[DFTF] Bumper Tips...

[Joe Johnson]

This is part of a series of posts called Drinking From The Firehose on getting Dr Joe back up to speed on All Things FIRST.



Today's topic:
Bumper Tips

Bumpers are old hat to all ya'll but I've never had to design and build a set from scratch.

We've put one on our practice chassis (yeah, we're a rookie team with a practice chassis... ...you got a problem with that? ;-)

But I am not happy with the results.

Let me start by saying our robot this year is a 27"X37" rectangle -- a very simple Frame Perimeter.

I am THINKING that we will make a 1 piece bumper (a boxy skirt that drops on from above... ..Well, we'll make 2, one red, one blue).

Is this a dumb thing to do?

How do folks attach the noodles to the board? Zip ties seemed a good idea in principle but it didn't come out great on the prototype.

Would it be legal to fill the space defined by the noodles and the plywood with "great stuff" (or some other foam)?

Is there a better way to make this work?

Oh, I am planning on having the robot chassis tightly integrated to the bumper, each giving the other strength. So... ...keep that in mind when you make suggestions (and feel free to comment on that idea too)

So... ...All things bumper.

Have at it.

Joe J.

[Al Skierkiewicz]

Joe,
The general way to accomplish this is pick a color and make that the permanent construction. The fabric holds the noodles in place, so make it tight when assembling. A staple gun works great to hold the fabric in place, add aluminum angle if you feel a need for extra hold. Then make the skirt out of the other color. Don't use foam to hold everything in place it is not an allowed bumper part and it will permanently crush when hit.

[Alan Anderson]

Originally Posted by Joe Johnson

How do folks attach the noodles to the board?

We don't attach them at all. The fabric keeps them in place.

Would it be legal to fill the space defined by the noodles and the plywood with "great stuff" (or some other foam)?

I think that would violate the rules. The components of a Bumper are well defined and don't include filling the void between noodle and plywood.

[engunneer]

as a former iRobot intern of 2.5 years, I welcome you (back) to FRC

Most teams will make each bumper segment separatly. Dropping the whole thing over the top is not a bad idea, though, and is perfectly legal.

All the teams I have been on do not attach the noodles to the board. The fabric holds them in place. Actually, using anything except the fabric might be illegal (look up the list of "soft" parts that can be between the backing board and the rest of the world.)

One year (2008, team 1318), we successfully "chained" each segment to neighboring segments with a door hinge. A wing nut on each far end was all we needed to hold the bumper on. once the nuts were off, the bumper could be unwrapped from the robot.

[Kevin Sevcik]

First things first, you might want to look into making reversible bumpers instead of two sets. Flipping a cloth panel is loads faster and more reliable than swapping entire bumpers. We actually missed a match at Lone Star thanks to a stripped riv-nut that made a bumper near impossible to remove.

We've never bothered trying to firmly attach the pool noodles to anything, if you do a good job pulling your cloth tight when you're stapling it down, then the noodles center up nicely and stay put just fine.

The GDC is pretty picky about the bumpers and what can go in them. I haven't seen anyone ask about adding Great Stuff to fill in bumpers, but I'm going to bet the reply would be along the lines of: "A Bumper consists of 3/4" plywood, two pool noodles, and a tough cloth covering, such as....". That's how they've answered question about having slightly less than spec bumpers. I can't see them looking kindly on bumpers that you're adding stuff to, even if it's just Great Stuff.

I like the idea of gaining strength from your bumpers. I'd look even harder at reversible bumpers in that case. Also remember that you have to pass inspection for size and weight without bumpers, so your bot needs to stay together without them.

