I have a 2004 E350 Superduty that I usually drive empty. I load it up and pull a trailer twice a year to go camping. I never noticed any shimmy for the first 140K miles. In winter I switch to Firestone Winterforce studded snow tires, which are lower load rated tires, and I assume lighter weight than 2,600 lb tires. The factory tires were Michelin, which I replaced at 140K miles. That was when my problems started. Before that, there was no noticeable shimmy with the Michelins or the lighter load rated studded Firestone snow tires. I got Dunlop Rovers to replace the worn out Michelins, then had shimmy problems. Replaced lower ball joints, all the tie rod ends, did the road force balance according to TSB 05-24-8, installed hydraulic steering shimmy damper, replaced I-beam pivot bushings, sway bar end bushings (which help tie the I-beams together through the sway bar), got new aftermarket wheels with new Hancook tires, added Centramatic balancers, then recently replaced all the ball joints at the same time. For two years this was a problem with the summer high load range tires, not with the Firestone snow tires.
Increasing the caster made the problem go away!!!!!!!!!!!!!!!!!!!!!!!!!!! Do not pay attention to anyone who posts "too much caster causes shimmy" or that there is a Ford truck TSB that says to decrease caster to eliminate shimmy.
I do not know what the alignment was set at for most of this time because the mechanic set the caster bushings with the slots toward the inside when replacing the ball joints, which was just recently at 195K miles. The bushings that were in it were 1-1/2 degree bushings and with the slots to the inside the caster is set to the middle of the range and camber is the most negative. A Firestone shop measured the alignment with this setting and it was like this:
left caster 2.2 left camber -.5
right caster 3.4 right camber -1.2
I told them to adjust the caster to as high as it could go while keeping the camber within spec and they said the factory original bushings are not adjustable and wanted to sell me bushing-in-bushing kits with the numbers on them for several hundred dollars. So I had to adjust the nonadjustable bushings myself.
I went home and set the left one for the most caster it would do, with the slot facing back and drove it for a week while I ordered new bushings. That made it like this and it still shimmied:
left caster 3.7 left camber 1
right caster 3.4 right camber -1.2
I got a 3-1/2 bushing for the left and a 2-1/2 for the right from NAPA, Moog bushings for about $20 each.
Now it is at:
left caster 5.7 left camber .7
right caster 5.9 right camber .7
Shimmy is gone and with nearly equal caster on each side it goes straight. I guess the factory setup is to have 1/2 degree less caster on the driver's side so that it pulls left to counter road crown on a two lane road. Most of my driving is on the highway in the left lane so the crown is to my right. The bushing replacement is easy, but you need to have an alignment first and keep track of the angle of the bushing that is installed so you can adjust from there.
Thanks Harvard for starting this post. Now I love my van again. 195K miles, alternator just went, and #7 spark plug boot have been the only problems other than this shimmy after replacing the factory tires.
