You're absolutely right — ball plungers alone won't hold under hard riding. You need an auto-engaging catch that locks the cartridge shut the moment it closes, with no user action required. The cam lock then adds anti-theft security on top.

The Fail-Safe Approach: Layered Retention

Total add: ~€15. The over-center latch is your "never forget" safety net.

Here's where we stand and what's left:

✅ Decided

• Battery: 15S1P, 15× 21700, 54V, 270Wh

• Down tube: 50mm OD, 435mm cartridge cutout

• Cartridge: hinged at BB, cam lock at HT, ball plungers, USB-C PD

• Electronics: BMS in cartridge, controller in sealed BB section

• Motor: Bafang G020 48V, pre-built 700c wheel

• Controller: KT T06S 48V sine wave

• Full BOM: $1,262

• Lock: keyed quarter-turn cam + ball plunger snap

🔲 Still Need

Here's where we stand and what's left:

✅ Done

• Frame geometry (Detroit clone, 135mm rear dropout)

• Down tube battery design (50mm OD, 50/50 split, removable cartridge)

• Battery config (15S1P, 3×5 × 21700, 54V / 270 Wh)

• Cell retention (knurled caps + wave springs, gravity-swap)

• Lock & snap mechanism (ball plungers + keyed cam lock)

• Connectors (XT30AW power + JST-SM signal + USB-C PD charge)

• BMS (15S smart, UART + BT, self-contained in cartridge)

• Sealing & weatherproofing (IP54)

• Full BOM with Oslo-friendly sourcing (~€296)

• SVG technical drawing

🔲 Still To Do

What do you want to tackle next?

Good thinking. Here's everything you need to know spatially before you draw. Detroit vibe, Omera design.

Geometry (Detroit-Inspired, Not Copied)

The Detroit runs 74° HT / 76° ST / 382mm chainstay — aggressive pursuit geometry. Your Omera keeps the same spirit but the 50mm downtube is wider than Detroit's 42mm Columbus Airplane tube, so the frame silhouette will already look distinct. That fat downtube is the design identity.[ppl-ai-file-upload.s3.amazonaws]​

Down Tube — The Main Event

This is where almost everything lives. 581mm center-line length (M size), 50mm OD, 47mm ID, 1.5mm wall.[ppl-ai-file-upload.s3.amazonaws]​

Three zones from HT to BB:

1. HT sealed zone (0–35mm) — cam lock + over-center latch mount, ball plunger holes, frame-side XT30 female + JST-SM female socket

2. Cartridge cutout (35–470mm, 435mm long) — the 50/50 split, underside, hidden from view. Hinge rails along both long edges

3. BB sealed zone (470–581mm, ~110mm) — KT T06S controller PCB, PAS wiring, motor cable exit hole toward rear dropout

Inside the cartridge (435mm long):

• 351mm cell zone: 3 channels × 5 cells, PETG plastic insert glued inside the alu shell[ppl-ai-file-upload.s3.amazonaws]​

• 95mm BMS/port zone (HT end of cartridge): 15S BMS (65×30×3mm), USB-C PD port with rubber cap, XT30 male + JST-SM male (auto-mate when locked)

• 3 knurled screw caps at HT end for cell swapping[ppl-ai-file-upload.s3.amazonaws]​

When drawing the cross-section (looking from BB end):

text
     FRAME (top 50%, welded)
    ╭─────────────────────╮
    │    lip    lip        │
    │  ○   ○   ○          │  ← 3× 21700 triangle pack
    │    ○   ○            │     fits in 43.5mm envelope
    ╰─────────────────────╯
     CARTRIDGE (bottom 50%, removable)

BB Junction — Crowded Node

Draw this area carefully, five things converge here:[ppl-ai-file-upload.s3.amazonaws]​

• Down tube enters from above-front (at ~41° from horizontal)

• Seat tube rises at 76°

• Chainstays exit rearward (both sides)

• BSA 68mm shell sits at the center

• 2× pin hinges for cartridge mount on the underside of the DT/BB junction

What fits inside the sealed BB zone of the downtube (~110mm):

