I am attempting to set up a model for a suspension on a mass of 100kg with k=17kN/M and a damping ratio of 0.5. The system is modelled as a simple spring damper system.

The model below is attempting to model a base excited spring damper system. I feel like the issue is that the initial sinusoidal model is time dependent whilst it should be dependent on position, however I don't know how to resolve this.

clear
clc
t=0:0.1:15;                                          %time peroid
Y0= input('wave amplitude ') ;                        %Wave amplitude
l= input('length of wave ') ;                        %length of the wave
u=input('speed of boat');                                                %Boat Velocity
w=u/l;                                               %frequency of wave
y=Y0*sin(w*t);                                       %wave height model

Dr=0.5;                                              %Required Damping Ratio
k=17000;                                             %Spring Constant of suspension
m=100;

wn=(k/m)^0.5;
wd=wn*(1-Dr^2)^0.5;

if (wd<6)
    error('Suspension excessively soft')            %test  for suspension softness
end

r=w/wn;                                             %Frequency ratio 
b=2*Dr*(k*m)^(0.5);                                 %Damping coefficient
i=(1-r^2)^2+(2*Dr*r)^2;                             
X0=(r^2)*Y0/(i^0.5);                                %Maximum amplitude of displacement
T=atan((2*Dr*r)/(1-r^2));                           %Phase Lag

x=X0*sin(w*t-T);                                    %Displacement of sprung mass
x1=X0*w*cos(w*t-T);
x2=-X0*w^2*sin(w*t-T);


plot(t,x);
X1mean=mad(x1);
Xmax=max(x);
X1max=max(x1);

This code outputs values, however they aren't what I would expect to see. I am looking to get the amplitude of excitement as well as a velocity profile.