Mostly solid, a few things I'd push back on:

✅ Good

Knurled screw caps with wave spring washers as negative terminals — this is clever. The spring maintains constant pressure on the cell stack as the bike vibrates, preventing intermittent connections. Elegant dual-purpose design.

XT30 auto-mating connector — clean. When the cartridge clicks shut, the XT30 male/female engage automatically. No user wiring. Rated for 30A continuous which is way more than your ~5A draw. Cheap, proven, weatherproof if greased.

1mm drainage hole — small detail, huge practical value for Oslo weather.

USB-C PD charge port with rubber cap on the cartridge itself — means you can charge in-frame or on the kitchen table. Exactly the original concept's vision.[ppl-ai-file-upload.s3.amazonaws]​

⚠️ Concerns

60/40 split is too aggressive. 50/50 is already pushing it structurally. At 60% removed, the remaining top shell is a shallow channel that will flex and eventually fatigue-crack at the cut edges under braking loads. I'd go 50/50 — still plenty of access to slide cells in, but significantly stronger remaining section. The 3× 21700 triangular pack is only ~43.5mm wide — you don't need 60% of a 50mm tube open to access it.[scienpg]​

Piano hinge is overkill and fragile. A full-length piano hinge along a vibrating bicycle tube will eventually work-loose or corrode. Two discrete pin hinges (one at each end of the cutout near the BB) are stronger, simpler to replace, and easier to seal against water.

BMS behind the cells (in the cartridge) — this means the BMS is removable with the cartridge. That's fine for the battery, but the BMS also needs to talk to the controller which lives in the frame's electronics bay. Every time you remove/insert the cartridge, the BMS-to-controller data connection must mate cleanly. The XT30 handles power, but you'd need a second connector for the BMS balance/comm wires — or put the BMS in the frame-side electronics bay instead, with only the raw cell taps coming through the connector.

I'd actually keep the BMS in the cartridge as proposed — it means the cartridge is a self-contained smart battery that protects itself regardless of what it's plugged into. Just add a small 4-pin signal connector alongside the XT30 for BMS→controller comms.

"Genuinely patentable" — probably not. EP2134592B1 covers bicycle frames with integrated detachable batteries in structural tubes. There's extensive prior art. But that's a branding concern, not an engineering one.[patents.google]​

Revised Spec

✅ Good

Knurled screw caps with wave spring washers as negative terminals — this is clever. The spring maintains constant pressure on the cell stack as the bike vibrates, preventing intermittent connections. Elegant dual-purpose design.

XT30 auto-mating connector — clean. When the cartridge clicks shut, the XT30 male/female engage automatically. No user wiring. Rated for 30A continuous which is way more than your ~5A draw. Cheap, proven, weatherproof if greased.

1mm drainage hole — small detail, huge practical value for Oslo weather.

USB-C PD charge port with rubber cap on the cartridge itself — means you can charge in-frame or on the kitchen table. Exactly the original concept's vision. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/29f8498e-62b4-487c-8778-4da05be1fbe4/emotor.pdf)

⚠️ Concerns

60/40 split is too aggressive. 50/50 is already pushing it structurally. At 60% removed, the remaining top shell is a shallow channel that will flex and eventually fatigue-crack at the cut edges under braking loads. I'd go 50/50 — still plenty of access to slide cells in, but significantly stronger remaining section. The 3× 21700 triangular pack is only ~43.5mm wide — you don't need 60% of a 50mm tube open to access it. [scienpg](https://scienpg.com/scs/index.php/engineering/article/view/scse.2025.3.149)

Piano hinge is overkill and fragile. A full-length piano hinge along a vibrating bicycle tube will eventually work-loose or corrode. Two discrete pin hinges (one at each end of the cutout near the BB) are stronger, simpler to replace, and easier to seal against water.

BMS behind the cells (in the cartridge) — this means the BMS is removable with the cartridge. That's fine for the battery, but the BMS also needs to talk to the controller which lives in the frame's electronics bay. Every time you remove/insert the cartridge, the BMS-to-controller data connection must mate cleanly. The XT30 handles power, but you'd need a second connector for the BMS balance/comm wires — or put the BMS in the frame-side electronics bay instead, with only the raw cell taps coming through the connector.

I'd actually keep the BMS in the cartridge as proposed — it means the cartridge is a self-contained smart battery that protects itself regardless of what it's plugged into. Just add a small 4-pin signal connector alongside the XT30 for BMS→controller comms.

"Genuinely patentable" — probably not. EP2134592B1 covers bicycle frames with integrated detachable batteries in structural tubes. There's extensive prior art. But that's a branding concern, not an engineering one. [patents.google](https://patents.google.com/patent/EP2134592B1/en)

Revised Spec

Updated Connector Spec

When the cartridge locks shut, two connections mate automatically:

Both connectors are cheap (<$2 total), waterproof variants exist, and they're polarized so you can't plug them wrong.

