Friction 101

The maximum force a wheel can push with (or resist being pushed with) is governed by:

Total robot push force is the sum across all grip-engaged wheels:

Why omnis don't resist sideways pushes

An omni wheel has perpendicular rollers around its circumference. When force is applied perpendicular to the wheel's drive direction, those rollers spin freely. Effectively, μ_sideways ≈ 0 for an omni. Push an all-omni 4WD sideways and you slide it like a hockey puck.

Traction wheels are solid rubber: μ_sideways ≈ μ_forward. Same grip in every direction.

Why traction wheels scrub on turns

When a robot turns, wheels at different positions follow different arc radii. Inner wheels travel a shorter distance; outer wheels travel longer. Different distances mean different speeds — but if all wheels are powered together, they all spin at the same rate.

An omni wheel resolves this by allowing perpendicular slip — the rollers spin sideways to make up the speed difference. A traction wheel can't slip. Instead it scrubs against the carpet, dissipating energy as heat and losing motor torque.

This is why all-traction 4WD is so bad. All four wheels scrub on every turn. Motor power is wasted, the robot fights itself, and the carpet wears out faster.

Why the center is the right place for traction

A 4-wheel rectangle pivots around its geometric center. Wheels at the corners trace large circles when turning. A wheel placed exactly at the rotation center traces a circle of radius zero — it doesn't move at all relative to the pivot.

So a 6WD with traction wheels in the center: when the robot turns, the center wheels are at (or near) the pivot point. Minimum scrub. You get the grip benefit of traction with almost none of the turning penalty.

Combine that with the 1/16″ drop (only the center wheels touch during straight runs and turns) and you have an architecture that turns like all-omni but defends like all-traction. That's why this is the meta.

Layout

• 4 omni wheels, one at each corner
• 2 motors per side
• Same wheel diameter throughout (3.25″ or 4″)
• No drop, no traction wheels, no complications

How it feels

Driver feel: light, snappy, very responsive on turns. Drives like a go-kart. Drivers love it.

Defender feel: trivial to push sideways. Hit it from the side and it slides like it's on ice.

When this works

• Rookie season — learn drive code, learn driver fundamentals before complexity
• Non-contact games — if defense isn't a thing in this year's game
• Pure scoring specialists with strong alliance partners who handle defense

When this fails (Override)

• Defenders push you off stacks — you lose possession runs
• Toggle contests — opposing robot bumps yours sideways, controls the toggle
• Stack stability — omnis roll-back slightly when you stop. Stack tips.

Layout

• 4 traction wheels, one at each corner
• 2 motors per side
• Same diameter throughout

Why this is bad

On a turn, the inner wheels and outer wheels can't slip. They scrub against the carpet, wasting motor torque as heat and friction. You're actively fighting your own drivetrain.

The scrub force scales with chassis width. A 12″-wide robot scrubs less than a 18″-wide robot. But it's always wasted energy.

The math

If your motors put out 1.0 N·m of torque, an all-traction 4WD might lose 30–50% of that to scrub during a 180° turn. Your effective drive power is half what it could be.

When this might work

• Almost never in V5RC.
• Pure pushing-only robots that don't need to turn (defensive bots that just hold ground)
• You've been told this is wrong but you want to learn for yourself — build it, drive it, feel the scrub. Then upgrade.

Layout

• 4 omni wheels at corners
• 2 traction wheels in center
• All 6 wheels at the same height — no drop

Why this is the wrong stop

You took the trouble to add traction wheels for defense, but because all 6 wheels touch the ground at all times, the traction wheels scrub on every turn. You pay the scrub penalty and you don't get the rocking benefit (where corner omnis lift slightly during straight runs).

This is what happens when teams add traction wheels but skip the CAD step where the drop is modeled explicitly.

How to upgrade to center-drop

• Drop the center wheel mounts by 1/16″ (0.0625″)
• Verify in CAD by measuring wheel-to-ground distance
• Verify in physical assembly: lift the chassis by the corners; the center should be the lowest point
• The robot should rock slightly side-to-side when you push it

If you have a 6WD flat now, the upgrade is mostly a frame redesign in CAD plus reprinting/recutting the chassis plates. Worth the time — you're already paying the weight cost of 6 wheels.

Layout

• 4 omni wheels at corners — 3.25″ or 4″
• 2 high-traction wheels in center — same diameter, mounted 1/16″ lower
• 4 motors, chained per side (all 3 wheels on a side spin together)
• Standard chassis dimensions: 12″×14″ or similar for V5RC

How the rocking works

When the robot is at rest on a flat surface, the center traction wheels touch the ground and the corner omnis hover ~1/16″ above the carpet (or barely brush it).

