Monday, February 7, 2011

Tour test of "aero" mass-start frames

Many years ago, Kestrel Bicycles came out with the "Talon" road frame which was targeted as spanning the bounds between triathlon and mass-start racing. There were no wind tunnel data, no scientific tests: it was simply designed to look aero, and who knew if it was or not? Kestrel, who was among the early groundbreakers in building and selling carbon frames, was sold by a Japanese group. However, the Talon survives, both with a standard model and a lighter a Talon SL. It got redesigned for 2009 (see video review). I've always found the bike attractive, in part because of its Santa Cruz roots.

Years later Cervelo, a long time leader in time trial frames, came out with their Soloist aluminum-frame bike. CSC showed the bike to be quite race-worthy at the highest level, notably with Bobby Julich winning the 2005 Paris-Nice. Then they developed the SLC, a carbon version, for that year's Tour de France. An SLC-SL followed in 2006, and for the 2008 Olympics, the S3 was released. It is still sold today.

Cervelo did do wind tunnel testing. They compared their SLC to their R3, the two sharing very similar geometries, and reported in a white paper, Col de la Tipping Point, that the SLC offered an advantage in coefficient of drag of 0.0045 m². This is out of a typical CdA for a rider and bike of around 0.31 m² (see later), so the advantage is only 1.5% in power, and that translates into a speed advantage of no more than 0.5%. See my analysis on the topic here.

Riders continued to show considerable success on Cervelo aerodynamic road frames. However, they certainly didn't win everything, and riders who switched from Cervelo didn't seem to do particularly worse when they switched to other bikes, typically designed for stiffness and weight but generally flaunting aerodynamics. You'd expect that from an only 1.5% difference in wind resistance power. Of course, every bit helps, but you can gain as much benefit from taping the front vents shut on your helmet, keeping your jersey zipped, pinning numbers carefully, etc.

But then the aero bike thing became more popular, at least among bike companies always looking for that extra edge to excite the huddled masses. With more competition, claim inflation began. For example, Felt produced the AR debuted for the 2008 Tour de France, but it was hard to find any photos of riders actually using it. Felt claimed initially it offered a 2% reduction in power versus a similar yet non-aero frame (one supposes the Felt F1). That's 2% of total power, so is closer to 2.2% of wind resistance power, considering rolling resistance. Felt has since redesigned the frame to stiffen it up, so at least it was getting occasional use last year, the last year Felt sponsored the Slipstream boys.

Also in 2008 Ridley debuted the aerodynamic Noah frame (see CyclingNews article). They claimed in tests that at 40 kph on a test track, Cadel Evans and Robbie McEwen averaged "12-15 watts" less on the Noah than on the control Ridley frame. Assuing a baseline CdA of 0.31 m², that's between 5% and 6% of total power.

So what's up? There's no way I believe the Ridley was three times more aerodynamic than the Cervelo or twice as aerodynamic as the Felt. Were they using the same wheels on the two bikes?

Litespeed, famous for its Ti frames (the light but whippy Ghisallo, the super-smooth but heavier Archon) came out with its Archon C1 last year, a road frame for which they put up some extremely impressive numbers (click to zoom):



In this case it was confirmed via an on-line forum the same wheels were used in each case.

20 watts saved?!? Granted, Litespeed has taken the revolutionary step of designing their frame around an assumed round waterbottle attached to the downtube, so that offers some advantage versus a frame which isn't optimized for the typical water bottle, but these number just seem huge.

Finally some independent data: Germany's Tour Magazine, famous for rating bike frames by how much they bend while ignoring how they actually ride, did a really nice wind tunnel study of the following:
  1. Kestrel Talon SL
  2. Cervelo S3
  3. Felt AR1 (nice design this year)
  4. Canyon Aeroad CF
  5. Stevens SLR
  6. Merida Reacto
  7. Kuota Kult
  8. Cannondale Super-Six
This last bike, the Cannondale, is the non-aero "control". Cannondale used to be called "Cannonwhale" because of it's fat-tubed frames. Such fat tubes, however, no longer look quite so fat because that design paradigm, started earlier by Gary Klein when he was building Al frames at MIT and copied by Cannondale, has become the norm.

