Specialized’s new Sirrus Carbon hybrid loses half of its seat tube to improve comfort

The Sirrus Carbon's radical frame is designed for compliance on city streets and gravel, according to Specialized

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By Paul Norman

Published: March 28, 2023 at 5:00 pm

Specialized has launched the new Sirrus Carbon, a premium hybrid bike that dispenses with the lower half of the seat tube in a radical 1x-specific design said to improve comfort.

In a design similar to that we saw from the prototype Rondo Ruut gravel bike, the Sirrus Carbon's dropped seatstays join midway down the seat tube.

However, instead of the seat tube then continuing to the bottom bracket, it splits off to join the down tube.

Specialized calls this design a Compliance Junction and says "it's been designed to have just the right amount of flex and forgiveness across the carbon frame without sacrificing performance and efficiency".

There are two bikes in the range at launch, the Sirrus 6.0 priced at £2,400 / $3,000, and the Sirrus X 5.0 priced at £1,950 / $2,250.

A few weeks ago, when the Sirrus Carbon's patent application surfaced, we speculated that the radical frame design might be a new Specialized Roubaix.

Well, we’ll hold our hands up because we were wrong, so what's the deal with the Sirrus Carbon now it has been launched officially?

The bike's comfort features don't stop at the radical seat tube design.

The Sirrus Carbon has Specialized's Future Shock 1.5 suspension headset to help smooth out vibrations before they reach the handlebars.

Specialized says you can fit tyres up to a 42c width in the Sirrus Carbon frame, or 38c with mudguards.

That means it's not confined to riding on the road, which Specialized has exploited to offer the all-road-capable Sirrus Carbon X 5.0 spec, alongside the more road-going Sirrus Carbon 6.0.

With no lower portion to the seat tube, the Sirrus Carbon is 1x-specific. Specialized says this is a benefit for the bike's intended use.

"By eliminating the front derailleur, a 1x drivetrain results in less ‘oops’ moments – less cables and parts, less shifting required, and less things to go wrong, with a gear range as wide or wider than many 2x drivetrains," reads the brand's press material.

In terms of gear range, the Sirrus Carbon uses an 11-50t cassette, paired to a 38t single chainring for the Sirrus X 5.0 and a 40t chainring for the Sirrus 6.0.

Although there's nowhere to fit a front derailleur, the Sirrus Carbon offers many bottle cage mounts.

The smaller XS and S frames have three: one on top of the down tube, one below the down tube and one on the top tube. Larger-sized frames add a fourth to what remains of the seat tube.

You can also fit a rack and mudguards, although both need to be of specific design for compatibility. The rack needs to use the MIK system and the mudguards need to be Specialized Dry-Tech fenders.

Specialized has been pushing the boundaries with bike design recently, with the Diverge STR gravel bike also having a go at abolishing the traditional seat tube by using the new Future Shock Rear to decouple it from the rest of the frame and provide up to 30mm of fore/aft movement.

Could the Sirrus have implications for the future of frame design? BikeRadar's Liam Cahill has given his take here.

Below is the geometry table for the Sirrus Carbon X 5.0. The 6.0 model differs slightly, for example with longer stems on some frame sizes.

The Sirrus Carbon is designed to fit both women and men. "Because body proportions vary as much within a gender as between them, Sirrus is designed as a shared platform," according to Specialized.

Kayla Clarot, Specialized's leader of brand concept, adds: "The Sirrus’ look is a reflection of its purpose – get out of the way and enjoy your time on the bike. Timeless and classic, the colours convey a level of understated refinement that says ‘fast without the flash’.

"The look is also a reflection of our Beyond Gender approach to bike design. We design for the individual and don't produce male- or female-specific products unless there's a true performance benefit and data to support the decision."

All figures in mm unless stated.

The all-road Specialized Sirrus X 5.0 (£1,950 / $2,250) has a SRAM NX Eagle 12-speed drivetrain with a 38t chainset, paired with an 11-50t cassette and 38mm tubeless-ready tyres. Claimed weight is 24lb (10.9kg).

The Sirrus Carbon 6.0 (£2,400 / $3,000) offers a 12-speed SRAM GX Eagle drivetrain with a 40t chainring and 11-50t cassette with Specialized Roubaix Pro 30/32mm tyres.

Paul has been writing about bike tech and reviewing all things cycling for almost a decade. He had a five-year stint at Cycling Weekly and has also written for titles including CyclingNews, Cyclist and BikePerfect, as well as being a regular contributor to BikeRadar. Tech-wise, he's covered everything from rim width to the latest cycling computers. He reviewed some of the first electric bikes for Cycling Weekly and has covered their development into the sophisticated machines they are today, on the way becoming an expert on all things electric. Paul was into gravel before it was even invented, riding a cyclocross bike across the South Downs and along muddy paths through the Chilterns. He dabbled in cross-country mountain biking too. He's most proud of having covered the length of the South Downs Way on a crosser and fulfilling his long-time ambition to climb Monte Grappa on a road bike

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