Our team is building a double reverse 4 bar this year and as you can probably tell, it is leaning forward. It is just tall enough for the tall post (34in.) when no weight is applied to it but when it is holding a cap it leans forward too far and it is no longer tall enough. Any solutions?
Pictures would be appreciated, but here’s some suggestions…
My first suggestion would be to watch this (Emphasis on 2:25):
My next suggestion is make sure you have a very firm and stable tower. For support, I generally avoid using 1x1 plates, and instead I would make and use 1x1 L-Channels. This would provide support on both push and pull with the addition to help prevent torsion.
Hi there!
DR4B leaning is actually a lot more common than you think, and there are a few easy ways to sort it out.
First of all, you will want to make sure your parallel bars on your 4 bar are at least 7 or 8 holes apart. The further apart they are, the less wiggle there will be. In the attached image, the left DR4B would have a lot of wiggle, but the right DR4B would not. 1961U comes to mind as a team that really nailed their lift spacing last year: look carefully at their lift.
The other thing you can do is to attach a gear to both bars on the bottom 4 bar to help keep them parallel. 202Z used this technique last year to help them build (to my knowledge) the most stable lift in the world. You can see this really well at 0:39 when you have a clear shot of their lift. Pay attention to their lift tower and look closely at the gearing.
I’ve also attached a second image that more clearly points to exactly what I’m talking about.
If you’re still having an issue, try putting extra rubber bands on the bottom 4 bar and taking them off the top 4 bar. This is a much more slapdash solution, but it was used by some teams last year as a successful last minute fix (notably including 6007X.)
Let me know if anything wasn’t clear, I can help clarify further if you need. Good luck!!
I don’t believe this was already mentioned but another thing you want to do is make sure your towers are square to each other and to the drive base. The most common way to ensure this (from my knowledge) is to make a triangle with your tower, drive, and a third bar going between them. You can see this demonstrated in the 202Z reveal video linked by @Anomaly above. Hope this helps 
I don’t think this was motioned and I think it is crucial to a good DR4B (and most common cause of this issue). Make sure the angle of the (2) 4 bar linkages (bottom and top stage of the lift) are geared and positioned so that they are as close to perfect reflections of each other as possible. Most of the time, a rookie teams will build a DR4B with the second stage at a steeper angle than the bottom stage and it causes the entire lift to lean foreword when the lift hyperextends. you also have to insure that the gearing between the bottom and top stages are not shifting under force.
One of the keys to help with this is to pay attention to where the holes on the gears are. You don’t actually want the gears where the two link to be reflections of each other because the teeth will not mesh with everything else being a reflection. You want to rotate either the top or the bottom gears where they meet so that they are reflections in location but not quite in orientation so that everything else can be reflected properly.
Good point Callen. my team normally rotates the top gear up slightly so that it meshes with the gear below then we rotate the arm (or linkage bar) down in the square holes to compensate. Most of the time any other symmetry problems past that can be solved by balancing rubber bands.
If you’re using the high-strength 60-tooth gears (Really any one with holes at 45 degrees because dividing any of 12, 36, 60, or 84 by 8 leaves a 0.5 remainder; I just know the high-strength 60-tooth gears have the right holes.), rotating the upper one 45 degrees compared to the lower one will allow you to make all the arm parts simultaneously horizontal. If you’re using lock bars to help connect, you can pop out the little metal part, rotate it 45 degrees, and reinsert it to make the lock bars match the gear, though this only works with the black plastic lock bars, not the all-metal lock bars.