2 replies to this topic

[Ninja2]

greetings!

Was wondering if any of the people here could confirm or disagree with my theory on this. Its pretty basic stuff, but would appreciate if someone could clarify.

Im modelling a quarter car incorporating suspension and tyre springs and dampers. The tyres spring and damper are attached to the road and the unsprung mass, and the suspensions spring and damper are attached from the unsprung to the sprung mass. Im using matlab to analyse this model to give plots of the unsprung masses vertical acceleration and the roads vertical acceleration, where the road input is a simple sine wave.

My results show that the sprung masses acceleration is exactly 180 degrees out of phase with the roads vert accel, and has a reduced amplitude. Is this a reasonable result?

In my discussions about this, Im hypothesising that the springs are primarily responsible for the change in phase, and the dampers are primarily responsible for the change in amplitude. Is this reasonable? And if not, can anyone else give a correct explanation? Would it be incorrect to assume that then acceleration of the unsprung mass is 90 degrees out of phase with the road functions acceleration?

Cheers

Cb

[Greg Locock]

I suggest you look at some other frequencies. Your answer is possibly correct for one small range of frequencies, but even then it is unlikely.

Reference to a book on dynamics might help to sort this one out, I have no particular recommendation there, but William Thomson seems to be a common choice.

[ttgh]

It all depends on the frequency of the inputs. If you're using MATLAB (are you using Simulink as well?) try inputting a variety of frequencies for the sine waves. You should find that you get body bounce at around 1.2Hz and wheel hop around 10-12Hz. Body bounce is where the body moves a lot, and wheel hop is where the wheel moves a lot. You might also want to look at the response of the system to a step input, and a white noise input. Evaluation of eigenvalues and eigenvectors should also yield interesting results for analysis. Once you're happy with doing the analysis, try putting the simulation in a loop and storing the results. You can then generate 3d plots to show how altering the damping on the body changes another result. Remember the limitations of the model you are using, sounds like there are no non-linearities such as bump stops, and the biggy is the assumption that the tyre maintains contact with the road. On the last point, you can keep an eye out for this by looking at the tyre displacement - much over 5cm is going to give unrepresentative results ;)

Even with such a simple model as this there is a lot of scope for analysis - have fun!

HTH

Cheers
Graham