# Hood Compression

So I’ve been doing a lot of looking on chief delphi and have seen a lot of insane shooters. Right now we have 2.5" in of compression, 2 falcon 500’s at 90%, (2) 4 in stealth wheels sandwiched by 4 steel round plates to help keep inertia up. We can shoot at about 32 ft currently without missing a shot but the shots kind of “lob in” not a crazy amount but still do.

What I am wondering is should the compression on the ball be the same until the ball leaves the shooter completely to allow a more direct line of flight with less of an arc or is there something else we can change to do so?

Here is a pathetic drawing to show what I mean

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Off the top of my head here are a few thing to consider based on our own testing:

-increasing the compression will result in more surface contact with the wheel and should cause the a greater speed but will cause greater current draw and more drag. so it’s not exactly perfect linear correlation.

-Check your electric logs from your motors and work backwards through the JVN calculator to see what speed you are getting (Using the current draw from the readings). We found we were getting much lower speeds than what we had initially calculated using theory.

• It could be your hood is causing excessive drag on the ball and slowing it down. It could also be that your ideal angle is lower than what your hood is currently releasing at.

Just a few ideas to play with, Hope it helps!

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Have you played with using different compression at different points in the ball path?

If you want a more linear shot you need to increase the surface speed of your flywheel, you can do this in two ways:
1 - Increase your flywheel Diameter
2 - Increase your flywheel RPM

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They are.

It gradually comes out at about quarter inch less compression at the end. If that makes sense.

The biggest problem is we are already using the 2 falcons at a 1:1 and with our current design we can’t fit 6 inch wheels or that is one thing we probably would have done. This is the first shooter the team has ever been able to make so it’s a lot of learning and trying to keep everything compact like we want.

So make a new design!

Not to put too fine a point on it, but perhaps a shot that is accurate beyond 32 feet is too lofty of a goal for your team’s first ever attempt at a flywheel shooter?

32 feet is a HUGE distance for a foam ball, farther than most teams have ever shot similar balls. Maybe you’re trying to do a bit too much.

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Can’t really make a new design at this point as we don’t have the resources to do so this far in…Even with no bag day we are limited. Could look at gearing it up for sure though.

I mean like I said it doesn’t miss at the 32 ft and it really isn’t much of a arc. We were just looking at a more direct flight and I know it’s possible from videos I have seen.

Also I don’t think FIRST is a sport/competition where you should limit yourself and what you think you are capable of. If the kids on the team have a goal we are going to do what we can to try and reach it.

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These are two conflicting points. I don’t know your resources, but I know that there dozens of public designs for 4" and 6" wheeled shooter designs.

We’re just under halfway through build season and all things considered two side plates for a hooded shooter are quite quick and simple things to manufacture, even by hand. On 103 we have often printed out a layout sketch with hole positions and outlined geometry, glued/taped it to a piece of stock, and drilled and cut everything by hand to great success.

If you’re having trouble coming up with a design I will happily help draw one up given your current geometry and what resources you have. Please PM me if you’re interested, and we can maybe do it over a skype session.

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When I say resources are low. I mean like we can’t really scrap a whole design as other parts of our robot are already built and we can’t afford to remake all of those as well. I will see what my team thinks. Since what we have does work I don’t know if just trying to get a direct angle is worth scraping what we have if that makes sense.

Thank you for the offer for sure!

Looking through what you said. I think you have too much compression. The general consensus from the plethora of teams I’ve talked to and from my teams own experiments 2" of compression should be the sweet spot for you.

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This is a good observation, and something to pay attention to, not even specifically in this context but going forward in life in general. Sometimes when you’re on a set path, you can rationalize a lot of concerns away really easily, even to the point where rationales conflict with each other. You have to be wary that you haven’t overconstrained yourself and aren’t seeing a problem from a too-limited lens. (Or else you might end up doing something silly and building a wall of hooks in 2015… long story)

Back to the original problem: If all of these things are true:

• Increasing the flywheel RPM
(by adding an over-gear gearbox) isn’t possible
• Changing the flywheel diameter isn’t possible
• Using more or less compression does not change the distance you can achieve

Then, you can’t achieve a faster / flatter shot with your current design.

“Anything is possible, we should try to achieve anything” is a nice sentiment, but it’s not the most valuable lesson FRC

can teach. Fundamentally this is a competition about cost-benefit analysis. What kind of a shot are you specifically trying to do? Is it just “as far as possible”, or is there a particular target? And what are you willing to compromise to get there? What I’m trying to gently nudge you toward is to decide whether or not you have reached the point where the returns of your efforts have diminished enough that the extra work stops being worth it. I’m not saying it with certainty, but if the ways out of your current design problem are all too much work / too late to do at this point, then I guess you have already answered this question.

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What does the backspin look like coming out of the hood?

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You can increase your exit velocity by making all non-powered surfaces as slick as possible (try PTFE) and your shooter wheel as grippy as possible.

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do you have a CoF for the power cells on the PTFE?

No, but we could test that at our next meeting. I’ll put it on the list.

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Never actually looked at it. We will have to do that tonight maybe.

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So serious…

I started by saying we are hitting the shot we want just not 100% the way we want to hit it because If there isn’t a simple way to change it to what we want there isn’t a point for us to scrap things completely or change them a ton to get it to work 10% better since we are already at 90%.

I get what you are saying 100% but, I don’t get how telling a team that something is too lofty since it’s their first shooter is ever going to help them become better as a team or as individuals. All that is going to do is put words in their head such as “maybe we can’t” “maybe we don’t have the ability” “is it even worth trying” Everyone starts somewhere and I think that is something to notice no matter the size of team, resources, money, etc.

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