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Calibration: Bowden Length

Bowden length is the one calibration value no MMU type ships a default for - unlike gear rotation distance or gear current, there's no Kconfig setting for it anywhere in the installer. It's purely physical to your build (the actual tube length between selector and extruder), so until it's measured - by you, or automatically - Happy Hare simply doesn't know it.

MMU_CALIBRATE_BOWDEN picks one of three strategies automatically, based on what extruder_homing_endstop (in mmu_parameters.cfg) is set to:

Sensor-based (the best method, if you have one)

If extruder_homing_endstop is extruder, mmu_gear_touch, or filament_compression, this is the most accurate method - a real homing sensor gives a precise stop point:

MMU_CALIBRATE_BOWDEN

Calibrating bowden length for gate 0 (automatic method) using mmu_gate sensor as gate reference point
Filament homed to extruder after 724.5mm movement
Calibrated bowden length is 724.5mm

You can give an approximate length to bound the search, though it's rarely needed since Happy Hare defaults to a generous 2000mm:

MMU_CALIBRATE_BOWDEN BOWDEN_LENGTH=1500

Encoder collision method

If extruder_homing_endstop is encoder (filament is driven into the closed extruder gears and the collision is detected by the encoder no longer counting movement), supply a BOWDEN_LENGTH slightly shorter than the real distance - a rule of thumb is your measured selector-to- extruder distance minus 40-50mm:

MMU_CALIBRATE_BOWDEN BOWDEN_LENGTH=650

Pass #1: Filament homed to extruder, encoder measured 682.3mm, filament sprung back 3.2mm
Pass #2: Filament homed to extruder, encoder measured 681.8mm, filament sprung back 3.1mm
Pass #3: Filament homed to extruder, encoder measured 680.6mm, filament sprung back 3.4mm
Recommended calibration reference is 680.2mm. Clog detection length: 16.8mm
Bowden calibration and clog detection length have been saved

This method also produces a starting value for FlowGuard's clog-detection length (flowguard_encoder_max_motion) as a side effect, from the same telemetry - see Feature: FlowGuard to tune that further. Happy Hare may briefly heat the extruder for this method, so the motor has enough force to resist the collision realistically. REPEATS= (default 3) controls how many passes it averages over.

Manual method

If you have neither a homing sensor nor an encoder - just a mmu_gate sensor, as on Tradrack - push filament by hand until it reaches the extruder gears, then reverse-home off the gate sensor:

MMU_CALIBRATE_BOWDEN MANUAL=1
MMU_CALIBRATE_BOWDEN BOWDEN_LENGTH=1500 MANUAL=1

This uses Klipper's own measurement of stepper movement during the reverse-home, rather than a sensor detecting the far end.

Common to all three

SAVE=0 reports the measured length without persisting it. RESET=1 clears the saved length for the current gate, which also re-enables first-time auto-calibration if autocal_bowden_length is on (see below). HOMING_MAX= (default 150mm) bounds how far the final homing move to the extruder is allowed to travel.

Most MMUs share one bowden length across every gate; a design where gates genuinely differ needs each one calibrated separately with the selected gate changed between runs.

Auto-calibration and autotuning

Two settings in mmu_parameters.cfg can reduce or remove the need to run this command by hand:

  • autocal_bowden_length runs this same calibration automatically the first time it's needed, instead of requiring you to run MMU_CALIBRATE_BOWDEN up front. It defaults on for Box Turtle, EMU, KMS, and BTT ViViD - their stock builds ship with an extruder-entry sensor or a sync-feedback buffer with compression/proportional sensing, either of which this needs to work reliably. It's off for every other type by default, but fitting an extruder-entry sensor unlocks it on any MMU, regardless of vendor.
  • autotune_bowden_length (off everywhere by default, fully opt-in) keeps refining the bowden length continuously during normal use afterward, rather than settling on one first-measured value. It works best alongside an extruder or toolhead sensor, since that gives an accurate homing point close to the point being measured.

See Calibration for how these two sit alongside the other autotuning settings.

See also