# Hurdy-Gurdy Design Study ## Design Thesis This concept treats the hurdy-gurdy as a friction instrument first and a keyboard instrument second. The wheel, rosin, string wrapping, tangent geometry, and trompette bridge are coupled setup problems. A successful build packet has to make those adjustments visible instead of pretending the body outline alone defines the instrument. ## Mechanism Overview The player turns a crank that rotates a wheel through an axle. The wheel edge is rosined and contacts the strings like a continuous bow. Bowing pressure comes from string height, bridge geometry, cotton wrapping, wheel surface condition, and the player's crank speed. A wheel cover protects the player's hand and keeps rosin dust contained while still allowing inspection and service. Melody strings, often called chanterelles, pass through a keybox. Each key carries a tangent that stops the melody string at a setup point. The tangents need lateral position, height, and contact surface adjustment. The key action must return reliably and avoid rubbing neighboring tangents or strings. Drone strings bypass the tangent keyboard and speak continuously against the wheel. They need their own bridge, nut, contact, muting, and setup decisions so the instrument can shift between full drone, reduced drone, and practice states. The trompette is a special drone string that can drive a loose buzzing bridge. The player can trigger the buzz through crank acceleration. The design problem is not just making the bridge loose; it is balancing string tension, bridge mass, wheel friction, and crank impulse so the buzz is controllable instead of random. ## Subsystems ### Wheel Bow And Crank - A true wheel is the central fabrication challenge. - The wheel edge needs a serviceable rosin surface and a way to inspect flatness, roundness, wobble, and string contact. - The crank and axle need low backlash and enough stiffness that crank pulses do not create unwanted wheel wobble. - Bearing, bushing, and wheel-cover choices are pending measurement and prototype testing. ### String Contact And Wrapping - Cotton or similar wrapping on bowed strings is treated as a setup variable, not a fixed design fact. - Contact pressure, rosin loading, string material, wrapping density, and wheel condition interact. - First tests should separate wheel truing from string setup so a bad wheel is not misdiagnosed as a bad string choice. ### Tangent Keyboard - The keybox needs a stable reference plane for keys and tangents. - Tangents should be adjustable during setup because final speaking points are pending measurement. - The concept favors visible, serviceable tangent attachment over hidden mechanisms during the first prototype. - Key return, lateral play, and tangent clearance are first-order risks. ### Drones - Drone strings should be laid out as independent voices with their own muting and contact setup. - Drone placement must leave room for wheel service, keybox service, and hand clearance at the crank. - Tuning, string material, and gauge are intentionally not specified in this L1 packet. ### Trompette And Buzzing Bridge - The trompette bridge should be designed as an adjustable experiment. - Setup variables include bridge seating, string break angle, contact point, string wrapping, rosin, and crank impulse. - The first prototype should log when the bridge stays silent, chatters controllably, or buzzes continuously. - The desired behavior is a rhythmic accent under player control, not permanent rattling. ### Body, Soundboard, Pegbox, And Tailpiece - The body and soundboard must support wheel, bridge, keybox, pegbox, and tailpiece loads without freezing out service access. - The pegbox needs enough adjustment range for multiple string families, but no final string schedule is claimed here. - The tailpiece should support separate setup paths for melody, drone, and trompette strings. - Body outline, bracing, soundhole treatment, and material choices are concept only until measured or reviewed. ## Parametric Intent All future dimensions should be named as parameters before fabrication work: - `estimate_wheel_diameter_pending_measurement` - `estimate_body_length_pending_measurement` - `estimate_key_count_pending_measurement` - `estimate_string_count_pending_measurement` - `estimate_keybox_reference_height_pending_measurement` - `estimate_trompette_bridge_mass_pending_measurement` - `estimate_crank_throw_pending_measurement` These names are placeholders for future reviewed data. They are not values. ## First Measurement Gates - Wheel trueness: measure runout before any string setup judgment. - Wheel surface: record rosin behavior and contact consistency. - Melody action: verify tangent adjustment, return, and clearance with a test string before a full keyboard is built. - Drone setup: test continuous speech and muting independently of melody keys. - Trompette response: record crank impulse, buzz onset, controllability, and false chatter. - Service access: confirm wheel, string wrapping, tangents, and bridges can be adjusted without disassembling the whole instrument. ## L2 Engineering And Acoustic Reasoning The acoustic behavior starts at the wheel/string interface. The wheel must deliver a repeatable stick-slip cycle across melody, drone, and trompette strings, while the soundboard and bridges convert that friction into useful motion. For L2 review, the wheel is treated as a controlled surface rather than as a decorative round part: future work must define truing method, edge material, rosin behavior, cotton wrapping, and contact-pressure adjustment as separate parameters. The tangent keyboard is a pitch-selection mechanism, but no pitch layout is released here. The L2 model only asserts relationships: key travel must move a tangent into a repeatable string contact point; tangent lockup must survive setup changes; return force must clear the string without bouncing; and keybox geometry must stay stable relative to the melody-string path. Tolerances are questions to measure, not numbers to invent. The drone and trompette voices are separate review branches. Drones require continuous, stable speech and a muting plan. The trompette requires an adjustable buzzing bridge whose response changes under crank impulse. The design should therefore separate normal drone testing from trompette testing so a successful drone setup is not confused with a successful buzzing-bridge setup. ## Material Reasoning Candidate material choices remain pending measurement, but the subsystem requirements are clearer: - Wheel material must hold a true edge, accept rosin, resist dust loading, and be serviceable after wear. - Keybox and key materials must favor dimensional stability and repairable bearing surfaces over decorative complexity. - Tangent material must be smooth enough not to damage strings while holding a repeatable contact shape. - Bridge and soundboard materials must be selected together because wheel friction, drone coupling, and trompette buzz all load the same acoustic structure. - Cotton or equivalent string wrapping is treated as a consumable setup material with its own test log. ## Failure Modes To Review - Wheel runout causes uneven speech or false trompette chatter. - Rosin dust contaminates bearings, keys, or tangent returns. - Tangents creep, rotate, or fail to return clear of the melody strings. - Drone strings overdrive the soundboard or mask melody response. - Trompette bridge buzz is either absent, continuous, or too sensitive to crank motion. - Crank backlash turns rhythmic accents into wheel wobble. - Service access requires removing too many coupled setup parts. ## Tolerances As Questions - How much wheel runout is audible on wrapped strings? - How much key side play can the tangent tolerate before pitch contact becomes unstable? - How much crank backlash is acceptable before trompette control suffers? - How repeatable must string wrapping density be for a useful setup log? - What inspection access is needed to adjust wheel, bridges, tangents, and wrapping without resetting the whole instrument? ## L2 Boundary This file is an L2 engineering-risk map and prototype-planning scaffold. It does not provide a fabrication drawing, tuning plan, acoustic model, measured wheel geometry, or measured string schedule.