No U.S.-based Litespeed here, but Tour is a German magazine, so prioritizes testing German bikes.

Tour tested the frames with a "dummy" rider in the wind tunnel. I think this is a good approach. Some argue the legs should be spinning as well, but assuming 50 kph speed and 90 rpm cadence, the legs have time to spin only 15 degrees in the time it takes relative wind to go from the bottom bracket to the rear hub. So it's not as if the legs are churning much during the time it takes wind to interact with the legs and frame: modeling the rider as stationary is probably just as good. And the advantage of a dummy is his position is perfectly predictable on a given frame. Of course, it's challenging to match position from one bike to another: I'll need to study the German in the article (Google Translate is my friend) to see how they dealt with this. For example, it's meaningless to say you're going to choose the same "size" of bike from different brands. Even ignoring the fact that bikes are only sold in a few sizes, so a match in any given parameter is unlikely, there's no widespread agreement on how to even measure a bike's size

But these concerns aside, here's Tour's front page from their web site:



Hopefully they'll offer the article for on-line purchase: it's hard to find in the US. Anyway, here's a plot of their main result: CdA for each of the tested bikes as a function of yaw angle.... drum roll....



Finally the fog starts to lift.

Look first at zero yaw. This means the relative wind is straight head-on. The Cannondale control bike actually does quite well: CdA = 0.312 for the Cannondale, 0.310 for the Felt, and 0.308 for the Kestrel. All of the other aero frames are actually slower, with the Stevens a total dog at 0.325. Shocking, really: the "aero" frames actually appear to have more drag than the 'whale.

Go to 5 degrees yaw, and the aero frames start to move ahead. The Cannondale has fallen back to be tied with the Merida at 0.325. The Canyon still trails the Cannondale with 0.328 while the Stevens is still the dog of the pack at 0.332 The Kuota now edges past the Cannondale, however, with 0.322. The Felt is a bit faster with 0.321. The Cervelo surges dramatically,, actually reducing its value from zero yaw, with 0.317. The leader remains the remarkable Talon, however, with 0.315.

So here we have the Kestrel 3.1% lower drag than the Cannondale, the Cervelo with 2.5% less drag, and the Felt with 1.1% less drag. These numbers are proportional and comparable to the early claims on the Cervelo and Felt of 1.5% and 2.2%, respectfully (assuming 10% rolling resistance power in the case of Felt's claim). Kuota now has an outside claim to the "aero" table, but none of the other frames can claim an advantage over the Cannondale yet.

Moving to 10 degree yaw, the aero frames start to really shine. The 'dale is now tied with sad Stevens for last @ 0.336. The Felt's surged ahead @ 0.322, then the Kestrel @ 0.324, then the Cervelo @ 0.325. The Merida, Kuota, and Canyon are all stuck in no-mans land between 0.331 to 0.332. Too bad, at 5 degrees the Kuota looked as if it might have been a contender.

The Felt leads with a 4.3% power advantange @ 10 degrees. The ancient Kestrel is surprisingly strong with 3.7%. Cervelo here shows a 3.4% power advantage versus the Cannondale. The rest of the pack is far behind: less than a 2% advantage for the Merida and Stevens and Canyon, with the Stevens still unable to pass the Cannondale.

Out @ 20 degree yaw, I'd be worrying about the cross-wind handling of the aero frames. But here is where the Felt and Cervelo really show the big power gains: 0.305 versus 0.330 for the 'dale (finishing ahead of 0.331 for the Canyon, amazingly). So if you want to claim big advantages for an aero frame, do it at 20 degrees yaw. But is 20 degree yaw realistic? I'll address this in a bit.

