Cervélo S3 Ultegra Di2 Disc review

Cervélo's 'baby' aero bike has less integration than the S5, but is that such a bad thing?

cervelo s3 ultegra di2 disc
Aero shaping makes the S3 a slippery customer
Cycling Weekly Verdict

If you are looking for a bike with impeccable aero credentials whilst enabling you to get to the end of a long day in the saddle without feeling battered and broken the Cervélo S3 might just be for you. Cervélo's quest to balance stiffness with comfort has yielded a machine that is ruthlessly efficient at translating your power into sustained momentum albeit with some loss in out and out performance. If you want world class sprinter levels of BB stiffness or mountain goat climbing prowess you might want to look elsewhere, but if long solo miles are your thing or you fancy yourself as a breakaway specialist then this might just be the perfect bike for you.

Reasons to buy
  • +

    Aerodynamic advantages are truly noticeable

  • +

    Improved comfort over previous version

  • +


Reasons to avoid
  • -

    Quill stem design is a little tricky to work with

  • -

    RAT axles begin to frustrate over time

You can trust Cycling Weekly. Our team of experts put in hard miles testing cycling tech and will always share honest, unbiased advice to help you choose. Find out more about how we test.

Cervélo has given its S-series bikes an overhaul, redesigning both the S5 and the S3 with a particular focus on frame stiffness and aerodynamics. The S3 is only available in two complete builds or a frame only package. We tested the top-end electronic Ultegra Di2 equipped version.

The frame

Unlike the new Cervélo S5, the latest S3 is available as both a rim brake and disc brake bike. The new disc brake version uses Cervélo's True Aero technology, which is the combination of specific tube shapes, sizes and designs to improve the aerodynamic flow across the bike. It's a similar story at the front end, and the bike has new frame and fork shaping and all the cables are tucked neatly inside the cockpit – possible because of a new upside down quill stem.

>>> Best road bikes

Cervélo has quoted numbers for the reduction in drag over the old frame with a saving of 13 watts.

Aero shaping makes the S3 a slippery customer

So, just like with the Cervélo R5 and the Cervélo R3, Cervélo has tried to create a distinct ride quality between the more aggressive S5 and the new S3. For example, the S3's stack is 16mm higher than the S5. The new bike also has slightly skinnier seat stays and wider tyre clearance, both for improved vibration dampening. In fact, the S3 has identical geometry to the Cervélo R3, a bike we rate for it's ride position and performance.

Additional frame changes include a dropped down tube and the rear stays have also had a tune up to help make it more aerodynamic.

>>> Which Cervélo road bike is right for you: here's our handy buyer's guide

cervelo s3 ultegra di2 disc

Dropped seat stays and aero profiled tubing abound on the S3

Cervélo has also stuck to it's BBright press-fit style bottom bracket arrangement. We have had mixed experiences with this system and have found, like almost all press-fit bottom bracket that it would develop a creak after a few months of testing. Options are available to improve on this but it would have been good to see Cervélo reverting to a threaded bottom bracket to increase reliability (and adding to customer reassurance).

The Components

Cervélo has decided that Shimano's second-tier electronic groupset is more than good enough for it's top step S3, a spec choice that we are inclined to agree with. Ultegra 8070 Di2 works flawlessly and it's only a few material and manufacturing process changes that separate it from Dura Ace. A blindfold test would make it very hard to differentiate between the performance of the two.

There are no odd gear ratio choices either. A semi-compact (52/36) chainset and 11-30 cassette make for a very good choice for a bike of this calibre and speed potential.

Ultegra Di2 works flawlessly. Gear ratios are befitting a bike of this calibre.

As the name denotes, this particular S3 is the disc specific version and as such comes with Shimano Ultegra hydraulic disc callipers. These are flat mount for ease of adjustment and combined with the 160mm rotors provide plenty of stopping power. A little brake rub was noticeable when riding hard out of the saddle so there is some flex in the system somewhere, however this is more likely to be the frame than the brakes themselves. They have proved to be noisy when wet or dirty so it's imperative that you keep the disc rotors clean to ensure good performance without embarrassing your riding buddies.

Disc brakes have more than enough power but can be noisy.