• KT T06S controller bare PCB (~80×40mm)[ppl-ai-file-upload.s3.amazonaws]​

• Motor cable exits through a grommeted hole in the chainstay toward the rear dropout

• PAS sensor cable exits at BB shell (left side, crank-mounted sensor)

Head Tube Junction

The clean end — this is what people see first:[ppl-ai-file-upload.s3.amazonaws]​

• Cam lock keyhole on the underside of the DT, near the HT junction — hidden from normal viewing angle

• Over-center latch lever also underside, blocked by cam lock tongue when locked

• USB-C charge port on the cartridge surface, accessible through a small hole in the frame when cartridge is installed. Rubber cap flush with frame surface

• 3 knurled cell-swap caps face toward the HT end — accessible when cartridge is swung open

When locked, this junction should look like a clean weld — the seam, lock keyhole, and USB-C port are all on the underside, invisible from standing height.

Rear Triangle — Belt Split + Motor

Two critical constraints:

• 135mm horizontal/sliding dropout for the Bafang G020 hub motor (not track 120mm)[ppl-ai-file-upload.s3.amazonaws]​

• Gates CDX belt needs a frame split point — either the right seatstay or right chainstay must be splittable to thread the belt through. Most belt-drive frames split the right seatstay with a coupler

• Disc brake mount on the left chainstay (IS or post mount for 160mm rotor)

• Motor cable runs inside the right chainstay from BB area to rear dropout

Seat Tube — Keep It Simple

Nothing special here anymore — all electronics moved to the downtube:[ppl-ai-file-upload.s3.amazonaws]​

• 31.7mm OD, standard 27.2mm carbon seatpost

• Seatpost clamp at top

• That's it. Clean.

What Makes It Omera, Not Detroit

When you draw, these are the visual differentiators:

• The 50mm downtube — 8mm fatter than the Detroit's 42mm. This is the design statement. It's bold, hints at something inside, but doesn't scream e-bike[ppl-ai-file-upload.s3.amazonaws]​

• Disc brakes — the Detroit runs rim brakes. Omera has hydraulic disc, so you need fork/stay mounts and rotor clearance

• 135mm rear — wider than track 120mm, changes the rear triangle proportions slightly

• The seam line — a hairline laser-cut line on the underside of the downtube. Your signature detail. On matte black, nearly invisible

• No top tube battery — Detroit has a standard 35mm TT. So does Omera. The difference is all in the DT

Drawing Checklist

When you sit down on iPad tomorrow, draw these views:

1. Side profile — full bike silhouette, get the geometry angles right, show the fat DT proportion relative to the 35mm TT and 19mm stays

2. DT underside — the seam line, keyhole position, USB-C port, drain hole

3. DT cross-section at BB end — 50/50 split, 3-cell triangle, lip overlap, gasket

4. DT longitudinal cutaway — the three zones (HT sealed / cartridge / BB sealed), cell rows, BMS pocket, controller pocket

5. BB junction detail — how DT, ST, and chainstays meet, where the hinge pins sit, cable exit holes

6. Rear dropout — 135mm horizontal slot, disc mount, belt path, motor cable entry

7. Cartridge removed — the cartridge as a standalone object with USB-C port, caps, XT30/JST connectors visible

Yes — essentially. The process goes like this:[youtube]​qualityinspection+1

The Actual Process

Step 1: You send them drawings
Not final production CAD at first. Most factories start with:

• A 2D geometry drawing with all dimensions (tube lengths, angles, BB type, dropout spacing, head tube size)

• Reference images of what you want (your iPad drawing is perfect for this)

• A spec list (material, wall thickness, seatpost diameter, etc.)

Step 2: They make the CAD
The factory's engineer converts your drawing into their own technical production drawing and sends it back for you to approve. This is a 1:1 blueprint showing every tube, every angle, every weld point. You check it, request changes, iterate.[vpic-group]​[youtube]​

Step 3: You approve, pay deposit
Typically 50% upfront, 50% before shipping.[youtube]​

Step 4: They manufacture (4–8 weeks)
Tube cutting → bending/shaping → welding → heat treatment (T6) → alignment check → powder coat → ship.pinheadbikelocks+1

What You Actually Need to Send

The Actual Process

Step 1: You send them drawings
Not final production CAD at first. Most factories start with:

• A 2D geometry drawing with all dimensions (tube lengths, angles, BB type, dropout spacing, head tube size)
• Reference images of what you want (your iPad drawing is perfect for this)
• A spec list (material, wall thickness, seatpost diameter, etc.)

