Couple questions about the bicycle model (i.e. school me)

[fastback33]

So, coming form RCVD Chapter 5, we have our stability derivatives, and static margin, stability factor etc. When I plug these into response to control input, or response to force etc. How do I know what the actual body of the car is doing? or better yet what the tires reaction is? What is the advantage of quasi-steady state over transient response?

I have no real guide to how the model is supposed to work other then a book, so if someone has a good general outline of what we're supposed to get out of the model is that would be sweet and very helpful!

[Jersey Tom]

If you want to see dynamic response, you need a dynamic sim.

What's the advantage of steady state parameters? They're quick and easy.

[fastback33]

Thanks Tom, I was hoping you would reply!

So, what are my response to control ratio's telling? e.g. r/δ, (V^2/R)/δ....

Also I understand they give me a general summary of the different models and how the car UO wrt to the given speed and inputs. But, say I wanted to find the response just before a turn under braking. Wouldn't I just be able to calculate the the quasi-steady state reactions without having to do any kind of transient response data?

Is there a good combination of response to a force, and response to a control that will give me more accurate representation of vehicle stability over others?

EDIT: I realize these questions may not be as clear as they are presented in my head, but my general plan of attack with understanding vehicle stability is to use a control input and a force input combined at a given speed and given steering conditions, and then vary the radius and speed to see how the body attitude responds.

[Jersey Tom]

Sounds like we're chasing a number of things here, so let's back up.

Step 1 - What are you trying to achieve? What do you deem as important to know, and what isn't important?


Does a vehicle simulation have to by dynamic? No, certainly not. There are many ways of creating steady state simulations which are generally quick and easy. They will give you part of the answer. Maybe it's the part you want, maybe it isn't. That's for you to decide.

Let's use an analogy that's familiar to most if not all engineers - a spring / mass / damper system. If we want to know the system's final steady state response to an applied force, all we need is the spring rate. F = -kx. Easy! However there are a myriad of different damping rates and masses you can put in the system which will dramatically alter the way it responds to that applied force in the time domain.

Or think about it this way. For any arbitrary spring / mass / damper (or vehicle) system there are an infinite number of different dynamic responses it can have. For ALL of them, the elaborate dynamic response can be indicated by just three steady state "response derivatives" - the response against position (spring rate K), the response against velocity (damping rate C), and the response against acceleration (mass M).

That help at all? Bear in mind I'm not going to spoon feed all the answers of what derivatives are more meaningful than others or whatever.. but this should at least point you in the right direction.

[fastback33]

Yes, of course it helps, can't ask a book a question. In that regard I feel as though chapter 8 may shed more light as well as time to digest what else I have read. Ill return to this thread when the semester ends as I don't have enough time to read rcvd at will.

Thanks Tom

EDIT: Or when I have more time