the contact pad;
* the oscillatory mechanism is attached to the axle and it is adjusted until the desired preload is obtained;
* for 15 seconds tyre load, axle position and hydraulic motor angular speed are recorded;
* measurements are performed for different hydraulic motor speed, inflation pressure and preload.