Need help in a simple script

Question

I'm basically trying to create a java script that will help me calculate certain values after I input an initial value, in this case, the weight of the car.
I'm not really sure what's gone wrong, since I'm just a beginner at this. Could you all point me in the right direction?
import javax.swing.JOptionPane;
public class ForzaSpringCalculation
     public static void main(String[] args)
//This script takes the weight and front bias data of the car and outputs the various spring rates
          double weight;
          double bias;
          double frontSpring;
          double rearSpring;
          double frontBump;
          double rearBump;
          double frontRebound;
          double rearRebound;     
     //Collects weight and weight bias data of car
          String weight=JOptionPane.showInputDialog("Weight of car?");
          System.out.println(weight);
          String bias=JOptionPane.showInputDialog("Weight front?");          
          System.out.print("Weight front: ");
          System.out.println(bias);
     //Calculations for suspension
          //Front Spring
          frontSpring=weight*bias/100*0.3828;
          System.out.print("Front Spring: ");
          System.out.println(frontSpring);
          //Rear Spring
          rearSpring=weight*100-bias/100*.3828;
          System.out.print("Rear Spring: ");
          System.out.println(rearSpring);
          //Front Bump Damping
          frontBump=frontSpring*0.00734;
          System.out.print("Front Bump Damping: ");
          System.out.println(frontBump);
          //Rear Bump Damping
          rearBump=rearSpring*0.00718;
          System.out.print("Rear Bump Damping: ");
          System.out.println(rearBump);
          //Front Rebound Damping
          frontRebound=frontBump*1.97;
          System.out.print("Front Rebound Damping: ");
          System.out.println(frontRebound);
          //Rear Rebound Damping
          rearRebound=rearBump*2;
          System.out.print("Rear Rebound Damping: ");
          System.out.println(rearRebound);
          System.exit(0);
     Thanks a lot!

Answer 1

I see, so it's like double defining? But once I remove the 'double weight' and 'double bias', this is what I get.
ForzaSpringCalculation.java:26: operator * cannot be applied to java.lang.String,java.lang.String
frontSpring=weight*bias/100*0.3828;
^
ForzaSpringCalculation.java:31: operator * cannot be applied to java.lang.String,int
rearSpring=weight*100-bias/100*.3828;
^
ForzaSpringCalculation.java:31: operator / cannot be applied to java.lang.String,int
rearSpring=weight*100-bias/100*.3828;
^
3 errors
Is there another String thing I can use?