The Specialized Sirrus has traditionally been a fairly innocuous choice of bicycle. The positives it brought – familiar handling, decent build kits, and a carbon frame option to add lightness – were certainly standouts. But this latest Specialised Sirrus Carbon stands out just by looking at it.
I mean, look at it! Have we really seen a mass-market frame design quite like this?
Here’s everything we know about the new Specialized Sirrus Carbon bike.
It’s not a seat tube, it’s called a Compliance Junction.
The most distinctive part of the Sirrus is what looks like a floating seat tube. Outside of something like a Trek Y-Foil or a non-UCI-compliant triathlon bike, most bikes feature a seat tube that goes from the saddle down to the bottom bracket area. Much of that has to do with the inherent strength that comes from the front triangle and rear triangle coming together. The Sirrus Carbon eschews that idea.
This angled strut cuts the seat tube about halfway down and leaves that open space to float in the ether. The seat tube is then supported by the seat stays and a new angled strut that connects from about halfway down the downtube to this new floating seat tube.
Why would Specialized pursue a fundamentally different frame design? Specialized claims it reduces vertical stiffness substantially. Vertical stiffness isn’t how the bike feels as you try and power through a stop light. Rather, it is how muted the little bumps and imperfections in the road are once they get to your saddle.
The way brands like Trek have introduced greater comfort was by isolating the seat tube and allowing it to move independently of the rest of the bike. They call this their IsoSpeed system, and it’s found on their endurance bikes like the Domane. Other brands have dropped the chainstays in hope that the seat tube might flex partway down. Specialized themselves even introduced a complicated rear suspension in their Specialized Diverge STR.
Isolating the seat tube to form a Compliance Junction requires less maintenance than a bearing, shock, or pivot of some kind. And if Specialized’s claims of the Sirrus X offering substantially more ride comfort are true, then they’ve found a way to make your ride smoother without introducing more proprietary parts.
It also looks really, really distinctive.
Specialized Sirrus Carbon features
The new Sirrus Carbon isn’t just about the peculiar frame design, and Specialized goes to great lengths to show off what else is going on here. After all, this bike is much more likely to be a commuter bike than an example of cycling modernism.
Each Sirrus Carbon frame size uses a unique carbon layup and tube sizing. The idea here is to keep the frame consistently rigid for small riders and for tall riders alike based on what their riding weight is likely to be. While the idea to design a bike around expected rider weight is starting to become a more common idea for mountain bike suspension, it is nice to see this level of thought put into a bike that might be used as a daily commuter.
The Sirrus Carbon receives some front-end bump-smoothing in the form of Future Shock, their way of integrating a small amount of suspension just below the stem. Future Shock offers an estimated 20mm suspension travel, though I wouldn’t recommend hitting any rock gardens with the bike.
While Specialized makes a flat bar gravel bike called the Diverge EVO, we suspect there will be many folks who want to use the Sirrus Carbon as a flat bar gravel bike. And it looks like it will do that well. The Sirrus Carbon fits up to a 700c x 42mm tire, or a 38mm tire with fenders.
Additionally, the Sirrus Carbon features a range of mounts. There are the aforementioned fender mounts front and year, a top tube mount for small bags, and a mid-fork mount for additional front rack support.
One downside to the unique frame design is a lack of water bottle mounts. Sizes XS and S receive just one water bottle mount inside of the frame, while sizes M through XXL receive two. All sizes receive a mount under the downtube by the bottom bracket, like a gravel bike. And those larger sizes that have a water bottle on the seat tube will likely want to use a side-loading water bottle cage to get the bottle in and out more easily.
The Sirrus Carbon is available in six sizes (XS – XXL). Specialized makes a big deal of not having men’s and women’s specific sizing, stating that will “only make male- or female-specific products when there’s data to support the decision and a true performance benefit.” Standover heights are respectably low, but no lower than anything else on the market. Riders concerned about standover height are best suited by visiting their local Specialized dealer.
Sirrus Carbon 6.0 vs Sirrus Carbon X 5.0
Specialized launched the Sirrus Carbon with two model variations. Think of them as a flat bar road bike version and a flat bar gravel bike version.
The Specialized Sirrus Carbon 6.0 bike features a 32mm Specialized Roubaix tire, making it closer to a road bike than an all-terrain bike. The lack of a seat tube means that Sirrus Carbon is 1x-only. This bike uses a SRAM GX Eagle mechanical drivetrain as a result, with a 40T SRAM Rival crank and an 11-50t SRAM NX Eagle cassette.
Quoted weight is 20.3 lbs, though without a specified size. Expect to pay $3000 / £2400 / AU $4200 for the Sirrus Carbon 6.0.
Those looking for more of a gravel bike with flat bars will be happy with the Sirrus Carbon X 5.0. This one swaps for a SRAM NX drivetrain, a 38t chainring, and 700c x 38mm Specialized Pathfinder Pro tires commonly seen on their gravel bikes like the Diverge.
Quoted weight is 24 lbs without a specified size. Expect to pay $2250 / £1950 / AU $3200.
Sirrus Carbon: a one-off or sign of things to come?
The new Specialized Sirrus Carbon is a distinctive bike, but we expect that angled strut and floating seat tube to show up on other bikes in the future. The 1x-only design is limiting for something like a Roubaix road bike, but perhaps Specialized will make the move to 1x-only on something like the Diverge gravel bike.
Until that comes, learn more about the Specialized Sirrus Carbon at specialized.com.
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