Overall, only the Cervelo, Felt, and Kestrel really score conclusively as "aero" frames. The others are all marginal or full-on faux. Filling in the seat tube and declaring aerodynamic advantage isn't doing much if you're leading off with a big fat head tube and down tube. Well, it may trigger the placebo effect, the rider convinced he's slicing through the wind faster than ever before. And maybe he is: placebo is among the most effective performance enhancers.

The rankings don't surprise me, really. The head tube of the Cervelo is a thing of beauty: slim and contoured. The Felt is also a pretty thing to behold. The Euro frames, though, subscribe to the "fat head tube, fat down tube" school of stiffness-über-alis, and since the head tube is the first thing seen by the wind, if you lose the head tube/down tube battle the war is lost. The seat tube isn't going to save you.

But what about the wind yaw number? Wind yaw was tested out to 20 degrees. A 20 degree wind yaw with a rider going 45 kph corresponds to a cross-wind of 16 kph. If weather stations are reporting winds this strong, that's substantial enough that handling from cross-winds becomes a concern such that I think I'd be more worried about dicey handling from broad frame tubes than I would about a few percentage difference in wind drag.

Even so, meteriological stations are at a standard height of 10 meters above the ground, and we care more about the 0.26 to 0.9 meter altitude where the frame lives. To translate wind speeds higher up to those closer to the ground we need to account for shear, and for that we can use the Hellman formula:

(v / v10) = (h / 10 meters)α,

where v10 is the wind speed at 10 meters, and v is the wind speed at the desired altitude. This model roughly applies up to a few hundred meters above the ground.

In the model, α describes how rapidly the wind speed drops off approaching the ground, and depends on the nature of the terrain. For "neutral air above human inhabited areas", which I think describes much bike racing, α = 0.34 is recommended. So if I want to evaluate the average of the square of the wind speed from 0.26 meters to 0.9 meter, I integrate (z / 10 meters)0.68 from 0.026 to 0.09 and divide by (0.09 ‒ 0.026), then take the square root, yielding 0.38. So the "effective" crosswind at the height is more like 38% of the wind at 10 meters altitude (assuming the wind is partially blocked by ground objects like trees or buildings, etc). This means that 16 kph wind, quite strong, is more like 6.0 kph, yielding a yaw angle of 7.6 degrees.

Here's a plot showing the yaw angle (x-axis) plotted versus height above the ground (y-axis) where I assume a 20 degree yaw angle at 10 meter elevation. The bike is shown to same height scale. You can see the yaw angle at the height of the bike frame is well below the 20 degree value calculated from meteriological wind speed, and indeed below the value associated with the rider's body.

yaw angle versus height


Sanity check on this wind speed: if I'm riding @ 45 kph on the flats, and then I suddenly need to deal with this sort of wind, to what will my speed drop at the same power? Well, first the wind resistance to be considered needs to include the human body (before I was focusing on the yaw angle at the bike, which on the road, unlike in a wind tunnel, will vary with height). The body is around 2/3 the total, so the "center of wind resistance" will be higher around a 1 meter altitude. Here the Hellman formula gives 7.3 km/hr of wind. Neglecting difference in rolling resistance, I solve to get my new speed at the same wind resistance power: v ( v + 7.3 )² = 45³, yielding v = 40.3 kph. So that's a fairly significant breeze, dropping my speed 5 kph, yet is enough to produce only an 8 degree yaw at 45 kph.

So I think the 20 degree yaw angle is way beyond typical conditions. Even the 10 degree yaw angle is on the high end (like riding next to open fields in a stiff breeze, or along an unobstructed coast road). 5 degree yaw is probably more applicable in a cross-wind, with a zero degree yaw often applicable as well. The important thing is to remember yaw at the bike isn't the same as yaw at the body or yaw up at a 10 meter reference altitude.

That old Cervelo number of 1.5%, or even the original Felt number of 2%, is looking a lot better with these assumptions.

