Re: Sudden violent vibration …
That is usually caused by play in the front steering or suspension. With a little movement allow it allows a vibration that grows and quickly becomes violent. It usually takes a combination of things for that to happen. Possibly loose parts, bad shocks and tires out of balance. It's hard to believe a vehicle can go from being perfectly smooth to that violent from one bump in the road.
With the tries on the ground, have somebody turn the steering wheel from side to side and look for play in the tie-rod, draglink, steering box, spring perches. front spring U-bolt clamps, kingpins. loose wheel bearings, broken spring leaves broekn fron cross member.... Then jack up the frontend and spin the tires to see if there are any bulges in the sidewalls or tread (separation) or if they are badly out of round. Bias ply tires get flat spots from sitting too long and you have to drive them miles to get them round and running smooth again. Most tires will have be slightly out of round but it should not be easily noticeable. They might also be out of balance. If you do not see any weights on them, check the front and back of the rim. I would definitely have them balanced. Weak shocks can also contribute to "speed wobble". They allow the movement to get larger and larger.
At the phone company we had F-350 aerial boom trucks that were loaded to the maximum gvw, a lot of weight on the rear axle. If everything was not perfect in the frontend, they would go into violent speed wobbles. When the drivers complained, I would test drive them at about 50 mph across some nearby railroad tracks, if I went across them at just the right angle, I'd better be holding on, it was liking a bucking bull. No stopping the shaking without coming to a complete stop. It's literally yanking the steering wheel out of your hands because of those heavy front tires, it continues as you slow down until you finally come to a complete stop. Then it is fine again, maybe fine for months. Usually that was from loose bushings in the front panard bar, from the frame to the straight axle. This would allow the axle with the parallel front springs to move from side-to-side, it did not take much play in those busking. Every service we checked them. We stocked a panard bar so we could quickly replace it during the service and then later we could press in and out the bushings and have it ready for the next truck. Some of those trucks, the only way to stop the speed wobble were with new panard bushings, the addition of a steering stabilizer, take any play out of the steering box, set the toe-in, new front shocks and new front tires. Because the rear weight could very on those trucks daily the caster was constantly changing which also might have contributed to it. Everything had to be absolutely perfect. As soon as the front tires got the least little uneven wear pattern the speed wobble would start again. We would mount new tires and balance them and save the slightly worn ones to run on the rear on something else. A couple years later the F-350's started coming with factory steering stabilizers so we knew Ford was having problems with them. That didn't stop the problem either. Then a couple years ago I went out and bought one of these 1994 F-350 aerial trucks with brake problems, (no brakes at all) for $2300.00 so I can trim my trees, roof the house and paint the second story and not fall off a ladder. I don't plan to be going over 50 mph in the yard so I'm not worried about speed wobbles. I looked under it the panard bar bushings are shot.