Step 2: They make the CAD
The factory's engineer converts your drawing into their own technical production drawing and sends it back for you to approve. This is a 1:1 blueprint showing every tube, every angle, every weld point. You check it, request changes, iterate. [vpic-group](https://www.vpic-group.com/bicycle-manufacturing)

Step 3: You approve, pay deposit
Typically 50% upfront, 50% before shipping. [youtube](https://www.youtube.com/watch?v=8t1yex_8VjY)

Step 4: They manufacture (4–8 weeks)
Tube cutting → bending/shaping → welding → heat treatment (T6) → alignment check → powder coat → ship. pinheadbikelocks

What You Actually Need to Send

You Don't Need

• Full SolidWorks/Fusion 360 CAD (they make their own) [youtube](https://www.youtube.com/watch?v=8t1yex_8VjY)
• Engineering stress analysis (they know tube specs for bikes)
• Manufacturing drawings (that's their job)

Your iPad drawing tomorrow + the spec sheet we've already discussed is genuinely enough to get quotes started. The factory turns your sketch into real engineering. The key is being very precise about dimensions and special machining (the downtube cutout, hinge mounts, cartridge tolerances) — that's the non-standard part they'll need clarity on. qualityinspection

For your workflow — CAD modelling + photorealistic renders + factory-ready STEP export — all free on Mac:

Best Free Option: FreeCAD + Blender

FreeCAD 1.0 (released late 2025) for the actual engineering model:[eufymake]​[youtube]​

• Fully free, including commercial use — no subscription, no limits[youtube]​

• Parametric — change one dimension, everything updates

• Exports STEP files which factories read directly[youtube]​[wiki.freecad]​

• Runs natively on Mac including Apple Silicon

• freecad.org[eufymake]​

Blender for renders:style3d+1

• Import the STEP/STL from FreeCAD

• Cycles renderer produces photorealistic studio shots — Instagram-ready

• Free forever, open source

• blender.org

Alternative: Onshape (Free Tier)

Cloud-based, runs in the browser, no install. Professional parametric CAD with STEP export. The free tier requires your designs to be public (anyone can see them) — which may or may not matter at concept stage.[youtube]​

• onshape.com

Comparison

Best Free Option: FreeCAD + Blender

FreeCAD 1.0 (released late 2025) for the actual engineering model: [eufymake](https://www.eufymake.com/ca/blogs/printing-guides/3d-modeling-software-mac)

• Fully free, including commercial use — no subscription, no limits [youtube](https://www.youtube.com/watch?v=DYTuXe4k4vg)
• Parametric — change one dimension, everything updates
• Exports STEP files which factories read directly [youtube](https://www.youtube.com/watch?v=sDxoJrs0yAM)
• Runs natively on Mac including Apple Silicon
• freecad.org eufymake

Blender for renders: style3d

• Import the STEP/STL from FreeCAD
• Cycles renderer produces photorealistic studio shots — Instagram-ready
• Free forever, open source
• blender.org

Alternative: Onshape (Free Tier)

Cloud-based, runs in the browser, no install. Professional parametric CAD with STEP export. The free tier requires your designs to be public (anyone can see them) — which may or may not matter at concept stage. [youtube](https://www.youtube.com/watch?v=DYTuXe4k4vg)

• onshape.com

Comparison

Recommended Workflow

1. iPad — sketch proportions and vibe in Procreate/Concepts
2. FreeCAD on Mac — build the precise 3D model with real dimensions, STEP export for factory
3. Blender on Mac — import model, add materials (matte black alu, carbon fibre texture), studio lighting, render hero shots for website/IG

FreeCAD has a learning curve but there are tons of bike frame tutorials on YouTube. Since you're already technical, you'll pick it up fast. [youtube](https://www.youtube.com/watch?v=CwjfDEKbqUk)