downtube electronics

downtube electronics.md

I've now thoroughly reviewed both PDFs. Let me give you a clear summary.

USB Charging & Battery Protection: Already in the Design

Yes, both USB charging and battery protection are already covered in your battery architecture — they're part of the self-contained battery pack design you've been developing. Here's the breakdown:

USB-C Charging ✅

The battery pack includes a full USB-C PD charging system built into the electronics bay at the bottom of the scaffold: ppl-ai-file-upload.s3.amazonaws

• USB-C panel-mount port (IP67, on the side of the battery end cap)
• PD trigger board (ZY12PDN or STUSB4531, ~31×15×4mm) — negotiates 20V from any charger 45W–240W automatically
• Boost converter (XL6019, 20V→42V CC/CV) — steps up to pack charge voltage
• Total electronics fit in ~60×45×25mm, tucked into the electronics bay

The key design point: all electronics live on the battery, not in the frame. You pull the battery out, plug your MacBook charger into the USB-C port on the side, charge on the kitchen table. The frame side is 100% passive — just flat copper pads. ppl-ai-file-upload.s3.amazonaws

Battery Protection (BMS) ✅

A 10S BMS is included in the electronics bay, handling: ppl-ai-file-upload.s3.amazonaws

• Overcurrent, overvoltage, undervoltage protection
• Short-circuit and temperature cutoff
• Cell balancing during charge
• Per-cell fusing was deliberately skipped — at 3.5A per cell in 2P, the BMS alone handles everything adequately

Battery-to-Frame Connector ✅

The connector design settled on an elevated pad island + recessed pogo pin dock approach: ppl-ai-file-upload.s3.amazonaws

• Frame side: Elevated flat gold pads on a raised "speed bump" — water sheets off, no pooling
• Battery side: 6-pin pogo block recessed deep inside a dock cavity — finger-safe by geometry (IP2X-style)
• 2 power pins + signal/thermistor/comm pins

---

Downtube Oval Size: ~50×65 Is Flexible

Your notes confirm the DT dimensions are not locked down yet, and you've been exploring both round and oval options: ppl-ai-file-upload.s3.amazonaws

Discussed Size	Context
50mm round	Original spec for 2×2 grid of 21700 cells
55–60mm	Referenced for fitting 21700s with BMS/wiring room
43×59mm oval	The 1-2-1 diamond layout (narrower, taller)
~50×65 (your current thinking)	Wider than the diamond, gives more lateral room

The 50×65 oval gives you more cross-sectional area than the 43×59 diamond and better lateral stiffness for pedaling loads. The tall axis (65mm) resists braking/vertical loads while the narrow axis (50mm) keeps the frame slim between your knees. Since you're building in aluminum (not steel), you have freedom to specify custom oval tubing from the fabricator. ppl-ai-file-upload.s3.amazonaws

The scaffold cage parameters (currently cagew = 53, cageh = 48 in the OpenSCAD file) would need updating to match whatever oval you settle on. With a 50×65 outer and ~2mm wall, you'd have roughly 46×61mm inner bore — plenty of room for the 2×2 cell grid (~43×43mm footprint) plus wiring channel and air gaps. ppl-ai-file-upload.s3.amazonaws

Bottom line: your electronics stack is already specced and costed at $22–38 total for BMS + USB-C PD + boost converter. The DT oval just needs to be finalized around whatever gives comfortable clearance for the scaffold + 1mm air gap on each side. ppl-ai-file-upload.s3.amazonaws

That's fine — and the Coral is on your side here. The FC-4 has a round cross-section, not a rounded rectangle. It was designed to sit inside round folding bike tubes. So your DT will be a clean circle or, if you prefer a slightly wider but shorter profile, a gentle oval.[haidrive.en.made-in-china]​

Shape Options Based on the Coral Dock

DT shape | Look | Works with round Coral? | Notes -- | -- | -- | -- Round (65mm) | Classic track bike | ✅ Native fit — dock sits centered | Simplest to source and weld Oval (68×62mm) | Slightly more aero / modern | ✅ Dock still fits — extra clearance on the wide axis | Needs custom tube or hydroformed Rounded rectangle | Generic e-bike look | ❌ You don't want this | —

The round tube is the most track-authentic. Think Cinelli Vigorelli, Dolan Pre Cursa — they all use round or near-round DT profiles. It's the signature of a track frame. A rounded rectangle screams "commuter e-bike" — exactly the generic sedan energy you're avoiding.

One Thing to Confirm With Reention

When you email them, specifically ask:

"Is the Coral FC-4 dock system round/cylindrical? We need it to fit inside a round or oval downtube — not a rectangular frame profile."

