One look at the new Specialized Diverge STR may leave you puzzled. The centerpiece of the bike is a new rear suspension system that sees a flexible frame upright connected to a shock absorber in the top tube, via an exposed aluminum ‘tendon’.

The design is called Rear Future Shock and it is said to “suspend” the rider. It reduces vibration at the saddle while retaining the stiff “double diamond” frame of the bike and therefore the acceleration and handling you’d expect from many of the best gravel bikes.

Chris D’Aluisio, one of Specialized’s concept engineers and the man behind the brand’s Smartweld technology, started working on the idea more than five years ago, building prototypes at the House.

D’Aluisio’s prototypes reveal how Future Shock Rear came to be and provide rare insight into the work, breakthroughs and dead ends behind bike design – something you will rarely see.

Origins and first prototype

The first prototype has an air shock mounted on the seat mast.
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Chris D’Aluisio first came up with the idea of ​​”suspending the cyclist” above a bicycle frame in 2014. While riding down Eureka Canyon near his home in California, D’Aluisio realized that his body provided the suspension he needed; he could stand on top of the bike and bumps below him were no longer an issue.

This realization led to the development of Specialized’s Future Shock, which debuted in 2016 on the Specialized Roubaix Bike of the Year.

The design isolates the cockpit, protecting riders from impact at the front of the bike.

The technology quickly made its way onto the first-gen Diverge in 2017, with Specialized relying on a compliant seatpost and dropper post clamp to soak up rear bumps.

This design worked for small bumps, but Specialized says it needed more deflection for rougher terrain to keep riders in the saddle for more traction and power.

The bottom bracket is held in its own linkage.
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D’Aluisio says the way around this problem was to come up with a design that would see the rider’s weight move in a downward fore-aft curve, rather than up and down, as happens with suspension components. typical telescopes. .

That’s the idea behind the first Diverge STR prototype. In order to achieve back-and-forth motion, the bike has a seatpost attached to the bottom bracket. A linkage is then at the top of the frame, with damping controlled by a standard air shock.

The bottom bracket is attached to a separate linkage on the underside of the downtube.

“We wanted the bottom bracket to drop down and forward to get some chain growth so that when you’re out of the saddle and pressing the pedals, the chain will suspend you – [this prevents ] suspension of compression,” says D’Aluisio.

D’Aluisio says the prototype worked well, but having the bottom bracket in its own linkage meant the bike didn’t have the pedaling efficiency of a stiff frame that he wanted.

A cleaner solution

The second prototype has a much cleaner look.
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D’Aluisio took the concept forward from the first prototype, introducing a rotating bottom bracket shell and hiding the air shock in the downtube of the frame.

D’Aluisio, who has been with Specialized for over 20 years and was at Cannondale before that, says this home-built prototype is his favorite bike he’s ever built because “it did everything I wanted ‘ and that it was ‘so effective’.

The air shock sits in the downtube—a neat but complicated fix.
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Others at Specialized liked how clean this prototype was. The brand decided to try the design in sizes larger than D’Aluisio’s 52cm frame, but it revealed limitations.

“The 56cm [frame] feedback was good. But with the 58 the feedback wasn’t so great and there were some questions about what was going on. Those riders didn’t feel like they had the saddle advantage,” D’Aluisio says.

Taller riders liked the design when seated, but not out of the saddle. D’Aluisio says that on the larger frames, Specialized had the air shock “so pumped up to try to accommodate the needs of the rider in the saddle that the bottom bracket was out of phase.”

Look closely and you can see a magic marker line indicating how far the bottom bracket rotates.
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This led to a buildup of friction—the friction that can keep static surfaces from being set in motion—and meant the bottom bracket wouldn’t move for taller riders.

D’Aluisio says that led him to go ahead with the seat tube idea and leave the rotating bottom bracket, even though he found the performance on the 52cm frame.

He also says that this design with the air shock was too complex and that Specialized could never do it on an industrial scale.

Squint and it looks the same

D’Aluisio’s third prototype closely resembles the final design.
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Abandoning the moving bottom bracket and attempting to simplify the design led D’Aluisio to develop this third prototype, which bears a striking resemblance to the final Diverge STR design.

“If you squint, it looks the same,” D’Aluisio says.

This prototype has a pivot on the seatpost, but later versions had tendons.
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The prototype has a flexible seatmast with a Thomson seat tube attached to a shock in the top tube and a boot that keeps debris and water out of the system.

The design also introduced adjustability, which the old Diverge’s flex seatpost didn’t have.

You can see how D’Aluisio has grafted carbon parts together.
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D’Aluisio says later versions of this prototype had tendons, like those seen on the final Future Shock Rear.

The flex in the tendon means the shock can be clamped into the top tube, rather than needing a pivot, and ultimately minimizes complexity, play and friction.

The final design, sort of

One of the final prototypes used a 2017 Diverge frame.
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The next step for D’Aluisio and Specialized was to build a test model. Specialized moved the pivotless design in Diverge frames from 52cm to 61cm.

The version pictured here is a 2017 Diverge, from when the bike hadn’t even been updated the second time around, revealing how long that process had been going on. It has the end shock but not the end tendon.

The hydraulic shock absorber is quite small.
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At this point, the project was a co-development, with the frames being machined at Morgan Hill, Specialized’s California headquarters, and its factories developing the carbon layups for the bike.

Specialized road and engineering manager Luc Callahan also began testing and developing 352 sample frame posts over three years. This is “the longest committed development period for a single road or gravel technology”, according to the brand.

Fast forward to now

The Specialized Diverge STR completed.
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Fast forward to now and Specialized says the Future Shock Rear will offer up to 30mm of travel in the out-and-back curve, which was essential all along.

Callahan’s testing led to nine different frame posts to suit different frame sizes and rider weights.

The final version of the Specialized Diverge STR is visually similar to D’Aluisio’s fourth prototype with its sinew exposed.

The appearance of the Future Shock Rear will likely divide opinion, but it might be worth getting used to.

When asked if we can expect to see Future Shock Rear on other bikes, Specialized Road and Gravel Manager Stewart Thompson said, “This is an innovation we really believe in. If we can find other applications and opportunities, I think we would go for it. ”

The design falls into the gravel suspension category, for now, but we might see it on the cobblestones of Paris-Roubaix in the future.