Cervélo S5: The Bayonet Fork, the V-Stem and 6.3 Watts

The Front End: Bayonet Fork and V-Stem

The S5 keeps the two features that defined it and rebuilds the rest around them. The first is the bayonet fork. On a conventional road frame the steerer passes through the head tube and the upper and lower bearings sit inside it, so the head tube has to be wide enough to house both and the fork crown has to be shaped around that constraint. On the S5 the steerer runs outside the head tube, forward of it, and the fork legs and head tube can be shaped as one continuous surface. Cervélo has used the arrangement for a decade because it removes a compromise rather than a gram.

The second feature is the V-stem, now a one-piece bar and stem rather than two parts. Integration removes the clamp, the four bolts and the turbulence they generate, and it lets the bar's profile continue into the stem without a transition. The cost is fit: an integrated cockpit cannot be adjusted in length, so Cervélo builds the HB19 in 15 size combinations and resolves the choice at the point of purchase, through a dealer, rather than afterwards.

That is a commercial decision as much as an engineering one. A rider who buys the frame and fits a standard cockpit loses the aerodynamic work the front end was designed around. A rider who buys the complete bicycle and later wants a different stem length is buying a new cockpit.

6.3 Watts and the UCI Section Rules

Cervélo measures the current S5 at 6.3 W faster than its predecessor and 124 g lighter. Both figures belong to the complete bicycle, and both are small enough that the method matters. A 6.3 W gain at racing speed is roughly the difference between two tyre models of the same width, or between a clean front end and one with exposed cables. It is worth having and it will not be felt as a change in speed; it will be felt as a change in the effort required to hold a wheel.

The gains are concentrated at the front. The fork legs are deeper, the head tube is deeper, and the cockpit is integrated. The rear contributes through asymmetric seat tube and rear wheel cutout profiles, which is a departure: an asymmetric section can manage the flow on the drive side, where the chainring and crank disturb it, differently from the non-drive side, where they do not.

The sections themselves are constrained by the UCI's dimensional rules, which allow a maximum depth relative to width. Within that envelope Cervélo has increased surface area while reducing mass, which is only possible if the layup is doing more work per unit of area. The manufacturer states the frame sections are shaped to the current regulation rather than to a target coefficient.

Asymmetric Rear Profiles and the 57|64 Wheelset

The wheelset is the Reserve 57|64 Turbulent Aero, developed alongside the frame and built on DT Swiss 180 hubs. The name refers to Reserve's programme of modelling the flow as it leaves the tyre and rim rather than only as it meets them. A 57 mm front and 64 mm rear is a deeper pairing than the 48|58 of an all-round frame, and the differential is deliberate: the front wheel steers, so it is shallower; the rear does not, so it takes the deeper section.

Tubeless compatibility matters here more than on a climbing wheelset. A deep rim at 29 mm of tyre width carries enough air volume to run lower pressure, and lower pressure is what keeps a deep section from feeling harsh on the road surfaces where an aero bicycle spends most of its time.

The frame is designed around 29 mm tyres rather than the 26 mm of the previous generation. That change is not cosmetic: a wider tyre changes the rolling radius, which changes trail, which changes steering. Cervélo adjusted the geometry to keep the trail figures where they were, so the handling of the current bicycle is intended to match the previous one despite the wider tyre.

Fit, Mass and the Purchase Decision

The reference build is 7.25 kg without pedals in size 56, on SRAM RED AXS E1. That is not a light figure for a bicycle at this price, and it is the honest consequence of a frame built for aerodynamic shape rather than for minimum mass: deeper sections carry more material, and the integrated cockpit and deep wheelset add where a climbing build would subtract.

Fit is the decision that matters most with this frame. The integrated cockpit means the position is set when the bicycle is ordered, and the 15 combinations exist so that it can be set correctly. A rider who is between sizes should resolve the cockpit length first and the frame size second, because the frame can be made to fit with spacers while the cockpit cannot be lengthened.

At $13,000 or €12,999 the S5 is the least expensive way into a current WorldTour aero platform with a wheelset designed for it. The question to settle before ordering is not whether the bicycle is fast. It is whether the position the cockpit locks in is the position the rider will still want in three seasons.