Inspectors please help! Protrusion from frame perimeter.

I am working with a team that has bearings holding the the wheel shafts in the robot. The bearings are currently sticking out about an 1/8th of an inch past the frame perimeter. I don’t believe this is a problem and should be legal due to 1/4 protrusions being “ignored” also our frame is a bit under 112.

The real questions lies in the bumpers. To make our bumpers work well we were planning on cutting 3/16ths deep slots about 1.5 inches up from the bottom of the bumpers so the bumpers hug the bearings. This will also allow the bumper to be right up against the frame instead of the 1/8th inch. Will the slots be a problem or are they legal?

Please correct me if I said anything that is wrong and will prevent passing inspection.

The first part of R21 reads:

BUMPERS must be constructed as follows (see Figure 4-8):

A. be backed by ¾ in. (nominal) thick by 5 in. (± ½ in) tall plywood or solid, robust wood. 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.

Emphasis mine. You should be good to go, as long as they’re small pockets and don’t comprise a large portion of the bumper backing.

You should review the Q&A answers to questions Q231 and Q240. Q231 considers the method of determining the frame perimeter. It rules against a wheel being .08" outside the frame perimeter. The inspectors may not consider a bearing as allowed as a protrusinon. Q240 did not consider a slot as a small clearance. Granted the question asked about a 1-1/2"x 1/2" slot.

You should ask in Q&A for an official ruling.

The protrusions language is meant to allow for bolt heads, welds, rivets, etc. Bearings are pushing that definition. However, the FRAME PERIMETER as determined by the string method may include the bearings and still be under 112", your OK. I would be more inclined to pass your bumper design if if had shallow holes where the bearings are located rather than one long slot. From your description, it sounds like one or two of the plys in a plywood backing would be compromised. That is pretty significant.

Al, am I missing something here? 1/8" protrusion of bearings through the AM14U chassis outer plates will be a common feature of many robots this year, because that is the way the kit chassis was designed.

Do you really think a bumper will be stronger if it is located on the bearing protrusion without a pocket? It seems to me that a clearance pocket, or slot, actually makes the system stronger by enabling more surface contact with the chassis sheet metal.

I completely agree about the kit chassis design. We are not using it, but we had some of the new kids put together, wire up and program the kit chassis as a learning (for the new kids) and teaching (for the experienced kids) exercise.

I will let Al answer as to his intent, but I tend to agree with what he said that shallow holes are much better than a slot for preserving bumper integrity. Cutting out a slot will greatly increase the chance of a fracture of the bumper at that point compared to a shallow hole.

The protrusion of the bearings on the kit frame would require a slot of slightly less than 1 ply on standard 3/4" plywood. If that puts the bumpers tight against the frame it would be stronger than leaving an unsupported 1/8 gap for the remainder of the frame. Do you want the bumpers to be supported by the frame, or by 12 bolt heads and a bearing? Leaving the bumpers supported by such a small surface area will place all of the force onto your mounts and is more likely to compromise a less than “robust” attachment design. In my mind this presents a safety hazard. To me, Slots are the way to go, but FIRST and FRC inspectors have some rules that only make sense in FRC, not necessarily the real world. Even then some of them don’t make sense in FRC…

In short, keep slots shallow and blind and make sure your mounting points take all of the forces straight to the frame.

The OP stated a slot 3/16" deep, giving my worst case 1/4" with inaccurate routing. I do like tight bumpers but I do not want teams to try and satisfy an inspector and then bind bumper against bearings. Holes just seemed like a better solution. I hate the smell of burning wood in the morning.

Ditto here…

We initially considered altering our design to avoid the bearing protrusions but once we realized our properly assembled kit chassis was built that way, we elected to leave our custom chassis that way as well.

I’d suspect quite a debate from a large number of teams if inspectors rule this “illegal”.

We’re using the KOP chassis, and while I’m reasonably comfortable we have enough wiggle room to pass the frame perimeter test even if they count the bearings, I’d be rather surprised/frustrated/angered if this were counted as part of the frame perimeter, given the design of the kitbot and the wording of the rules (bearings certainly seem “minor” to me).

What would the point of ruling this illegal actually be, other than possibly disenfranchising a lot of teams for absolutely no reason?

One could also state that the bearings define the frame perimeter in that area and manage that issue. Sure, it is not a fastener. Nobody says the Frame Perimeter must be straight. Bumpy is OK (within reason)

With 4" wheels, the bumper zone starts at the centerline of those bearings. That means that a slot in the backing to straddle the bump doesn’t seem to be a significant alteration of its strength. It seems pretty radical to consider the bearing protrusion to be part of the perimeter. With a cutout, there would be a lot of backing board inside the perimeter. How much of the backing board is allowed to be inside the frame perimeter?

Bill et al,
The bump of a flush mounted bearing is not at issue and should be viewed in terms of the “minor protrusion no greater than 1/4"” that a bolt head or weld produces. I do not want teams to think that pillow blocks or other mounted bearings mounted on the outside of the frame will be looked at in the same light. “Minor” is the operative word here.
These are a sampling of what I am thinking might need to be included in the Frame Perimeter determination, depending on use and orientation.

The pillow block / mounted bearing would protrude way past a 1/4 inch clearly ceasing to be minor even in the most liberal reading of the rule? You then run into issues with 1/2" support on either side of the vertex as well.

In the past (Just for you Al :slight_smile: ) In this case the rule wording is similar though. We drilled thru holes in the bumpers to give clearance for axle bolt heads. 1 inchish Dia to allow for sloppy location. The little decrease in plywood strength was more than made up by solid contact with the frame.

Does gussets fall in the minor protrusion category assuming the gusset + rivet is less than 1/4 inch?

Gusset on the frame or on the bumper? We will see both.

If the protruding bearings are defining the frame perimeter and pockets are being cut in the bumpers, then the bumper is being mounted inside the frame perimeter, correct? Can bumpers be mounted inside the frame perimeter? Or am I completely misunderstanding something?

The minor protrusions (including the bearings being discussed such as supplied with the AM chassis kit) are not defining the Frame Perimeter. Major protrusions, structural materials and bearings I linked above may define the Frame Perimeter. In those cases, the Frame Perimeter as determined by the string method, becomes the reference for all the other rules dependent on the Frame Perimeter including bumpers and the 20" extension.

Thank you! That makes sense.

Gussets on the frame as part of the frame. Total protrusion (with rivet) less than 1/4". Nothing to do the bumper.

Without actually seeing them, I would make a general determination that they would be considered as part of the Frame Perimeter and the rivets would be ignored.