The wheelset is a notable part of the build kit as it features Cervélo's in-house 1AER D5 carbon rim. The rims are 48mm deep, tubeless ready and have a middle-ground rim width that works well with the Vittoria Rubino Pro 25c tyres fitted as standard. The rims are laced to ultra-reliable DT Swiss hubs which have not missed a beat and have kept true during the testing period.

I'm not normally a fan of the Vittoria Rubino as there are much better tyres on the market but they have proved to be a good fit for the S3. I took the liberty of setting up the wheelset tubeless, a task that was fairly straightforward and it has proved to be reliable. Case in point was riding the UKCE Wiltshire Wildcat sportive recently, where the recent bad weather and beautiful but narrow country lanes turned it into a puncture-fest for many riders. I happily sailed through the worst of the debris without worry or any damage to the tyres.

>>> Converting your bike to tubeless: out handy guide

The S3 makes use of Cervélo's unique ST29 stem and fork combination to create an incredibly clean front end in terms of integration. It works in  way similar to the quill stems of old, in that the stem itself wedges into the shortened steerer tube of the fork and uses an expander system to hold things in place. This allows the stem to adjust stem and bar height without needing to cut the steerer tube, so you can run a slammed setup for racing or raise the height for longer rides. Split spacers can be placed underneath the stem when a taller position is required. It is a slight faff to adjust as the internal bolts can be difficult to locate but it does prevent an unsightly spacer stack on top of the stem.

cervelo s3 ultegra di2 disc

Cervélo's ST29 stem acts like an old school quill stem enabling the user to adjust bar height without leaving an unsightly spacer stack on show. Split spacers are easy to remove.

The stem and the comfortably shaped carbon Cervélo handlebar have a standard fitting so you can swap to any other brand of handlebar if you so wish, so the integration doesn't go too far in my opinion.

The fly in the ointment is the adoption of the RAT axle design. When it works, this system is excellent but with continued use it begins to be less effective and on some occasions downright infuriating to correctly engage the axle and get the bike rolling again. This is down to the slotted end of the axle wearing if not engaged perfectly upon each use - something that sometimes can't be avoided. Going to a standard axle arrangement or even the Mavic style would be better for long term reliability.

RAT axles are a little hit and miss.

The ride

I was expecting the S3 to be pretty speedy, it is after all an aero race bike, but to be honest I have been blown away by the sheer rapidity of the package. Even on the first shakedown ride around my normal test loop I gained a few PBs almost without effort and every time I looked down at the Garmin I seemed to be a good 1-2mph faster than the effort level warranted. This continued on every ride. The boost is provided in a different guise to the advantage of a lightweight race bike such as Cannondale's Supersix EVO; a bike that encourages you to push harder all the time. The S3 just has the knack of relentlessly maintaining speed and efficiently helps you to maintain this momentum.

I took the S3 on a 75 mile lone break at the recent UKCE Wiltshire Wildcat sportive event.
(Image credit: Andrew Hobbs Photoqraphy)

I wouldn't say the S3 is the most engaging bike I have ridden, it offers a slightly delayed response time when sprinting hard out of the saddle that could be down to Cervélo trying to balance the stiffness to comfort ratio of the S3's frame. This slightly watered down level of frame stiffness also translates into a less responsive climbing performance when out of the saddle too. That's not to say it's a poor climber, not by any stretch of the imagination, but it certainly rewards a steadier in-the-saddle climbing technique. The overall weight does come into play a little with regards the S3's climbing prowess but it certainly isn't overly bloated when compared to other brand's equivalent aero machines.

Sit on the saddle and keep the power on is the best way to get the S3 climbing fast.
(Image credit: Andrew Hobbs Photoqraphy)

Probably the simplest way to sum up the S3's performance is to say that if I were a breakaway specialist at a stage race then this would be the bike I would choose. It's not light enough to be a climber's bike and it's not stiff enough in my opinion to appeal to a big sprinter. What it is is incredibly efficient at translating effort into momentum and maintaining that speed better than most bikes in its class. Not only that but the S3 has to be one of  the most comfortable aero designed bikes I have ridden long distances on - trait that certainly boosts its appeal for the average rider not keen on raw, brutal power delivery.

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