Rollers & Compliance
The roller surface is what actually touches the game piece, so what you put on your rollers directly determines how reliably the intake grabs and controls game pieces. The right choice changes every season depending on the game piece material, weight, and shape.
Roller types
Polycarbonate tube with grip tape
Medium to high
General purpose, most game pieces
A polycarb tube with grip tape (cat tongue tape, McMaster grip tape, or similar) wrapped around it. Cheap, easy to build, easy to replace the tape when it wears. Very common in FRC.
Compliant wheels (flex wheels)
High
Soft or deformable game pieces (inflated balls, foam)
Rubber wheels with a hex bore that flex around the game piece, increasing contact area. Stack them on a hex shaft to make a full-width roller. Available from AndyMark, WCP, TTB.
Compliant stars
High
Rigid or odd-shaped game pieces
Star-shaped soft rubber pieces on a hex shaft. The arms of the star flex around the game piece. Very effective for grabbing objects that aren't perfectly round or flat.
Polycord on a 3D printed hub
Medium
Light game pieces, indexing, conveying
Round elastic cord stretched between grooved hubs. Gaps between cords let debris fall through. Easy to make, easy to replace.
Choosing the right material
The decision comes down to what the game piece needs:
Use compliant materials. Flex wheels or surgical tubing work well because they deform slightly around the game piece, increasing the contact patch and improving grip.
The softness of the roller should roughly match the softness of the game piece. An inflated ball against a hard polycarb tube has a tiny contact patch (just a line). The same ball against a flex wheel has a wide contact patch because both surfaces deform into each other.
Polycarb tubes with grip tape or polycord both work well. You don't need as much compliance because the game piece doesn't deform, so grip comes from friction rather than conforming contact. Polycord is especially good here because the individual cords create discrete contact points that pull the piece in effectively.
Compliant stars are also excellent for rigid pieces because the star arms wrap around edges and corners.
When the game has multiple game piece types (like 2023 with cubes and cones), you may need to compromise. Compliant wheels and stars handle multiple shapes reasonably well because they conform to whatever they touch. You can also use different roller materials on different parts of the intake (grippier material at the leading edge for initial grab, smoother material further in for conveying).
Building rollers
Cut a polycarbonate tube to the width of your intake. Press or glue 3D printed endcaps with bearings into each end of the tube (for dead axle) or with hex bores (for live axle). Wrap the outside with grip tape.
Tube sizing: Common ODs are 1.5" and 2". Thinner tubes are lighter and fit tighter spaces, thicker tubes have a larger contact surface.
Grip tape: Cat tongue tape is popular in FRC. McMaster also sells adhesive-backed grip tape in rolls. Wrap it around the tube with no gaps or overlap, and replace it when it wears smooth.
Onshape: Use Andrew Card's Configurable Rollers featurescript or model your own with the tube OD, endcap geometry, and bearing bore.
Slide flex wheels directly onto a hex shaft. Stack multiple wheels side by side to create a full-width roller. Add spacers between wheels if you want gaps for compliance or to reduce weight.
This is the fastest way to build a roller because everything is COTS. No printing, no fabrication, just slide wheels onto a shaft and add retention.
Available in different sizes and durometers from AndyMark, WCP, and TTB.
Slide star-shaped compliant pieces onto a hex shaft, similar to flex wheels. The star profile means the arms flex independently when they contact a game piece, which lets the roller conform to irregular shapes.
Stars are especially useful when the game piece is rigid and you need the roller to "grab around" it rather than just press flat against it.
Available from AndyMark.
Print a hub with grooves spaced to match your polycord diameter. Stretch the polycord around the hub so it sits in the grooves. The hub rides on a hex shaft (print it with a hex bore and a metal insert for durability).
Polycord comes in different diameters. 3/16" and 1/4" are common for FRC. Thicker cord grips harder but wears faster. Cut the cord slightly shorter than the groove circumference so it stretches on with tension and doesn't slip off during use.
Wrap surgical tubing around a hex shaft or a 3D printed core. Secure the ends with zip ties or by trapping them under a shaft collar. You can wrap in a spiral or in parallel loops depending on the coverage you want.
This is the cheapest and fastest roller to build, which makes it ideal for early prototyping. If it works well enough during testing, you can keep it on the competition robot or upgrade to a more durable option later.
Roller speed
Spin the rollers faster than the game piece needs to travel. A surface speed of roughly 2x the desired game piece speed gives a margin for slippage and ensures positive control on first contact. The goal is "touch it, own it," meaning the game piece is captured the instant the intake contacts it, not after the robot aligns and nudges it in.
If the rollers are too slow, the game piece bounces off or gets pushed around instead of being grabbed. If they're absurdly fast, the game piece can skip or bounce out of control. Start at 2x and adjust from there during testing.
Durometer (hardness)
Durometer measures how hard or soft a material is on the Shore A scale. Lower numbers are softer, higher numbers are firmer. This matters when choosing between different flex wheel or compliant star options.
30A to 40A
Very soft (like a rubber band)
Maximum grip on soft game pieces. Wears fast.
50A to 60A
Medium (like a pencil eraser)
Good balance of grip and durability. Common range for FRC intake rollers.
70A to 80A
Firm (like a shoe sole)
Lower grip but very durable. Better for rigid game pieces or high-wear applications.
If the game piece is soft, go softer on the roller. If the game piece is rigid, a firmer roller is fine because grip comes from surface friction rather than deformation.
Maintenance
Roller surfaces wear down during a competition event, especially grip tape and compliant materials. Grip decreases as the surface gets smoother and thinner. Check rollers between matches and swap them if they're visibly worn. Keep spare grip tape, flex wheels, polycord, or tubing in your pit box so you can replace rollers quickly.
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