Power Hop in the 1970s (39.4MB wmv): Power hop is documented for a variety of tractor configurations (MFWD, 4WD, and 2+2), several different tractor brands, and different chassis sizes being field tested by John Deere engineers in 1978 and 1979. Note that in the final scene of the IH 2+2 tractor, power hop occurs as the tractor climbs a slope without pulling any implement. The down slope component of its weight alone is enough “draft load” to induce power hop on this soil and slope. The poor quality of the video is the result of many generations of copies that preceded the final version on this DVD.

Deere Power Hop Test Highlights (Part-1, 37MB wmv Part-2 41MB wmv ): This is a compilation of major events in the extensive power hop testing efforts jointly conducted by John Deere and the tire companies starting in September 1989 and continuing through October 1991.

Agtech Power Hop Test Highlights(37.9MB wmv): This reviews the equipment and techniques used by the Alberta Farm Machinery Research Centre (subsequently the Agricultural Technology Centre) to evaluate and control power hop on tractors in farm fields. It also shows power hop of varying degrees on MFWD and 4WD tractors.

4WD Power Hop Control: Two Solutions (27.2MB wmv): Power hop of this John Deere 8970 4WD tractor could be controlled either with high front inflation pressures or with high rear inflation pressures in the soft soil conditions found at this central Iowa location in 1991.

Power Hop with Suspended Front Axle MFWD (68.7MB wmv): This shows the effect front axle suspension has on power hop on MFWD tractors. On unsuspended tractors, power hop is typically more vigorous and tends to come from the front of the tractor. With front suspensions, power hop is somewhat more muted and tends to come from the rear of the tractor but can cause problems for the suspension system. The last shot shows a vertical front suspension system working just opposite to its design intent on an MFWD tractor experiencing power hop. The suspension arms are pivoting at their outer ends and moving up and down at their inner ends instead of pivoting at their inner ends and moving up and down at their outer ends. As a result, instead of the wheels moving vertically while the tractor chassis motion is attenuated, the wheel motion is attenuated while the tractor chassis moves vertically.

The Ultimate Goal: A Stable Tractor (14.7MB wmv): This shows a correctly set radial tire equipped 4WD tractor operating in the field as the load is slowly increased from 0 until the tractor is brought to a  complete stop (100% slip). The tractor remains stable through the complete pull spectrum,   showing no tendency to power hop at any pull or slip level and presents the ideal case.

4WD Power Hop Simulation (16.5MB wmv): ADAMS, Early 1980s-These 3D simulations of a John Deere 8640 4WD tractor were produced at the Deere & Company Technical Center, Moline, Illinois, in 1981. Nicky Orlandea’s original version of ADAMS was used to create the simulations. The three-dimensional graphic displays with hidden line removal were produced by a program named HAL created by Tibor (Ted) Berenyi. The animations were created by Bernard Romig by filming individual frames to display at a rate of 30 frames per second. The model was fully 3D and included tire, power train, and engine elements. The tiresoil interactions (traction and stiffnesses) were essentially the same as those used later in the two dimensional model developed in the Distinguished Lecture paper. The 3D simulation clearly is capable of modeling power hop, but it required an enormous amount of effort and computing power (for its day) toproduce each test run sequence.

Comparison of ADAMS Simulation to Colorado Field Tests in the Early 1990s (54.7MB wmv): These 3D simulations of a John Deere 8760 4WD tractor were produced by David Smith of the Deere & Company Technical Center in 1992. Using the commercial version of ADAMS, he simulated conditions corresponding to some of the field tests that had been conducted in Colorado in the fall of 1991. For the cases simulated, the model and field tests agreed on which inflation pressure combinations led to power hop and which resulted in a stable tractor. An animated wire frame model of the tractor geometry is shown in the video. Note that the forces acting on the wheels and the drawbar load are shown as animated arrows whose length is proportion to force magnitude.

