Truck shakes violently when hit bump

  • 5.4L
  • V8
  • 4WD
  • 294,649 MILES
Truck shakes violently when hit bump at fifty mph. Had the upper and lower ball joints replaced, front and rear shocks replaced and left wheel bearing replaced and control arms replaced
I am very confused since that did not fix the problem. What can it be and what can I do? I am afraid of an accident and getting hurt. Please help
Do you
have the same problem?
Friday, August 31st, 2018 AT 3:53 PM

1 Reply

The loose and worn parts in all front suspension systems will not cause that, but when the problem exists, those worn parts can allow it to occur more violently or more often. There is two common things to look for to solve this. The first is one of the three main alignment angles called "caster". That is seeing the upper ball joint back further than the lower ball joint as viewed from the side of the vehicle. That equates to the angle the fork of a bicycle sits at where it's angled toward the rear on top. A little caster creates the tendency for the steering wheel to want to return to center by itself after you go around a corner. More caster, meaning that upper ball joint is back even further, provides more directional stability so you do not have make as many steering corrections on the highway, but when caster is adjusted too high and you hit a bump, one tire momentarily tugs to one side, then when it wants to correct and come back to center, it comes back with so much force, it overshoots and goes the other way, then does the same thing coming back again. The rolling resistance of the tires keeps adding energy to that overshooting, and you typically cannot stop it by holding tightly to the steering wheel. The only thing that stops it is slowing down.

Ford is one manufacturer that often leaves off some of the alignment adjustments to save money. For some of the other angles that means less-than-perfect tire wear, but caster has very little effect on tire wear. If it is not adjustable on your vehicle, it is not too high now. It will not have changed on its own. Where caster causes this problem is when the alignment specialist adjusted it too high.

The next thing to look at is a lot of truck models need a steering stabilizer to prevent this from naturally occurring due to the design of the suspension system. That stabilizer looks exactly like a shock absorber, and one end bolts to the truck's cross member, and the other end attaches to the steering linkage. A regular shock absorber pushes together real easily so a tire hitting a bump on the road does not push the vehicle up with a jolt, and it needs much more force to pull it apart so it reduces a tire's tendency to drop into a pot hole. While a steering stabilizer looks the same, it pulls apart and pushes together will equal force both ways. Just like with a shock absorber, hydraulic damping oil can leak out, then the part loses its effectiveness.

Sometimes this wobble first becomes a problem with age and/or high mileage. Worn control arm bushings do not hold the suspension system parts in perfect geometry and they let the wheel wander around a little. Sagged springs, which happens to all vehicles based on age, lowers ride height which also changes the geometric relationship of all the suspension parts. These things can invite the wobble to occur rather than work to reduce that tendency. Experienced alignment specialists will always measure the ride height before taking your money for an alignment. A second problem with sagged ride height is due to the changed geometry of the parts, you will have accelerated tire wear even though the alignment computer says all the angles are adjusted perfectly. The computer simply assumes ride height is correct. If it is low, it can be adjusted on vehicles with torsion bar springs. On others with leaf springs or coil springs, those must be replaced before performing the alignment.

If your truck has a steering stabilizer, check if it is wet from the oil leaking out. Replace it for that. If it looks dry, disconnect one end and verify it takes a lot of effort to extend or collapse the shaft. If you can move it freely with one hand, it is too weak to be effective. Also, it is not that uncommon to need to add a second, aftermarket steering stabilizer in addition to the original one. Trucks with over-size tires are especially prone to this wobble and the need for two stabilizers.

Check this article out for more information related to this problem:
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Friday, August 31st, 2018 AT 8:42 PM

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