The overall design is sound. The other agent did good work — just needs the 50/50 correction and the signal connector addition. Want me to update the BOM with the revised cartridge spec?

All your revisions are incorporated. Here's the updated BOM:

Cost Breakdown

€296 total with primary choices (€280 with cheapest alternatives). The spot welder is a one-time tool purchase — subtract €45 if you already have one or borrow from a maker space.

Sourcing for Oslo

• nkon.nl (Netherlands) — cells + nickel strip. Best EU prices, handles Li-ion shipping properlynkon+1

• electrokit.com (Sweden) — wire, heat shrink, nickel strip backup. 1–3 day delivery next door[electrokit]​

• Biltema / Clas Ohlson (local Oslo) — gaskets, O-rings, adhesive, misc hardware

• euro-locks.com — quarter-turn cam lock, IP65 rated, keyed alike option[euro-locks]​

• AliExpress (2–3 week lead) — XT30AW waterproof connectors, spot welder, USB-C panel mount[aliexpress]​

One Flag: The Knurled Caps

The CNC thumbscrew caps (€36 for 3) are the most custom part. Options to reduce cost:

• Shapeways / JLCPCB CNC — upload a STEP file, get 6 machined in aluminum for ~€50–70 total

• 3D print in brass-fill PETG as prototype, then CNC the final version

• Or simplify to a quarter-turn bayonet instead of threaded — cheaper to machine, faster to operate, no cross-threading risk

The BMS with Bluetooth is a nice touch — you can monitor cell voltages, temperature, and charge state from your phone via the app. The UART output goes through the JST-SM to your motor controller for SOC display / low-voltage cutoff.greenbikekit+1

Much tighter now. Everything fits with the reduced 35mm sealed section at the head tube end:

The BMS runs 4mm past the cutout edge into the sealed zone — that's actually fine since the hinge is right there and the BMS just needs to physically sit in that space. The controller lives in the sealed BB section, permanently wired to the motor.

No wasted space. Everything packed tight from head tube to BB.s3.amazonaws

There are three solid options, each with a different UX feel:

Option 1: Quarter-Turn Cam Latch (Recommended)

Southco E5 Mini or generic equivalent — the industry standard for panel access.southco+1

• Turn 90° with a coin, hex key, or custom tool to open

• Ramped cam pulls the cartridge tight against the frame (anti-vibration)[southco]​

• Flush mount, ~16mm diameter cutout

• Stainless steel or zinc alloy, IP66 rated when sealed[alibaba]​

• ~$2–5 on AliExpress for generic clones[aliexpress]​

This is what most e-bike battery locks use (Bosch, Shimano, etc.) — proven in bikes. You could use a keyed version for anti-theft, so the battery can't be removed without the key.[support.biktrix]​[youtube]​

Option 2: Push-to-Close Rotary Latch

Southco R4 Micro — push the cartridge closed, it clicks and locks.southco+1

• No tool needed to close — just push the cartridge up and it latches

• Release via a small lever or cable pull

• Completely concealed — nothing visible from outside

• Tolerates misalignment (good for a hand-fitted cartridge)

• Anti-vibration bumper built in

• ~$5–10 on AliExpress

This is used in automotive panels and EV battery doors. More elegant UX but slightly bulkier internally.

Option 3: Push-Push Spring Latch

Simple spring-loaded plunger — push to close, push again to pop open.fornd+1

• Simplest mechanism, zero moving parts on the outside

• No key, no tool — completely invisible flush surface

• Cheapest: ~$1–2 on AliExpress

• BUT: no anti-theft. Anyone who knows where to push can open it

• Weaker holding force — may rattle on rough roads

Comparison

Option 1: Quarter-Turn Cam Latch (Recommended)

Southco E5 Mini or generic equivalent — the industry standard for panel access. southco

• Turn 90° with a coin, hex key, or custom tool to open
• Ramped cam pulls the cartridge tight against the frame (anti-vibration) [southco](https://southco.com/en_any_int/e5-1-125-170)
• Flush mount, ~16mm diameter cutout
• Stainless steel or zinc alloy, IP66 rated when sealed [alibaba](https://www.alibaba.com/pla/Southco-E5-Quarter-Turn-Cam-Lock_1600267310866.html?mark=google_shopping&biz=pla&searchText=locks&product_id=1600267310866&pcy=gb_en)
• ~$2–5 on AliExpress for generic clones [aliexpress](https://www.aliexpress.com/w/wholesale-southco-latch.html)

This is what most e-bike battery locks use (Bosch, Shimano, etc.) — proven in bikes. You could use a keyed version for anti-theft, so the battery can't be removed without the key. [support.biktrix](https://support.biktrix.com/kb/guide/en/replacing-a-hidden-battery-lock-cZ7hhQN5t9/Steps/1116481)

Option 2: Push-to-Close Rotary Latch

Southco R4 Micro — push the cartridge closed, it clicks and locks. southco

• No tool needed to close — just push the cartridge up and it latches
• Release via a small lever or cable pull
• Completely concealed — nothing visible from outside
• Tolerates misalignment (good for a hand-fitted cartridge)
• Anti-vibration bumper built in
• ~$5–10 on AliExpress

This is used in automotive panels and EV battery doors. More elegant UX but slightly bulkier internally.

