The P4 Fork - a Prodigious Father

With the P4, Cervélo introduced a new aero fork: the FK25. Since the P4’s launch in 2009, that fork has fathered several children that benefit from its great aero genes. This article explains the differences between those forks.

Cervélo Internal Engineering Codes
FK25 is Cervélo’s internal engineering project code for the P4 fork. “FK” stands for Fork, and 25 means this is the 25th design. FK numbers are assigned at project initiation, not completion. This is why FK numbers don’t always match the order in which they were released. In several cases, different alternatives were tried to improve performance but the quest for speed always lead back to the FK25. In the end, all the forks in this article ended up having a common parent in FK25.

The Cervélo P4 fork: FK25
The P4’s FK25 fork represents the culmination of three years of aero engineering research, which is a whole story in itself. With the help of MIT aerodynamicist Dr. Mark Drela, we started with the Wolf series of forks and performed multiple CFD analyses to benchmark existing aero forks both UCI legal and not, both new and old. (Some of these forks are 3T, Blackwell, Carbonaero, Edge, Hotta, Lotus, Oval, Reynolds, Trek, Wolf, etc..)
 
Testing new fork concepts required the creation of a series of specially made prototype forks constructed with a Rapid Prototyping process called stereolithography (SLA). SLA allows us to start from a computer design and build a complete fork for windtunnel testing in a few hours (as opposed to days or weeks to machine a mold and build a production sample). We made many trips to the wind tunnel to understand the multitude of parameters affecting a fork’s aero performance. We studied the effect of spinning wheels and how the drag was affected by fork blade spacing, crown width, how the fork & wheel matched the frame, dropout offset, fork blade airfoil cross section shape and how they should vary along the blade length, variations in thickness and chord, as well as various degrees of trailing edge truncation. We also designed, prototyped, tested and patented several forks with integrated brakes of various designs.
 
At each stage we tested, analyzed, and selected the lowest drag combination of features and dimensions. The resulting FK25 fork is the lowest drag fork we’ve ever measured, with an improvement of 15-20 grams less aerodynamic drag than the previous generation of aero forks (including UCI non-compliant forks), which results in 1.5 to 2 Watts power savings at 40km/h. This is the FK25, the fork used on the P4 and the “father” of a family of super-aero Cervélo forks.

 P4 Fork fig 1

Figure 1 Cervélo’s P4 aero fork, project code FK25

P4 Fork fig 2

Figure 2 The leading edge profile of FK25’s fork crown matches the P4’s aero head tube profile exactly.

 

Cervélo P3: FK26
With so much technical effort invested in the P4’s FK25 aerodynamics, it was obvious we needed to pass that advantage on to other fork models. Starting in 2009 we designed those aerodynamics into a fork for the 2010 P3, and in 2011 the P2 got it as well. FK26 has exactly the same aero shapes as its father including the dropouts, the blades, and the sides and back of the fork crown. The only shape difference is the front of the crown, which matches the P3’s round head tube, whereas the FK25 leading edge matches the P4’s. There is no measurable difference in aero drag between these two forks since the front brake caliper obscures this part of the fork. This fork is highly interchangeable and makes a great aero upgrade for any bike. Ask your Cervélo retailer to order one for you!

 P4 Fork Fig 3

Figure 3 Cervélo’s FK26 aero fork. Note the front of the crown has a normal round shape, not the P4 frame’s shape.

P4 Fork fig 4

Figure 4 Cervélo’s FK25 (left) and FK26 (right) differ in the crown’s leading edge: FK25 matches the P4’s head tube, and FK26 matches round head tubes like the Cervélo P3, P2 and many other bikes.

Cervélo S5: FK26SL
The Cervélo S3 aero road frame had established itself as the gold standard in aero road bikes against which all others were measured. So when we came to design the next generation aero road bike, it had to be a significant leap forward in aerodynamics but still had to be laterally stiffer and lighter than the last generation. It is no surprise that we used the most aerodynamic FK25 fork we had as the starting point.

The S5’s FK26SL fork shares the same mold (and therefore the same aerodynamics) as the P3’s FK26, but the type of carbon fiber and layup are tuned to save weight (SL = Super Light) while still retaining the same stiffness. With the SL, we created a 53mm offset fork used on the 48 and 51cm sizes of the S5. The 43mm offset is still used on other frame sizes.

Cervélo S5 Team & S5 VWD: FK26UL
Like the SL, the Ultra Light version also shares the same mold (and therefore the same aerodynamics) but uses further material and layup developments to create the lightest fork in the series, still with no compromise in stiffness compared to the standard FK26. Just like the SL, the 53mm offset is used on the 48 and 51cm S5.

P4 Fork fig 5
Figure 5 Cervélo’s FK25, FK26 and FK24: P4, P3 and T1 forks. FK24 (far right) is for track bikes, so has no brake hole.

P4 Fork fig 6

Figure 6 Another detail trackies appreciate: the aluminum dropouts on the T1’s FK24 fork (far right) are tough enough to survive multiple wheel changes and big enough to fit 22mm diameter washers on track axle nuts.

Cervélo T4: FK27
For the Cervélo T4 carbon track frame, we again designed another derivative of the P4’s FK25. This one keeps a “standard” 43mm fork offset to match the T4’s head angle and has an aero leading edge on the crown to match the T4’s aero head tube (but there’s no brake hole in this fork). FK27 also uses aluminum dropouts (to survive multiple wheel changes) and has enough room for large 22mm OD washers on track axle nuts. And like FK24, it features the same laterally stiffer lay up as the T1’s FK24.

Interchangeability
Since the steerer tubes of all these forks are the standard 1 1/8” diameter, they make a great aero improvement to virtually any bike. If you are upgrading your Cervélo, you only need to know that because the back of the crown is the same shape as the P4’s FK25, they can all be installed in the P4 and S5 frames with no mechanical interference and perfect aero integration.

Summary
The original FK25, which was designed for the P4, has now spawned a family of 6 different forks which share its super-aerodynamic design. The family’s aero performance advantage is sizeable--approximately 2 Watts versus the last generation of Cervélo aero forks. That advantage was achieved through years of work in the wind tunnel and behind the computer using CFD modeling. All of the designs are UCI legal, and are still faster than any other fork we have ever measured, legal or not. The result is that this family of forks has dominated competition in all levels of triathlon and most recently helped Team Garmin-Cervélo to 3 stage wins at the 2011 Tour de France. Certainly, a design for the ages!

Summary Table

Fork Aerodynamics Model Offset, mm  Brake Hole? Max. axle nut diameter, mm Crown Leading Edge Shape Crown Trailing Edge Shape Layup Dropout
FK25 Optimum P4 43, 650Cx40 Yes 20 P4 aero P4/S5 P4 Carbon
FK26 Optimum P2, P3 43 Yes 20 Round P4/S5 Heavier Carbon
FK26SL Optimum S5 43, 53 Yes 20 Round P4/S5 Super light Carbon
FK26UL Optimum S5 Team, S5 VWD 43, 53 Yes 20 Round P4/S5 Ultra light Carbon
FK24 Optimum T1 35 No 22 Round P4/S5 Stiff Aluminum
FK27 Optimum T4 43 No 22 T4 aero P4/S5 Stiff Aluminum
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