To ditto & expand upon the comments above:
- in each segment of your lift you need cross-bracing. Just bracing once in one segment isn’t going to do it.
- on our lift, my team has 1 or 2 c-channels spanning the gap (in different areas) as well as 1x pieces that cross in an X shape, attaching left side to right side. The 1x pieces are great because they’re lightweight and flat, and are still effective.
- my team also has “self”-bracing on each vertical segment (from chassis to first gear; from 2nd gear to top gear), so that we have teeny 1x “X” bracing between the 2 c-channels comprising each side of the lift. Example: there are 2 c-channels going up from the chassis on the left side of the robot, with the gear sandwiched in between them. Where we can fit it, we have little 1x "X"s connecting these c-channels.
- rubber band triangle won’t help your lift go up straight, but it will help your lift go up, period. The concept is that the triangle method provides continuous tension through the whole range of motion of the lift, not more when it’s folded & less in the middle. My team had to add a little appendage to one of the c-channels for the top point of the triangle; you’ll just have to play around with this to find a setup that gives you “neutral buoyancy” – the lift stays wherever you put it, without motor help.