Specialized Aethos: lightest ever disc brake production frame and unrivaled ride quality

The Aethos frame weighs in at 585g - making it the lightest ever disc brake production chassis - but it's about so much more than that

If you get paid to ride your bike, or your sole motivation for cycling is bagging Strava bragging rights, stop reading now. The brand new Specialized Aethos – the world’s lightest ever production disc brake road bike – is probably not for you.

>>> How does it ride? Read the Specialized Aethos review

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If you ride a bike because it makes your heart feel full to bursting, cresting climbs sends fireworks around your nervous system and the feeling of being connected to fast-moving tarmac beneath you makes your soul sing, then read on. Round tubes are back, my friends.

A number of cycling’s favourite GC bikes have had an aero facelift this year. Engineers across the board have figured out how to make a bike hit the UCI weight limit of 6.8kg, whilst also borrowing tube shapes from their aero brethren. From a technical perspective, I’ve been unable to argue with the premise of these ever converging machines: physics says aero tubing is faster, aero tubing no longer comes at the expense of low weight; low weight minus drag equals speed.

However, in my opinion, many of these bikes have carried a theme: maximised forward propulsion at the expense of the very top slice of ride quality. It was a comment I made about the Specialized Tarmac SL7, ditto the BMC Teammachine SLR01 2021, and my colleague made similar observations of the Giant TCR SLR 01.

They’re all excellent bikes, I can’t fault them (I gave the Tarmac SL7 a 9/10 score and I wholly stand by that) – but a little bit of magic was missing. I was starting to wonder if I needed to recalibrate my expectations. Thankfully the antidote is here, and I couldn’t be happier about it. Enter: Aethos.

It looks a bit like the old SL4, but it’s not.

Image: Chris Sansom

At 6kg for a built size 56, the Aethos is under the UCI weight limit and it’s not aero, so Specialized is not calling the Aethos a race bike. However, it’s stiff, lively and responsive and the geometry is in keeping with the Tarmac. It looks like a bike from the last decade, except it’s mortifyingly light. It’s a home mechanics dream. The only obvious quibble is: who is this bike for?

All about ride quality with a side order of weight savings

The Aethos was born of a research project. Its goal was to redefine tube shapes in pursuit of the ‘ultimate ride quality’.

By happy coincidence, the result also broke new ground in bringing the frame weight down to 585g – in the lightest ‘Jetfuel’ paintjob (other colours add 25g-45g) – whilst bettering the stiffness to weight ratio of the outgoing SL6.

Until the arrival of the Aethos, the Canyon Ultimate CF Evo 10.0 was the lightest ever disc brake production frame, weighing 668g according to an independent Zedlar test. Its stiffness to weight is 141 vs 162 for the Aethos and 132 for the Tarmac SL6.

The size 52 Specialized S-Works Aethos Dura Ace Di2 which I have on test weighs in at 5.95kg. The Canyon Ultimate CF Evo I tested earlier this year, in a comparable size X-Small, was 5.98kg with SRAM Etap AXS (which is heavier). However, the Aethos features a standard Power saddle, not a naked carbon saddle as per the Canyon, and there’s no light-but-hard-to-live-with integrated seat post as per the Giant TCR.

Why does it look like a bike from 10 years ago?

‘Ride quality’ is a pretty subjective beast, but Specialized needed to make it quantitive. The brand says that its engineers – headed up by Peter Denk (formerly of Cannondale) – requested funding for a ‘supercomputer’ that would go on to analyse over 100,000 frame designs, learning how stress travels through a frame and how that frame can be optimised in line.

The result is a frame with traditional round tubes that wouldn’t look out of place 10 years ago. Actually, it looks a bit like a Tarmac SL4. It also reminds me of the custom Werking frame I had built for me, following an extensive interview about two years ago, so I suppose the fact I like it isn’t a big surprise.

