Cervélo S3 Disc Ultegra Di2 2017

Aero performance meets disc brake stopping power with Cervélo's S3 Disc. If you want an aerodynamic road bike with disc brakes, you now have a few options to choose from, a significant increase from just a few years ago. The Cervélo S3 Disc is a fast choice: it has an insatiable appetite for speed and razor-sharp handling, but the ride is far from smooth.

Canadian brand Cervélo has long been at the forefront of aerodynamics. Its Soloist was arguably the first aero road bike and spawned many imitations over the years. Its aero road bike range is now spearheaded by the S5, the choice of Mark Cavendish, and is its most aerodynamic offering.

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The S3 trades some of that aero-ness for a more rounded package that balances weight, stiffness and comfort. And now it's available with disc brakes, as the company slowly but surely rolls out more disc brake options – first with the R3 Disc then the all-new C5 endurance model.

Frame design

Rather than just slap on a set, Cervélo has reimagined the S3 Disc to smooth out any penalties that might occur from adding disc brakes. The result is a frame that Cervélo claims is 9% stiffer, more aerodynamic to the tune of a 2-watt saving, and lighter by as much as 40g compared with the regular rim brake model. Small numbers they might be, but to have a disc brake bike that is lighter, stiffer and more aero than the same bike with rim brakes is impressive.

"We've taken everything that makes the S3 such an aero class leader and made it stronger, stiffer and ultimately, faster," says Cervélo. "Cervélo only launches a new bike when significant performance gains have been made, and by integrating our disc-brake technologies into the world championship-winning S3 we have done exactly that."

There are the now standard 12mm thru-axles at the fork and rear dropouts, coupled with flat mount disc brake callipers. The switch from quick release to 12mm thru-axles has, unsurprisingly, resulted in more stiffness. Cervélo says there's an 8% increase in head tube stiffness and 9% bottom bracket stiffness.

Another benefit of going to disc brakes is increased tyre clearance, with 25mm tyres fitting with ease (though it's specced with 23mm). You may wonder why not wider, but remember this is a race bike not an endurance bike, and 25mm is still the current standard for race bikes in the professional peloton (and where pros lead, amateurs faithfully follow).

Because disc brakes do undoubtedly add drag to a bike, Cervélo has focused on reshaping the frame to ensure it can level out the aero penalty. It has refined some of the key profiles to help the bike slice through the air more cleanly. Most of the saving comes at the front, with a redesigned fork, because it's the frontal surface area of the bike that is critical to good aero performance. Cervélo has reshaped the profile of the fork blades to reduce air pressure around the back of the crown and improve airflow.

At the other end of the bike, the seatstays and chainstays have been completely redesigned, taking lessons from the Rca and R3 Disc frame, along with new dropouts that accommodate the standard 142x12mm thru-axle setup. One thing that remains the same is the Dropped Downtube – with a chopped tail and the water bottles partially shielded from the airflow.

The result – as I said earlier, and according to Cervélo's claim – is a bike that's more aero than the rim brake version it's based on. Cervélo isn't alone in achieving this: Specialized has produced similar findings with the disc version of its Venge ViAS. So while disc brakes do add drag, savings can be made in the frame.

Internally routing the cables reduces drag, and here the hydraulic disc brake hoses are also hidden inside the fork and frame, only reappearing just before reaching the brake callipers. The cable routing has been future-proofed so any current or future groupset should fit with no problem. Because of the skinny aero shaped seat tube, the Di2 battery is concealed inside the down tube, with the added benefit of keeping the weight low in the frame.

The list of changes is impressive, but one area I'd like to see Cervélo improve is in its choice of 12mm thru-axles – the levers are large and ugly and there are more aesthetically pleasing options available. It's the one wrinkle on an otherwise good looking setup.

Away from the headline changes, there are some familiar Cervélo features: the BBRight asymmetrical bottom bracket, which is 11mm wider on the non-drive side and identical to BB30 on the drive side to make the frame tubes as wide as they possibly can be; there's also a tapered head tube, carbon fibre dropouts, and the aero seatpost is secured in place via an internal seatclamp.

Ride and handling

There's no doubting that all the development that has gone into new Cervélo S3 Disc has resulted in a bike that offers truly scintillating pace when you stand on the pedals. The high level of stiffness is detectable when you push the bike into a corner or leap out of the saddle to cover an attack; it responds with virtually no lag or hesitation.

Its mix of ultra-sharp responsiveness and pure speed is intoxicating. Pace comes easily to the S3 Disc and it winds up superbly, the speed ratcheting up with the sort of acceleration that only comes from the stiffest and more aero road bikes. Get into a fast pace line and the bike feels right at home, stable at high speed even in gusty conditions, and nimble enough to react to sudden changes of direction.

