# Wheelharp Design Study ## Design Thesis The Wheelharp is best treated as a controlled-friction keyboard rather than a normal plucked or struck string instrument. The player presses keys, but the sound depends on how each string enters a moving rosined wheel, how quickly it speaks, how it damps when released, and how consistently the wheel bank runs. The engineering challenge is not simply adding motors to a keyboard. It is making a large number of small bowing contacts behave like one playable instrument. ## Mechanism Overview A motor drives a bank of wheels. Each wheel presents a rosined edge that can bow a string when the string is brought into contact. The keyboard maps player motion into controlled engagement: a key may lift a string toward a wheel, lower a wheel toward a string, or actuate an intermediate pressure element. This L1 packet does not choose the final linkage. The instrument needs a note-on path and a note-off path. Note-on requires repeatable contact pressure, a clean attack, and minimal mechanical noise. Note-off requires the string to leave the wheel and be damped without bouncing back into contact. If damping is ignored, the wheel bank can turn every note into a persistent blur. The semicircular keyboard can make the instrument visually and physically distinct, but it also complicates routing. Strings, key levers, dampers, and wheel-service paths must not fight for the same space. ## Subsystems ### Wheel Bank - Each wheel edge must run true enough for stable bow contact. - Wheel material, edge finish, rosin compatibility, cleaning method, and replacement method are pending measurement. - Shared-drive and individual-drive architectures are both open. - The wheel bank needs guards for fingers, hair, loose clothing, and rosin dust. - L2 evidence target: a one-wheel coupon with recorded edge material, truing method, rosin behavior, noise observations, and service access notes. - Authority boundary: no wheel count, wheel diameter, axle size, bearing type, or spacing is released for fabrication. ### Motor Drive - Motor type, drive belt, gear train, pulley path, controller, and noise isolation are unresolved. - The drive should support consistent wheel motion before notes engage. - Electrical isolation and service access should be designed before any powered prototype is built. - No motor speed, torque, current, voltage, or power values are claimed here. - L2 evidence target: a drive architecture decision comparing shared shaft, belt train, and modular motor approaches by safety, noise, serviceability, and controllability. - Authority boundary: powered design remains pending electrical and pinch-point review. ### Key-To-String Engagement - The key action must convert finger motion into controlled bow pressure. - Candidate actions include lifting the string to the wheel, lowering a pressure wheel toward the string, or moving a small shoe that controls contact. - The action should separate travel feel from final contact pressure so a playable key does not automatically imply excessive bow force. - Key return, bounce, lost motion, and quiet operation are first prototype questions. - L2 evidence target: one removable key-action mule that can test string lift, contact shoe, and damper sequencing without committing the whole keyboard. - Authority boundary: key spacing, lever ratios, pivot positions, and spring forces are pending measurement. ### Strings And Damping - String material, gauge, pitch layout, and tension are pending measurement. - Idle strings need damping before and after bow engagement. - String spacing and routing must account for wheel access, rosin maintenance, and damping hardware. - A future test rig should measure attack, release, squeal, chatter, and unwanted sympathetic response. - L2 evidence target: a string/damper test log that records whether release is clean before any keyboard layout is scaled up. - Authority boundary: no tuning table, string scale, gauge, tension, or damper material is released. ### Frame, Soundboard, And Resonance - The frame must hold wheel-bank alignment, keyboard action, string load, and motor vibration without turning into a noise source. - Soundboard or pickup strategy is intentionally unresolved. - A purely acoustic body, hybrid amplified body, or mostly electric pickup architecture are all still concept options. - L2 evidence target: a structural interface map that separates wheel-bank support, string load path, keyboard mounts, service openings, and acoustic or pickup provisions. - Authority boundary: frame stock, body dimensions, bracing, and resonance targets remain pending measurement. ### Maintenance And Setup - Wheel truing, rosin application, string replacement, and damper adjustment must be reachable without dismantling the full keyboard. - Rosin dust and motor debris need containment and cleaning paths. - A setup log should track wheel condition, string wrapping or surface prep, engagement feel, noise, and response. - L2 evidence target: a service checklist covering wheel replacement, rosin cleanup, string replacement, damper adjustment, and guard removal. ## Parametric Intent Future work should name parameters before assigning values: - `estimate_key_count_pending_measurement` - `estimate_wheel_count_pending_measurement` - `estimate_string_count_pending_measurement` - `estimate_wheel_edge_material_pending_measurement` - `estimate_key_travel_pending_measurement` - `estimate_contact_pressure_pending_measurement` - `estimate_damper_clearance_pending_measurement` - `estimate_motor_drive_architecture_pending_measurement` These are placeholders only. They are not build dimensions or performance targets. ## First Measurement Gates - Wheel truing: define a repeatable method to inspect wheel runout and edge condition. - Bow contact: test one string against one wheel before scaling to a bank. - Damping: verify that release is quiet and complete. - Drive noise: separate wheel tone from motor and belt noise. - Rosin management: log dust, buildup, cleaning, and reapplication behavior. - Keyboard feel: confirm that engagement can be controlled by a player without excessive force or bounce. - Safety: review guarding, electrical isolation, heat, and pinch points before powered multi-wheel tests. ## L2 Packet Gates | Gate | Current authority | Promotion evidence | | --- | --- | --- | | Wheel coupon | pending_measurement | One-wheel test with material, edge condition, rosin, noise, and wear notes. | | Key-action mule | pending_measurement | Bench linkage that proves note-on contact and note-off damping sequence. | | String setup | pending_measurement | Measured response log with attack, release, squeal, chatter, and sustain observations. | | Drive safety | pending_measurement | Electrical, guarding, heat, pinch-point, and service-access review. | | Frame interfaces | pending_measurement | Reviewed layout separating wheel bank, string path, key action, damping, and maintenance access. | | CAD/drawings | pending_measurement | Reviewed CAD, DXF, or drawing authority after measured parameters exist. | ## L2 Boundary This design study is a build-packet candidate. It names subsystems, gates, and planned fabrication classes, but it does not define a scale, tuning system, wheel size, motor specification, CAD model, DXF, string schedule, or fabrication release.