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FM336 Carbon Gravel Frame Review – 1.3x ISO Tested
The PermeantsBikes FM336 is a premium full-carbon gravel frame weighing just 850g, built from Toray T800/T1000 carbon and tested to 1.3 times ISO 4210 standards. With progressive geometry, T47 threaded bottom bracket, and extensive customization options, it delivers exceptional performance for gravel racing, bikepacking, and all-road riding.
The PermeantsBikes FM336 is a premium full-carbon gravel frame built for riders who refuse to compromise between speed, stability, and versatility. Whether you’re charging across loose gravel roads, entering an all-road race, or assembling a custom dream bike from the ground up, the FM336 delivers an exceptional foundation backed by modern geometry, deep customization options, and Toray carbon construction that punches well above its price point.
FM336 at a Glance
- Progressive race-ready geometry (comparable to Trek Checkpoint SLR)
- Ultra-lightweight carbon frame — approximately 850g in size Small
- Toray T800 with T1000 reinforcements for optimal stiffness-to-weight ratio
- T47 threaded bottom bracket — silent, creak-free, easy to service
- 27.2mm round seatpost for added compliance on rough terrain
- Fully internal cable routing for a clean cockpit and improved aerodynamics
- 12x100mm front and 12x142mm rear thru-axles for precise wheel tracking
- Flat-mount disc brakes (140/160mm) — the modern braking standard
- UDH hanger for future-proof drivetrain compatibility
- Maximum 47mm tire (700c) / 2.1″ (650b) clearance — official spec (rider-reported up to 50mm)
- 1.3x ISO 4210 tested — third-party fatigue and impact certified
Progressive Geometry — Built for Speed and Stability
Frame geometry is the single most important factor in how a bike actually rides. The FM336 features a progressive all-road geometry that strikes an excellent balance between aggressive road positioning and off-road composure. Riders get a fast, responsive feel on pavement without sacrificing the confidence needed on gravel, dirt, or broken terrain.
What sets the FM336 apart from many gravel frames is its longer front center and lower bottom bracket. This combination virtually eliminates toe overlap — a common complaint on shorter, more road-biased gravel frames — while delivering exceptional stability at speed. The stack-to-reach ratio ranges from 1.43 on the XS to 1.50 on the XL, giving each size a natural fit profile that blends race efficiency with endurance comfort.
| Size | XS | S | M | L | XL |
|---|---|---|---|---|---|
| Stack (mm) | 535 | 549.5 | 570 | 584 | 608 |
| Reach (mm) | 375 | 382 | 392 | 399 | 404 |
| Stack-to-Reach Ratio | 1.43 | 1.44 | 1.45 | 1.46 | 1.50 |
| Seat Tube (mm) | 440 | 460 | 480 | 500 | 520 |
| Top Tube (mm, effective) | 529 | 540 | 559 | 572 | 583 |
| Head Tube Angle (°) | 71 | 71 | 71.5 | 71.7 | 72 |
| Seat Tube Angle (°) | 73.9 | 73.9 | 73.7 | 73.5 | 73.6 |
| BB Drop (mm) | 80 | ||||
| Chainstay (mm) | 426 | ||||
| Wheelbase (mm) | 1008 | 1017 | 1032 | 1042 | 1052 |
Why Choose the FM336 — Key Features and Benefits
T47 Threaded Bottom Bracket
Press-fit bottom brackets have frustrated cyclists for years with creaking, loosening, and difficult servicing. The FM336 sidesteps all of that by using a T47 threaded bottom bracket — a modern standard that threads directly into the frame for a secure, silent, and easily serviceable interface. It also permits a wider down tube, which improves aerodynamic shaping without adding weight.
27.2mm Round Seatpost
The choice of a 27.2mm round seatpost is deliberate. Smaller-diameter posts flex more than 30.9mm or 31.6mm options, which translates directly into added comfort on rough gravel roads. The round profile also makes it simple to swap seatposts or install a dropper post for technical descents — something proprietary aero seatposts can’t offer.
Maximum Tire Clearance
Official tire clearance is 700x47mm or 650B x 2.1″. That’s enough room for wide, grippy gravel tires with adequate mud clearance. Multiple riders have confirmed fitting up to 50mm tires, which opens up even more versatility for bikepacking and mixed-terrain adventures.