It's clearly a bike made for racing and riding extremely fast, and its razor-sharp handling demands that you have your wits about you. The geometry promotes a very head down position ideal for charging along at full gas. It's definitely not a bike for cruising around the lanes admiring the views and talking about the cricket.

It's right up there with the fastest aero bikes like the Specialized Venge, Canyon Aeroad and Trek Madone. Of course, road.cc has no wind tunnel to independently test aero products, but it's clear from my testing on familiar roads and set loops using power and heart rate data that the Cervélo S3 Disc is up there with the other aero bikes I've tested.

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Where the S3 Disc comes unstuck is in how it copes with rough road surfaces. While the aero gains are highly appreciated when you're riding flat out, attacking and hanging onto a fast group, it's simply too harsh on anything but a billiard smooth road surface. It's a jarring ride, and will have your fillings working loose on a long ride. It's such a shame as it's a flaw that makes the bike a little too demanding to live with on a daily basis – it feels like strapping on a rocket when all you want to do is go for a steady ride.

In an effort to improve comfort, I swapped the 23mm tyres for 25s (on Roval CLX 50 wheels that I'm currently testing) and ran them at a lower pressure. It made a small measure of difference but the overriding sensation was still one of a hard and overly firm ride on anything but the smoothest tarmac. And around where I live there isn't much of that at all.

And that's a real shame because the speed and responsiveness provide a real thrill for anyone who likes to ride fast, with the added security of brakes that work in all conditions. I suspect the unique 'hammerhead' handlebar might be contributing to the front-end harshness, and a change there would be an interesting comparison, but this is the bar it's sold with and there are only so many changes you can make while testing.

Compared with the Canyon Aeroad or Specialized Venge, the S3 Disc just isn't as accomplished at dealing with badly surfaced roads. There's little to separate the three bikes when it comes to pure speed and sprinting prowess, and each would be a fantastic race bike, but the Canyon and Specialized easily shade the Cervélo in how composed they are away from the smoothness of a dedicated race circuit, providing a more rounded package for when you don't have a number pinned to your jersey.

Equipment

You have a choice of two full builds: mechanical Ultegra with HED wheels for £4,249, or the bike tested costing £6,199, which upgrades the groupset to Ultegra Di2 with R785 brake levers and RS805 flat mount disc callipers with 160mm rotors.

The crankset deviates from the Shimano theme: a carbon fibre FSA SLK 52/36t specially adapted with a 5mm offset to provide the correct chainline with the S3 Disc's short 405mm chainstays. A KMC chain and 11-28t Ultegra cassette complete the groupset specification.

No expense is spared with the wheels. US-made Enve SES 3.4 wheels (not the brand new ones that have been launched this year though) certainly look the business and provide top-level performance, with a high degree of stiffness and superb aerodynamics. They're not the deepest rim that Enve offers, but the 1,421g weight is an added bonus when the road points up, contributing to the bike's all-round performance.

They're fitted with 23mm Continental Grand Sport Race tyres, but given there's space in the frame for 25mm tyres I'd seriously recommend upgrading them to eke out a bit more comfort. The Grand Sport Race is a mid-range tyre that doesn't cost the earth thanks to Conti's PureGrip compound, which makes the tyres more affordable than the excellent BlackChilli rubber. Rolling speed is good, traction is good in the dry and okay in the damp, and puncture protection is adequate. Still, good as they are, I'd rather see Continental's best tyres at this price – it's no place for skimping.

The bike is finished with a comfortable Fizik Antares saddle and FSA SL-K stem, and the slightly mad looking Cervélo Aero handlebar. Aero handlebars are all the rage right now and a common addition to an aero bike, as reducing frontal surface area is key to reducing drag.

Rather than opt for a one-piece bar and stem and accept its limitations in terms of adjustability, Cervélo developed an aero bar that fits a regular stem. That means you can easily change the stem length to suit, but does result in the odd 'hammerhead' looks. I didn't find the top of the bar the most comfortable place to lean my hands on either; maybe it's better if you have spades for hands.

Conclusion

There's a lot to like about the Cervélo S3 Disc. If you want pure speed with the reassurance of hydraulic disc brakes, it's a very good option: it's extremely fast and the handling is lively and direct – just what you want from a race bike – but its composure on rough roads falls some way short of its key rivals. If you're willing to overlook its lack of comfort, it's an explosive bike.

Verdict

Faster, stiffer and more responsive than the rim brake version, but comfort is sacrificed