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Pushing Force & Traction Limit

How hard your drive can actually push, and whether your motors or your wheels give up first. Most teams add motors when the answer is grip.

Inputs

The full size V5 motor. Most drivetrains are built from these.

The smaller V5 motor. It gives roughly half the torque, so two of them are worth about one of the big ones.

Diameter across the wheel. Mecanum and flex wheels share a diameter with the plain ones, so pick by name rather than by size. Anything not listed can be typed in.

lb

Including battery. Weigh it rather than guessing.

Sets a starting grip figure below. Override it if you have measured your own.

Traction 0.8–1.4, omni 0.5–0.9. This is the least certain number here.

%

100 if every wheel is driven. Less if you have undriven casters or wheels.

Results

Pushing force

—lbf

What limits you

—

Motors could give

—lbf

Wheels could hold

—lbf

Gear ratio

—

Spare capacity in the other limit

—lbf

Motor force, against what the wheels can hold—

Filling the bar means the motors can deliver exactly what the tyres can grip. Past it the extra torque only spins the wheels, and more motors will not help.

How this is calculated

A drivetrain has two separate ceilings, and you only ever get the lower one.

What the motors can deliver

force = motors × stall torque × gear ratio ÷ wheel radius

More motors, more reduction, or smaller wheels all raise this. Note thatsmaller wheels push harder, because the same torque on a shorter lever arm gives more force at the floor. It is the same reason a low gear works.

What the floor can hold

force = grip × weight on driven wheels

This one does not care about your motors at all. Past this point the wheels simply spin. The only ways up are more weight, more grip, or getting more of the weight you already have onto wheels that are actually driven.

Why this matters more than it sounds

These two limits respond to completely different changes, so which one binds tells you what to do next, and the intuitive answer is often wrong.

  • Traction-limited? Adding motors changes nothing. You are already able to spin the wheels; more torque just spins them harder. Add weight, switch omnis for traction wheels, or move weight onto the driven wheels.
  • Torque-limited? Better wheels change nothing either. Gear down, fit smaller wheels, or add motors.

A team that gears down a drive which was already traction-limited has made the robot slower for no gain in pushing power at all. That is the single most common wasted rebuild in VEX, and it is what this calculator exists to prevent.

A useful sanity check

Because traction force is grip × weight and the accelerating mass is that same weight, the best acceleration a robot can manage is roughly grip × g, independent of how heavy it is. Weight helps you push other robots. It does not help you get going.

Sources & assumptions

Stall torque is published for the 100 RPM cartridge (2.1 N·m) and derived for the others by scaling inversely with speed, since the motor is unchanged and only the gearing differs. A 600 RPM cartridge therefore delivers about a sixth of the torque, and using the headline 2.1 figure for a blue cartridge overstates force sixfold.

The friction coefficient is estimated. No published VEX figure. Ranges reflect commonly reported community measurements and vary considerably with wheel wear and tile condition. Measure your own robot where the answer matters.

Assumes all driven wheels share load evenly, no drivetrain losses, and a rigid robot on flat tiles. Real gearboxes lose several percent to friction, so the torque figure is optimistic.

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Collects what you entered, what came out, how it was worked out, and anything the tool flagged, with a timestamp and a version so someone else can reproduce it.

This is evidence, not a notebook entry. It deliberately does not write your problem statement, your reasoning, or your conclusion, because under RECF rules an Engineering Notebook has to be the students' own work and no tool may generate or organise its content. Take the numbers, decide what matters, and write it yourself.