There are differences though. Areas of the frame which respond to stress have been altered to be more conical. This design – combined with the lack of requirement to create aero shapes – meant that engineers could use larger, longer and more continuous plies.

Aethos frame (left) vs standard approach (right) – in typical manufacturing, carbon is thicker at stress points

It wasn’t necessary to build up layers of carbon at stress points, as is customary for example at the bottom bracket – hence the low weight of the frame. The Aethos frame includes 11 per cent fewer plies when compared with the S-Works Tarmac SL6.

Explaining why the frame shape harks back so much to heritage designs, Richard Salaman, Specialized’s UK head of product, said: “75 per cent of the ‘ride’ of a bike comes from the frame shape. The aero arms race means that frame shape has had to change – once you know that dropped seatstays are more aero, you’re going to use them. It’s why so many frames look similar these days. But those shapes don’t move and flex in the same way,” he says, holding up a butter knife and demonstrating it’s lack of vertical compliance.

“A square or a rectangle is going to perform in a different way to a round tube,” he says. So, in other words: squarer tubes are faster, but rounder tubes ride better. Both can be designed to be laterally stiff and thus responsive. It’s up to riders what they prioritise more.

>>> Is a stiffer bike really faster?

The Aethos is unique in its frame design – and the weight drop absolutely represents new ground for mass produced bikes. However, checking the price tag of the Aethos S-Works models (£10,750), it’s worth noting that if you’re a fan of traditional round tubes and the ride quality they provide, you could go custom for less. Sure, you won’t get this brand new carbon tech, and the frame won’t be as light – but you can have it built exactly to your measurements if you’re prepared to put the time in. Aethos is a family of bikes, so more conservatively priced options are expected.

In terms of justifying the cost, the Aethos was three whole years of development in the making. Its RRP will be out of reach for many. However, in the same way that Shimano Claris has benefited from updates at Dura-Ace level, trickle down technology means that learnings made here will impact future design.

Will it crack?

History has taught us to associate low weight with high fragility. However, according to its creators, the manufacturing process – eg using unbroken plies – means that this bike is just as strong as any other in the range. The maximum rider weight is exactly the same as that of any other Specialized frame: 125kg.

At this point, we can only take the brand’s word for it. However, we have a model on test, and we’ll no doubt smash into a few potholes during the process in the interest of bringing you our own assessment. Watch this space.

Tarmac geometry

Image: Chris Sansom

The ‘fit geometry’ of this frame, reportedly, matches the Tarmac SL7. The front-centre, in a size 52, comes in at 477 on both, wheelbase matches at 975 and they both share the same head angle of 72.5º. The stack/reach of the Aethos is 527/380 vs 517/382 on the SL7. The brand says the differences in stack/reach are accounted for in the build – the Tarmac has an unmovable spacer at the bottom which makes the difference.

Check out the full review for more on the geo and set up.

Traditional set up

The Aethos has been designed to appeal to a discerning rider, and that discerning rider is more likely to do their maintenance at home.

As a result, you’ll see that whilst the cables are internal, they’re not routed through the handlebars or stem in any kind of head-scratching jigsaw manner. Personally, I’m in two minds here – yes, the build is easier, but fully hidden cables are a thing of beauty that I’ve become accustomed to.

The brand has used a round bar and separate stem on the S-Works model, only opting for the new ‘Alpinist’ handlebar on the limited edition ‘Founders’ edition. Presumably, this is a cost focused decision, however, personally I think the one piece solution would have resulted in a much cleaner look.

On a wholly positive note, the bottom bracket is threaded. Opting for press fit will have saved weight, but at the sacrifice of easy maintenance.

The seatpost is a standard diameter 27.2, making it easy to swap should you wish. In another win for simplicity, I noted that nearly every commonly adjusted bolt require a standard 4mm Allen key – there’s no infuriating random Torx bolts to interrupt your flow.

Interestingly, the fork has been designed only for 160mm rotors, in a move which dropped a few grams and represents rider preference. The Alpinist wheels, and 26mm Turbo cotton tyres, are not tubeless ready – a choice that goes against the grain – though there is space for 32mm tyres.

