How low can your seatstays go?
Why seatstays have done the limbo, dropping from the traditional junction at the top tube.
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The traditional, two-triangle bicycle silhouette triggers happy thoughts. It’s classic and hasn’t changed much since the birth of the machine itself. But in recent years this old friend of ours has undergone so many changes that it was inevitable to see its basic shape change.
It may be slight, but the dropped seatstay design has altered that silhouette perhaps more dramatically than any other design feature — much to the chagrin of traditionalists. But it serves an important function, both for aerodynamics and comfort. Whether that’s worth it depends on how you ride.
Where did the idea come from? And what exactly does it do? As with most advancements in the bike industry, the answer to both of those questions is, “Depends on who you ask.”
Dropped for gains — and for cushion
BMC often takes credit for being the first bike brand to introduce dropped seatstays to traditional road bikes. Indeed, the BMC Teammachine road bike looked awfully different when the curtain peeled back on the 2010 model, which featured the dropped seatstay design for the first time. The design placed BMC ahead of its competitors, even though Trek had already introduced the design for its Speed Concept time trial bike.
BMC originally adopted the design in order to allow other tubes to flex more easily in an attempt to up its compliance quotient. “In the development of the Teammachine SLR from model year 2011, we had the plan to build vertical compliance into the frame,” says Stefan Christ, head of research and development at BMC. “It was the first time this was considered in the requirement specifications for a competition road bike, since before all our developments were driven to improve stiffness to weight. The project started when Cadel Evans joined the BMC Racing Team — in the first exchanges we had regarding the requirements for his race bike, it became evident that it would be beneficial to add compliance in order to reduce rider fatigue.”
Christ says the idea came about simply from bending various tubes and considering load paths, rather than running computer simulations. “By dropping the seatstays relative to the top tube,” says Christ, “you create an offset in the load path. The seatstay/seat tube/top tube node is rotating counterclockwise when loaded. It’s this rotation that allows for significantly more compliance when seated compared to the deflection of the seatpost itself.”
The Teammachine has featured lowered stays since then — Cadel Evans went on to win the 2011 Tour de France on those dropped seat stays — and the design has crept into other bikes in the BMC road lineup. It has even been implemented in BMC’s hardtail mountain bike, the Teamelite, for years now. And it should come as no surprise that dropped seatstays factored into BMC’s design when it launched its URS gravel bike in 2019.
Comfy, but fast?
Before too long, other brands had caught onto the design as various testing verified that dropped seatstays were indeed a faster solution than more traditional stays that met the seat tube at the top tube junction. Dropped seatstays ruled the roost during the “aero is everything” era of bike design that has persisted up until now.
“It’s a big driver for aero performance,” says Cameron Piper, Specialized road product manager. “We’ve learned over time and simulations that there’s a sweet spot that’s maxed out to be almost the lowest, without being a detriment to rear end stiffness and performance.”
That of course means it’s entirely possible to go too low. Sure, that low position would probably maximize aerodynamic performance, but it would end up sacrificing lateral stiffness in the pursuit.
To find the sweet spot, Specialized uses its in-house Win Tunnel, as well as computational fluid dynamics (CFD) modeling. The former is an exclusive perk for Specialized; the latter is used by most of the big players in the bicycle design space. CFD allows engineers to analyze how air flows over hypothetical tube shapes from various angles, without having to go through the process of building a prototype and testing it in a wind tunnel.
And it seems generally agreed upon that testing proves lower seatstays are more aerodynamic. It’s no coincidence that brands across the board have adopted the design.
Are the aero gains worth it?
Sometimes it pays to stand out.
While the dropped seatstay craze has seemingly blazed out of control, Trek has taken a completely different approach to the back ends of its frames, particularly when it comes to compliance. And while the company acknowledges that dropped seatstays are indeed the faster design option, that choice comes with trade-offs.
“We investigated deeply if dropped seatstays were the right solution” for the 2021 Trek Emonda SLR, the company’s flagship all-around bike. That’s according to Jordan Roessingh, Trek’s director of road bikes. “From a CFD perspective, we tested extensively and it came back that dropped stays are faster, but they’re also structurally less efficient.”
For Trek, the trade-off of that stiffness wasn’t worth the aero or comfort benefits, particularly since this aerodynamic design is at the rear of the bike. It is indeed possible to gain back some of the aerodynamic advantages in the front of the bike, where such tube shapes are more critical and impactful. According to Roessingh, when the stays attach at a higher point on the seat tube, the rider gets more torsional stiffness, and you can essentially reduce weight since the seatstays don’t need to be as thick, or as reinforced, for strength.
“This was the lighter weight solution, and we still got to our aerodynamic goals,” Roessingh says. Consequently, the Emonda’s frame looks more traditional, with the seatstays joining the seat tube at the junction with the top tube. While true purists may still balk at the unique tube shapes on the Emonda, the more traditional silhouette is certainly gratifying to behold.
Christ doesn’t argue with the notion that lowered seat stays present other design challenges. “The new bike architecture created additional deflection and stress in the frame structure,” he says, “which we first had to get familiar with, and develop specific testing for, in order to ensure strength and safety. My education as a composite engineer gave me confidence that by proper layup development, those challenges could be solved.”
In order to solve those challenges, it became necessary to set targets. And those targets can vary wildly from brand to brand, as well as between goals for specific models. Once BMC knew it wanted to add compliance without sacrificing too much lateral stiffness, the company turned to computer software to find the perfect seatstay positioning. Because dropped seatstays add weight and reduce torsional stiffness, BMC had to make trade-offs. In most cases, being the lightest bike out there got dropped further down the list behind comfort and aerodynamics.
BMC calls its software ACE Technology. “When we introduced the ACE Technology computer simulation, the concept of dropped seatstays was confirmed by computer simulation, and the position of the attachment of the seatstays was optimized,” says Christ. “We gave the software the full freedom to put the connection anywhere from in-line with the top tube to the lowest UCI-legal position, and the solution was actually very close to the position we initially had. So simulation was reconfirming intuition.”
The dropped seatstay trend exists because it allows designers to come close to roping a unicorn: a bike that combines aerodynamics, stiffness, and heaps of comfort. But whether the design is worth the sacrifice — namely, light weight — largely depends on the rider’s goals. As all-around bikes become more aero, it’s safe to say dropped seatstays aren’t going anywhere for a while.