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The P4 Fork - a Prodigious Father

October 15, 2011 - Filed under: Ask the Engineers

Comments (34)

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 a 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 and 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.

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

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!

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

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 more 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.

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.

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

Table 1, List of FK25-derived Cervélo fork technical attributes

Written by Damon Rinard

34 responses for this Entry

Damon Rinard, Cervelo Engineer says:

April 09, 2012

Hi Samuel, The P5-Three fork is both similar and dissimilar to the FK26UL. The two forks are similar because they share the same low-drag fork blade cross section, so both have excellent aero performance in the legs. They are dissimilar though, in that the P5-Three fork is designed to work aerodynamically with the P5 frame, so the shape of the fork crown is aero matched with the P5's head tube and down tube. In contrast, the FK26UL is matched to the S5's head tube and down tube. They are not interchangable: each fork would interfere with the other frame. Cheers, -Damon Rinard

Damon Rinard, Cervelo Engineer says:

April 09, 2012

Hi Paul, Yes, yes and yes: Yes you can upgrade your P3 to the P4 fork, yes it will fit and yes you will benefit. The only thing is, the P4 fork has the cosmetic point on the front of the crown. It matches the P4's head tube profile, not the round profile of your P3's head tube. There's no functional downside to installing the P4 fork in your P3 (the front brake hides the fork crown from the wind anyway) but there is a better fork for your P3: FK26, identical in shape (so identical in aero performance) except the front of the fork crown matches the round profile of your P3 perfectly. Cheers, -Damon Rinard, Cervelo Engineer

Samuel Lai says:

April 06, 2012

May I ask how the Three is different and similar to the FK26UL? Is the former better suited to the P5? Please reply. Thanks!

Paul Ganniclifft says:

April 05, 2012

Hi, I have a cervelo p3c and need new forks, can i upgrade to p4 forks? will they fit? Will I Benefit? I would be very grateful of any feedback. Kind regards, Paul.

Damon Rinard, Cervelo Engineer says:

March 05, 2012

Hi Samuel, The P5 Three fork uses the same 3:1 (hence the Three in the name) aerodynamic fork blade profile as the P4 family of forks. It's made in only one offset, 43mm. It's drilled for a standard brake hole and accepts up to 20mm axle nut and skewer nut diameters. The crown leading and trailing edges are aero matched to the P5 frame and the dropouts are carbon. The P5 Six fork is the same, but the fork blade chord is twice as long, reaching 6:1 aspect ratio (hence the Six in the name) to save 10 more grams of aero drag. Cheers, -Damon Rinard, Cervelo Engineer

Samuel Lai says:

March 02, 2012

May I ask for the summary table of comparative data on the Cervélo P5 Six?

Damon Rinard, Cervelo Engineer says:

December 20, 2011

Hi Justin, We haven't tested them back to back, but based on other testing we've done I'd say they're about tied. Of course on a P3 you'd want to get into a more aero position, which then makes it clearly faster. Cheers, -Damon Rinard, Cervelo Engineer

Justin Huang says:

December 20, 2011

S5 and P3 with same rider, riding in the same position, which one is faster?

Damon Rinard, Cervelo Engineer says:

December 09, 2011

Hi Samu, Based on our wind tunnel testing your girlfirend could save about 0.1 second per kilometer. Your experience with the fork offset and handling is normal: we tune the dimensions for best handling, but even large deviations (10mm for offset, for example) are often no problem once the rider adjusts and develops a few new steering reactions. This happens within a few minutes to an hour or so, if you take the bike to all expected riding situations (low speed, high speed, etc.) After all, the rider steers the bike! The different frame and fork dimensions just make it a little easier. Cheers, -Damon Rinard, Cervelo Engineer

Samu Ilonen says:

December 09, 2011

Hi. I have put order for T4 since I sold already my T3. I really hope get it...but my girlfiend is more serius about track and she has 51cm T3. I had FK24 already in my previus T3. Ok, it makes it faster to turn but it was ok to me anyway. So now we changed FK24 also to my girlfiends T3 from Funda. Can you give any estimation how much time it should save /km, say @ 45km/h? Ok, I should order also T4 also for her but, I'm slightly counting money here... :D

Damon Rinard, Cervelo Engineer says:

November 23, 2011

Hi Neil, You're right that changing fork length changes head angle. However, every millimeter changes the angle just 0.057 degrees. But the FK25 (& family) forks are not shorter than normal, it's the FK30 family of forks forks that are *longer* than normal. So the FK25 series forks can be freely interchanged with all Cervelo road & tri bikes with 1 1/8" steerers, and many other brands of bikes too. For more info, see "Effect of Changing Fork Length" on Sheldon Brown's web site http://sheldonbrown.com/rinard/forklengths.htm Cheers, -Damon

Neil says:

November 22, 2011

Just read the article and comments. Seems to me that knowing the S5 fork is 5mm shorter, it may not be wise to put this on earlier version of the S series bikes or at least not without calculating trail. I've always thought every mm shorter your fork is, your head tube angle gets .1 degree steeper. Probably not bad on the larger sizes but 54 and down could be noticeably worse.

Damon Rinard, Cervelo Engineer says:

November 07, 2011

Hi Glen, Thanks for the love and I hope you keep enjoying our Ask the Engineers posts! -Damon

Glen says:

November 04, 2011

Hey Damon, It sounds like you guys over there at Cervelo have got your Stuff together. Keep up the good work and I love the "Ask the Engineers" section on your web page.

Damon Rinard, Cervelo Engineer says:

November 02, 2011

Hi Justin, We have no plans for an S5ca. Instead, the S5 VWD uses all the Project California technology to be the lightest S5 ever. Cheers, -Damon

Justin Huang says:

November 01, 2011

Hi, Damon. Will cervelo release S5 California or something like that?

