[NoSkaOnTheRadio]

The biggest thing I can advise you keep an eye on is where your designs put your bot's CG... too high and you're in danger of tipping, and unless your manipulator is also able to reorient your bot, your match will be a disaster. The inner tubes from last year weren't all that heavy, but the trackballs this year are pretty big; the difference between the CG of an unladen robot with its manipulator at full vertical extension and the CG ofa laden one in the same configuration is probably going to be substantial.

Anyway, your team has probably already thought of most of what I'm about to say, but just in case...

The scissor lift is good if you want your bot to have a low profile without a trackball; you can get a lot of vertical height out of an extended scissor lift for relatively little height in its starting configuration. The main drawbacks are power and speed, though. It takes a lot of power to get the lift going, and you'll probably have to gear down a motor (maybe a worm gear or something similarly drastic) or use a lead screw, both of which sacrifice a lot of speed. It's also a pretty tough task to build a robust scissor lift that is also mass-efficient.

The four bar linkage sounds viable, but depending on where it sits on your bot it could wreak havoc on its balance. Hold the trackball too far out in front of the bot, and you're all but asking it to fall forward. On the plus side, this sounds much easier to implement than the scissor lift.

The crane seems like a good idea balance-wise, but it would need to be robust. Team 1757 used a crane-like system last year with a claw on the end; we'd definitely be able to lift an inner tube over the overpass, but I don't think it's tough enough to lift a trackball.

My last word of advice is not to combine lifters! Not only does that fly completely in the face of K.I.S.S., I would bet you'd see a significant rise in man-hours required both for design and fabrication, and then for integration. It could certainly be done, but I think you'd need to be pretty confident...