[Jon Stratis]

Also keep in mind that you need to be able to quickly take the bumpers off and put them on (for inspection at minimum, if you have reversible bumpers or a bumper cover). The best way we've found so far:

Have a couple short sections of c-channel attached to the back of the bumper. These slide around the kit-chassis c-channel, and allow you to either drop a pin in to attach, or use a 1/4-20 to go through the whole thing. If using a 1/4-20, it's helpful to use a PEM nut or rivet nut on the bottom side so you don't have to get a wrench in there. Having a drill-powered Allen wrench to tighten the bolts makes the process quicker too. Of course, this assumes you can push the bumpers on from the side - if you're going with a single contiguous bumper that drops over the top, you can't use c-channel... maybe some L brackets?

Make sure you follow the bumper rules to the letter... it's amazing how much of the inspection checklist is taken up by the bumper rules!

Also, reversible bumpers are awesome. We used them last year... you can actually see us changing them while on the field at a pre-ship scrimmage:
http://www.chiefdelphi.com/media/photos/36403

[Cal578]

I did not see anything in the bumper rules prohibiting other materials or parts such as Great Stuff. I wouldn't use it structurally, but using a little to just hold the noodles to the plywood seems like a great idea (it's amazingly sticky). I don't think you'll get any advantage by trying to fill the empty space. In the end, the fabric will really be the main method of holding the noodles to the plywood.

For our prototype bumpers, since the fabric wasn't ready yet, we just used some industrial "Saran wrap" to hold the noodles to the wood. Quick, cheap, easy, works great. That alone is not ready for collision events, but it's good for testing the design. When the fabric covering is installed, we can trim off any exposed plastic, and we'll be good to go.

I saw a couple of teams last year with one-piece bumpers like the OP mentioned, one red and one blue. They worked very well, and changed out faster than most teams using covers. It all depends on your method of fastening the bumpers to the frame. If you have something that is quick-release, you'll be very successful with the separate red and blue bumpers.

[artdutra04]

For quick and easy swapping of bumpers, nothing is easier than using using studs and hairpin cotter pins. 228 has used this method since 2009, experimenting with different methods of alternately having the studs attached vertically to the robot frame or having them attached horizontally to the bumpers, depending on the bumper configuration.

As long as you don't lose any of the cotter pins, you can swap bumpers in 20-30 seconds.

Here's a photo of 228's 2011 robot with the bumper mounts in the background.

[Brandon Holley]

Joe-

I believe Great Stuff would not be legal, but is an interesting idea. Maybe worth a proposal to the GDC for next season.

I would recommend making two sets of bumpers for changing colors. We tried the reversible bumper and while it worked, we found it to be a little more cumbersome than a nicely designed quick change set.

I would also encourage you to continue on the path towards a bumper set that is a total of one or two pieces. I'm partial to the two piece "U" assembly which is basically like two c's sliding on from each side and connecting in the middle.

Pulling the fabric tight helps to hold the noodles in place, and with the angled aluminum, the fabric will not tear after repeated impacts.

-Brando

[Jon Stratis]

Originally Posted by Cal578

I did not see anything in the bumper rules prohibiting other materials or parts such as Great Stuff. I wouldn't use it structurally, but using a little to just hold the noodles to the plywood seems like a great idea (it's amazingly sticky). I don't think you'll get any advantage by trying to fill the empty space. In the end, the fabric will really be the main method of holding the noodles to the plywood.

Check R28 - it tells you exactly what to construct your bumpers out of. Especially for the "soft" parts of the bumpers, you want to make sure there's nothing else there, or a quick inspector will tell you to take them apart and fix it according to the rules.

[Tristan Lall]

Use good-quality hardwood plywood. It's pretty dense (more free weight) and very durable. You could easily use it to span large gaps in the structure (even though the 2012 rules limit this).

I've often used it as a structural member—it's very stiff and tough, so it makes a good outer frame rail reinforcement, when properly supported. The actual outer rail can be relatively lightweight as a result.

Use some sort of quick-release mechanism. FIRST says it should take at most 10 min to change bumpers; I say each bumper segment should take no more than 10 s, and design accordingly. (This is useful in the elimination rounds.)

