Monday, March 22, 2010

Mostly Linux: Dodge Ram 2500 Death Wobble Cure. Updated

One post from John Molidor has some good insight. Mostly Linux: Dodge Ram 2500 Death Wobble Cure. Updated

"These problems are all very serious and it is a terrible shame that Dodge has not stepped up and addressed these issues.
Play in front end components is not the cause of these problems. It is the forces that the front end components are being exposed to.
Wear and tear in Ball Joints and bushings is and should be expected. The fact that any marginal amount of wear results in these issues and escalates to the death wobble means that there is a serious inbalance in the forces being applied to these front end components.
These forces are primarily the result of a balance between Caster and Scrub radius. Both these measurments are the result of the Steering Axis Inclination Angle.
The pivot points for the spindle or the upper and lower Ball Joints are not perfectly vertical. The upper ball joint is always slightly rearward and inward of the lower Ball Joint. The amount of tilt back of this angle creates Caster and the amount of tilt inward at the top helps determine the Scrub radius.
Caster creates a self centering force much like the front casters on a shopping cart.
Scrub radius however is much different. Positive and negative Scrub radius results in a force that either forces the front of the wheel in or out as the vehicle travels forward. The faster you travel the higher this force is.
Under normal driving conditions this is the force that is always pushing or pulling on your front end parts. This force is also what causes the steering wheel to be pulled put of your hand when one of your tires hits a big bump. This is usually countered with a steering stabilizer.
Any car or truck that needs 6 degrees of caster to drive stable is probably suffering from a poor Scrub radius.
It is very likely that a poorly engineered Scrub radius is what is causing the majority of these issues.
Scrub radius is where that pivot point makes contact with the ground at the contact patch of your tires. It can be changed slightly by changing your static Camber or with wheel offset and changes in tire height.
If the pivot point is at the center of your tires contact patch there will be no adverse forces applied to the front wheel. If the pivot point is inside of the center of contact area the tire will want to turn out and the opposite is true for the pivot point being outside the center, the wheel will want to turn in.
The further off of center that this pivot point is the higher the forces that are being applied to your very expensive front end components.
If you can minimize the Scrub radius you will minimize these forces and it will take less Caster, less steering stabilizer and you won't have to have brand new steering components to avoid the dreaded Death wobble."
John Molidor
Molidorjohn@yahoo.com 

3 comments:

  1. Hey Jeff,its John Molidor. I am sorry to hear about your death wobble problems.
    While helping my brother with his 2006 I did some basic measuring and was quite surprised with my findings.
    It appears that from the factory the truck was not square. My measurment from the rear wheels to the fronts wheels showed that one side was about an inch long or short, however you want to look at it.
    This wood cause the truck dog or drive up the road with the rear tracking off to the side and could also be applying unneeded forces to the front end alignment.
    I checked the front adjustable lower radius arm bolts and they were slightly different from right to left. It seems possibly to try to counter the out of wack rear end.
    I centered the front end as best I could and then loosened the rear leaf springs at the axle and loosened the bolt holding the spring pack together.
    I then measured from front to rear on both sides and and used whatever play I found in the rear to get the measurments as even as possible.
    At the end of the day I was able to get the measurments to within 1/8 of an inch without tweaking at the front.
    If these measurments are way out of wack it could be contributing to your problem.

    If you want to get an ideal of your static scrub radius you can try this.

    With your truck level mark the inside and outside edges of your tires on the gound.
    measure the height of the center of your front wheels. Jack up the front axle and remove the front tires then with the jack set the center of the wheel mounting surface at the same height.
    you can make a fixture or approximate by sight with a straight edge or yard stick.
    hold up the yard stick and center it with the upper and lower ball joints. Extend the stick down to the ground where the tire contact patch marks were made.
    Where this pivot point line makes contact with the tires contact area is what gives you a scrub radius. If it is centered you would have zero. If it is in or out you have a pos or neg scrub radius.
    This is the point that the wheel actually pivots or rotates on.If it is inside of the center the force pushing on the front of the wheel is directed outward and if it is on the outside the opposite will be true.
    it is not hard to see if spacing the wheel out or increasing tire height is needed to center the pivot point.
    You will also notice the pivot point will be foward of the contact patch center. This is a result of the caster angle.
    Also keep in mind that any movement in the either ball joint will change this angle and contact point.
    If a ball joint is worn it could be moving under load.

    ReplyDelete
  2. Thanks John, I'll try to walk through your recommendations this weekend if we aren't snowed in again.

    Jeff

    ReplyDelete
  3. Thank you for the share. You are helping others to grow their knowledge by sharing such a valuable information very helpful

    Thanks Again
    Dodge Steering Stabilizer

    adjustable control arms

    Solid Steel Industrial

    ReplyDelete