Browsing by Subject "Dynamische Radlast"

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Einfluss dynamischer Radlastschwankungen und Schräglaufwinkeländerungen auf die horizontale Kraftübertragung von Ackerschlepperreifen
(2006) Schlotter, Volkmar
Given the large variation of application conditions of tractors and agricultural machinery, the force transmission between tyre and ground is of particular importance for the efficiency in agricultural use. For a proper assessment of the tyre the exact knowledge of the tyre force behaviour in vertical, longitudinal and lateral direction is required as well as their inter-dependences. Besides the steady-state behaviour the dynamic behaviour of the force transmis-sion is of high relevance. This thesis centres on the investigation of the lateral-force behaviour which is highly important for cornering, for driving on slopes, and also for vehicle roll. The steady-state and transient behaviour of the lateral force has been investigated using the Single Wheel Tester of Hohenheim University. Comparative tests for the longitudinal force transmission have been carried out. The focus was the assessment of dynamic behaviour under the influence of transient parameters particularly on rigid surfaces. The investigations for the steady-state lateral-force transmission show higher lateral forces for wider tyres especially in case of small slip angles. Tyre load has a strong influence on the cornering stiffness. A large increase in tyre load can lead to doubling the cornering stiffness. For small slip angles the lateral force tends to a maximum or even declines with increasing tyre load, so that for the same lateral force larger slip angles are required. The test results with non-steady-state slip angle excitation reveal a time delay in the lateral-force generation. This behaviour can be described by a time constant. An increase of the tyre inflation pressure from 0.5 bar to 1.6 bar leads to a decrease of the time constant by more than one third. A hyper-bolic decrease of the time constant with increasing driving speed confirms a stroke-dependency and leads in consequence to the introduction of a relaxation length. The relaxa-tion length derived from the measurements shows an increase with reduced tyre inflation pressure and rising tyre load. The values for the relaxation length are in a range of approx. 0.5 m to 1.6 m. A strong dependence on the respective method of measurement was found this means that measured values should always be reported in connection with the employed measurement method. With longitudinal force measurements the longitudinal stiffness, analogous to the cornering stiffness, was determined. For the longitudinal stiffness of the traction force values nearly twice as high are determined compared to the longitudinal stiffness of the braking force. For both parameters a nearly linear increase with the tyre load was found. The comparison of lug-excited and surface-excited tyre load fluctuations indicates a dominant influence of the lug excitation and the radial run-out of the wheel compared to stochastic excitations on road surfaces. For stochastic excitations in form of a bump the tyre load fluc-tuations are strengthened. A different behaviour of the lateral force with positive and negative gradient of the tyre load can be seen. Therefore the mean lateral forces are decreased for stochastic excitations. With low frequency excitations of the tyre load up to 2 Hz realised with the Single Wheel Tester the mean lateral force decreases by up to around 6%. The transient response of the lateral force has been determined using sinusoidal excitation of the tyre load. The relaxation length of 0.40 m to 0.75 m for tyre load excitation is lower than the relaxation length for slip angle excitation. Here as well a decrease with increasing tyre inflation pressure can be observed. Comparison with the results for the longitudinal force transmission under the influence of tyre load fluctuations shows that the time constant of the traction force amounts to only approx. one third of the time constant of the lateral force: the traction-force transmission reacts faster to tyre-load fluctuations than the lateral force transmission. Simulation of stationary lateral forces with various models shows good accordance with the measured results. For interdependent longitudinal and lateral forces the Slip-Drift-Model with appropriate adaptation of the input parameters shows good results. For modelling the non-steady-state lateral force transmission the Maxwell model was used. The spring and damping parameters have been determined with the results of measurements. An increase of the lateral tyre stiffness due to increasing tyre inflation pressure or reduced tyre load can be seen. The damping constant is strongly reduced with increasing driving speed, whereas the tyre load and the tyre inflation pressure have only a slight influence. The lateral damping constant is considerably higher than the vertical damping constant.