Remedy a too dropped center?

Our robot features drop center for various reasons and it has been very advantageous, but the team has come to the conclusion that the drop center might be too dropped. During Peachtree Regional we duct taped on 4 boxes of roller chain, an extra sim, and an air tank. The ballast was very effective but we noticed that it was effecting our ability to move quickly across the field so all of the ballast got removed. Suggestions to adjust the drop center slightly with less janky solutions?

Depending on what wheels you use:

Just turn down the OD of the center wheels.

I don’t have a suggestion on the wheels but I most certainly have one for ballast. Duct tape is not a fastener! In the future, ballast must be attached with standard fasteners and be inspected prior to match play in which it is used.

Are you using the kitbot frame? If not, how much is your center wheel dropped?

What wheels are you using?
You may be able to reduce the OD a little bit on the center wheel or increase the OD on the outer wheels.

How are your axles mounted to your frame?
If you are using pillow block bearings, the answer is pretty simple. Add a shim underneath the bearings of whichever wheels you want to raise.

How close is your CG to center?
You could move your CG towards one end of the robot so that it always (or almost always) “sits” on one set of four wheels.

Again, depending on wheels, you could use a larger tread (if you are using tread wheels) on the outside, or smaller on the center wheels, or both!

Can we get a picture to help us help you out?

We fixed a not-dropped enough center this morning. VexPro traction wheels. We pulled the old tread and put on new tread, but with a strip of 3/4" wide urethane belting under it to increase the diameter by 1/8" or so. That plus the newer, stiffer nitrile tread got us enough rock that we’re turning smoothly. You could do the same on your outside wheels to increase their diameter a bit.

How are a ballast and the center drop related?

It would be helpful if you could describe what you didn’t like about the movement of the robot with the ballast compared to before it was added.

We ran a 5/16" drop this year, which is one of the larger drops I have ever seen, and have no issues @114 pounds + battery and bumpers.

Add a bunch of zip ties in loops around the treads of the four outer wheels. This will do two things: increase the OD and decrease the side-to-side traction. I like to call these “poor man’s omniwheels”.

In our defense it was one of our quick fixes between matches. :stuck_out_tongue: But yes 4080 uses way too much duct tape!

To answer some of the questions asked so far in this thread:

wheels- http://www.vexrobotics.com/217-2903.html

Al- we did get it cleared by an inspector before going into our next match. Where it was located it was set on a frame member and had a secure frame member both on the back and a side. Regardless, point received and understood, we will be sure to use a more solid fastening system next time, thanks for your concern.

I could be wrong but I’m pretty sure it’s a 1/8" drop right now.

As to how the wheels are currently mounted, it’s a similar setup to the way the kitbot wheels are mounted (dead axle through holes in our c-channel frame.)

Lil’ Lavery- I couldn’t give you an exact location but it is close to the center of our robot, so moving the CG forward or backward slightly is something we can look into. Our CG vertically is another point of concern (hence the discussion of ballast) as our robot is underweight, about 95 pounds without battery and bumpers. The majority of that weight is very low, but the shooter assembly (25lbs of weight centered around 40") as well as our hanging system (simple pneumatic cylinder with hooks) and the supporting structure all may be contributing to the tipsiness by raising our cg.

As for a picture, I don’t have one on hand that clearly shows our wheel setup and our robot is in a crate currently.

1/8" isn’t a whole lot of drop, though it depends on the layout of the chassis. What is the wheelbase (distance from front axle to back axle)? Is the dropped wheel perfectly centered?

Having 25 pounds 40" off the ground will certainly contribute to tippiness, and would be very difficult to offset by changing the center drop or other drivetrain geometry.

Normally I’d ask for a picture, but in this case I’ll ask for video, with narration describing what it is you are attempting to cure.

Here is a link to us (4080). The rocking occurs a lot throughout the match, but it you can get a good gist of the rocking at about the 5:00 min mark.

Casey,
Future inspectors should point at duct tape and make the same warning. What we are most fearful of is a hard crash launching ballast at people along the side of the field and if you climb, we don’t want ballast falling on someone that does not have a hard hat. Since I have seen some incredibly hard hits this year, (enough to tip robots) launching is a big concern.

Looking through the videos available, these are the best I could find.

If you look from ~4:50-5:00 you can see quite a bit of mild rocking.

From ~1:15-1:40 and then again a little bit as the robot leaves the feeder station around 2:45.

In both of these examples the severity of the tip is somewhat mitigated as by this point the driver was reluctant to switch to high gear as much because of the rocking.

@GUI- yes, it is an unfortunate amount of weight up there. We are also looking into ways to reduce the weight of our shooter assembly (our withholding allowance) but hemorrhaging it entirely for the sake of maneuverability doesn’t seem like an optimal tradeoff unfortunately. :rolleyes:

A powerful drivetrain, grippy wheels, an 1/8" drop, and a heavy shooter up very high can produce those same results. With short wheelbases, I would probably drop less than 1/8", or may not even drop at all (depending on the exact wheelbase configuration).

Since you are only 95 lbs, add weight as low as you can and as centered as you can in the robot. A massive steel part an inch above the ground in the middle of the robot would be ideal.

Another potential cause is to check your drivetrain speed controllers, and move the brake/coast jumpers to coast mode. Also, check your driver to make sure they are not slamming the joysticks to full forward and reverse, you want to smoothly ease into all movements.

@artdutra04 In at least one of the videos we were definitely doing that, but that was to resolve an jamming problem so that accentuated the rocking. Typically the joystick motions are much smoother, but it still resulting in tipping. Hence the thread :slight_smile:

It looks like you have a wide, tall robot, which is pretty much gonna be a tippy robot, no matter what you do with the center drop. Most wide robots have only 4 wheels, since you don’t really need dropped center wheels to make them turn reasonably well.

If I’m mistaken about the design of your robot, please describe it better (including dimensions for wheelbase, track width, etc), post some pictures, or something…it’s hard to see it in the videos, I had to watch them a few times just to find a robot that looks like it might say 4080 on the bumpers.