# How to make a linear dr4b

Hey guys!

I was just wondering how it would be possible to make a perfectly linear dr4b lift without having both the top and bottom bars be the same length? I’ve seen teams do this (especially in China) however I have been unable to achieve this’s. Any tips? Thanks!

Could you provide an example of a lift like this? Maybe a video or something? Because as far as I know that’s not possible. Two different length bars means two different arcs for each four bar, so it should always move at some sort of net arc overall if they aren’t the same length.

Well, you can always do a 4B on a 4B, driven independently and a bit of math in the controller.
Buth the uneven RD4Bs I have seen, they aren’t perfectly linear. They are just linear enough to be well usable in limited range of heights. You can compensate the rest in your top 4B/chainbar.
You can partially compensate by uneven gearing between the parts (remember, “sin(x) = x” for very small x, so “sin(2x) = 2x” for small x) and that might get you linear enough lift within the tolerances you lay with.

If you watch to the end of the video where they show their most recent robot, it looks like the chain bar goes back the same distance everytime meaning that the lift must be linear even though it is evident that the top bar is longer than the bottom. How is this possible?

Their top bar isn’t too much longer than their bottom bar, so it may just move linearly enough so that it doesn’t matter, because the cones are funnels, so you don’t have to be too accurate when stacking them.

Another possibility is that they may adjust their chainbar position slightly based on lift height, in order to account for this disparity. My team’s robot for states had an even more non-linear lift, and I took that into account when coding my chainbar macros. Whenever the button to move the chainbar was pressed, I took the lift height (based on a potentiometer reading) and plugged it into a custom parabolic function that I created that spits out a chainbar potentiometer value that keeps the overall movement linear. It’s a pain to do, and I wouldn’t recommend it over just making a linear arm, but it’s doable.

Funnily enough, our dr4b has equal length arms and i feel like its not perfectly linear at all. When its all the way down, our 4b is like 1 - 2 inches ahead from when it is all the way up. The gears are as close to perfectly aligned as i can (its always at least 1 tooth off since vex gears are like that).

You could just use the same length bars then use standoffs to extend the top part out far enough to put motors on the outside.

Thank you so much for all the help guys! I think we will try to use 30 and 34 hole bars to hopefully make it linear enough to still work lol

There might be some slop in your system; the gears can perfectly align (from my experience).