Damon Rinard, Cervelo Engineer says:

October 25, 2011

Hi Andrew, I don't know yet when the details will be up on the web - but keep checking back. The T4 uses the same mold as the P4, just as the T3 uses the same mold as the P3, so the aero improvement also transfers. I can confidently say the T4 will be the fastest track bike in the world. Cheers, -Damon

Andrew says:

October 25, 2011

Hi Damon - are you able to share any info on the T4 - when will the details be up on the bikes page? Is there a significant reduction in drag over the T3? Thanks

Damon Rinard, Cervelo Engineer says:

October 24, 2011

Hi Justin, We're molding T4 framesets now and expect the first batch to be ready to ship in Q1 2012, in time to supply a few national federations' Olympic squads. We've planned 100 extra framesets beyond that number so there should be one available for you if you act quickly. Thanks for the Cervelo love, -Damon

Damon Rinard, Cervelo Engineer says:

October 24, 2011

Hi Ron, Aerodynamicaally, the Funda is about the best fork short of the P4's FK25 & descendants. The stiffness is about the same too, so aero is the best reason to upgrade. -Damon

Damon Rinard, Cervelo Engineer says:

October 24, 2011

Hi Gord, Depends on how close the weight and aerodynamic differences are. Remember, even in the shelter of the peloton you still feel 65 to 75% of the aero drag you 'd feel alone. For example, instead of saving 9 Watts riding an S5 versus an S3, you'd save about 6 Watts. (The numbers are closer to 30 and 20 Watts versus a typical non-aero road bike.) You'd have to have a frame that's a lot lighter to save 6 Watts. -Damon

Justin Huang says:

October 22, 2011

When will cervelo T4 release? I have got a T3 already, i love cervelo so much.

Ron says:

October 21, 2011

Hi Damon I assume based upon your comment and the article you tested the Funda Pro 3T fork. It sounds like the differenceis small, about 1 watt at 40 kph. From other fork characteristics is there any significant difference to up grade from the Funda to one of the FK series?

Gord says:

October 19, 2011

Other companies are now offering aero road bikes that claim to be almost as fast as a Cervelo yet much lighter than the S5. If you are riding in a group, wouldn't you want the lighter option that is close in aerodynamics?

Damon Rinard, Cervelo Engineer` says:

October 19, 2011

Hi Andrew, This thread on the Cervelo Forum http://forums.cervelo.com/forums/p/5694/40713.aspx has some photos of the T4 track bike Theo Bos rode at the Rotterdam 6 day when he rode for the Cervelo TestTeam. The production version coming up uses the same mold, which has been remachined at the bottom bracket gusset to smoothe it into the down tube a bit, just like the Cervelo P4's water bottle area. We're molding T4 frames now, and expect to be able to have enough to supply a few national federations in time fot eh 2012 Olympics. I hope we will have enough to sell some to individuals! Check with your Cervelo retailer to have them contact us if you're interested. Cheers, -Damon

Damon Rinard, Cervelo Engineer says:

October 19, 2011

Hi Alex, Yep, all these forks would fit your 2009 Cervelo S2. Obviously you won't want the track forks as they can't accept a front brake, and the P4's FK25 would work mechanically but would have a cosmetic mismatch at the head tube. The best candidates are the P3's and S5's forks: FK26 and the SL and UL variants, all in 43mm rake.

Damon Rinard, Cervelo Engineer says:

October 19, 2011

Hi James, I'm glad you asked about putting a brake hole in a track fork. Don't do it! The brake hole is co-molded in the road forks and includes carbon reinforcements that are integral with the fork's structure. Simply adding a hole (e.g., by drilling) would weaken the fork. Instead, may I suggest using the road version so you can install a front brake. back in my university days, I did the same as you propose - ride the track bike to race at the track - and it worked well for me. I simply unbolted the brake and lever after I arrived at the track. But please use a road fork for this. Cheers, -Damon

Damon Rinard, Cervelo Engineer says:

October 19, 2011

Hi Dalton, Your 2010 Cervelo S3 came with a 3T Funda fork, also a quite a good design. In fact the Funda shape is among the slipperiest of the last generation of aero forks. Upgrading to an FK26 (or SL or UL) would save about one Watt or so at 40 km/h. -Damon

Andrew says:

October 18, 2011

Hi Damon - you mention in the article the exisitence of the T4 track bike. Are you able to share any details? Post any pictures?

Alex says:

October 18, 2011

Hi Damon, Would this fork fit my 2009 Cervelo S2? Alex

James Liu says:

October 18, 2011

How much would it hurt to put brake holes in the track forks? I want to be able to ride my track bike to the track, kind of hard to do in traffic without brakes. (Not impossible, I know, but my track is 20 something miles away)

Dalton says:

October 17, 2011

Hi Damon, What fork came on the 2010 S3 SRAM red edition? What would the watts saved or drag improvements be if i upgraded from the stock fork to the FK26UL at 40km/hr? Thanks

Damon Rinard, Cervelo Engineer says:

October 17, 2011

Hi Ricky, Yep, any of these forks could be installed in an S3. Avoid the track forks (they have no brake hole!). The P4 fork, FK25, works fine on an S3 mechanically, but the shaped leading edge would be a cosmetic mismatch. But all the other forks would work perfectly. FK26, FK26SL and FK26UL are the prime candidates. Your S3 came with 43mm offset for perfectly stable handling, but some riders (Carlso Sastre for one!) prefer the 53mm offset for a little quicker steering response. -Damon Rinard, Cervelo Engineer

Ricky Garcia says:

October 16, 2011

Could any of these forks be used to upgrade an S3 frameset?