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

Compliance Zone on the Specialized Sirrus Carbon X 5.0 hybrid bike

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.

Wait, wasn’t this a new Roubaix?

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?

Specialized calls this the Compliance Junction.
Robyn Furtado / Our Media

Urban and gravel capabilities

Specialized adds front-end vibration damping to the Sirrus Carbon with its Future Shock 1.5 suspension headset.
Robyn Furtado / Our Media

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.

The Sirrus Carbon can fit tyres up to a 42c width (or 38c with mudguards).
Robyn Furtado / Our Media

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.

1x-specific and lots of mounts

Want a front derailleur? You’ll need to look elsewhere.
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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.

The Sirrus Carbon X pairs a 38t single chainring with an 11-50t cassette.
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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.

Despite the missing seat tube, Specialized has managed to squeeze in a decent number of mounts.
Robyn Furtado / Our Media

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.

What does this mean for the future of the seat tube?

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.

Specialized Diverge STR Expert gravel bike review BikeRadar
Jack Luke / Our Media

Specialized Sirrus Carbon geometry

Specialized says the Sirrus Carbon is designed to fit both women and men.

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.

The frame is made from carbon fibre.
Robyn Furtado / Our Media

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.”

Headtube length 107128151176208237
Headtube angle (degrees)69.569.57070.570.571
BB height268268268268268268
BB drop808080808080
Fork length (full)395395395395395395
Fork rake/offset515151515151
Chainstay length440440440440440440
Top tube length (horizontal)521547568590614644
Stand over703.3748.3780.5807.1847.7872.5
Seat tube length355416453483533563
Seat tube angle (degrees)7473.573.573.573.573
Handlebar width680680680680680680
Stem length60708090100110
Crank length165170170175175175
Seatpost length300300350350400400

All figures in mm unless stated.

Specialized Sirrus Carbon specs and prices

The Sirrus Carbon is offered in two specs at launch.

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.

Specialized Sirrus Carbon X 5.0

Specialized Sirrus Carbon X 5.0.
  • Frame: Sirrus FACT 9r Carbon
  • Fork: Sirrus FACT 9r Carbon with Future Shock 1.5
  • Groupset: SRAM NX Eagle 12-speed/SRAM S650 chainset
  • Gear ratios: 38t, 11-50t
  • Wheels: Alloy rims on alloy hubs
  • Tyres: Specialized Pathfinder Pro 2Bliss Ready 38mm
  • Brakes: Tektro TKD-148 hydraulic disc
  • Bars: Specialized alloy
  • Stem: Future Stem, Comp
  • Saddle: Body Geometry Power Sport
  • Seatpost: Alloy 27.2mm
  • Sizes: XS, S, M, L, XL, XXL
  • Claimed weight: 10.9kg / 24lb
  • Colours: Gloss white mountains, Gunmetal, Satin white mtns reflective
  • Price: £1,950 / $2,250

Specialized Sirrus Carbon 6.0

Specialized Sirrus Carbon 6.0.
  • Frame: Sirrus FACT 9r Carbon
  • Fork: Sirrus FACT 9r Carbon with Future Shock 1.5
  • Groupset: SRAM GX Eagle Lunar 12-speed/SRAM Rival chainset
  • Gear ratios: 40t, 11-50t
  • Wheels: DT Swiss R470 on Specialized hubs
  • Tyres: Specialized Roubaix Pro 2BR tubeless-ready 30/32mm
  • Brakes: SRAM Level TLM
  • Bars: Specialized alloy
  • Stem: Future Stem, Comp
  • Saddle: Body Geometry Power Sport
  • Seatpost: Roval Terra Carbon
  • Claimed weight: 9.2kg / 20.3lb
  • Sizes: XS, S, M, L, XL, XXL
  • Colours: Gloss black, Pearl, Metallic white silver, Black reflective
  • Price: £2,400 / $3,000