Originally Posted by Cal578

I did not see anything in the bumper rules prohibiting other materials or parts such as Great Stuff. I wouldn't use it structurally, but using a little to just hold the noodles to the plywood seems like a great idea (it's amazingly sticky). I don't think you'll get any advantage by trying to fill the empty space. In the end, the fabric will really be the main method of holding the noodles to the plywood.

For our prototype bumpers, since the fabric wasn't ready yet, we just used some industrial "Saran wrap" to hold the noodles to the wood. Quick, cheap, easy, works great. That alone is not ready for collision events, but it's good for testing the design. When the fabric covering is installed, we can trim off any exposed plastic, and we'll be good to go.

You should probably ask the Q&A about using materials not specifically listed. In the past, that's typically been impermissible, but this year's interpretation may vary, because of some changes in the way the rule is presented.

[Cal578]

Check R28 - it tells you exactly what to construct your bumpers out of.

What part of R28 says what you can't use? The closest I see to that is section C, which says:

use a stacked pair of 2-½ in. “pool noodles” as the bumper cushion material which completely covers the plywood.

Any adhesive you might use is not part of the "bumper cushion material". I wasn't suggesting, and I don't think the OP was either, that anyone use Great Stuff instead of the pool noodles, just in addition to.

[EricH]

Originally Posted by Cal578

What part of R28 says what you can't use? The closest I see to that is section C, which says:

Any adhesive you might use is not part of the "bumper cushion material". I wasn't suggesting, and I don't think the OP was either, that anyone use Great Stuff instead of the pool noodles, just in addition to.

Try the part about "must be constructed as follows (See Figure 4-4)" from the very top of the rule (though I'm 99% certain they meant Figure 4-5). Or the part about hard stuff not extending past 1" beyond the frame perimeter (though if Great Stuff isn't hard, then that won't prohibit it).

Let's put it this way: If your inspector determines that having Great Stuff or some other material in the bumper is illegal, as of right now, you have no way of proving that it is legal to any inspector's satisfaction--they'd have to contact HQ for guidance. If you ask Q&A, and they say that it is legal, bring the Q&A with you to convince skeptical inspectors.

[Cal578]

Thanks for the answers, EricH. And I hope I don't appear stubborn about this (we're not even planning to use Great Stuff anyway, that was just the OP's idea). I am trying to understand the rules, and I think our discussion may be useful for other readers as well.

Try the part about "must be constructed as follows (See Figure 4-4)" from the very top of the rule (though I'm 99% certain they meant Figure 4-5).

I just read thru that section again, and looked at figures 4-4 and 4-5, and I still don't see many limitations on what we can use. I see a bunch of "optional", "etc.", and "e.g.", which looks to me like the rule makers were deliberately giving us quite a bit of discretion in what we use for much of the bumper. We must use plywood, noodles, and "rugged, smooth cloth", and we can't use velcro for attaching to the frame, but otherwise we have lots of options (as I read it).

Your advice about making sure we can pass inspection is sound, and I've said the same thing to my team and others. Arguing with inspectors is rarely a win-win scenario.

[EricH]

Originally Posted by Cal578

I just read thru that section again, and looked at figures 4-4 and 4-5, and I still don't see many limitations on what we can use.

I'm going to assume that the first line of [R28] references Figure 4-5, despite it saying 4-4 (which figure is referenced by [R28-B], and illustrates that rule).

"Bumpers must be constructed as follows (see Figure 4-X)" is what the rule says. So, if we scroll down to Figure 4-5, the cross-sectional view of the bumper is shown. It shows the 1" limit for hard parts, the aluminum angle (optional), an example attachment method (TBD by the team), and the plywood (required). It also shows the cover (fabric type TBD by team within tough/smooth and color) and the pool noodles (size spec'd by section C).

What I don't see is anything (other than possibly fasteners, which are mounting-method dependent, and of course pool noodles) inside the fabric/plywood area. Does that mean it's not allowed? Not necessarily. Does that mean it's allowed? Not necessarily. So I think we're in limbo here until someone either a) asks Q&A or b) tries to pass inspection.