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Rotational force

John.t.little1

Aluminum
Joined
Oct 10, 2014
Location
St. Augustine, FL
Hey y'all I've been scratching my head and trying to figure out this problem. A friend has a high horsepower truck and the steering is a week link he is planning to change the tie rod position to lower and straighten the angles but we are concerned that the tie rods being lower will cause rotational force to be applied to the idler arm and center link. Any thoughts on how I can figure this out? Thanks in advance -John

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If you know "the number" what will you do with it?

Most forces (vectors) get resolved into vertical and horizontal components as applied to the planes of reference that are relevant. there really isn't anything called a "rotational force" . Though torque is close, it is just force and a distance.

The laws of sines takes care of it. Look up Vectors, and maybe "resolving force vectors" for detailed and lengthy explanations and examples. The test will be open book ;-)
 
Front end geometry is involved here. Any changes, like the one you are proposing causes all kinds of issues and can create binding forces that can be equal to many, many unintended tons of force. In practice, the steering gear is positioned height wise on the chassis in the center of wheel travel between maximum bounce and jounce. This is done to center wheel toe change which always occurs during suspension travel. Without considering limit issues during max wheel travel, which MUST be checked, your change will cause an excessive toe in condition during spring compression. That excessive toe will scrub your tires, create excessive compressive loading on the tie rods, their joints and the steering rack. None of this is healthy. The only reason to want to do this is a change in chassis height of 2". Be advised that these components are composed of heat treated special alloy and designed for these forces. A simple weldment is down right dangerous and ill advised.
 
Front end geometry is involved here. Any changes, like the one you are proposing causes all kinds of issues and can create binding forces that can be equal to many, many unintended tons of force. In practice, the steering gear is positioned height wise on the chassis in the center of wheel travel between maximum bounce and jounce. This is done to center wheel toe change which always occurs during suspension travel. Without considering limit issues during max wheel travel, which MUST be checked, your change will cause an excessive toe in condition during spring compression. That excessive toe will scrub your tires, create excessive compressive loading on the tie rods, their joints and the steering rack. None of this is healthy. The only reason to want to do this is a change in chassis height of 2". Be advised that these components are composed of heat treated special alloy and designed for these forces. A simple weldment is down right dangerous and ill advised.

It's not suspension lift Need to make some changes to current centerlink to address bump steer but I'm afraid it will add in additional leverage to tie rods and make the centerlink want to twist.
That's what he told me. The tie rods are adjustable and I do see what you're saying about the toe in condition when suspension is at full compression. I'll advise against it.
 
Truck builders have engineering design and development departments that, in the aggregate, know what they are doing. I suggest that it is better to trust the engineers than for non-professionals to try to improve their work.

Larry
 
Good for you!

Just keep in mind that every arrow that goes off in some whacky direction can be replaced by one that goes up and down, and one that goes left and right ;-) OH also, unless the parts that those vectors are attached to are accelerating out into space, there are a pair of vectors that oppose the applied force, Mostly the systems are assumed to be in equilibrium, and both sets of vectors are not shown.

Statics, as opposed to dynamics.
 
First, I have to agree with others above who caution about changing the OEM design. I am sure a lot of engineering thought went into it. Their engineers probably considered things that you, I, and none of the others here will even think of.

That being said, I find your sketch very lacking for any real analysis. With my limited knowledge of steering mechanisms, I am assuming that the part you call the centerlink is moved left and right by the idler arm pin. And it moves the tie rods (one on each end but only one is shown) which, in turn are attached to arms on the wheels that turn them. But you do not show how that center link is constrained except for the pin on the idler arm. There must be additional constraints on it's motion or it will flop around and never do it's job. These additional constraints will be, MUST be considered in any analysis of the forces involved because they will be supplying some of them. Also some of the critical dimensions of the components, especially the center link, are not given.

From a glance, I think the rotational force you are talking about may not be the biggest problem. By lowering the point of attachment and moving it forward, you are also apply rotational forces on the centerlink that are on axis that are perpendicular to the plane of the drawing and that are vertical in that plane.

Another dimension that is not shown is the length and angle of the arm on the wheel that the tie rod is attached to. This arm rotates about a vertical axis or on an axis that is at an angle to vertical. So the angle of the tie rod will change as the wheel is turned. This angle would not show (very much) in your front drawing so an above view would also be needed.

All in all, there is way too little information to provide any meaningful analysis, even if you do know vector math. To properly analyze this mechanism you would need to know the exact dimensions and angles for it in several positions, not just centered. I would do a minimum of five: full-left, half-left, center, half-right, and full-right. Each of these will present a different mix of the angles involved. Oh, and you will have to consider both wheels and their linkages, not just one because the other wheel will be providing additional forces on that centerlink at the same time.



Hey y'all I've been scratching my head and trying to figure out this problem. A friend has a high horsepower truck and the steering is a week link he is planning to change the tie rod position to lower and straighten the angles but we are concerned that the tie rods being lower will cause rotational force to be applied to the idler arm and center link. Any thoughts on how I can figure this out? Thanks in advance -John

6d889c87e745f2b8a31e8573e9204a83.jpg
 
Thanks Guys and no Ron it's for a buddy's race truck

not done all the calculations, but moving the attack point will increase the vertical component of the vector by about the sin of 15 degrees. or so. that is just from adding a new vertical component of the force at the center link to the added torque is about 25% of the force form the connecting rod or so...may not be a real issue. you also lengthened the torque arm by about 7%....

..that is a very rough estimate.

in real terms, if the rod has 100 lb on it you will have about 35 lb pointing up twisting the center link, as opposed to about 10 lb in the first case. does this make sense to you?

dee
;-D
 








 
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