Reason for dr4b extension piece

Doing the dive back into the vex archives I noticed a lot of robots using a dr4b used an extension linkage to get extra height to the top 4 bar… I was wondering if anyone knew the reason why these teams (2915, AURA, 118, or even teams like 202Z in ITZ) kept this little extension piece on the top instead of just making the top piece a 6 bar? I have included a crude drawing and a screenshot from AURA in the skyrise world final to get an idea of what I am talking about… The only reason I could really think of was maybe added stability? But not sure…Capture Capture2


I don’t personally know, but I assume it was for strength.

Crude drawing indeed! :rofl:
There must have been some advantage to it rather than the other said option of

I would suggest PMing these teams about it.

But if you just cut off the piece I am referring to it would turn into a 6-bar linkage and have the same height. However, what do you mean by the bottom bar taking up space?

Ooooh wait i didn’t see the middle part

The other thing (in my mind anyways) is that doing it this way reduces the friction of adding more gears.

Yeah I figured I would try that unless anyone had any ideas, I have seen a lot of teams do it so let’s see lol. And yeah I use SolidWorks if I need to really draw something but otherwise it will be really bad :joy:

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Ok new idea. I would assume for stability and strength. Since it has to carry all those blocks and stuff. Gives it more durability. Again I’m not entirely sure.

Yeah that’s a stability thing. Having those unpowered contacts with the back really helps with wobbling and rigidity. You really want to have 2 points of contact back there for stability.


Yeah that is what I figured too, I also thought it might be because having 1 really long bar might bend? But that makes sense too, thanks!

Could it also be friction management? I mean, using a 4bar versus tons of gears would definitely reduce the friction.

I don’t think so, cause if you cut that piece off there would be less moving parts and more likely less friction. See how 62 did their linkage in skyrise for the 4-6 bar method.

That can be another thing if you are lifting heavier objects. Having those bars attached via screw joint will help keep the whole system from sagging when Lifting heavier objects. You can Do with out it but you will just need some solid build quality to compensate.

Ignoring slop, the extension would create more friction than a DR6B without it, since it adds an extra joint and more weight. This is insignificant though (if built well), and the reduced slop is worth it. I’m not sure where you get the ‘tons of gears’ idea from, since you don’t need extra gears to make larger linkages off a DR4B.


Yeah also if I’m correct that powered arm creates more torque than gears alone. I could be wrong but it’s the same principle that tilters used last year. Short powered arm -> Long driving arm linked to whatever system you are moving. We did a similar thing with our Tray bot arms except it was just the 2 bar not the full 4 bar. What they are doing is maxing the mechanical advantage on there system without doing some crazy gear ratio.

Probably got it from QCC2 in skyrise lol, that thing had a lot of gears.

it made the overall linkage much more rigid because the bar wasnt just “floating”, it had a fixed piece to pivot off of. This technique was really necessary in skyrise because of the load and height u had to lift to. The games after skyrise have had nowhere near the amount of stress on the lift if there was one.


This is incorrect. In a parallel linkage, the mechanical advantage from difference in size of the arms will always be 1, since both the driving arm and driven arm have the same angular displacement. A shorter driving arm in a parallel linkage would actually technically output less torque because it applies force closer to the fulcrum of the lever. The reason shorter driving arms on tilters resulted in greater mechanical advantage was because they were offset linkages, so the driving arm had to cover a greater angular displacement than the tray did in order to tilt forward.


For the skyrise robots - it was mainly to get the additional height.
this is especially needed for vertical intake designs.


Ah that makes a lot of sense. I didn’t think about that before.