This is a pretty simple, totally k'nex shifting gearbox with 3 gears. Hope you can use this in some of your models.
I now have an updated version of this.
Step 1: 3 Different Gears
(notes in the pictures) watch for the difference in each picture
Step 2: Watch This Movie of the Different Speeds
Step 3: Now Start to Build. Step One: the Frame
copy the pictures
Step 4: Step Two: the Axels
once again, just follow the pictures
Step 5: Step Three: Put It Together
follow the pictures more, then your done