When Specialized introduced the S-Works Tarmac SL7 this summer, all but making the Venge obsolete with a bike that is nearly as aero but meets the UCI weight limit, we naturally had a question: When can we get our hands on one? But then another question quickly came to mind: Now that the fastest aero bikes can be made at the 6.8kg UCI weight limit, what can Specialized do with all the concerns of a rule book thrown out the window? Luckily, Specialized was way ahead of us. Deep inside the brand’s HQ, a secret project was already years in the making, one that no one outside the immediate group developing it knew existed until the design was already finalized. That project became the S-Works Aethos, the lightest production disc-brake frame ever at 588 grams (painted, size 56), with builds available as low as 5.9kg (13.01lbs).
Let’s face it, the vast majority of us are never going to need a bike that conforms to the UCI weight limit. Yet there’s still this fascination many hold of riding the same bikes that the pros use. But the truth is, unhindered by any rule book, we can be riding bikes that are lighter and that ride better than those available to the world’s top riders. The S-Works Aethos is a project simply to see what is possible. And it’s aimed at riders who may not necessarily race, but who still love riding, going fast and pushing new limits.
New Tube Shapes
Developing such a light bike with a focus on excellent ride quality required some big breakthroughs in design, on par with the leaps in aero gains experienced in the Venge in the past few years. In fact, Stewart Thompson, the road and gravel category lead at Specialized, doesn’t think we’ll see a leap in performance like the one in the Aethos project ever again. One such breakthrough was a rethinking of how forces flow through the frame. Specialized discovered that there is more stress applied to the front end of the frame than the rear, making the head tube and top tube more important to resisting bending than previously thought. With the aid of computer modeling, engineers developed round tube shapes that are subtly conical to impart more stiffness, with as little material as possible. The tubes then taper from larger in the front to smaller in the rear to account for the differing forces applied to different parts of the bike. The result is a frame that is stiff and provides a new level of ride quality for Specialized bikes. But for the rest of the bike, Specialized keeps things simple with a standard stem, bars, seat post and seat post collar. The Aethos also uses a 68mm threaded BSA bottom bracket.
Carbon Layup Removes Redundant Material
Beyond tube shapes, there are some innovations in the carbon layup itself. Carbon frames tend to have extra plies of material, called “stiffness layers,” beyond what is necessary, which means excess material. By using larger and longer plies, Specialized was able to reduce the number of needed plies by 11 percent over the S-Works SL6 generation bikes. This isn’t a process that results in a weaker frame; Specialized describes the tube wall thicknesses of the Aethos as “substantial,” with the bike being rated for up to a 275 pound rider.
The reduction in unnecessary layers leads to a much lighter frame. Made with FACT 12r carbon introduced earlier this year with the S-Works Tarmac SL7, the S-Works Aethos frame weighs a scant 588 grams, painted, in a size 56. That also means a stiffness to weight ratio that blows other bikes out of the water, besting the last generation S-Works Tarmac SL6 handily. Our size 54 weighed 6.27kg (13.82 lbs) without pedals or cages, but it is possible to go even lighter. Specialized will be offering a limited edition “Founders Edition,” with an integrated Roval Alpinist bar/stem and CeramicSpeed bottom bracket and pulley wheels, which weighs a claimed 5.9kg. Only 300 of those will be made, so get in contact with your bike shop ASAP if you have to have one.
From a geometry standpoint, Aethos features an identical fit to the Tarmac SL7, making it very familiar to Specialized riders. And even though the frame is extremely light, making it a slight challenge to build it up at the UCI weight limit, it still conforms to all UCI rules and can be raced in UCI events, if you so choose. But, again, this bike really isn’t aimed at racers, and a bike with more aero features like the Tarmac would probably be a better choice for those riders.
The Aethos features a radical departure for Specialized in one very conspicuous department: branding. This bike bucks the longtime practice of plastering every available square inch of space with logos, and instead opts for just a subdued logo on the top tube in addition to the head tube “S” logo. There are multiple lightweight paint options available (our bike’s paint is so minimal you can see the carbon layup beneath it), the lightest of which is Satin Carbon/Jetfuel. Satin Carbon/Chameleon Red Gold and Satin Carbon/Gloss Black Chrome add about 25 grams while Gloss Clay/Flake Silver and Satin Flake Silver/Chameleon Red Gold add about 45 grams.
Just as the knowledge gained from developing a super-aero bike can make all bike designs more aero, Specialized says the new approach to designing frames developed in the Aethos will find its way into many products down the line. But for now, the privilege of trying these new tube shapes and build processes is limited to the Aethos, and will set you back $12,500 for a Dura-Ace Di2 or SRAM Red eTap AXS build with Roval components and wheels. A frameset costs $5,200. The Founders Edition costs $14,500.
It speaks to the quality of a super-light bike when the thing you notice about it is not so much the low weight (though you certainly notice it), but instead is the overall dialed, sublime ride quality. The focus on new slightly conical, tapered tube shapes in the Aethos seems to have paid dividends in creating an extremely responsive bike that feels more part of the rider than even already excellent race bikes. It reacts quickly but never feels twitchy. And it goes exactly where you want it to.
Going uphill it of course feels super capable and light, a sub-14-pound build will do that, and the stiffness makes ride feedback so instantaneous that you can’t help but rise up out of your saddle and dig in. All that we kind of expected after looking over Specialized’s goals for this frame. But the surprising thing was what happens after you reach a summit. On descents, we thought the Aethos would carve well, which it did, but the bike has some otherworldly speed on the negative gradients for a design without an ounce or aero attributes to its name. It pushes you to find new limits, but not in a way that makes you reckless or dangerous. It’s a faster experience, but one that never feels off the rails, helping you expand your limits, within control.
And perhaps most importantly, nothing about the bike feels fragile or under-built. There’s never a sense that compromises have been made anywhere on the bike just to reach a weight benchmark.
The question of the Aethos comes down to how much you like to play around in hills and technical roads. Riders in pancake-flat areas can still appreciate this bike, but won’t get to experience it on the terrain where it excels the most. Many riders will probably prefer the aero advantage of other bikes. But there is no denying that the Aethos is a hell of a lot of fun, and makes sense for the riders out there who don’t race but who want the best ride quality and lowest weight.
$12,500 as tested
13.82 pounds (6.27kg) (without pedals or cages)
Dura-Ace Di2 (52/36T crankset w/ power meter; 11-30T cassette); S-Works handlebar and stem; Roval Alpinist Carbon Seatpost; Roval Alpinist CLX wheelset; 28mm Specialized Turbo Cotton tires