Just yesterday the Tour of Qatar held a prologue time trial. To reduce travel costs, they require the riders to use their mass-start bikes in the race. Garmin-Cervelo was the only top team there with aerodynamic frames: the Cervelo S3. Even better, they had Jack Bobridge, who had just remarkably beaten Chris Boardman's 14+-year-old pursuit record. A super-lock, right? Except Bobridge, the top finisher for his team, could place only thirteenth, ten seconds down over the 2 km course. The race was won by Lars Boom on his very "non-aero" Giant frame.

So the message from the Tour test: it's all about the yaw angle. And in the real world, yaw angles tend to be fairly small. Maybe the Chung-on-a-stick will give us some decent real-world data of what the wind is like from the level of a bike frame.

So while aero bike frames may well make a difference of 1-3% in power, and that's a wonderful advantage to have, these claims of 10% or more of power claimed appear to be unjustified. Well, there are those Litespeed numbers.... we'll need to see what happens with those. Maybe VeloNews, which promises to do more analysis this year, will conduct a similar analysis, testing the LiteSpeed and hopefully the Scott F01 which is in pre-production.

The Tour test also looks at stiffness, in the Tour magazine tradition of immediately dismantling test bikes and bending them. Here the faux-aero bikes do better, in particular the Canyon, which is generally considered to be designed specifically to do well in the Tour tests which are important for German bike companies. The Talon, on the other hand, finishes last in the stiffness tests. Yet reviews generally rave about how smooth the ride is on the Talon. I think this is no coincidence: a bit of compliance, not just vertical, goes a long way to smoothing out the ride. But that's another topic.

So which do I like? Of the Tour test bikes, the Cervelo gets the nod for its combination of aerodynamics and race-proven ride quality. Next probably the Kestrel, although it's at least 100 grams heavier than the Cervelo. Third place to the Felt, which has exceptional aerodynamics and looks really good, but is heavier even than the Kestrel. But looking beyond the test, that Litespeed C1 is intriguing, especially since it's now available in black, which should take a bit of weight off (white paint is heavy). And the Scott F01 is still pre-production, but by using the Kamm tail design approach popularized by Trek, Scott claims to attain aerodynamic efficiency without the elongated cross-sections we've come to associate with aerodynamic frames like the Felt. It claims to come in under 900 grams, and Scott, with its Addict, has a proven record of producing frames in the sub-800 gram range (real weight, not a fairy-tail number like you see from Pinarello, for example). And HighRoad riders used it extensively in the Tour last year.

Neil Pryde also has an intriguing design, which Cycling Plus liked (see PDF) in their Feb 2011 article. However, Cycling Plus didn't evaluate aerodynamics, just perceived ride quality.

My vote, if one is willing to wait, goes with the Scott. Not that I'm in the market for a new bike, my engine presently much more neglected than my chassis. The way for me to get faster is to "ride bike".

15 comments:

Jim said...

The Cervelo tested was an S3 not an S1!

And I'm not sure the Tour of Qatar prologue is a very fair test - the average speeds were low and it was much more about cornering ability than aerodynamics - I don't think it was surprising that it was won by a guy used to riding hard technical courses on a road bike!

djconnel said...

Thanks! I'll fix that typo (S1).
So many companies have the "1" be the top-of-the-line frame, I sometimes forget Cervelo is different. Felt F1, AR1, Scott F01, but then Specialized SL3, so it goes both ways.

Last year there was a TTT: Sky finished ahead of Cervelo (later penalized) and Garmin. I'm not sure which bike Garmin was on, but I think it was the F1. In any case, Sky was on tubby Pinarellos.

In 2009 Garmin beat QuickStep in another TTT, with Cervelo far back. This time I'm sure Garmin wasn't on the AR: instead on the F. Quickstep was on Specialized. Katusha was third, on the Noah, which is nominally aerodynamic.

