Integrated Bumpers

[FrankJ]

I know there are a lot of attachment failures including attachments pulling out of the plywood. I am interested in structural breakage like Spencer's.

[Andrew Schreiber]

Originally Posted by Al Skierkiewicz

Due to the repeated impact and the energy imparted this year, fields regularly grew by up to three inches each weekend, as the player stations were driven apart.

Wouldn't this count as a field fault? What is the tolerance on that dimension of the field?

[techtiger1]

This is why my team insists on building overly rigid frames. I think in 09 we had a small part of the frame bend but it was above the bumper zone anyway.

[Al Skierkiewicz]

Frank,
Yes, several times, as evidenced by splinters on the field. I saw somewhere between 10 and 20 bumper systems sheared off the robot this year. Some of the damage was surprising as threaded fasteners were pulled through the plywood, wood screws were stripped out, and mounting brackets broken. In at least one case there was subsequent robot contact on the unprotected portion of the robot that damaged the frame.
The crash tests were reported to LRIs and have been the topic of discussion with LRIs for several years. We continue to discuss this with new LRI trainees. Anyone can duplicate the test with a section of plywood, some pool noodles and a support that has variable width. What I would call "blunt trauma" would repeatedly cause cracking or complete failure of the plywood when supported at the ten inch interval. "Blunt trauma" would be a robot corner or other geometric protrusion, coming into contact with the bumper assembly. As I remember, Dave's tests were run with a stationary object and with a full robot with variable spacers behind the bumper. The test was run on their practice field.
Andrew, there was no way to run an event without this occurring. Any drive team will tell you that they were standing on rippled carpet by the third day of competition. Most events tried to make adjustments each day. It required peeling the driver's stations away from the carpet and then pushing the stations back into position and then reseating to the carpet. Of course this varied with location as some regionals are way more aggressive than others.

[Gregor]

Originally Posted by Andrew Schreiber

Wouldn't this count as a field fault? What is the tolerance on that dimension of the field?

Nope. See page three of this document.

I've seen this happen at every event I've been to. Ever notice the bunched up carpet (that I always manage to trip on) behind the glass?

[Tristan Lall]

Originally Posted by Al Skierkiewicz

???

Perhaps you missed the report on tests performed by Dave Lavery and his team that tested various materials, methods of attachment and robot structure backing. The current rules are an outgrowth of that testing. Yes, even 3/4" plywood cracks and fails when struck by a 150lb robot running at the speeds we encounter except when supported by substantial robot structure at least every 8". Even then, repeated hits, the angle of the collision and a variety of other factors (i.e. running into or being forced into the low goal corner) can lead to bumper failure and damage to the robot. Anyone who witnessed this past game, especially in the early weeks of competition, know that even accepted practice in bumper construction failed from time to time. Teams that used a particular style of support (typical in WCD) found that repeated hits to the bumper system caused the standoff style support to punch holes in the plywood or fail altogether. The resulting failure damaged drive axles and wheels.
Due to the repeated impact and the energy imparted this year, fields regularly grew by up to three inches each weekend, as the player stations were driven apart.

I'm saying that as long as your bumper is considered supported (according to the R26 definition), you have satisfied the rule, and have nothing to fear from an inspector, no matter how weak your frame. Furthermore, as long as your bumper is also constructed well, you have little to fear in a collision—but what risk exists is yours to take, and will depend on your design.

Given these constraints, it's silly to believe that a robot's frame must always be the main support member for the robot, when there's also a perfectly good bumper there.

Originally Posted by 2014's R26

BUMPERS must be supported by the structure/frame of the ROBOT (see Figure 4-10). To be considered supported, a minimum of ½ in. at each end of the BUMPER must be backed by the FRAME PERIMETER. Additionally, any gap between the backing material and the frame

1. must not be greater than ¼ in. deep, or
2. not more than 8 in. wide.

If the bumper is legally mounted, it is supported more robustly than the one in Dave's test. Did 116 perform other, more rigourous tests that I don't recall? Also, the bumper support rule (in general form) appears to predate the test.