MFWD Hop Mode Shape Animation, 1989 (5.7MB wmv): This animation shows the hop mode of the John Deere 4450 MFWD tractor, discussed in detail as Case 2 in the paper, just after hop starts. It was produced by Bernard Romig using Mathematica. The elongated oval in the video is the path of the tractor center of gravity. Note that a mode shape represents only relative amplitudes-not absolute displacements. Thus the motions were created larger than might actually occur for the purpose of clarity of the animation. This animation is very similar to the motions exhibited by real tractors just after power hop begins.

Tacoma Narrows Bridge (7.6MB wmv): The Tacoma Narrows Bridge scenes in this familiar clip show self-excited torsional vibrations induced by a steady 42 mph wind just prior to failure on November 7, 1940. For several weeks prior to this date, the bridge exhibited large vertical bending vibrations in winds of less intensity. These were not self-excited vibrations, but, rather, forced vibrations due to vortex shedding. Many textbooks have incorrectly described the Tacoma Narrows Bridge failure as being the result of a forced vibration. See the references on this in the Distinguished Lecture for a detailed discussion.

Galloping Power Lines (6.4MB wmv): The very large amplitude oscillations of power transmission lines shown in this clip occurred after an ice storm. Wind blowing across the resulting “ice airfoil” cross sections of the wires created the self-excited vibrations. The ice on the wires is not visible due to the distance of the lines from the camera, but it is visible on the ground and at the base of the towers at the very beginning of the video clip.  Near the end of the clip, bending trees illustrate the effect of strong crosswind.

Galloping Bridge Cables (42.8MB wmv): On February 22, 1974, at approximately 7:30 a.m. as the first author crossed the I-280 Bridge over the Mississippi River at Davenport, Iowa, he noticed large amplitude vibrations of the suspension cables. A sleet storm the night before had created “airfoil” shaped cross-sections on the cables. Then the wind direction changed, resulting in aerodynamic flutter, a self-excited vibration. This tied-arch suspension bridge had just been opened for use the previous summer. At each panel point there are four cables each 2.25 inches in diameter spaced on a 12 inch by 16 inch pattern. The longest cables are approximately 100 feet in length. Of noteworthy interest, one of the four cables in each cluster usually exhibited far greater amplitudes than the other three. Movies of the phenomenon filmed from about 8:30 to 9:30 later that morning were sent to the Illinois Department of Transportation. Ultimately, that department designed and installed a series of cross-clamps with five rubber isolators connecting the cables and cross bars to force all cables to move together and to provide some damping.  These were positioned at two-thirds the height of the cables to provide asymmetry of cable length as well as to blend with the arches. No instability has been observed following sleet storms since the clamps were installed.

Airplane Nose Wheel Shimmy (2.2MB wmv): This is a single photograph showing the wreckage of a burning aircraft on a runway. The path of the shimmying nose wheel that failed is clear from the wavy rubber marks on the runway. Caster wheel shimmy and trailer weaving are common examples of self-excited vibrations.

Grocery Cart Caster Wheel Shimmy (7.8MB wmv): One of the most ubiquitous examples of self excited vibrations is the shimmying of caster wheels. This video clip shows the vibration on a common grocery cart.

Firestone Demo of Road Lope on MFWD Tracto (35.6MB wmv): This video was provided by Ken Brodbeck of the Firestone Agricultural Tire Company. Road lope is a forced vibration that occurs when the rotation frequency of a wheel with sufficient geometric out-of-roundness of the tire, rim, dish, hub, axle, or a combination of these is near a pitch or bounce natural frequency of the tractor on its tires. In the video, the front wheel dishes are initially not centered relative to their rims, and the tractor exhibits road lope at the critical speed of 22 mph. The second section of the video illustrates how the wheels were reassembled to minimize the offsets. The third section shows the tractor running smoothly at 22 mph with the improved assemblies. A final section shows smooth ride at 25 mph.