Suspension location for suspension pedestal

[Bart van Driessche]

For my thesis at the department of Industrial Design Engineering, I am developing a suspension pedestal. These kind of products are used regularly in boats (link) or truck seats and make use of two diagonal beams and a shock absorber. The vertical excitation can originates from waves or road undulations. A simple FBD can be found below.

I was wondering if the location of the shock absorber, relative to the rear pivot points (A), affects the functioning/effectiveness of the system? If I am correct, placing the shock absorber more forward or backwards only puts more or less stress on the rear pivot points. Yet, by using proper bushings this may not affect the functioning/effectiveness of the system. Assuming the construction is stiff enough to withstand the weight (Fw).

Or, is it better to decrease the distance between the center of gravity (C), in relation to the rear pivot points (A)? E.g. placing the center of gravity at location C'.

Or, e.g., placing the shock absorber more forward at location B', so that the center of gravity (C) is located between point A and B'?

 

[jrmichler]

You have two variables: Distance AB, and distance AC. You can calculate the force on the bushings for any distance AB and AC. You also know, or can establish the maximum and minimum distances AB and AC based on the size of a typical seat.

The challenge is find the best combination of AB and AC, and to show that it is the best combination. The tool for doing this is a contour plot. Make, for example, the horizontal axis the distance AB, and the vertical axis distance AC. The bushing force is the plotted variable, with contour lines of constant bushing force.

Such a plot would be a very solid part of a master's thesis, because it naturally leads to a discussion of seat suspension optimization.

 

[Bart van Driessche]

Thank you for the reply jrmichler.

Yet, I think making a contour plot exceeds the scope of my project. Is it also possible to explain the best relative position based on basic theory or assumptions? Some one recommended me to place the center of gravity (C) as well as the position of the shock absorber (B) at or close to position A. Though, he didn't explain me why this would be beneficial? What I can image is that if the total length of the whole upper beam is smaller, and therefore the distances AB and AC too, the deviation of this beam due to play at the pivot points at location A is less.

 

[jrmichler]

A master's degree is an introduction to research degree. This is a simple problem, and one that you can and should solve for yourself. We at PF are more than willing to help you. In this case, help consists of suggesting a path to a solution.

Do the calculations in a spreadsheet. Hand write the results on a hand sketched graph. Hand draw the contour lines. If you are up to speed on free body diagrams, it should take less than three hours. And the resulting plot will fully answer all of your questions, including some that you have not yet thought of.

 

[Bart van Driessche]

I appreciate the help and quick answer. Thank you.

But why using the bushing force in the pivot points, as this is not the main parameter that affects the effectiveness of the system, right? Perhaps my topic question was not fully clear. With 'system' I meant the effect of the distances AB and AC on the functionality/effectiveness of the shock absorber. This is the determinative parameter for a comfortable ride. And thus, therefore, a comfortable system as a whole.

 

[jrmichler]

Complex problems are solved by solving part of the problem, then using the results to solve further, then repeat until the entire problem is solved. In this case, you cannot completely ignore the bushing loads. High bushing loads require strong (heavy) parts, and have high(er) bearing friction. Friction is bad. You also need to fully understand how the loads on the various pieces change with changes in dimensions AB and AC. Start there. Then use what you learned to investigate shock absorber location.

Hint: If you do as I suggested earlier, the answer should be obvious.