Option 3: Push-Push Spring Latch

Simple spring-loaded plunger — push to close, push again to pop open. fornd

• Simplest mechanism, zero moving parts on the outside
• No key, no tool — completely invisible flush surface
• Cheapest: ~$1–2 on AliExpress
• BUT: no anti-theft. Anyone who knows where to push can open it
• Weaker holding force — may rattle on rough roads

Comparison

My Pick

Quarter-turn cam latch, keyed — specifically the Southco E5 Mini or a generic clone. It's what every major e-bike brand uses for removable batteries because it solves anti-theft and anti-vibration in one $3 part. The small keyhole on the underside of the down tube is invisible from normal viewing angles, and you already carry a key for your bike lock anyway. [southco](https://southco.com/en_any_int/e5-m-142-uu1)

Want to go with the keyed quarter-turn, or do you prefer the push-to-close for the cleaner UX?

The term you're looking for is "keyed to differ" — each lock gets a unique key combination, so no two customers can open each other's battery.

How It Works

Lock manufacturers offer two keying modes:

At scale (100+ frames), you can get Euro-Locks to supply a master key system — each customer has their unique key, but you hold a master that opens all units for warranty service.[euro-locks]​

Now I have a crucial finding. The Dosnoventa Detroit specs say "Columbus Compass 1-1/8" Carbon" headset dosnoventabikes+1. This is the straight 1-1/8" version (IS42/28.6 both top and bottom), which uses a head tube with 46mm OD and 41.2mm ID fishtailcyclery+2. However, looking more carefully at the newer Dosnoventa Detroit spec (), the fork is listed as "TAPERED 1-1/8"", and the Cinelli Vigorelli version (same Columbus Airplane) uses IS 42/28.6 | IS 52/40 (tapered) [scalibruno]​. Let me verify which version the current Detroit actually uses.

Looking at the data from (the current Dosnoventa Detroit frameset page), it clearly states: "FULL CARBON FORK . TAPERED 1-1/8"" and "COLUMBUS COMPASS . 1-1/8" CARBON". The "Tapered 1-1/8"" description on the fork means the fork steerer starts at 1-1/8" and tapers, but the frame headset listed is "Columbus Compass 1-1/8" Carbon" which is the straight version (IS42/IS42). This means the head tube is straight 1-1/8" (not tapered).

Now I have enough data to compile the answer. Let me consolidate everything.

Now I have all the data I need from the Columbus Airplane aluminum catalog and the Dosnoventa Detroit specifications. Here's what I found from the actual Columbus Airplane aluminum 7005 catalog (the framebuilder catalog from Columbus):[framebuilding]​

Columbus Airplane 7005-T6 Tube Specifications

The Columbus Airplane is an OEM aluminum tubeset — it doesn't appear in the standard Columbus steel catalog. The specifications come from the Columbus aluminum catalog:[framebuilding]​

Key Clarifications

• The down tube at Ø42mm is the standard Airplane road/track DT — it's round triple butted, not the Ø48 oversized option. The Facebook post mentioning a MASH Cinelli Airplane frame listed "down tube 45mm", but that likely reflects an ovalized/hydroformed profile measurement rather than the nominal tube OD.facebook+1

• The top tube at Ø35mm is consistent across the Airplane catalog — both listed TT codes are Ø35. This is oversized compared to standard steel (25.4mm) but standard for modern aluminum track frames.[framebuilding]​

• The seat tube at Ø31.7mm is standard for 27.2mm seatpost frames, confirmed by Dosnoventa's own spec.dosnoventabikes+1

• The head tube is more complex. The Columbus catalogs list raw head tubes (CYRA18 series at Ø38mm) that accept brazed-in IS cups. The finished head tube with integrated bearing seats has a 46mm OD at the bearing race and 41.2mm ID, per Columbus Compass 1-1/8" headset specs.fishtailcyclery+2

• The chainstays at Ø26mm are the track-specific Airplane stays (AT78 series), shorter and lighter than the Ø28mm road versions. These get ovalized/shaped toward the BB end.[framebuilding]​

• The seatstays at Ø19mm are larger than typical steel track stays (14mm) because aluminum requires larger diameters to achieve equivalent stiffness.[framebuilding]​

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.

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)