Would I recommend this bike?

It’s probably been clear from the outset that I’m a fan of this bike.

Whilst I’d make a few small spec tweaks myself, the Aethos’ frame responds exactly how I think frame should. For more detail, check out my full review here. 

This statement would logically lead to the expectation that I’d recommend this bike. And I would – unless you’re looking to win races. The Aethos ignores aerodynamics, and like the effects of aging, gravity and law of thermodynamics – we can’t deny physics.

If you don’t mind having to put out a few extra watts to pay for a joyful ride, I reckon you’ll love the Aethos. If you’re after raw speed, the Tarmac SL7 has to be the one. And if you want a compromise between the two? Well, I can’t help but feel that the sale rails of the outgoing Tarmac SL6 represent a very good hunting ground for a deal savvy shopper.

Prices and spec

The Aethos is a new model family. Specialized has initially announced only two price point options: the Founders (£13,000) and S-Works (£10,750). However, fear not – no family is complete without more accessibly priced models and we can expect these to land in the not too distant future.

Specialized Aethos – Founders Edition

  • Frame: S-Works Aethos FACT 12r Carbon, Threaded BB, Electronic cable routing only, 12x142mm thru-axle, flat-mount disc
  • Fork: S-Works FACT Carbon, 12x100mm thru-axle, flat-mount disc
  • Handlebar: Roval Alpinist Cockpit, one-piece bar/stem combo, carbon
  • Saddle: Body Geometry S-Works Power Arc
  • Seatpost: Alpinist Carbon Seatpost
  • Groupset: Shimano Dura-Ace R9170, hydraulic disc (inc Ceramic Speed jockey wheels, power meter, 52/36 chainrings)
  • Wheelset: Roval l Alpinist CLX, 21mm internal width carbon rim, 33mm depth,Win Tunnel Engineered, Roval AFD hub, 21h, DT Swiss Aerolitespokes
  • Tyres: Specialized Turbo Cotton, 26mm

Specialized Aethos – Dura Ace Di2

  • Frame: S-Works Aethos FACT 12r Carbon, Threaded BB, Electronic cable routing only, 12x142mm thru-axle, flat-mount disc
  • Fork: S-Works FACT Carbon, 12x100mm thru-axle, flat-mount disc
  • Handlebar: S-Works Short & Shallow
  • Stem: S-Works SL, alloy, titanium bolts, 6-degree rise
  • Saddle: Body Geometry S-Works Power Arc
  • Seatpost: Alpinist Carbon Seatpost
  • Groupset: Shimano Dura-Ace R9170, hydraulic disc (inc power meter, 52/36 chainrings)
  • Wheelset: Roval l Alpinist CLX, 21mm internal width carbon rim, 33mm depth,Win Tunnel Engineered, Roval AFD hub, 21h, DT Swiss Aerolitespokes
  • Tyres: Specialized Turbo Cotton, 26mm

Specialized Aethos – SRAM Red eTAP AXS

  • Frame:-Works Aethos FACT 12r Carbon, Threaded BB, Electronic cable routing only, 12x142mm thru-axle, flat-mount disc
  • Fork: S-Works FACT Carbon, 12x100mm thru-axle, flat-mount disc
  • Handlebar: Works Short & Shallow
  • Stem: S-Works SL, alloy, titanium bolts, 6-degree rise
  • Saddle: Body Geometry S-Works Power Arc
  • Seatpost: Alpinist Carbon Seatpost
  • Groupset: SRAM Red eTap, hydraulic disc (inc power meter, 52/36 chainrings)
  • Wheelset: Roval l Alpinist CLX, 21mm internal width carbon rim, 33mm depth,Win Tunnel Engineered, Roval AFD hub, 21h, DT Swiss Aerolitespokes
  • Tyres: Specialized Turbo Cotton, 26mm