On the general issue of failure modes, I think we've discussed this before. No matter how they're legally supported, hardwood plywood bumpers will rarely fail catastrophically when struck with other legal bumpers (and many other robot mechanisms), especially if they're constructed using the permitted aluminum clamping angle (e.g. 0.125 in thickness and 1 in leg length). When they do break, the damage is typically delamination and partial cracking. That kind of damage is not even a minor (human) safety risk. The field damage and robot damage risks are not unusually large, and are handled the same way as always: penalize it and/or kill it remotely. Bumper repairs will in most cases be simple, legal and mechanically adequate. Teams obviously expose themselves to some risk by legally building their frame weaker than the bumper itself, but that's no different from any other mechanical optimization that a team may elect.

[Al Skierkiewicz]

Tristan,
Your link to Dave's comment was a rehash of the actual testing.
To remind everyone why we have these rules. Prior to bumpers (yes there was such a time), robot frame damage that was severe enough to knock a robot out of competition occurred regularly. This fact was enough to make people at the top (Woody and Dean) cringe and look for a better solution. Dave's test and the current bumper rules are a direct result of that. Woody saw that the bumpers reduced damage to robots and that was good enough for him. I worked two double regionals and three single regionals plus the Champs this year. I witnessed bumper damage at every event. Rarely was the result simply a bumper being ripped off or dragging on the field. Rules being what they were, in some cases the robots were disabled.

If you have ever seen a student's face after their robot has been rendered useless, you know why I support the bumper rules. I am not interested in merely satisfying a rule, I want the students to drive in as many matches as they possibly can and good bumpers help them do that. Ike said it pretty well above. If you spent so much time building what you think is the best robot you can build, why would you sacrifice your creation by mounting a substandard or ugly bumper on it? Make it look pretty, make it functional and robust and don't let it fall off.

[dtengineering]

Having built robots without bumpers, I'll second Al's comment about their value on the playing field. They have also eliminated the "wedge" robot designs that would just go around tipping other robots... there weren't many of them, but I'm glad they are gone from FRC. (Don't get me wrong... I think they are great for Battlebots, but FRC is "non-contact like basketball" as I often describe it.)

I also wanted to add that the requirement for bumpers has probably saved several thousand dollars in damages to school walls, doors, and the shins of slow-moving humans. For most of our robots, they spent far, FAR more time doing demos, test runs, and R&D back at the school and in the community than they did in competition. The bumpers make it easy to let kids take the robot for a spin, and turn the occasional error in autonomous mode testing from an "Oh... that's bad." to an "Ooops."

But on the main topic of the thread... we built our robot almost entirely of baltic birch plywood one year... the bumpers were backed by 1/2" ply. We never did intentional destructive testing with that setup... but I'm pretty sure that it was bulletproof as far as FRC applications are concerned.

Jason

[Tristan Lall]

Originally Posted by Al Skierkiewicz

If you have ever seen a student's face after their robot has been rendered useless, you know why I support the bumper rules. I am not interested in merely satisfying a rule, I want the students to drive in as many matches as they possibly can and good bumpers help them do that. Ike said it pretty well above. If you spent so much time building what you think is the best robot you can build, why would you sacrifice your creation by mounting a substandard or ugly bumper on it? Make it look pretty, make it functional and robust and don't let it fall off.

I'm suggesting that teams consider strengthening their robots by building robust bumpers (taking advantage of all that free weight and the inherent strength, stiffness and energy absorption of the plywood), and rely less on heavy frames to meet their robustness targets. I don't think that's at odds with the core objective you articulated.

[BJC]

Originally Posted by Joe G.

Typically, robots with "structural bumpers" aren't at a complete loss of structure without their bumper -- they are still held together as one piece, and don't use the bumper to bridge frame members together. Instead, the bumper, or more specifically, their fastening system, is used to dramatically reinforce or prevent bending of frame members made with extremely light material, which would never stand up to the forces of FRC on their own, and the presence of the bumper is taken into account when sizing other frame members and connections between them. Bumpers may resist any kind of bending, including upwards bending as in the 179 flat panel chassis, torsional bending, or inwards bends from impact.

See here for a whitepaper on 33's system.

We ran this system in 2013 and it worked very well. The only reason we didn't run this system in 2014 was our drivetrain choice made torsional stiffness unnecessary. For 2015, if we choose to use a drivetrain that requires high torsional stiffness (such as 6wd or 8wd) we will definitely be using the bumpers as a central component of the frame. It's simply too much free weight to waste.

Cheers, Bryan