I haven’t seen any transmissions that use more than 2 ratios, is there a reason for this? If a (~5 speed) transmission could be built using only 2 pistons, would there be enough field space for this to really be useful, or is a 2-speed the maximum useful transmission? What limits the top gear ratio of a transmission?
The top gear ratio is limited by robot weight and motor power corresponding to that. In my opinion, there isn’t really ever a need for a two speed transmission. One of our teams has insisted on building one on their robots the last few years, and I think its next to useless. Unless you have a specific application, like green egg’s descorer bot this year, it isn’t every too helpful and in many cases just adds unnecessary complexity and weight.
Because adding more ratios adds more space to the system. Pneumatics are the primary way to shift transmissions, and you can’t make a piston fire halfway - it’s either pushing out or pulling in. With 2 pistons, you can only really have 4 ratios to shift (each piston shifting a different gear: 2 pistons x 2 different ratios per gear).
Transmissions themselves already take up space. Adding a second piston to the system to shift another gear would have to be spaced a significant distance from the first.
You could potentially do more ratios with a servo motor (or normal motor + potentiometer/encoder + PID), as you can control its target destination, but then your gearing system just gets wider and wider.
Just to expand you mind a little bit, Pistons can be fired at variable distances.
[Variable Pneumatic Piston Set Up - YouTube
As for shifters, anything more than 2 in a robot competition is wayyy overkill, I’ve seen 3 speed shifters in even VEX but was it really worth it, Not really.
[Variable Pneumatic Piston Set Up - YouTube
As for shifters, anything more than 2 in a robot competition is wayyy overkill, I’ve seen 3 speed shifters in even VEX but was it really worth it, Not really.
The piston itself cannot be fired variably with a single solenoid. You can use devices such as that regulator, which uses the dial (potentiometer) to provide pressure to extend to a point, then keep the pressures on both sides of the piston even (so it doesn’t move either direction anymore). Such devices would not be legal in the VRC (not sure about VEX U), which is probably the application of the question (correct me if I’m wrong).
Space is not a big issue, and I’ve thought of a way to use 2 pistons to get to multiple positions.
Space is always an issue, especially with us losing about 1/3rd of our effective space in this years game.
Well look at it this way… What does a transmission do?
- changes the speed
- changes the torque
So how would you use it in Toss Up with the limited amount of space in the Toss Up arena plus the bump?
Well…that’s for you to decide and figure out 
I honestly don’t see a point for the transmission other than the fact that it may help you get over the bump but other than that there isn’t a need for speed because there really isn’t room for you to gain speed 
I hope that helps Good luck!
The best place to use a transmission in this game is probably for hanging. Switch motors off of the drive onto the arm. Also, speed has always been a key component of vex competition, and I doubt that will change in Toss Up.
While speed may be a key component, high pushing power probably will not be in most cases. I see no reason to have a transmission in a standard scoring robot this year, aside from the hanging. I’ve yet to do any math but I’m interested to see how hanging without a lift transmission will work out.
with applications and practicality aside
2 pistons can give you a very wide range of controls
if you use timing and have the second piston act as a latch as the first piston extends, the timer resolution will be the bottleneck for the amount of control you have on the primary piston position
to clarify, the setup will be:
[LIST]
*]primary piston starts to extend at time t=0
*]at some arbitrary time, the secondary piston kicks in and stops the primary piston from extending any further (mechanical stop)
[/LIST]
an important consideration in this design, though, would be to make the relative reaction time of the secondary piston to be instantaneous
this can achieved by one of many ways including:
[LIST]
*]make the secondary piston range short
*]make the primary pressure lower than the secondary
*]start the secondary before the primary (this is a pretty severe case, though)
*]add a air flow regulator on the primary to reduce the rate of extension (or just bend the tubing going into the primary)
[/LIST]
this is possible using Vex stuff if you really want to try…
That’s exactly what I was thinking, using standoffs to stop the movement.
I think that, during a match, it wouldn’t be all that helpful, except for having short bursts of super speed when nothing is in your way maybe, but it could definately be helpful for judged awards. So I think whether or not it is worth it is up to your ability to build it in the time you have left, the amount of money it will cost, the weight it adds to the robot, and the space it takes up. Since you seem to have a plan ready, and space isn’t an issue for you, it would seem that you need to ask yourself if the potential judged awards will be worth the cost of the parts. You should also keep in mind that such a build could take a large amount of time. I think it would be a good idea to take some time to write an evaluation of the cost and effort vs. the reward (Obviously, don’t write, “to impress judges”, but something like, “to have a more adaptable robot” will do.) in your notebook and decide for yourself. This way, even if you decide against the transmission, you have proper documentation and you can prove that you had a plan to build one at some point. Have fun!
Fighting over stashing goals might be easier with high torque gear ratio.
It will always be appealing to have a transmission. Arm to drive transmission will probably be the most popular this year as teams need significantly more torque when hanging than any other time.
That would be called a PTO my friend. 
- Andrew