# Polycarb backing on hooded shooters

Our team hasn’t built a successful hooded shooter in active memory. Looking around and analyzing other team’s CAD models and robot pictures, I’ve noticed that the overwhelming majority of shooters are built with a polycarbonate backplate. What is the benefit of that over a set of two rails with a high-friction material lining, like this:

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This varies year to year and is the kind of thing you can only really figure out by prototyping.

In 2012, the two rail system was very valuable for centering shots. I have heard that in other years a flat backing made shooting compression more consistent. Try both, see what works.

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Depending on the spacing of the two rails, the ball rotates on a smaller radius than it would on a fully-backed shooter. Intuitively, I would expect this to affect the rotational velocity to linear velocity ratio of the ball.
On the other hand, my main takeaway from this season so far is to never trust shooter math. No matter how I’ve tried to model that sort of thing, it hasn’t come close to the empirical measurements; there’s too many parameters to a given shot.

Also: power cells are a lot more compress-able than 2012 balls, at least the ones we still have in our shop. I accidentally shot one through a 2-inch gap making a drill prototype.

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I’d say one of the biggest differences is manufacturability. I can make a polycarb back plate pretty simply. Grab your side plate, draw a semicircle X inches from your center of rotation, and drill through the line a few times. Bolt in some churros to those holes, and ziptie the polycarb to each side of each churro. Add your wheel, and you’ve got a shooter. I can do it all with hand tools and get some decent results (better results with CAD and CNC, but that’s true for just about everything!).

To make those rails… I’m not sure I can realistically do it with hand tools.

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What are you doing for your shooter maths?

I think a curved sheet metal or sheet plastic hood might work better for the very soft Power Cells we’ve been given this year.

As Lytigas wrote, it’s kinda hard to calculate the exact dimensions or model the perfect radius in CAD. It will take experimentation. If you have access to a waterjet cutter or CNC router, you could make a few sideplates with different radii. Or make one set a little large, then add layers of high density foam to get the “squeeze” that works best.

This might seem obvious, but a hooded single-wheel shooter will have half the exit velocity of a non-hooded 2-wheel shooter. So plan your speed accordingly.

Pushing a ball through a shooter will slow down your motor, so the first shot might go further than subsequent ones. There are 3 ways to compensate.

1. Wait a couple seconds for the motor to get back up to speed.
2. Add a flywheel to the shooter shaft
3. Add an extra motor (or motors) and an encoder to ensure constant speed no matter how quickly you fire Power Cells.

Our plan for our prototype (which might be together by the end of today) is to use a similar design to yours, as far as the wood goes…but add a couple more 2x4s between the plywood end plates, and screw an old piece of polycarb to it in an arc. We can then easily move the 2x4s around to vary compression, entry/exit angles, etc.

The one you designed is beautiful, see if you can figure out a way to accurately prototype it, and see how it works.

We actually already built it and are having problems with it that you can check out in this other active thread

Since the rails are simple arcs, they can be manufactured with a handheld jigsaw and a drill press. The backing of the handmade rail may not be as elegant as a CNC’ed rail, but the rail itself is just as effective.

oh…that explains everything! We all assumed you had a nice polycarb backed mechanism in the other thread, now we see it’s a rail system, and the ball is getting stuck in it, causing the breaker popping. Try a polycarb back, and see if it fixes your problem

I wish, but the breakers pop while the wheel is spinning up, before the ball is even put into the system. Oddly enough, when you ramp it up really slowly it works fine when you put balls into it

For me, parabolic arcs, with and without air resistance, using the popular ballistics calculators on Chief have worked fine. What has not worked fine is predicting exit velocity based on flywheel RPM and dimensions. With no slip, the system should behave like a epicyclic/planetary gearing, but shooters are under very different kind of loads, and that math has consistently over-estimated ball exit velocity, almost 2x. We are working on different materials to contact the ball with, which may help.

Plywood and AM churro spacers. Polycarbonate backing cable tied to the spacers and it’s done. Hole pattern to match what compression range and contact arc you want to test.

Successful 6m range achieved with this test rig.

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