The importance of an accelerator wheel

Recently I was scrolling through chief Delphi and YouTube for 2020 shooters and I was wondering how necessary an accelerator wheel would be. So far I know that most setups are meant to get the power cell to a certain velocity and some setups also incorporate a way to center the power cells Before reaching the main fly wheel. What is your opinion on it?
(Some people may refer to this as a kicker wheel)


I think an accelerator wheel can be very helpful when you only have 4 inch wheel powering your shooter or you have some kind of non-hooded shooter, however you will probably be fine without one if you use a bigger (6 inch or 8 inch) wheel on a hooded shooter.
Even a 4-inch wheel without any accelerator will probably work fine for front-of-trench shots, it would be more likely to struggle making behind-the-control panel shots.
It is worth noting that some teams seemed to have success putting an accelerator wheel after the main shooter flywheel, so you may want to experiment with that.
Of course, the best way to figure out what works is to prototype mechanisms, however I understand how hard that is right now.
Good luck with your shooter design!


Would there be a way to have a 4inch shooter wheel without an accelerator wheel. Like maybe if the feeder feeding balls into the shooter was fast enough or if the shooter had enough inertia to not drop as much rpm after 1 or 2 balls?

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I’d say absolutely, especially if you got high-friction wheels, like the fairlane wheels some teams were using. I’d recommend 1.5-2 inches of compression.
Most wheels would be almost guaranteed to work from the front of the trench, some just might get a bit inconsistent for behind-the-control panel shots (based on my team’s prototyping).

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We had a 4” hooded shooter with just the flywheel. It worked well for us. We could shoot from behind the wheel of fortune until the balls deteriorated and became inconsistent.

Edit: photo


Would it be possible to obtain more consistent shots by centering the ball before hand to the center of the fly wheel?

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The hood is just wide enough for the ball, so it was pretty well centered as long as the ball was complete. There might be value in making some small amount of side-to-side compression (probably with PTFE to reduce friction like we used on the curve of the hood).

Without proof I think a lot of the shot variability with worn balls was aerodynamics.

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Here’s the other part of the equation: horsepower.


My plan was to use two falcons which should be enough. By any chance do you know how long it took you guys to empty a full 5 balls while keeping decent accuracy?

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One more thing I notice about the 3928 shooter is that you have the ball take about 180 degrees of travel around the wheel (or are “front fed,”). This is a great idea!
It gives you twice as much flywheel contact distance to input energy from the flywheel from the ball than a “bottom fed,” shooter, where the ball is only in contact with the flywheel for a little less than 90 degrees of travel.

So would you say 150-160 degrees of you “touch” would be enough for consistent long shots? Also if your up close and you want to unload as many balls as possible in the shortest amount of time does the amount of contact really matter if the accuracy isn’t as much of a problem.
Edit: close as in from auto line

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I’m sure that Mr. Hascall from 3928 can give a better opinion, but I’d guess that any consistent long shots in competition are very difficult due to varying ball compression/wear, but 150-160 degrees of “touch,” would give you enough power for consistent long shots with consistent balls for sure.
In terms of shooting many balls fast, I’d say as long as you have a decently heavy flywheel you can shoot as fast as you want, the only limit is how fast your hopper can feed the shooter.


If you want to lighten the flywheel, but still get a lot of inertia, you can gear it at a faster speed than the actual shooter wheel, I believe 118 did this on their 2017 robot.

Depends on your wheel recovery time. Not sure what other teams use to get this info, but we graph everything under the sun. Our setup (2 Falcons) recovers in way less than 200ms so we can fire 5 per second. As Alexander mentioned, it probably will come down to how fast the hopper can move the balls.

The proper answer here is to measure your recovery time then make sure your hopper doesn’t feed faster than that.


Ours is bottom fed but the balls do come up directly under the flywheel, so we do get around 160° of contact. This pretty much falls out of having a turret, you want the ball to come up in the same location no matter which way the shooter is facing.

Speed of shooting is OK, but it is currently 2 buttons, one to spin up the flywheel and the second to push to top ball in the elevator up into the flywheel. I’m thinking this might be an area where a single shoot button and software can manage things faster and more consistently. The elevator already has sensors to move the next ball up to the top staging position. As @Mark_Wasserman suggests, if you automate then you’ve got to make sure you don’t feed subsequent balls before your flywheel recovers.

These threads are bad.

Anecdotes between different teams aren’t even enough to make rank-choice qualitative comparisons because you have no idea how teams executed one design versus another. There’s too many independent variables and they’re horribly controlled.

A) Someone builds a ton of shooter prototypes with the same care and roughly same construction (e.g. same wheels, stiffness, hood material, number of motors and gearing), tests them with a suite of equivalent and comparable conditions, documents their practices, and reports back the results
B) Do some math/physics modeling to help steer you in the right path and actually understand what is going on in a pitcher

I like option B, but I love option A.


Wow what an amazing paper you’ve made
and I see you’re a fellow men of culture


We used a wheel (4 in) to feed balls from the hopper into our shooter. This was done because we didn’t have an active mechanism to store balls (big mistake). We would run the feeder wheel only when we wanted to fire and when the flywheel (6 in) was up to speed. Unfortunately, we found at competition some balls were able to roll under the feeder wheel because they were slightly smaller than the ones we tested with, so we lowered it. We found that the wheel itself was actually accelerating the ball quite a bit and made our shots a LOT more consistent, even when the ball quality varied. Here is a video of it (at 1:19)

To elaborate a bit and bring this back to the main topic:

You could either expand the model I’m proposing, or you could look at the effects that are occurring and consider “what does an accelerator wheel do?”.

The effect is somewhat like extending the distance where the ball can be accelerated, creating two different surface speed regimes, and modifying the ball’s path through the pitcher. If you can understand those interactions, you could see the directions (maybe not magnitudes) in which an accelerator wheel would affect nominal backspin/exit velocity, as well as the error on these outputs.

Now, what do you need? That’s a great question, and partly why I like (A) since it will account for many values (friction, stiffness, etc) which are guesses in a mathematical model. But from a higher-level perspective, you need to be thinking in terms of end results: recovery time, shot accuracy, shot range, shot adjustability.

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From my limited understanding, the accelerator wheel also helps take the load off your main shooter control loop. This is (apparently) better than the equivalent length of acceleration mechanically coupled. Although a beautifully modeled flywheel and kalman filter would likely preform equivalently, who wouldn’t pass up simplicity of just shooting in open loop (254 2017, 4414 2020) not to mention many others with small p gain flywheels. We were already planning on implementing the new wpi lib state space stuff once we make it back into the lab. We might try some side by side comparisons between kicker wheel + open loop and slow kicker + better controls.