Ultra-Lightweight Toray Carbon Construction
The FM336 is built from premium Toray T800 and T1000 carbon fibre — the same material grades used by major brands costing two or three times as much. At approximately 850g for a size Small, it ranks among the lightest gravel frames available. Lower frame weight means faster climbing, quicker acceleration, and noticeably less fatigue over long days in the saddle.
If you’re comparing carbon options for road builds, PermeantsBikes also offers the ST-09 one-piece road disc frame at just 680g — another standout in the lightweight carbon category.
1.3x ISO 4210 Testing
Every PermeantsBikes frame is independently tested to 1.3 times ISO 4210 standards. This goes beyond the industry baseline with rigorous impact and fatigue testing that simulates years of hard riding. Third-party test reports are available on request — a level of transparency that’s rare among open-mold frame manufacturers.
Custom Paint and Geometry — Make It Your Own
Customization is where the FM336 truly stands out from the competition. PermeantsBikes offers two primary ways to tailor the frame:
- Custom paint and finish: Submit your desired RAL or Pantone colour codes and choose from matte, gloss, satin, metallic, or chameleon (colour-shift) finishes. Custom logos are applied via water-transfer decals under the clear coat for a seamless, durable result.
- Full custom geometry: For brand partners, OEM customers, or riders with unique proportions, PermeantsBikes provides complete custom geometry services. The FM336 can serve as the starting point for a frame tuned precisely to your body and riding style.
Custom paint turnaround is typically around 12 days from sample approval to production completion. The process is straightforward — send your colour codes via WhatsApp or email, review the sample, and approve for manufacturing.
What Riders Are Saying — Real Feedback and Comparisons
The FM336 has generated extensive discussion across cycling forums, and the feedback paints a consistent picture of quality and value:
- “After waiting three weeks for delivery to the UK, I was impressed by the build quality and packaging. The carbon layup is flawless, and the bike rides incredibly well for the price. The geometry is spot on — the longer reach and lower bottom bracket make it super stable at speed while still feeling responsive on climbs.”
- “I went for a custom chameleon paint job — deep purple fading to black. The finish is stunning and gets compliments on every ride. The process was straightforward: I sent my Pantone codes via WhatsApp, and the team provided a sample before production.”
- “Compared to my brother’s Specialized Diverge, the FM336 feels more planted at high speed, especially on loose gravel descents. I’ve had no toe overlap issues thanks to the longer front center — a problem I had on my previous bike.”
- “A word on sizing: I’m 180cm with an 83cm inseam, and after posting on the forum, several owners recommended the Large with an 80mm stem. It’s a balance between race and endurance fit, but the versatility is excellent for the varied terrain I ride.”
Understanding Gravel Frame Materials and Construction
Selecting the right gravel bike frame involves more than just picking a brand name. The material, geometry, and construction quality each play a significant role in how the bike performs across varied terrain — from smooth tarmac to loose, rocky trails. Here’s a closer look at the options available today.
Steel Gravel Bike Frames
Steel remains a popular choice for riders who prioritize durability and ride feel. Modern chromoly steel framesets are crafted to be lighter than older designs while retaining the material’s natural vibration damping. Steel frames are also easy to repair in the field, making them a solid choice for remote bikepacking routes. The trade-off is weight — even the best steel gravel frames run 1.5 to 2kg heavier than carbon equivalents.
Carbon Gravel Bike Frames
Carbon fibre dominates the performance end of the gravel market for good reason. It offers the best stiffness-to-weight ratio of any frame material, allowing engineers to tune compliance in specific areas while maintaining pedalling efficiency. A high-quality carbon gravel frame like the FM336 can weigh under 900g — roughly half the weight of a comparable steel frame. Carbon also resists corrosion and doesn’t fatigue over time the way metal frames do.
Aluminum and Titanium Options
Aluminum frames offer a middle ground between weight and cost. They’re lighter than steel but can feel harsher on rough terrain unless the tubing is carefully butted and shaped. Titanium sits closer to steel in terms of ride quality but at a significantly lighter weight — though titanium frames are typically the most expensive option per gram saved.
Specialized Gravel Frame Features Worth Knowing
UDH (Universal Derailleur Hanger)
The Universal Derailleur Hanger standard has quickly become a must-have feature for modern gravel and mountain bike frames. A UDH gravel frame like the FM336 uses a single, standardized hanger that works across drivetrain brands. This eliminates the frustrating hunt for obscure proprietary hangers — particularly useful when you’re riding in remote areas far from bike shops. UDH also ensures full compatibility with SRAM’s T-Type transmission systems, which mount directly to the hanger.
