FrameStack

Plotting and comparing bike geometries to find the right frame size and stem/spacer combination to match your bikes. Highly experimental. Currently under development.

Parse and plot bicycle frame geometry from a simple text file. Produces detailed 2-D side-view diagrams with all key measurements annotated, and supports overlaying multiple bikes for comparison.

Features

• Renders a full frame side-view: tubes, wheels, saddle, stem, handlebars
• Annotates ~17 dimensions: reach, stack, wheelbase, seat tube, head tube, fork rake, stem length, spacers, and more
• Angle arcs for head angle and seat angle
• 3-D saddle-to-handlebar distance (accounts for bar width)
• Multi-bike comparison overlay with per-bike colour coding

Requirements

python >= 3.9
numpy
matplotlib

Input file format

A plain-text, tab-separated (or whitespace-separated) file with one measurement per line:

Head Angle      73.0
Seat Angle      73.5
Stack           590
Reach           385
Head Tube       140
Seat Tube C-C   530
Top Tube (effective)    560
Chainstay       415
BB Drop         70
Front Centre    595
Wheelbase       1010
Fork Rake / Offset  50
Seat height     720
Spacers         20
Stem length     100
Stem Angle      6.0
Bar width       420

All linear values are in mm; angles in degrees.

Usage

Single bike plot

from Geometry import Geometry
import matplotlib.pyplot as plt

G = Geometry('my_bike.txt')

fig, ax = plt.subplots(figsize=(18, 10))
G.plot_bike(fig, ax)
plt.show()

Multi-bike comparison

See Geometry_comp.ipynb for a worked example overlaying four bikes:

from Geometry import Geometry
import matplotlib.pyplot as plt

G = Geometry('bike_a.txt')

fig, ax = plt.subplots(figsize=(12, 6))
G.plot_comp(fig, ax, 'bike_a.txt', color='blue')
G.plot_comp(fig, ax, 'bike_b.txt', color='orange')
G.plot_comp(fig, ax, 'bike_c.txt', color='green')

ax.legend()
plt.show()

Coordinate system

Origin is the Bottom Bracket centre.

Axis	Direction
+x	toward front wheel
+y	upward

Key computed points

Point	Derivation
rear_axle	(-sqrt(chainstay² - bb_drop²), bb_drop)
front_axle	(front_centre, bb_drop)
ht_top	(reach, stack)
ht_bottom	ht_top + head_tube × (cos ha, −sin ha)
seat_frame_top	bb + seat_tube_cc × (−cos sa, sin sa)
seat_pos	bb + seat_height × (−cos sa, sin sa)
stem_end	spacer_top + stem_length × (cos α, −sin α)

File structure

Bike_geometry/
├── Geometry.py          # Geometry class: loader, point computation, plotting
├── Geometry_comp.ipynb  # Example notebook: 4-bike comparison
└── README.md