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What’s going on with that seat tube? Specialized unveils radical “Compliance Junction” on new Sirrus Carbon

Innovative frame design for urban and gravel riding is intended to provide ‘flex and forgiveness’ without affecting efficiency

Specialized has introduced a radical-looking Sirrus Carbon bike design that features a new “Compliance Junction” designed to provide “just the right amount of flex and forgiveness across the carbon frame without sacrificing performance and efficiency”. The Sirrus models come with Specialized’s existing Future Shock front suspension and are hybrid bikes intended for both city and gravel riding. 

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The innovative frame has a seat tube that doesn’t reach down to the bottom bracket. It’s held in place by low-slung chainstays and a strut that connects to the down tube.

We reported on a patent application for this new frame a few weeks ago and thought that it was probably for a new version of Specialized’s Roubaix endurance bike. We were completely and utterly wrong. Ah well, you can’t win ’em all.

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That patent application gives us an insight into the thought behind the Compliance Junction.

“These modifications advantageously reduce the vertical stiffness of the bicycle while substantially maintaining or reducing to a lesser extent the horizontal (eg lateral and/or torsional) stiffness of the bicycle, thereby increasing the comfort for the rider while substantially maintaining or only slightly reducing the handling of the bicycle and/or force transfer of the rider to the bicycle,” Specialized said.

It’s all about vertical compliance, then – a bit of give to provide extra comfort.

Specialized went well beyond most patent applications in providing data from testing to back up its concept, although the exact figures don’t necessarily apply to the production frame.

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In a vertical stiffness test with a static load of 1,200N applied, Specialized says that its frame allowed 7.9mm of vertical deflection compared with 2.9mm for an unnamed conventional frame. Horizontal deflection at the saddle was 7.2mm compared with 4.1mm for the conventional frame.

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In a horizontal stiffness test, where a 600N load was applied at a specific location, Specialized says that its frame deflected 5.1mm while the conventional frame deflected 6.7mm.

“As reflected in the test results, the [Specialized] main frame produced 172% greater vertical deflection and 75% greater horizontal deflection at the seat during the vertical stiffness test compared to the conventional frame,” says Specialized. “By increasing the vertical deflection at a greater rate than the horizontal deflection, ride comfort is increased without substantially increasing the rearward saddle tilt.

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“The horizontal stiffness test showed a decrease of 24% in the horizontal deflection for the main frame compared to the conventional frame which does not substantially impact the ride characteristics (eg handling, force transfer, etc) but it provides evidence that the frame design works well to handle the loads/stresses applied during the horizontal stiffness test, which is believed to correlate to real-world riding.”

As mentioned, these figures were intended to prove the concept; they don’t necessarily relate to the finished Sirrus Carbon just released. Specialized hasn’t produced these figures anywhere but on its patent application. What Specialized does claim for the finished article is that “the result is a ride so smooth and comfortable that it is truly unmatched in its category.”

Specialized says that the use of carbon is key to the new design.

“Specifically for Sirrus, carbon presents a unique proposition as it changes the way we approach frame design and ride characteristics holistically, says Specialized. “Using varying carbon layups to adjust strength and flex throughout the frame, we formed the Compliance Junction, where the frame flexes in just the right way to create an incredibly comfortable, bump-smoothing ride feel."

Trek has also interrupted the seat tube recently to improve comfort – and also aerodynamics – with the IsoFlow technology that features on the Madone SLR. However, that’s very different in that the interruption is at the top rather than low down.

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As well as the Compliance Junction, the Sirrus Carbon features Specialized’s existing Future Shock suspension technology within the head tube. Future Shock is a system that allows the handlebar and stem to move to reduce the amount of high-frequency vibration that’s transferred from the road surface to the rider.

“When the front wheel encounters rough terrain, the bike moves up towards your hands and preserves your forward momentum without slowing you down,” says Specialized. “Because the Future Shock is positioned under the stem, the bike's wheels are held together rigidly by the frame. In other words, because the wheelbase isn't changing throughout the suspension’s travel like with traditional systems, you get the added benefit of extremely predictable handling.”

The new bikes take tyres up to 42mm without mudguards or 38mm with mudguards, so comfort shouldn’t be an issue. If you want to carry bags, Specialized says, “Sirrus is MIK compatible, and racks can be used with 42mm [Specialized] Dry-Tech fenders [mudguards].” 

The shortened seat tube means that on the XS and S models there's only space for one bottle inside the front triangle, but you can carry others below the down tube, and on the top tube. Sizes M, L, XL, and XXL can hold four bottles, two of them inside the front triangle. 

Sirrus is designed as a shared platform – so there aren’t separate men’s and women’s models – and the bikes come with 1x (single chainring) drivetrains. There are two new models:

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Sirrus Carbon 6.0, £2,400
Fork Carbon Thru-Axle FS
Drivetrain SRAM GX Eagle Lunar mechanical 1x (40t chainring, 11-50t cassette)
Brakes SRAM Level TLM 160mm/160mm
Tyres Specialized Roubaix Pro 30/32mm
Weight 9.2kg/20.3lbs

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Sirrus Carbon X 5.0, £1,950
Fork Carbon Thru-Axle FS
Drivetrain SRAM NX Eagle mechanical (38t chainring, 11-50t cassette)
Brakes Tektro TKD-148 Brakes 160/160
Tyres Specialized Pathfinder Pro 38mm
Weight 10.9kg/24lb

Aside from the two new models, Sirrus and Sirrus X are offered with aluminium frames (without the Compliance Junction). Check out the full range here. 

Mat has been in cycling media since 1996, on titles including BikeRadar, Total Bike, Total Mountain Bike, What Mountain Bike and Mountain Biking UK, and he has been editor of 220 Triathlon and Cycling Plus. Mat has been technical editor for over a decade, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a winner of the Cycling Media Award for Specialist Online Writer. Now over 50, he's riding road and gravel bikes most days for fun and fitness rather than training for competitions.

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