Load Paths & Structural Thinking
A load path is the route that force takes through the robot's structure from where it's applied to where it's reacted. When your intake hits a game piece, that force travels through the intake arms, through the pivot, through the gussets, through the frame, through the swerve modules, and into the ground. If any link in that chain is weak, that's where the robot breaks.
You don't need to do stress calculations or finite element analysis for FRC. You need to be able to look at a CAD model and trace the force from where it starts to where it ends, and identify the weak points along the way.
The concept
Every force on the robot needs a path to the ground. Think of it like water flowing downhill: the force starts at the point of contact (an intake hitting a game piece, another robot ramming you, the weight of a climbing mechanism) and it needs to flow through structure all the way down to the wheels. If the path is interrupted (a missing gusset, an unsupported tube, a single bolt carrying the entire load), the force concentrates at that point and something bends, breaks, or loosens.
The question to ask about every mechanism: If I push hard on this, can I trace a continuous path of structure from where I'm pushing all the way to the wheels, with no weak links?
What makes a strong load path
Force transfers efficiently through material that's in a direct line. A straight tube from the point of load to the frame is the strongest path. Every joint, gusset, and bolt along the way is a potential weak point because the force has to transfer from one part to the next.
What to look for in your design:
Can you draw a straight (or nearly straight) line from the load to the frame?
How many joints does the force cross? Fewer is better.
Is any single bolt or gusset carrying a disproportionate share of the load?
Example: An arm pivot that bolts directly to a frame rail through a thick gusset has a short, strong load path. The same arm pivot mounted on a tall, unsupported plate that's bolted to a cantilevered tube has a long, weak load path with multiple flex points.
Rectangles are weak. Triangles are strong. This is the single most important structural principle in FRC frame design.
A rectangular frame (four tubes bolted at the corners with gussets) can rack, meaning it can deform into a parallelogram when hit from the side. A triangle can't rack because the geometry is inherently rigid. You can't change the angles of a triangle without changing the length of the sides.
How to apply this:
Add diagonal bracing to rectangular sections of the frame. One diagonal tube (or even a gusset plate across the corner) turns a rectangle into two triangles.
The bellypan is a giant triangulation member. A flat plate bolted across the bottom of the frame prevents racking in the same way a diagonal brace would.
Superstructure that forms a tall rectangle (two vertical tubes and a horizontal cross-member) needs a diagonal brace or an A-frame shape to be rigid.
A single bolt is a single point of failure. If the bolt loosens, shears, or the hole elongates, the joint fails. Distributing the load across many fasteners means each one carries less force and the joint survives even if one fastener loosens.
Rules of thumb:
Minimum 4 bolts per structural gusset connection (2 per tube)
High-load joints (arm pivots, elevator base, climber mounts) should have 6 or more bolts
Use through-bolts with nylock nuts instead of threading into thin tube wall for critical joints
If a joint is only held by 2 bolts and it's in a load path, add more bolts
Common weak points
These are the places where load paths fail most often on FRC robots. Check every one of these in your design.
Cantilevered tubes
A tube bolted to the frame at only one end acts as a lever. The longer it is, the more the free end flexes.
Support both ends. Add a brace or gusset to the free end. If you can't support the far end, keep the cantilever as short as possible.
Single-sided gusset joints
A joint with a gusset on only one side can twist. The tube rotates around the gusset as a hinge.
Add a gusset on both sides of the joint (front and back). Two gussets make the joint rigid in both planes.
Tall unsupported verticals
A vertical tube that only connects to the frame at the bottom will sway when the robot accelerates, decelerates, or gets hit.
Add a diagonal brace from partway up the vertical down to the frame. Or use an A-frame shape (two tubes angling inward to meet at the top).
Motor mounts with flex
If the motor or gearbox mount has any compliance, the output shaft deflects under load, causing binding and accelerated bearing wear.
Bolt the gearbox to a rigid plate. Support the output shaft at both ends. Don't mount motors to thin polycarb or unsupported 3D prints.
Elevator or arm base
The base of an elevator or arm pivot is where the highest forces concentrate. If this connection is weak, everything above it is unreliable.
Thickest gussets, most bolts at the base. Through-bolt into frame rails. Use tube plugs if bolting into tube ends.
Bumper brackets
Bumpers take the full force of robot-to-robot impacts. If the brackets are weak, the bumper tears off.
Multiple brackets per side. Through-bolt to the frame. Design brackets that resist pulling away, not just pushing against.
How to think about it during design
Identify where forces come from
For every mechanism, ask: what forces does this create, and where do they act? An arm pivot creates torque at the pivot. A climber creates tension at the hook mount. An intake getting rammed creates impact force at the bumper.
Trace the path to the ground
Starting from where the force is applied, follow the structure through every tube, gusset, plate, and bolt all the way to the frame rails and then to the wheels. Draw this path on your layout sketch if it helps.
Find the weakest link
Along that path, which connection has the fewest bolts? Which tube is the thinnest? Which joint has a gusset on only one side? Which section is cantilevered? That's where the failure will happen.
Strengthen or shorten the weak link
Add bolts, add a gusset, add a brace, or redesign the geometry so the load path is shorter and more direct. The goal is to make the entire path uniformly strong so no single point is dramatically weaker than the rest.
The push test
The simplest way to evaluate structural integrity on the real robot is to push on it.
Put the robot on the ground with the wheels locked (or on blocks). Push hard on every mechanism and every part of the frame by hand. Push sideways, push up, push down. If anything flexes noticeably, that's a weak load path.
Pay special attention to:
The top of the elevator (push sideways)
The arm at full extension (push down)
The superstructure (push sideways at the top)
The bumpers (push inward and pull outward)
Motor mounts (push on the motor body)
If you can feel flex with your hand, the mechanism will feel it during a match at much higher forces. Fix it before competition.
Designing for impact
FRC robots hit each other, hit field elements, and drive into walls at full speed. The structure needs to handle this.
Where impacts happen: The bumper zone (5" tall, at frame rail height) takes most direct hits. Above and below the bumper zone, the robot is more exposed.
What to do about it:
Make the frame rails at bumper height the strongest part of the structure (thick wall 2x1, well-gusseted corners)
Don't put fragile mechanisms at bumper height on the exposed perimeter
Intakes that extend past the bumper should be designed to take a hit or retract when not in use
The bellypan and frame corners absorb most impact energy, so gusset the corners thoroughly
When in doubt about whether something is strong enough, ask yourself: if another robot rams this at full speed, what happens? If the answer is "it bends" or "it tears off," make it stronger. If the answer is "the force transfers through to the frame and the robot slides sideways," that's correct. The structure should transmit impact forces into the mass of the whole robot, not absorb them by deforming.
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