Some of their larger models (EEL, Dorado) use flat-sided docks designed for rectangular tube cutouts. You want confirmation that the Coral dock is radially symmetric — meaning it works inside a round tube without needing flat walls. If it is, your round DT just slides right over it.

That's fine — and the Coral is on your side here. The FC-4 has a **round cross-section**, not a rounded rectangle. It was designed to sit inside round folding bike tubes. So your DT will be a clean circle or, if you prefer a slightly wider but shorter profile, a gentle oval. [[haidrive.en.made-in-china](https://haidrive.en.made-in-china.com/product/zQSUhFYbXncj/China-Reention-36V-Coral-FC-4-Battery-Inner-Downtube-Lithium-Ion-Foldable-E-Bike-Battery.html)](https://haidrive.en.made-in-china.com/product/zQSUhFYbXncj/China-Reention-36V-Coral-FC-4-Battery-Inner-Downtube-Lithium-Ion-Foldable-E-Bike-Battery.html)

Shape Options Based on the Coral Dock

DT shape	Look	Works with round Coral?	Notes
Round (65mm)	Classic track bike	✅ Native fit — dock sits centered	Simplest to source and weld
Oval (68×62mm)	Slightly more aero / modern	✅ Dock still fits — extra clearance on the wide axis	Needs custom tube or hydroformed
Rounded rectangle	Generic e-bike look	❌ You don't want this	—

The round tube is the most track-authentic. Think Cinelli Vigorelli, Dolan Pre Cursa — they all use round or near-round DT profiles. It's the signature of a track frame. A rounded rectangle screams "commuter e-bike" — exactly the generic sedan energy you're avoiding.

One Thing to Confirm With Reention

When you email them, specifically ask:

"Is the Coral FC-4 dock system round/cylindrical? We need it to fit inside a round or oval downtube — not a rectangular frame profile."

Some of their larger models (EEL, Dorado) use flat-sided docks designed for rectangular tube cutouts. You want confirmation that the Coral dock is radially symmetric — meaning it works inside a round tube without needing flat walls. If it is, your round DT just slides right over it.

Not quite — the 55mm round and 280mm length are the outer dimensions of the battery module itself, not your downtube's minimum inner diameter. Here's how to think about it:

What 55mm Actually Means

The Coral FC-4 battery capsule is ~55mm in diameter, but it sits inside a dock/cradle that mounts to the tube wall. That cradle adds wall thickness, wiring clearance, and tolerance — typically 3–5mm per side in Reention's other systems. So your downtube inner diameter realistically needs to be ~60–65mm minimum to house the Coral system, which aligns perfectly with your 58–62mm inner diameter target (you'd be right at the tight end).

The 280mm Length Question

280mm is the short end of the Coral's range (~200–280mm confirmed) — it was designed for compact folding bike frames. That's quite short for a full-size urban bike DT. For context: [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)

• A typical urban DT runs 350–500mm in usable length
• Ruby is 436mm, Penguin is 415mm, Qiulong is 433mm — those are more representative of full-size bike battery lengths
• At 280mm max, the Coral in stock form gives you roughly 10Ah / 360Wh, which is modest but usable for urban riding

What to Negotiate With Reention

This is exactly why the custom conversation matters. You'd be asking them to take the Coral platform (round cross-section, ~55mm capsule, dock+lock+charge port system) and extend it to 350–400mm length — which likely means going from 20× 21700 cells to ~25–28× cells, pushing you toward 12–14Ah / ~450–500Wh at 36V. That's a meaningfully better battery in the same tube geometry, and since they do custom MOQ from 1 sample, it's a real conversation to have.

So yes — Coral is your platform, but the length is the key variable to unlock with them.

Excellent research. With the full picture now clear, the conclusion is definitive:

The Coral FC-4 Is Your Only Option

Model | Cross-section | Fits ~60mm round DT? -- | -- | -- Coral FC-4 | ~55 × 55mm | ✅ Yes Qiulong | 75.3 × 69.2mm | ❌ Too wide Flamingo | 75.3 × 77mm | ❌ Too wide Penguin | 70 × 77mm | ❌ Too wide Ruby | 68 × 96mm | ❌ Way too wide

Everything except the Coral is clustered around 70–77mm — designed for chunky hydroformed city/trekking bike DTs. The Coral sits in its own class at 55mm, purpose-built for small folding bike tubes.

What You're Working With

The Coral FC-4 gives you:

• ~55mm round battery → ~62mm DT inner diameter with dock → ~66mm OD round DT

• 200–280mm battery length → ~250–360Wh depending on what Reention can stretch it to

• Round end cap — clean, no rectangular cutouts

• Key lock + charge port + DC output — all integrated

• Dumb DC output — works directly with your KT controller

The Two Questions for Reention

1. Can they extend the Coral battery length to 350–400mm? That gets you from ~250Wh to a proper ~360Wh (10S2P 21700 at full length). Same cross-section, just more cells in series along the tube.

