I walked by the P5, and was nearly sucked into the back wheel.

I've heard of several other athletes saying the same thing!

Interesting topic, but you already admitted that we're debating several seconds over 40k. I think I'd rather get back on my trainer and increase my cycling volume as it will most certainly benefit my season more than engaging in speculation on the P5's yet-to-be-published aerodynamic drag numbers.

Sorry if that sounded too "DesertDude/The Authority/ TriJerk9000" esque....

David,
What's your opinion, as an aerodynamics engineer, on TM01 profile (the features that BMC calls "truncated profile" and "trip wire")? Do you think they contribute to performance as BMC claims, i.e. layer separation point moved further back, and "better rigidity with the same aerodynamics"?

Hey David,

Interesting read for sure, thanks for your insight. Once again the conclusion seems to be that the fastest bike is the one that fits you best. But obviously all this marketing deserves a little scrutiny and analysis. What are your thoughts on P5 vs the Shiv Tri with respect to Aerodynamics?

Chris

PS, I'm currently a sophomore doing BS in AE at USC... Fight On!

As I am also an aeronautical engineer by trade, I'd like to point out several things. Aerodynamics for a bike are very different than they are to aircraft as the Reynold's Numbers are totally different. In that respect, understanding the rotational airflow aspects takes a lot of trial and error and not as much theory, although every implementation is based upon a concept. In Cervelo's findings, the fork profile on the inside and outside are likely optimized for a specific wheel or type of wheel that may be different to Trek's (HED 3?) while trying to find an optimal structural and aero width to integrate the Magura brake. The Cervelo P5 uses different airfoil sections depending on the local flow quality. In the front, sharp tailed airfoils abound, whereas the seat tube uses a Kamm tail. The flipped up seatpost clamp is also very, very interesting. CFD would be vital to see how this redirects flow upwards. The shape of the airfoil itself and the skin maybe treated specifically with regards to the quality of airflow that has become turbulent from frame/wheel/ and body parts from the front end of the bike. Logically, this makes a whole ton of sense, although it can be difficult to simulate and design. It takes a lot of specific good correlation from the CFD and wind tunnel. In Formula One, they'd be using gallons of flow vis to see how boundary layer reacts and where the air becomes turbulent.

Last but not least. Techniques such as trip wires and Kamm tails can be used to great effect within the regulations, but a good standard airfoil shape can be modified to have similar effects with different geometry. Some non-intuitive aero devices that come to mind are Gurney flaps

You're showing your colors there. Did anyone at Trek send you?

The Cervelo wins there. The most common yaw angles are very low, and the Cervelo is designed with that in mind. If the SC is designed for a wider range, then that means that is losing out on the optimization for the most common yaw angles. So bascally the SC is great for really slow people that ride with pependicular winds. Great...

Ummm...not really. That's a bit of a red-herring statement. What exactly does "fits you best" mean?

After all, "Bikes are highly adjustable, and humans are highly adaptable", and all that... ;-)

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http://bikeblather.blogspot.com/

Considering the size of the clamp and the severe turbulences that will take place in that zone caused by legs' movement , I don't think that the clamp could play a role in the direction of the flow. Maybe testing the bike with the DZ dummy, some flow is redirected away of the legs anf frame's rear end but in real conditions I don't see that happening.

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http://cds-0.blogspot.com

You can fit anyone to almost any bike. With an adjustable enough seatpost and the right stem, the bars and saddle can be positioned properly relative to the BB (this is basically what everyone did in the 80's while trying to get comfortable on the aerobars on bikes designed for drop bars). Granted cornering/handling will be sup par, but the bike will "fit."

Looking at "fit" from an aero perspective, frames are quite aero, but stem spacers are not. It's better to have a taller frame with the stem slammed down, than a smaller one with stacks of spacers (or a taller "stem system" in the case of the new superbikes). The frame that fits best from an aero standpoint is the one that (when you place the saddle and bars where you need them) allows the bar setup to sit as level as possible with the toptube. Since there is no standard for bike sizing, some frames will require more spacers than others; this damage to ideal aerodynamics usually exceeds the differences in the frames themselves.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Supposedly no longer true with the P5 + Aduro. Damon has posted multiple times saying they evaluated different sized frames for the same rider and there was no significant difference so you should however you like if you fit multiple frames.

