Well if saving weight is a concern (and it should be), there are a few options but most of these are pretty obvious.
Less framing == less weight
Forget aluminum (well actually don’t forget aluminum), point being even with aluminum framing, less is still better. Do you really really need 1x5x1 all over your robot? Probably not, yet I see this all the time. For that matter do you even need C-chan? Really can L-chan do the job just as well? If so, use it and don’t be afraid to cut it up either!
H4 took some pretty good measures to keep weight down, but with H5 we are really having to address this issue. We’ve even done something with framing that has never been seen or done before (I’m 99.5% sure of this) and this technique is high-school legal so when the robot is unveiled take a good look at the framing.
My last framing point is simple, polycarbonate is very very strong. Done right you most certainly can use it for framing needs (alongside good old aluminum of course). Andrew Remmers build an entire gearbox out of lexan sheet and from what I heard it stood up really well.
Pneumatics are heavy!
And yes you might be able to get away with cutting down the tank (please Q&A that) but I have safety concerns, and personally wouldn’t do it. Use pneumatics wisely, they are “free” in the sense that they are not motors which are limited, but there is a mass and volume associated with using them. Put the tank somewhere where the weight is going to help you (like as low as possible and close to the wheels) and keep your tubing runs as short as you can.
Wire is heavy!
Wire does add up, good wiring will save you a little weight done right but more importantly good cable management will make your robot more reliable so do a good job with wires and enjoy the perks.
Don’t be a hoarder!
If you don’t need it, get rid of it! Cut that framing out, remove that extra part and ditch the flag spinner.
Collars are the devil!
If you try really hard to remove as many shaft collars as possible, you will save a stunning amount of weight. Those things are really freaking dense and they tend to find ways into your robot all the time, four here another eight over there, six here, there and everywhere. Kill the collars and you’ll save a bunch.
Same goes for lock nuts and hardware. Try to avoid them where you can, but don’t skimp out too badly here, your robot still has to stay in one piece after all.
So as usual, good design tends to lead to a lighter robot. It all involves the creative thinking process of designing a robot, and CAD helps this processes take place. CAD can also help you measure those weight savings.
I know for some of us measuring things is a foreign concept, but try it out. I’m sure you’ll find the results to be interesting. I mean some results HAS to be more interesting than none.