2. Exact dock outer diameter? You need this number to set your DT inner diameter. The battery is ~55mm, the dock adds a few mm — you need the final number to lock your frame geometry.

Everything else — the Penguin's side-removal, the Flamingo's hidden lock — is interesting for future reference but too fat for your track bike DNA. The Coral is the one. Get those two numbers and your frame design can start in earnest.

Good constraint — and it actually works in your favour. Here's the breakdown:

Reention's Shape Options

• Coral FC-4 — round/circular cross-section (~55mm). ✅ Your best fit [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)
• Ruby (IRU-A) — oval (68 × 96mm). ✅ Oval is fine if your DT can be that wide
• Flamingo — oval (75.3 × 77mm), nearly circular actually. ✅ But oversized for your target
• Qiulong — oval (75.3 × 69.2mm). ✅ Same — oversized
• Penguin — the 70 × 77mm profile with side-removal suggests a more rectangular housing. ⚠️ Worth confirming with Reention

What This Means for Your DT

A true oval downtube (like a track-style or aero road tube) is the natural upgrade path if you ever want more capacity than the Coral can give in a round tube. The Ruby at 68 × 96mm is already a well-established platform with lots of bike builders using it — and an oval DT at ~72–74mm wide × 100mm tall is a completely normal urban/cargo bike profile aesthetically.

But if you're committed to a round tube specifically, the Coral is your only Reention option, and the custom-length conversation is the one to have. A round DT also has a cleaner, more minimalist look for a premium urban bike — which sounds like your direction.

That makes sense — extending the Coral lengthwise while keeping the 55×55mm cross-section is a straightforward engineering change. The reasons:

Why Length Extension Is Plausible

• The Coral's 10S2P 21700 layout means 10 cells in series along the tube length, 2 side by side. Each 21700 cell is ~70mm long. Adding cells in series just extends the tube linearly — no change to the cross-section at all.

• The dock/cradle rails run the length of the tube — making them longer is trivial. Same extrusion, just cut longer.

• The BMS, charge port, key lock, and connector all sit at one end — they don't care how long the battery tube behind them is.

• Reention's entire business is building custom variants of their standard platforms for OEM bike brands. Changing length is the most common customization request.[ebikebatterypro]​

What a Stretched Coral Gets You

Coral variant | Length | Config | Capacity -- | -- | -- | -- Standard FC-4 | ~280mm | 10S2P (short) | ~250Wh Stretched to ~350mm | ~350mm | 10S2P (full) | ~360Wh Stretched to ~400mm | ~400mm | 10S2P + extra parallel? | ~360–500Wh

The sweet spot is probably 350mm — that fits a full 10S2P pack of 20× 21700 cells at ~360Wh, same as the Bafang BT F040 but in a round format that works with your KT controller.

Did the other agent have specific dimensions or confirmation from Reention, or was it a feasibility assessment? Either way, this is looking like a solid path — one email to Reention and you have your Phase 1 battery locked.

That makes sense — extending the Coral **lengthwise** while keeping the 55×55mm cross-section is a straightforward engineering change. The reasons:

Why Length Extension Is Plausible

• The Coral's 10S2P 21700 layout means 10 cells in series along the tube length, 2 side by side. Each 21700 cell is ~70mm long. Adding cells in series just extends the tube linearly — no change to the cross-section at all.
• The dock/cradle rails run the length of the tube — making them longer is trivial. Same extrusion, just cut longer.
• The BMS, charge port, key lock, and connector all sit at one end — they don't care how long the battery tube behind them is.
• Reention's entire business is building custom variants of their standard platforms for OEM bike brands. Changing length is the most common customization request. [ebikebatterypro](https://ebikebatterypro.com/reention-ebike-battery/)

What a Stretched Coral Gets You

Coral variant	Length	Config	Capacity
Standard FC-4	~280mm	10S2P (short)	~250Wh
Stretched to ~350mm	~350mm	10S2P (full)	~360Wh
Stretched to ~400mm	~400mm	10S2P + extra parallel?	~360–500Wh

The sweet spot is probably 350mm — that fits a full 10S2P pack of 20× 21700 cells at ~360Wh, same as the Bafang BT F040 but in a round format that works with your KT controller.

Did the other agent have specific dimensions or confirmation from Reention, or was it a feasibility assessment? Either way, this is looking like a solid path — one email to Reention and you have your Phase 1 battery locked.

