How to Create a Custom Road Bike Frame in RattleCADDesigning a custom road bike frame is a rewarding blend of engineering, art, and practical cycling knowledge. RattleCAD is a free, open-source CAD program focused on bicycle frame design; it provides parametric tools, geometrical templates, and output suitable for builders and CNC/laser cutting. This article walks you step-by-step through creating a custom road bike frame in RattleCAD — from planning and measuring, through modeling and validation, to preparing production drawings and cut files.
Overview of the workflow
- Define goals and constraints (riding style, fit, materials).
- Gather measurements and component specs.
- Set up a new project in RattleCAD and choose templates.
- Input geometry and tweak tube lengths, angles, and offsets.
- Check clearances (wheels, tires, chainstays, brake calipers).
- Add details: head tube, seat tube, dropouts, cable routing, bottle bosses.
- Export development drawings, DXF/CSV for CNC, and a parts list.
- Build or send files to a framebuilder/CNC service; prototype and iterate.
Planning: goals, intended use, and materials
Before you open RattleCAD, decide:
- Purpose: racing, endurance/gravel, commuting, or touring. This determines geometry choices (aggressive vs. relaxed).
- Rider fit: reach, stack, saddle height, and preferred handling feel. Gather rider measurements and target bike fit numbers.
- Wheels and tires: road (700×25–28 mm), gravel (700×35–45 mm), clearance for mudguards if needed.
- Materials: steel, aluminum, titanium, or carbon — materials affect tube dimensions, tube shaping, and joining methods. RattleCAD models geometry but not the detailed laminate schedules of carbon.
Preparing measurements and component specs
Collect:
- Rider measurements: inseam, torso, arm length, effective torso length (or desired reach/stack).
- Component specs: fork rake, headset standard and stack, bottom bracket shell width/height/type, crank length, chainline, derailleur hanger position, brake type (rim, mechanical disc, hydraulic), axle standard (quick-release, thru-axle).
- Wheelset dimensions and axle spacing.
- Dropout type and offsets.
Accurate component specs are critical; download manufacturer PDF datasheets when possible.
Setting up a new project in RattleCAD
- Open RattleCAD and create a new project.
- Choose a basic template closest to a road frame (many RattleCAD versions include road/track/gravel presets). Templates speed setup by pre-filling typical angles and tube positions.
- Set units (mm preferred for framebuilding precision).
- Define reference elements: ground line, wheel diameter, and fork rake.
Inputting mainframe geometry
Main parameters to enter and iterate:
- Top tube length (effective/center-to-center) or reach/stack target.
- Seat tube length and offset (center-to-top or center-to-center).
- Head tube length and angle.
- Seat tube angle.
- Chainstay length.
- Bottom bracket drop (height).
- Fork rake (offset) and trail — use RattleCAD to calculate resulting trail from head angle and rake.
Workflow tips:
- Start with reach and stack targets rather than raw tube lengths if fit is a priority. RattleCAD lets you adjust effective top tube and stack/reach and will show resultant tube intersections.
- Adjust seat tube angle to position the rider over the bottom bracket for optimal pedaling geometry (steeper for racing, slacker for endurance).
- Iteratively tweak head tube angle and fork rake to achieve desired trail and handling stability.
Modeling the fork and headtube assembly
- Enter headset standard (1-⁄8”, 1.5”, tapered) and head tube external/internal lengths.
- Set fork crown race seat and axle-to-crown.
- For disc brakes, ensure axle spacing and rotor clearance; for thru-axles, define axle diameter and dropout spacing.
- RattleCAD can display steering geometry (trail, fork offset) to help you confirm handling characteristics.
Chainstays, dropouts, and drivetrain alignment
- Position chainstays to achieve desired chainline and wheel clearance.
- Set derailleur hanger location relative to rear axle centerline — check compatibility with intended derailleur (short/medium cage).
- If using internal routing, plan guide points and entry/exit angles.
- Confirm bottom bracket type and shell width to match crankset and bottom bracket standard.
Checking clearances and interference
Use RattleCAD’s visualization tools and layers to confirm:
- Tire clearance in chainstay/seatstay area and fork crown.
- Brake caliper clearance and cable/hydraulic hose routing space.
- Seatpost insertion depth and minimum seat tube length for clamping.
- Chainring and crank clearance at chainstay area.
- Bottle cage positions relative to seat tube and down tube curvature.
Always leave conservative clearance for manufacturing tolerances, paint, and mud.
Adding details: tube mitering, gussets, and fittings
RattleCAD supports adding:
- Mitering outlines for tube ends (useful for CNC cutting or manual filing).
- Gusset location and approximate size (especially for steel frames at head tube and chainstay junctions).
- Mounting points: bottle bosses, rack/fender mounts, cable stops, brake bosses.
- Seat clamp style and collar dimensions.
For carbon frames, RattleCAD helps with geometry but final laminate and ply shapes are designed in specialized composite tooling software.
Exporting drawings and CNC files
- Generate 2D development drawings: tube lengths, miter lines, and key measurements.
- Export DXF files for tubes, gussets, and template parts for CNC or laser cutting. Confirm units and layer conventions before sending files.
- Create a parts list including tube diameters, wall thicknesses (for metal frames), and fittings.
- Save multiple versions or iterations with clear naming (e.g., v1_reach520_stack560).
Prototyping and iteration
- Build a mockup jig or use a virtual assembly in RattleCAD to verify fit.
- If possible, make a wooden or steel mock frame to test rider fit and handling before final build.
- Expect several iterations: small geometry tweaks often produce large changes in handling or comfort.
Practical tips and common pitfalls
- Small angle changes (1–2°) in head or seat tube produce noticeable handling shifts. Change them conservatively.
- Overly long chainstays increase stability but reduce snappiness; too short can cause clearance issues.
- Don’t forget bottom bracket height/drop in stability and pedal-to-ground clearance.
- Always verify component standards (headset, BB, axle) — mismatched standards are a common source of errors.
- Keep build and paint clearances in mind (add ~1–2 mm clearance for paint and tolerances).
Example: quick parameter set for an endurance road frame (as a starting point)
- Wheel: 700c × 28 mm tire
- Effective top tube: 555 mm
- Seat tube (C-T): 520 mm
- Head tube length: 150 mm
- Head tube angle: 72.5°
- Seat tube angle: 73.5°
- Chainstay length: 410 mm
- Bottom bracket drop: 70 mm
- Fork rake: 45 mm
Use these as starting numbers and adapt to rider fit and component choices.
Final checks before production
- Reconfirm all unit settings and export scale.
- Share DXF and drawings with fabricator and verify their tooling requirements (file format, tolerances).
- Confirm weld/joining details with the builder (tube miter angles and allowances).
- Print templates at 1:1 and test-fit on tubes or mockups.
Conclusion
RattleCAD simplifies turning fit targets and handling goals into precise frame geometry and production-ready drawings. The key is careful planning, accurate component specs, and iterative testing — the software handles geometry, but rider comfort and handling come from thoughtful choices and real-world prototyping.
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