Installation Ratio Questions

[CU Matt]

I have been working designing the kinematics for our suspension system and am unsure about some of the commonly used terminology.
For instance, which of the following would be in the form of a linear installation ratio:

1. IR=constant
2. IR=m*x + b

Where x is the vertical movement of the wheel center from static resting position.
Furthermore, I have read in the book "Race Car Vehicle Dynamics" that the installation ratio is equal to the spring compression divided by the wheel center vertical displacement. What is unclear to me is if there a standard exists for defining the installation ratio for a certain amount of spring or wheel center movement.

For my first iteration of the suspension kinematics (one of many to come, for sure), I am finding that for the first inch of wheel center movement in bump, our springs compress 0.75 inches. On the forums, and on FSAE spec sheets I have seen various installation ratios. What determines the installation ratio that I should design for? My thought was that I would specify an installation ratio so that I could iterate through calculations to determine estimates of load transfer, ride frequencies, and other parameters and then the final installation ratio would be determined during testing, where it could be modified by changing the stiffness of our springs. In my current calculations, changing the IR most significantly impacts our roll gradient [(degrees of roll)/ (g of lateral acceleration)]. It had a slight affect on the distribution of the load transfer between wheels, but nothing significant. A IR of 1:1 gives us about 0.9 deg of roll per lateral g. Right now it seems that the significance of the IR is its impact on the roll gradient and potentially on the ride frequencies, as well; is this correct?

To summarize, the following are my questions:

1. Which of the following would be in the form of a linear installation ratio:
a. IR=constant
b. IR=m*x + b
Where x is the vertical movement of the wheel center from static resting position.

2. Is there a standard for defining the installation ratio for a certain amount of spring or wheel center movement?

3. Are there hard fast rules to determining what installation ratio should be designed for; or are the final installation ratios mainly determined during testing.

4. Is the IR influence on ride frequency and roll gradient its main significance, or is there more to the story?

Any input on these questions would be most appreciated. Thanks!

-Matt

[Thomas MuWe]

Matt, let me think about what you wrote here:

You stated that the IR is equal to the spring compression divided by the wheel center. How could you specify your IR with a spring stiffness?
Of course because you do not have that much bump travel if you have stiffer springs in static case and with a linear dependent IR you come to a different IR when the car is standing with driver seated.
But that's it.

1. IR = constant, than why should it be linear?

2. Your IR is dependent on your wheel movement. In an fsae specsheet I stated the Motion ratio for full bump and full droop.

3. You need to know how much your damper can travel. Without that you will never be able the determine what IR you would like to have. At some cars you can see different mounting points on the rocker for push, pullrod just to change the IR. In my oppignon it just makes all thing more complicated! :-)

4. IR is also mainly responsible for your wheels to stay on the ground or for your damper to lock in bump. :-)

Best of luck!

[CU Matt]

"You stated that the IR is equal to the spring compression divided by the wheel center. How could you specify your IR with a spring stiffness? "

Good point. You are right, the IR will not be modified by the spring stiffness.

Thanks for your input on the other questions and further input from others will be appreciated, as well.

[Bobby Doyle]

The installation ratio is a number, the ratio of spring/damper displacement vs wheel displacement. However it is not perfectly constant during the suspension's range of travel. Many teams try to minimize IR change throughout travel - i.e. pursue a constant IR. Geometry in which the IR rises during heave is called rising rate.

Your target IR is function of available damper travel, damping characteristics, packaging, and special concerns like rising rate, etc.

Assuming a proper damper, many people would agree that an IR of 1 would be a good starting point. This is IF you can achieve desired damping characteristics with an IR of 1.


In my current calculations, changing the IR most significantly impacts our roll gradient [(degrees of roll)/ (g of lateral acceleration)]. It had a slight affect on the distribution of the load transfer between wheels, but nothing significant.


Right now it seems that the significance of the IR is its impact on the roll gradient and potentially on the ride frequencies, as well; is this correct?

It seems that you are neglecting to note that changing the IR is simple changing your wheel rate. The wheel rate (and ride rate) directly affect your ride frequencies. Your IR and spring rate determine your wheel rate. In most cases, the spring is treated as the tuning variable when changing wheel rate - not the IR.

Hope this helps.

[STRETCH]

Let me simplify this massively...

choose an IR that will accomodate the expected wheel movement and use as much of the damper stroke as possible. this will only vary if you start using bumpstops. Other properties can be tuned from the spring and damper settings.