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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. 

2023 Specialized Sirrus X 5.0 - 5.jpeg

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.

2023 Specialized Sirrus Carbon X 5.0 - 2.jpeg

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.

2023 Specialized Sirrus Carbon 6.0 - 1.jpeg

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.

> Best commuter bikes

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.

2023 Specialized Sirrus Carbon 6.0 - 1 (1).jpeg

“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.

2023 Specialized Sirrus Carbon X 5.0 - 3.jpeg

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:

2023 Specialized Sirrus Carbon 6.0 - 6.jpeg

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

2023 Specialized Sirrus X 5.0 - 4.jpeg

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 road.cc 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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14 comments

Avatar
Mungecrundle | 11 months ago
0 likes

Seems like there are more elegant ways of achieving the same result.

Avatar
chrisonabike replied to Mungecrundle | 11 months ago
1 like

Pedersen (Dursley-Pedersen) - 1910...

You can get modern ones with all the trimmings too.

Back then though "carbon bike" probably meant it belonged to a chimney sweep.

Avatar
cyclisto | 11 months ago
0 likes

You can't have a decent commuter bicycle without being cheap enough not to worry locking it outside.

Industries should focus on making cheap commuters and save the weird and expensive ideas for the bling Sunday bikes.

Avatar
Rendel Harris replied to cyclisto | 11 months ago
0 likes
cyclisto wrote:

You can't have a decent commuter bicycle without being cheap enough not to worry locking it outside.

Industries should focus on making cheap commuters and save the weird and expensive ideas for the bling Sunday bikes.

I don't know, with increasing numbers of office buildings offering secure indoor storage for bikes there is definitely a place in the market for nicer than "doesn't matter that much if it's nicked" bikes for commuting. What's really unrealistic is the kit that model's wearing, try commuting in British weather and road conditions in a white jacket, helmet, socks and shoes...

Avatar
youngoldbloke | 11 months ago
0 likes

- but where do you put the chain-catcher?

Avatar
mark1a replied to youngoldbloke | 11 months ago
0 likes
youngoldbloke wrote:

- but where do you put the chain-catcher?

Probably not required on 1x with narrow-wide chainring and/or rear clutch.

 

Avatar
quiff | 11 months ago
1 like

The Sirrus feels an odd platform to launch this on. Were people crying out for an expensive, innovative hybrid?

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Global Nomad | 11 months ago
0 likes

feels like they were a few days early on the press release

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wtjs | 11 months ago
3 likes

Innovative means 'wait five years for them to start breaking' by which time this frame type will have disappeared

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Sredlums replied to wtjs | 11 months ago
3 likes

It won't matter because people who buy carbon bike see them as disposable anyway. They will be replaced wiyhin that period with a bike 'with an even better carbon layup'.

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Welsh boy replied to Sredlums | 11 months ago
2 likes
Sredlums wrote:

...people who buy carbon bike see them as disposable anyway.

What a stupid overgeneralisation.

Avatar
Sredlums replied to Welsh boy | 11 months ago
0 likes

Yes, it is of course, clearly, a generalisation.
But let's be honest, how many people who buy a new carbon road bike still ride that bike after 5 years? It's simply not the case that most of those people intend to ride that bike for many years to come.

And yeah, those bikes are often of course sold to someone else, and with luck they will ride it for e few more years, but cutting edge, lightweight carbon bikes are just not as durable as steel, titanium or even aluminium bikes.

So apart from that I should have said 'new carbon bikes' I still stand by my comment. 

Avatar
visionset replied to Sredlums | 11 months ago
0 likes
Sredlums wrote:

But let's be honest, how many people who buy a new carbon road bike still ride that bike after 5 years? 

Well me for one and plenty of mates. I've got an original Cervelo R3SL, it's 13 years old and I don't really see any reason to 'upgrade'.  Why would you? 10 speed is plenty, it's super reliable with none of that internal cabling rubbish & external BB.  

Avatar
levestane | 11 months ago
1 like

Softride will be back soon (may I be forgiven).

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