During straight runs: all 6 wheels engage normally. Push force is high.

During turns: the chassis pivots on the center traction pair. Corner omnis are nearly off the ground; whatever does touch can slip freely. Almost zero scrub.

Under sideways force: when an opponent pushes you sideways, the chassis tips slightly. The corner omnis on the pushed side engage the ground. Combined with the center traction, all 6 wheels resist. Max defense.

Why this dominates stacking games

• Cycle speed: turns clean = no power wasted on scrub = faster acceleration out of turns
• Defense resistance: all 6 wheels grip when shoved sideways = hard to displace
• Stack stability: traction wheels stop cleanly — no roll-back when you stop to score
• Motor budget: still only 4 motors total, leaving 4 for arm/manipulator/lift
• Field record: Tower Takeover, ITZ, High Stakes, Tipping Point — WCD won them all

Wheel size choice

Two common diameters for V5RC:

• 3.25″: lighter, lower ground clearance, better for low robot heights. Recommended for Override (18″ start box).
• 4″: faster top speed at same RPM, but adds vertical space. Better for tall mechanism robots.

For Override, 3.25″ is the safer pick — saves vertical space for the stacking mechanism inside the start box.

Why the exact drop value matters

Too small (or zero) and all 6 wheels touch all the time. You scrub on turns. This is the 6WD-flat failure mode.

Too large (>1/8″) and the corner omnis never engage even under sideways pushes. You lose your turning omnis entirely — back to scrubbing.

Sweet spot range

How to verify the drop in Onshape

1. In CAD, create a sketch on the bottom face of the chassis
2. Project the lowest point of each wheel onto the sketch plane
3. Measure the distance from chassis-bottom to each wheel's contact point
4. The center pair should be 0.0625″ lower than the corners
5. If not, adjust the wheel mount positions or chassis frame thickness

Physical verification (the rock test)

1. Assemble the chassis with all wheels mounted
2. Place on flat surface (lab floor or foam tiles)
3. Push down on one corner of the chassis
4. The opposite corner should lift slightly off the ground
5. Push down on the center; both corners should lift
6. If the chassis sits flat with all 6 wheels in contact, the drop is too small

The push test

The ultimate verification: push your chassis against another robot of similar weight. If it pushes you sideways trivially, your drop isn't engaging. If you can hold ground or push the other robot, the drop is working.

Bring this to scrimmages. A push test against another team's build will tell you in 30 seconds whether your drivetrain is competition-ready.

The optimal Override wheel config

Wheel sourcing

• Omnis: VEX 3.25″ Omni Wheel (276-1485) — or equivalent. Buy 6+ to have spares.
• Traction wheels: VEX 3.25″ Traction Wheel (276-9020) — same diameter as omni for symmetry.
• Locknuts and shaft collars: 2× standard parts list. They vanish.

Why this wins for Override specifically

• 1+1 possession cap — you carry valuable cargo. Center traction = no sideways shove costs you possession.
• Toggle contests — defenders will try to push you off toggles. All 6 wheels engaged under sideways force = you hold position.
• Stack stability — traction wheels stop cleanly. Stacking after braking doesn't roll the stack back.
• Cycle speed — corner omnis on turns mean no scrub waste. Cycle times stay tight.
• 18″ start box — 3.25″ wheels save vertical space for the stacking mechanism.
• 4-motor budget — same as 4WD, leaves 4 motors for arm/manipulator/lift/pneumatics.

Common mistakes to avoid

• Forgetting the drop in CAD — you'll build 6WD flat and pay scrub everywhere
• Mixing wheel diameters — if center is 4″ and corners are 3.25″, the drop math breaks
• Using regular chassis nuts — vibration loosens them. Use locknuts.
• Skipping the push test — you won't know your drop is working until comp day, when it's too late
• Running all-omni 6WD — if all 6 are omni, you skipped the entire point. Defense fails.

Build verification checklist

• ☐ CAD shows center wheels 0.0625″ lower than corners
• ☐ Physical chassis rocks on the center pair when pushed at any corner
• ☐ Push test against another robot: hold ground or out-push
• ☐ All locknuts present and torqued
• ☐ All 6 wheels can spin freely when robot is lifted (no chain binding)
• ☐ Center wheels are high-traction, NOT omni
• ☐ Corner wheels are omni, NOT traction
• ☐ All 4 motors present and chained correctly

Companion guides