Stuff like that is why I like to see the actual data. Suppose your position is a medium with a medium bar. You could easily fitr a large with a low bar or a small with a high bar. I have a hard time believing that all three bikes would have exactly the same drag curve at all yaws. I'm certain the differences would be tiny, but this is ST we argue over 2-3 seconds in a 40km race like its life or death.

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Styrrell

Nope nobody from Trek, although the sale price of my Madone was the biggest factor in my purchase and it's treated me well through thousands of miles. The Speed Concept was chosen based on quite a bit of research and also the price-point when I purchased it (on sale). Regarding Yaw angles "wider" means up to around 15-20 degrees, which is far from perpendicular as you seem to interpret it. And the difference at 0 degrees will me minimal, along the lines of ~1%, the Speed Concept had lower drag than the P4 even at 0 degrees so your claim is invalid.

justkeeppedaling, thanks for the input. I think the changes in fork design are partly due to the evolution of wheel shapes ( Zipp firecrest for example), and I'm looking forward to seeing any CFD or wind tunnel data on this. I also agree with your conclusions on trip wires and other aero modifications, there are many designs and methods that can be used to reduce drag and improve stability etc. the key is finding the design that meets the most amount of desired properties (aero performance, stiff, light, etc.)


And in my opinion, all the major brands coming out with super bikes is a great trend. Regardless of the argument of which is best, this is a great trend for TT/Tri bikes, the Shiv Tri, P5, TM01, and Speed Concept are ALL amazing bikes and I can't wait to see what the next generation will look like.

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David
2007 Trek Madone 5.2 Discovery Channel
2011 Trek Speed Concept 9.5

One can test a bike for aerodynamics. One can even test with a dummy (not me, a mannequin). Even switch to a moving dummy, and how about adding a rolling road rather than a fixed floor.

For the same bike, there will be dramatically different results.

Of course, different bikes are optimized for different yaw angles.

As far as I'm concerned, all modern properly aero bikes are waaaaaay better than bikes used to be. I see the development in several areas.

1) Tri position (that was a giant leap forward) (QR Zero Gravity)
2) Aero frame tubes (Cervelo P2, then P3)
3) Aero Frames such as the P4 and imitators
4) Aero bikes such as the P5, Shiv and such, complete with "integration" of cables etc.
5) Not sure, but I can hardly wait to see what is next.


However, when I think about bikes and their development, I can't help but think of local Legend Walter Thorburn. He held the 1/2 IM in Tauranga for I think 14 years. He rode that record on a round tube bike in a speedo! We have some pretty hot triathletes here in NZ that competed on that course, all without beating his record. With all the whizz bang investment in bikes worth the price of a not so bad car, it's amazing how little difference the flash new bikes have made. Check out the results (and bike splits in particuar) of your nearest long term race and see how much we've improved in 25 years, and it's kinda depressing. That's one of the reasons why I'm still riding my high mileage Cervelo P2K with HED3's because it ain't the bike that's holding me back, and off the podium. (I do lust for a P5 tho :-)

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TriDork

"Happiness is a myth. All you can hope for is to get laid once in a while, drunk once in a while and to eat chocolate every day"

Err, no it doesn't. Cervelo, Trek and Tour Magazine all agree that the P4 has lower drag than the SC when the P4 has comparable bars fitted. The only way you could conclude that the SC has lower drag is if you compare the SC9 in Trek's data against the P4, but that is clearly a totally invalid comparison, the only fair comparison is with the SC7 in Trek's data, as that had the same bars as the other bikes, and even the Transition beat it at 0 yaw.

This is correct. At low yaw, the P4 reigns king. Independent tests also seem to show the same result

2cm of spacer is less than 5g of drag.

I would only use the 'buy the one that fits best' when frame drag is otherwise equal, or you will need more spacers than the steerer tube is specd for

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

It's a lovely bike! Question - is there a way to increase the riser stack height and reach if you max it out on the current fit?

Thanks,

Kieran

Holy blast from the past Batman. I started from the first post, thinking this was the new P5... I got v confused!

There's a whole thread on Cervelo fit, over here.