This tutorial courtesy of Eric Poirer and XSIStation

This is my first tutorial so if you feel it's need more info please tell me

This is a tutorial to create a expression driven tank transport
for a project I'm currently working on called "Code name: THX"
(no relations with Lucas) I've always liked to work with 
expressions, Maya's expression are great BTW, so I decided to 
try out XSI expressions, unfortunately they are not as good as 
in Maya, they lake the algorithm like: if, while, for,.. but 
from what I've been told they will be greatly improve in the 
next version of XSI (v1.1 ??). Algorithmic expressions will also 
greatly improve XSI particle system... I hope !

On with the tutorial

1. Import you scene in XSI, in my case 80% was done in Maya, 
   the rest is currently being done in XSI, and since this
   is a tutorial on expressions I'm only going to used the 
   low rez version of CAT-17.

   XSI now support layers, so my suggestion is to created several 
   layers for different part of your scene e.i. Background, 
   Tank, Tank_details, Wheels, Treads. To created a layer click 
   on the Layers->New layer button on top of MCA (Main Command 
   Area). Select the object(s) you what in that layer, click the 
   layer you what with the drop down menu, just below the Layers 
   button in the MCA, then go to Layers->Move selection to current
   layer to put the object(s) in the selected layer.

   Maya users already knows the many advantages to having a well 
   organist layered scene. For regular Softimage user the layer 
   function is a vast improvement to the old hide/unhide and will 
   greatly improve your workflow.

2. Crated a curve around your wheels, that curve will be the path
   that the treads will travel around the wheels and name this 
   curve track_path_R  BTW having a good naming convention is your
   best allied in righting expressions and scripts, try to be 
   logical in naming objects, in my case R is Right, L is Left.

3. Select the tread and rename it tread1, the 1 is important you
   will see why later. Make sure that the center is in the middle 
   of the tread, click the Center button in the MCA and move the 
   center is you have to. If you imported your objects from Maya,
   you will probably have to.

4. Select tread1 and click on Path->Set Path in the Animate tool 
   bar (green colored). Then click on the curve track_path_R to 
   select it as the path. A window for PathCns should pop up, in
   the Tangency tab you may have to adjust the Axis to Align to 
   but you tread in the right position, by default it's X. BTW if 
   you lose the window you can get it back by RIGHT click on 
   Property->Kinematics in the MCA

5. Still in the same window, click on the Path tab then RIGHT 
   click on the little icon beside Path Percentage and click on 
   Remove Animation, the icon will turn back to green.

6. Still with tread1 selected click on Edit->Duplicate Multiple 
   and enter the number of treads you need to close your track loop, 
   in my case I needed 74 so I copied 73. It does not matter if you 
   duplicate more, at the end just delete them. You will get 74 
   objects, tread1 to tread 74.

7. Create a circle curve and rename it contrl_track_R. Move the 
   points around to create a star, this will greatly improve
   the visual interaction of the rotating circle. This curve will
   drive the treads around the path, from 0 to 100+%

8. Select tread1, then RIGHT click on Property->Kinematics in the
   MCA. In the Path tab RIGHT click on the Path Percentage green 
   icon and then on Set Expression. From there there is two ways to
   enter the expression, the first one is to click on Object and 
   go get the rotation of contrl_track_R, under the 
   Kinematics->Local->Ori->Euler->X. Or, and this where your good 
   naming convention come in to play, just type in the expression:
   (you may have to delete the 0.0)

   ( contrl_track_R.kine.local.rotx /360*100 ) + ( 1*1.351 )

  	contrl_track_R = The circle star
                kine = Kinematic
                rotx = Rotation in X 

                /360 = 360 degrees in the circle
                *100 = x100 is to transfer the result of (rotx/360) 
                       in a Path Percentage

               1*1.351 = 1.351 will change depending on the length
                         of your tread you have model, the best 
                         way to get to a number that will work for
                         you is to try out different numbers until 
                         you get the best result. The 1 will change
                         depending on the object 
                         e.i tread2->2*1.351, tread3->3*1.351...
                      + = this places the tread at the correct 
                          position on the path. 
                          tread1 = 1.351%, tread2 = 1.702%, ...

9. Once you have entered the expression: 
  ( contrl_track_R.kine.local.rotx /360*100 ) + ( 1*1.351 )
   you can click validate and then Apply.

10. Since they are 74 tracks, editing one be one every track 
    would be long, keep in mind there is a Right and Left side,
    148 treads. This is where the XSI VBscripting comes in to save 
    the day ! The first way would be to create a script that will 
    assign the expression to all the treads. The algorithm would go
    something like:

     space = 1.351;

      for I <= 74 do

      SetExpr "treadi.kine.pathcns.perc", 
      "( contrl_track_R.kine.local.rotx / 360 * 100 ) 
      + ( I * space )";

      I ++;

  Since I'm new to VBscripting I have not tried it, I'm not sure 
  if treadi would work in XSI.

  The way I did it was to copy the SetExpr line 74 times in the 
  script editor, I change the numbers from 1 to 74, and then just 
  do a find replace on 1.351 to test out the number in step.7

11. Now to set the expression to the wheels. Select one of the 
    wheels, make sure that the center is position correctly,
    RIGHT click on Property->Kinematics, click on the Local 
    Transform->Pose tab, RIGHT click on the Rotation X green 
    icon->Set Expression. Again here there is two ways to enter
    the expression: 


        Again here 3.7 is a number that fits my wheel size, play 
        with that number for your wheels so that they will be 
        synchronize with the treads.

12. Apply this expression to all the wheels

13. Apply the same procedure for the Left side track

14. Now we will control the drive of the tank with one clear 
    object, it's good to keep things clear because, in most 
    cases a TD (Technical Director) will created the expressions
    and will pass it along to a animator. So by keeping things 
    simple and clear it will cut time in the production pipeline,
    and what could be clearer that a big arrow !

15. Created a curve circle and move the points around to create 
    a arrow. Renamed it cat_17_direction (or name_direction)
    Again the naming convention come in to play, it's important
    to have a clear picture in a scene with 100 different things 
    going on at the same time.

16. Open the Kinematics window of contrl_track_R, under the Local
    Transform, Pose, Rotation X, open the Expression Editor and 
    enter this expression:


    Again here 14.35 applies to my tank, to find the correct your 
    correct number adjust it until you see that the treads don't 
    slip as you translate the arrow in Z.

17. Finally make sure that every object is parented correctly or 
    you will see pieces of your tank that will not move.

This tutorial courtesy of Eric Poirer and XSIStation