Browsing by Subject "Tyre"

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Dynamische Eigenschaften von Ackerschlepper-Triebradreifen bei höheren Rollgeschwindigkeiten
(1989) Schrogl, Horst; Kutzbach, Heinz Dieter
Da über das dynamische Verhalten rollender Ackerschlepperreifen bisher nur wenig bekannt war, war es das Ziel dieser Arbeit, die Einflüsse von Reifenabmessungen und -bauart, sowie der Betriebsbedingungen wie Rollgeschwindigkeit, Belastung und Reifeninnendruck zu ermitteln. Durch die umfangreichen Versuchsreihen mit einem neuen Flachbahnprüfstand konnten dabei zum großen Teil sehr eindeutige und neue Erkenntnisse gewonnen werden.
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.
Entwicklung und Applikation eines instationären Reifenmodells zur Fahrdynamiksimulation von Ackerschleppern
(2009) Ferhadbegovic, Bojan
The driving speed of modern agricultural tractors increased continuously during the last years. Nowadays, a maximum driving speed of 50 km/h is standard for agricultural tractors in Germany. To be able to drive safely at such high speeds, the agricultural tractors are equipped with a suspended front axle. However, a rear axle suspension for standard tractors is still too expensive and has thus not prevailed yet. The increasing percentage of transportation duties and the efforts to increase the productivity generate a high demand for even faster agricultural tractors. Therefore, the first standard tractor with a maximum driving speed of 60 km/h was presented in 2005. This speed was achievable without a major change of the vehicle?s basic design. However, even higher driving speeds will require substantial modifications of the vehicle?s concept and are thus not expected very soon. Since the agricultural tractors are designed mainly for the field work, their driving dynamics characteristics on the road are not always perfect. The high vehicle masses, possible insufficiencies in the construction and interaction of various vehicle components together with the missing rear axle suspension can lead to critical behaviour of the vehicle, especially during high speed cornering. The entire suspension on the rear axle is accomplished by the tyres. Since the agricultural tyres are mainly designed for carrying high loads and generating high draft forces on soft soil, their driving dynamics properties on the road are rather insufficient. They usually have a radial run-out with an amplitude of 1-2 mm. Due to the rather low damping of the agricultural tyres, this rather small run-out can cause vibrations with a quite high amplitude, especially if the excitation is close to the eigenfrequency of the vehicle. Furthermore, the tyre inflation pressure varies between 0.8 and 2.0 bar - which is a relatively large span - making the vehicle setup more difficult. To analyse the driving dynamics of agricultural tractors and to identify possible problems and insufficiencies at an early stage of development simulation models are used. A very important part of a vehicle model is the tyre model. Being the link between the vehicle and the ground, the tyre has a high influence on the vehicle behaviour. It is therefore essential to use an accurate tyre model. Most of the commercially available tyre models today are optimized for passenger car tyres and are thus of limited use for soft and large agricultural tyres. In this doctoral thesis at the University of Hohenheim a dynamic model for agricultural tyres on rigid surfaces was developed ? the Hohenheim Tyre Model. It is a hybrid model since it needs physical parameters like spring and damper coefficients as well as empirically obtained data for the tyre-ground-characteristics. The Hohenheim Tyre Model uses the wheel speeds and the speed of the tyre tread to calculate the deflection velocity and the deflection of the tyre and subsequently uses both values as inputs for spring-damper-elements. The force calculated in these so-called Voigt-Kelvin-Elements is finally used to determine the slip and the slip angle of the wheel. The tyre model was created in MATLAB/Simulink and can be coupled to any multi-body-software. The model allows to calculate the transient tyre behaviour in three dimensions with a low computational effort, which makes it adequate for driving dynamics simulation. It takes the nonlinearities of the tyre into account and calculates the forces for pure longitudinal or lateral slip as well as for combined slip conditions.
Experimenteller Beitrag zum Schräglauf- und Lenkmomentverhalten rollender Reifen landwirtschaftlicher Fahrzeuge auf starrer und nachgiebiger Fahrbahn
(1991) Heine, Adolf
Beim Einsatz landwirtschaftlicher Fahrzeuge können zwei Fahrzustände mit schrägrollenden Reifen unterschieden werden. Die Spurhaltung beim Fahren am Hang oder beim Pflügen erfolgt durch konstant (stationär) schräglaufende Reifen, wohingegen Lenkvorgänge zur Änderung der Fahrtrichtung als instationäre Fahrzustände anzusehen sind, weil sich der Reifenschräglauf hierbei ständig ändert. Um das instationäre Schräglaufverhalten der Reifen landwirtschaftlicher Fahrzeuge auf starren und nachgiebigen Fahrbahnen untersuchen zu können, wurde als Versuchseinrichtung ein Einachsanhänger mit stufenlos gegeneinander einlenkbaren Versuchsreifen und einer Fünf-Komponenten-Kraft- und Momentmessnabe entwickelt.
Untersuchungen der Kräfte an gelenkten und angetriebenen Ackerschlepperrädern bei Gelände- und Straßenfahrt
(1996) Barrelmeyer, Thomas
Das Fahrverhalten von Geländefahrzeugen wird durch das Zusammenspiel von Reifen und Fahrbahn beeinflusst. Am Beispiel des Ackerschlepper-Triebradreifens wird durch Messungen mit einer Einzelradmesseinrichtung belegt, wie Treibkraft- und Seitenkrafterzeugung einander beeinflussen. Reifenluftdruckverstellanlagen werden vor allem diskutiert im Hinblick auf Bodenschonung bei der Fahrt auf dem Feld und zur Erzielung eines verminderten Rollwiderstandes bei der Fahrt auf der Straße. Ebenso wichtig ist allerdings, dass eine hohe Reifenauslastung sich positiv auf die Triebkraftgenerierung im Gelände auswirkt, während sich das Lenkverhalten auf fester Fahrbahn, das durch die gemessenen Schräglaufsteifigkeitswerte gut beschrieben werden kann, verschlechtert. Die Messeinrichtungen zur Messung der Reifenkräfte und zur Ermittlung mechanischer Bodenkennwerte werden ausführlich beschrieben. Die Messungen werden mit Modellrechnungen verglichen.
Untersuchungen der Kräfte an schräglaufenden angetriebenen Ackerschlepperrädern
(1991) Armbruster, Karlheinz
Mit einer Einzelradmesseinrichtung wurden an Ackerschlepper-Antriebsrädern bei Radschräglauf und Schlupf gleichzeitig auftretende Längs- und Seitenkräfte an Reifen unterschiedlicher Größe auf verschiedenen Fahrbahnen gemessen. Die Ergebnisse wurden dargestellt und unter Berücksichtigung des Einflusses von Fahrgeschwindigkeit und Schlupf auf die Bodeneigenschaften durch Modellrechnungen beschrieben. Steigender Schlupf führt zu größeren Triebkräften, eine Zunahme des Schräglaufwinkels zu einem degressiven Anstieg der Seitenkräfte. Die Radführungskräfte nehmen vor allem mit der Radlast zu. Größere Raddurchmesser führen zu höheren Triebkräften, breitere Reifen zu größeren Seitenkräften. Eine Zunahme der Längskräfte führt zu einem Rückgang der Seitenkräfte. Das Maximum der Seitenkräfte liegt im Bereich geringen negativen Schlupfes und geringer negativer Triebkräfte.