So there's simply no proof the aero frames offer huge advantage in the race results. That doesn't mean they don't help, but it does make one skeptical of the more extreme advantage claims.

Bryn said...

I would have liked to see water bottles in the test setup. I'm curious to know what having two full sized bottles on the bike does the aerodynamics. My guess would be that it would level the playing field across the board.

djconnel said...

In the WeightWeenies Forum, Dave, an engineer @ Felt, wrote:
The effect of adding bottles adds ~55-80 grams of drag depending on yaw using the conventional round bottle/cage on the AR frames.

They standardize to 50 kph, I believe. To go from "mass-equivalent of force" to actual force multiply by g. So combining these yields 7.5 to 10.9 watts of drag from the round bottle. That's up to 2% of total wind power (CdA = 0.31 goes to 500 watts at the same speed).

If it would add essentially zero drag to the Cannondale, you'd be right.

raymond said...

Can you post the article in it's original content? You opinion is biased and it can influence others. As all, Tour Magazine is a respectable and credible magazine. So please let me magazine do the talking!

djconnel said...

I can't do that, unfortunately, due to copyright. It's okay to quote copyrighted material for the purpose of commentary or analysis but to reproduce the entire article wouldn't be right.

raymond said...
This comment has been removed by the author.
djconnel said...

Here's their net watts saved:

http://img69.imageshack.us/img69/3674/tourmagazineaerobikewat.jpg

raymond said...

Hi again. To test a bike, fair and square, you have to put many things into consideration. In this test, they test many aero bikes and I believe there are more than one category to rate all these bikes. For example, comfort, stiffness, paint...etc. I was asking you what is the "final" ratings for all the bikes according to the magazine instead of just one category-aerodynamic. Do you mind sharing with us what is the final predict of the magazine?

djconnel said...

The Tour stiffness testing is another issue. Send me an email, however.

raymond said...

Dan, all I'm trying to say is this. You can't just release part of the test. You have to give to people raw and uncut. If you don't, you blog is just based on your personal opinion,which is non-credible. There is more then one aspect to make a great bicycle. The stiffest bike doesn't mean that is the best one. The most aero bike also doesn't mean that it's the best. You have to look at the whole package and that is what Tour Magazine does. Before you make your comments, ask yourself, have I ridden all the bikes long enough to judge which is more superior then the others from this review? Please be objective! And guess what, opinion is like an a**hole, everybody has one. I know for a fact that Canyon came out on top from this test and this is what you should tell people. The final ranking looks like this.
1.Canyon
2.Merida
3.Felt
4.Cervelo
5.Stevens
6.Kestrel
7.Kuota
This is what you should tell people. Don't hide the truth.

djconnel said...

Nowhere did I make any discussion of the overall rankings or of the bending measurements. I'm not trying to summarize the Tour article, nor do I claim to represent its conclusions. I'm not in any way critiquing Tour's results on the bending tests. My focus in this post was in the choice of yaw angle.

Kai said...

Hi Dan,

I just stumbled upon your blog. Pertaining to the TOUR article on aero bikes, you wrote,"Hopefully they'll offer the article for on-line purchase: it's hard to find in the US." Well, it took awhile, but it has happened.

TOUR-Quarterly is now available as an iPad app in English. As the name implies, it'll be coming out 4 times a year. Many technical articles, such as the aero article you mentioned, will be in the app.

The first T-QTR came out August 31st for Eurobike, it has the aero article. The second will be coming out Dec. 21st. As the guy translating everything, I'd like to get the word out. More infos are at: http://www.tour-qtr.com/

Lemme know if you should have any questions.

Greets

Kai

Phil B said...

Thanks for posting DJ. It looks like you might have transposed your summary of Felt and Kestrel data at 5 degrees. It also appears from the Tour article that they used the current Talon model, not the original - aging version. HTH.

djconnel said...

Whoops! Thanks for that important catch! I'll fix it....even more impressive by underdog Kestrel.