Bumper Question

[Matt Daioh]

In the rules, they say that the unsupported gap between each bolt on the bumper should be 8'' apart. But on our bumpers the gap from the center of one of the bolts to the other is 8.5''. The gap between the edge of the nut to the edge of the other nut is 8". Is that okay for the 8'' rule? What do you all think? (We're rookies and the bumpers have been a bit of a fiasco to be honest).

[EricH]

Without seeing the design, I'd probably be inclined to say no right away...

BUT!

I think you may be reading the rules wrong. That's OK, you're rookies.

So, here's what we'll do: I'm going go through the bumper rules, noting things that can help you. I will also note that anything on CD is unofficial.

So, <R07>, here we come. Parts A through E are construction, not mounting (other than location).

F. Each BUMPER segment must be backed by a piece of ¾” thick by 5” tall piece of plywood. Small clearance pockets and/or access holes in the plywood backing are permitted, as long as they do not significantly affect the structural integrity of the BUMPER.

Those pockets are for bolt heads. If you're relying on bolt heads as structural backing for the bumper, I'd suggest making those pockets, and using the frame as your support.

Back to <R07>, parts G through J are construction/covering. So we skip those for now (though if you want clarification on those, speak up!)

Now, the part you're having trouble with, <R07-K>. There are two parts and some definitions that can be confusing, so I'm going to take this slowly.

BUMPERS must attach to the FRAME PERIMETER of the ROBOT with a rigid fastening system to form a tight, robust connection to the main structure/frame (e.g. not attached with Velcro). The attachment system must be designed to withstand vigorous game play. All removable fasteners (e.g. bolts, locking pins, pip-pins, etc.) will be considered part of the BUMPERS.

Frame Perimeter is in all caps, as are Bumper and Robot. That means that we go to Section 1.6 for the definition:

FRAME PERIMETER – the polygon defined by the outer-most set of exterior vertices on the HOSTBOT (without the BUMPERS attached) that are within the BUMPER ZONE. To determine the FRAME PERIMETER, wrap a piece of string around the HOSTBOT at the level of the BUMPER ZONE - the string describes this polygon.
Note: to permit a simplified definition of the FRAME PERIMETER and encourage a tight, robust connection between the BUMPERS and the FRAME PERIMETER, minor protrusions such as bolt heads, fastener ends, rivets, etc are excluded from the determination of the FRAME PERIMETER.

That, right there, is where you're making your mistake. You're off the Frame Perimeter because of those bolt heads, which are specifically excluded from the Frame Perimeter (but not the sizing box--two different things there; further explanation on request). See the note on pockets above.

<R07-K>, part 2:

The BUMPER backing must be supported by the structure/frame of the ROBOT (i.e. the gap between the backing material and the frame must not be greater than 1/4” and no section of BUMPER greater than 8” may be unsupported). See Figure 3-3.

Bolts do count as structure... but what they're attached to is stronger. Also note that Figure 3-3 shows a 10" long by 1/4" deep gap as being legal.

So, here's what I think about the bumper attachment currently: You're off the frame perimeter, I'm not certain you're adequately backed by frame, but you might pass inspection, depending on your inspector and their degree of leniency.

There is a quick fix, though, so if you can get to the robot before ship/bag, you should be able to be very legal in attachment. With the bumpers on the robot, mark where the bolt heads are. Take the bumpers off, get to the plywood backing, and take an appropriately-sized drill bit to the spots where the bolt heads are, going deep enough to allow the bolt heads to be completely inside the plywood. Do whatever slight tweaks you need to to make your mount match the new location, about 1/4" closer to the frame, and you should be legal just about anywhere, barring other bumper rules violations that you haven't spoken up about (hopefully none).

You may also want to get the Inspection Checklist and go over the robot to make sure that you're in compliance. It's on the FIRST site, alongside the Manual. (While you're at it, do a version check of your copy of the Manual--you'd be surprised at what has changed since Kickoff.) You'll still need inspection at competition, but if you catch stuff even a day before Ship Day, you can plan on how to get within the rules quickly.

[Colin P]

I thought he meant unsupported sections of bumper as in:

[[[[Bumper]]]]]]]]
I______I_______I <--Bolts to frame
---x---- ---x---- <--distance between bolts
x>=8" ?

[EricH]

Originally Posted by Colin P

I thought he meant unsupported sections of bumper as in:

[[[[Bumper]]]]]]]]
I______I_______I <--Bolts to frame
---x---- ---x---- <--distance between bolts
x>=8" ?

He did mean that.

Because that is what was meant, there are two roads to go down, or maybe more. Being off the Frame Perimeter by the width of a bolt head, OK, maybe an inspector will let that slide. The distance between the bolt heads, OK, probably (it won't be measured center-center, BTW). Depth of gap, Probably a pass anyway, considering that most bolt heads are less than 1/4" thick.

But, that being off the Frame Perimeter is the big issue here. The stress concentrators known as bolt heads could be a big issue; if there's a hard enough hit, the plywood could be damaged--nothing like a small object that's harder than you are to break you.

So, my suggestion is to make pockets for the bolt heads, and be unquestionably legal on that front.
__________________
____~______~_____ <--bumper backing with the bolt heads in pockets inside it

[Matt Daioh]

Thanks, you are right that we are resting on bolt heads. We're fixing that tonight.

but Colin P was right, our original question was about x being 8.5 instead of 8

[EricH]

Matt, if your frame where those bolts are attached isn't beyond the edge of the bolts, you have more serious problems to worry about than the bumpers! Once you're contacting that frame area, no problems there.

Also, like I said: they won't be measuring center-to-center. It'll be point-to-point, so you would probably have been able to pass as you were, if you could have convinced your inspector that the bolts were part of the Frame Perimeter. That convincing would have been the hard part...

[GaryVoshol]

I thought of another interpretation to your question. When you say your bolts are 8.5" center-to-center, are these bolts the ones you are using to connect the BUMPER to the ROBOT, the fasteners?

That's not what is meant by "supported" in <R07-K>. That rule is talking about the ROBOT parts behind your BUMPER, what most people think of as the frame. There can be no unsupported sections of plywood - no sections that don't touch the frame - greater than 8" long. As long as you don't have any gaps greater than 8" in your metal parts where the plywood touches, you should be OK. The other exception in <R07-K>, the 1/4" part of the rule, allows for vagarities of build - bolt heads sticking out, slightly bent metal or warped wood, etc.

Most teams use 2 or 3 fasteners per side to connect their BUMPERS to the ROBOT.

[Al Skierkiewicz]

If your bumpers are resting on bolt heads that are used for fastening frame parts and they are not more than 1/4" thick then that meets bumper rules. If the bolts extend beyond the allowable size of the robot STARTING CONFIGURATION then you have a different issue. Under all circumstances the robot must fit inside the box.
If your bolt heads are larger thicker than 1/4", Eric's tip is acceptable assuming all other rules are followed. Gary's interpretation of the 8" rule is correct. The gap in your supporting frame structure can be less than 8" long.

[MrForbes]

Originally Posted by GaryVoshol

Most teams use 2 or 3 fasteners per side to connect their BUMPERS to the ROBOT.

I think Gary is reading the original question the way it was intended...

You only need two bolts to hold on each bumper, they can be up to about 37" apart if you want them to be.

But you need something solid (like the robot frame) back there for the bumper to rest against, when the robot runs into something, to keep the bumper from deflecting in and possibly breaking. The 8" rule means the gap in whatever is behind the bumper cannot exceed 8 inches length.