X-Lab RT9 Review: The World's Lightest Disc-Brake Road Bike, Tested

Introduction

The X-Lab RT9 from XDS makes a bold claim: it is the world’s lightest production carbon road bike frame with disc brakes that regular riders can actually buy. The complete bike, built with top-spec components, weighed 6.39 kg (14 lb 3 oz) on our scales, an extraordinary figure even if it landed slightly above the brand’s own claim. The frame is listed at 550 g, comes with a carbon fork, and uses a universal derailleur hanger, a threaded BSA bottom bracket, and clearance for 32 mm tires.

The build is as serious as the numbers suggest: Shimano Dura Ace, a lightweight carbon crank with an integrated power meter, and a cockpit and wheelset from Branta, XDS’s in-house component brand. Over a few weeks of riding, the RT9 proved fast, stiff, and almost impossibly light. It also proved that chasing the lowest possible weight comes with real trade-offs.

Who This Bike Is For

The RT9 is a long, low racing bike in the classic sense. The handlebars sit far forward and quite low, so it rewards riders who can hold a stretched-out position. If you are young, strong, and flexible, this geometry will feel natural. If you are not, the bike is still rideable, but a couple of hours in the saddle may start to feel uncompromising. This is a machine built for speed, not all-day comfort.

It is also a bike for riders who prefer classic round tubes over futuristic aero shapes, and for anyone who carries their bike up several flights of stairs. At 6.39 kg, that daily chore becomes a lot easier, and the light weight is felt everywhere from lifting onto a rack to accelerating out of corners.

How X-Lab Got the Weight Down

The weight story starts with materials and process. X-Lab uses a mix of Toray T1100 and M40X carbon fiber, with M40X chosen for its lighter, stiffer character. The tube walls are extremely thin, down to about 0.4 mm in places, and a bladder-molding process removes excess resin during production, an approach similar to what Scott used on the superlight Addict RC.

The frame is a claimed 550 g, and no component is left untouched to save grams. The stock carbon crank weighs less than a Dura Ace crank without a power meter, and it includes a power meter as standard. The wheels, bars, and seat post all carry the Branta name, and the saddle is the Selle Italia Boost SLR S3. Every part of the build has been chosen with weight in mind.

Main Strengths

On smooth tarmac, the RT9 is a genuine joy. The frame is stiff and does not feel like it is about to flex or fall apart, and the bike responds eagerly on climbs. The round-tube design also pays off in crosswinds: with less surface area for gusts to grab, the bike feels steadier than many deep-aero rivals.

The aesthetic is part of the appeal. Matte black everything, round tubes, no dropped seat stays: the RT9 looks like a classic racing bike that happens to weigh almost nothing. The component sizing is thoughtful too. The stock cranks are short and the bars are narrow, which suits a lot of riders, and X-Lab allows free swapping of cockpit and cranks on the RT9 and AD9 after the stock bike is ordered.

Sizing and Fit

There are only four sizes, ranging from XS to L. The XS is genuinely small, which means X-Lab is catering to smaller riders better than most brands. Taller riders face a bigger problem: the largest RT9 is roughly the same size as a 56 cm Specialized Aethos. X-Lab says there are currently no plans for XL or XXL sizes, though the company would invest in new tooling if demand appeared. If you are on the tall side, check the geometry chart carefully before getting excited.

Ride Quality and the Stock Wheels

Here is where the RT9 gets complicated. On the smallest amount of rough surface, potholes, or loose gravel, the bike feels bone-shakingly harsh. The initial suspicion was the frame, but a wheel swap told a different story. With a set of conventional alloy DT Swiss rims fitted, the same frame rode significantly smoother. The frame is plenty stiff and handles great; the difference was dramatic.

The stock wheels are the Branta C50 ULR, made by XDS specifically for this bike. They are 50 mm deep with a 22 mm internal width and carbon spokes. The carbon spokes are the likely source of the harshness: every carbon-spoked wheelset we have ridden has felt extremely firm, and these are no exception. Even lowering tire pressure on the 28 mm Michelin Pro 5 rubber did not help much. With a steel-spoked wheelset, the RT9 rides the way a bike this light should ride. We would also note that the wheel branding looks very similar to another well-known wheel brand, which is a shame, because the wheel does not need to imitate anyone to stand out.

Things to Consider Before Buying

The frame finish deserves attention. The clear coat is extremely thin, so thin that a light tap from a track pump left a visible mark, and the surface can be scratched with a fingernail. The coating on the handlebars and seat post is much more durable. X-Lab told us the first batch was made with the fragile finish, and after the issue was reported, the engineers switched all new frames to a more durable regular clear coat. If you are buying now, it is still worth inspecting the finish and confirming which batch your frame comes from.

Price is the other major consideration. At $9,150, the RT9 is not cheap, but it is significantly less than the Specialized Aethos and the Scott Addict RC in top spec, the two bikes that previously held the world’s lightest production disc-brake road bike title. A frameset option is planned for the RT9 and a few additional models in the coming months, with pricing not yet confirmed. For riders who already own a wheelset and groupset, the frameset could make this an even more compelling package.

Buying Advice

If your priority is a production road bike with disc brakes that weighs almost nothing, the X-Lab RT9 delivers. The frame is stiff, the components are top tier, the weight is remarkable, and the price undercuts the usual superlight rivals.

Plan for the stock wheels. The carbon-spoked Branta C50 ULR wheels are the weakest part of the package for everyday riding, and factoring a steel-spoked wheelset into your budget makes sense. The frame finish fix is reassuring if it holds, and the limited size range means the RT9 will fit some riders better than others. For the right rider on the right roads, this is a stunning machine. For everyone else, it is a remarkable frame waiting for a better set of wheels.

Conclusion

The X-Lab RT9 is a bike of extremes: extreme lightness, extreme stiffness, and an uncompromising focus on weight that shows up in the details. The wheel swap proves the frame is the real star, and with a more durable finish on new frames plus a conventional wheelset, this could be one of the most compelling superlight road bikes available. As it stands, it is an impressive product with a few caveats that are easy to plan around.