Ive never built a transmission before, so i decided to give it a go. It uses 1 drive motor and a servo to move the middle shaft back and forth, engaging 2 different sets of gears. It is semi reliable, sometimes the servo doesnt push the shaft all the way. The hardest part was converting the rotational force from the servo into linear force for the shaft, it could still use some improvements.
I like the transmission and think the concept is nice for a VEX bot but if you were to bring it to a FRC comp, I see too many gears binding and breaking. I think you would have to shift it not on the fly. This is the only problem I see.
The tranmission works great actualy, though you are right about the gears breaking and binding. Its only a matter of time before they break, I actuly found some plastic shavings that fell off of it. Im probably going to rebuild it so it has at least 2 gears in contact together all the time so its a little more robust.
By the way, it HAS to shift on the fly, when the gears are rotating it helps mesh them together.
Looks good. Although the small gear on the bottom left looks like it is positioned too fr to the left.
yayy plastic gears 
Just curious, what did you use to convert the servo to linear motion?
Thats insane. My FRC team is still trying to complte a 2 speed for our first robot and here you guys have made a 2 speed for the vex. well we better hurry or else everyone will be blowing us away with speed! 
I am planning on making a really cool version of that just with the new Vex Pneumatics
a lock bar at the end of the shaft coming from the servo is attached to a piece of metal which is being held between 2 shaft colars, when they servo moves it pulls the piece of metal and the whole shaft with it. It had a problem with jamming occasionaly though so i redesigned it, which you can see posted here:
what are your gear ratios for each transmission level and could up send me more pictures from different angles of your 2 speed vex transmission
a 116 alumnus is actually building a 4-speed vex tranny. Of course, it alone is too large for the FVC size requirements and recquires all 8 pwm outputs on the controller for just one. But then again, it also geared to * theoretically * have an output speed of 60 mph with one of the knobby wheels (although I doubt it would have enough torque to overcome static friction :rolleyes: )
This inspired me to design my own two speed Vex tranny. I simplified to two shafts (<-- kudos if you figure out how) and started building it, but I never finished. Maybe I’ll take one of our Vex kits home and finish it under the guise of making an example to introduce the newbies to advanced mechanisms.
Your new design definetly looks more reliable but I noticed it used a spring. Since a spring isn’t a Vex part would that make it not competition legal? Nonetheless awesome job.