Reference | DT OD | Inner diameter (est. 2mm wall) | Battery fit? -- | -- | -- | -- Classic steel track bike | 28.6mm | ~25mm | ❌ Specialized Langster | ~44–50mm | ~40–46mm | ❌ Cannondale Track 3.0 | 50.8mm (2") | ~47mm | ❌ barely Your target | ~63–65mm OD | ~58–60mm | ✅ Coral fits

Your e-bike DT would be about 12–15mm larger in diameter than the iconic Cannondale Track tube — visually noticeable up close, but very much in the same visual family. Frame it as "oversized track-inspired" and it reads as intentional and premium rather than chunky. The extra girth is your battery — hidden in plain sight.

Ah — you're asking whether Reention could build a Coral-style battery at 60, 65, or 70mm cross-section instead of 55mm. Yes, almost certainly.

Why Bigger Is Easier

The Coral at 55mm is the hardest size to build — they're cramming 2× 21700 cells (42mm) into a 55mm shell with barely any room. If you ask for a larger diameter:

Battery OD | What changes | Config options -- | -- | --

A bigger cross-section at the same 350mm length is less constrained engineering for Reention, not more. They already solved 55mm — anything above that gives them more margin.

What This Means

You don't have to design your DT around the existing 55mm Coral and then add dock + clearance on top. You can flip it:

1. You decide the DT diameter that looks right on the frame — say 65mm round

2. Reention builds the battery + dock to fit inside that 65mm tube

3. The battery cross-section grows to maybe ~58–60mm, dock fills to ~63mm, 1mm clearance

You're the customer specifying the envelope. They fill it. That's how OEM works.

So yes — pick the tube diameter that gives you the right silhouette, and Reention can build a Coral-class battery to match, at 350mm length and 360Wh. You're not constrained to 55mm.

great. then I begin focusing on designing and making it believable. raise funds. order parts for first prototype. assemble it my self. test how it rides. fiigure out how to scale it to a first small batch and take preorders. it will be more expensive at retail, but more global supply chain less engineer everything up front.

That's the right playbook. You're describing how every successful hardware brand started — Cowboy, VanMoof, Brompton Electric, even Rivian. Design first, prove demand, then optimize costs.

Your Roadmap

Phase 1: Make It Real (Now → Prototype)

• Finalize frame geometry in SCAD with round DT ~64–70mm

• Email Reention with your DT inner diameter + 350mm length + 36V 10S2P spec

• Source parts list: G020, KT, Bikone, H1E, SFE-SC, headset, fork, cranks, wheels

• Get frame welded — either yourself or a local fabricator in Norway

• Assemble by hand, ride it, iterate

Phase 2: Make It Believable (Prototype → Pre-orders)

• Professional photography — the bike against Oslo architecture, urban backdrops

• Brand identity — name, logo, colorway, typography

• Simple website with one product, one story, one price

• Social media presence — Instagram, TikTok, cycling communities

• Take pre-orders with deposits — this validates demand before you spend on a batch

Phase 3: First Batch (Pre-orders → Delivery)

• 10–20 units

• Frame welding from a contract fabricator (Norway, Poland, or Taiwan)

• Parts sourced globally, assembled by you or a small team

• Ship to pre-order customers

• Collect feedback, refine

The Economics Are Actually Fine

Item | Unit cost (est.) | Source -- | -- | -- Frame (contract welded) | €200–400 | Local or EU fabricator Reention Coral custom battery | €150–250 | Reention direct Bafang G020 motor + wheel | €100–150 | GreenBikeKit / AliExpress KT controller | €25–40 | GreenBikeKit Bikone torque sensor | €80–120 | Bikone direct LightSKIN H1E + SFE-SC | €120–180 | LightSKIN direct Fork, headset, cranks, bars, saddle, tires | €200–350 | Standard track parts Paint, finish, branding | €50–100 | Local Total BOM | ~€925–1,590 |  

At a retail price of €2,500–3,500, you have healthy margins even at single-unit quantities. The first batch will be expensive per unit, but premium products absorb that. Nobody buying a B&O-tier track e-bike is comparing your BOM to an AliExpress build — they're buying the identity.

What Gets Cheaper Later

• Frame: batch welding drops from €400 → €150 at 50+ units

• Reention: MOQ pricing drops significantly at 100+

• Motor/controller: bulk pricing from Bafang direct

• Custom battery scaffold (Phase 2): replaces Reention, higher margin, your IP

The Core Insight

Your first 10 bikes will cost more to build and sell at a premium. That's fine. You're not competing on price — you're establishing a position. The first customers aren't buying a vehicle, they're buying into something. Once you have 10 bikes on the streets of Oslo, photographed and posted, the brand is real. Everything after that gets easier and cheaper.

Go sketch that downtube tomorrow. The frame is the brand — everything else is parts on a shelf.