Internal Cable Routing
Fully internal cable routing keeps cables protected from mud, grit, and debris while giving the bike a clean, streamlined appearance. On the FM336, all cables run through the down tube and chainstays, emerging only at their respective contact points. This design also provides a small aerodynamic benefit — something that matters more than most gravel riders realize once speeds climb above 30 km/h.
Finding the Right Frame Fit
A well-fitting frame is just as important as material choice or component spec. Poor fit leads to discomfort, inefficiency, and potential injury over time. Here’s what to focus on:
Frame Sizing
Manufacturers provide size guides based on height and inseam measurements, but these are starting points — not absolutes. Stack and reach are the two dimensions that most directly affect your riding position. A higher stack gives a more upright, comfortable posture for long rides. A longer reach produces a more aerodynamic, aggressive position suited to racing.
Reach and Stack in Practice
For the FM336, a rider who is 175–180cm tall with a standard inseam will typically fit the Medium or Large depending on preferred riding style. The Medium offers a slightly racier feel; the Large provides more stability and comfort. Stem length and handlebar choice can fine-tune the fit further without changing the frame size.
Build Kit Recommendations
The FM336 is compatible with a wide range of modern components. Here are practical recommendations for a well-balanced gravel build:
- Wheelset: 700c for speed-oriented riding or 650b for maximum tire volume and comfort. Both sizes work within the frame’s clearance specs.
- Groupset: Electronic shifting (SRAM AXS, Shimano Di2) or mechanical — the fully internal routing supports both without modification.
- Crankset: T47 bottom bracket is compatible with 24mm (Shimano) and 30mm (SRAM DUB, RaceFace) spindle standards.
- Handlebars: An optional integrated carbon aero handlebar is available, painted to match your frame for a cohesive look.
- Seatpost: Any 27.2mm round post works — from lightweight carbon compliance posts to mechanical dropper posts for technical terrain.
For riders exploring the full range of PermeantsBikes carbon offerings, the road bike frame collection and gravel bike frame category include options for every riding discipline and budget.
Ready to Build Your Dream Carbon Gravel Bike?
The PermeantsBikes FM336 sets a new standard for what a carbon gravel frame can deliver at its price point. You get a frame that’s light enough for racing, compliant enough for all-day comfort, and versatile enough to handle everything from bikepacking to fast group rides. It’s an ideal choice for experienced cyclists seeking a high-performance open-mold frame, as well as new bike brands looking for a reliable OEM partner.
Whether you’re building your first gravel bike or adding a dedicated all-road machine to your stable, the FM336 deserves serious consideration. Reach out to the PermeantsBikes team to request a geometry chart, discuss custom options, or get a quote tailored to your build.
FAQ
How does the FM336 geometry compare to other gravel frames?
The FM336’s geometry closely mirrors the Trek Checkpoint SLR and Orbea Terra H30, with a longer front center and lower bottom bracket than many competitors. This produces exceptional stability at high speed and on descents, while the progressive stack-to-reach ratio keeps the ride responsive when you push the pace. It never feels twitchy on loose terrain.
What warranty does PermeantsBikes offer on the FM336?
All PermeantsBikes frames carry a 2-year warranty against manufacturing defects. Since every frame is tested to 1.3 times ISO 4210 standards, you’re getting safety margins that exceed what most brands offer. Additional EFBE certification is available on request for EU market compliance.
Can I order a single FM336 frame for a personal build?
Absolutely. PermeantsBikes welcomes single-frame orders for personal builds. Standard paint colours are stocked with a minimum order quantity of just one set. Custom painting requires a MOQ of 30 sets, though samples are available to review before committing.
Is the FM336 compatible with SRAM T-Type transmissions?
Yes. The FM336 features a Universal Derailleur Hanger (UDH), which is the mounting standard required for SRAM’s T-Type Eagle Transmission systems. This also ensures compatibility with all current and future derailleur designs that use the UDH interface.
What is the maximum tire size the FM336 can fit?
The official maximum clearance is 700x47mm on 700c wheels or 2.1″ on 650b wheels. Several riders have confirmed fitting 50mm tires with acceptable clearance, though this depends on the specific tire model and rim width. For mud-prone conditions, staying at or below 47mm ensures reliable clearance.